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Biology > Cellular Control > Flashcards

Cellular Control Flashcards

0
Q

Locus

A

Specific position on a chromosome, occupied by a specific gene

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1
Q

Allele

A

Alternate version of a gene

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2
Q

Phenotype

A

Observable characteristics of an organism

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3
Q

Genotype

A

Alleles present within cells of an individual, for a particular trait / characteristic

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4
Q

Dominant

A

Characteristic in which the allele responsible is expressed in the phenotype even in those with heterozygous genotypes

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5
Q

Codominant

A

A characteristic where both alleles contribute to the phenotype

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6
Q

Recessive

A

Characteristic in which the allele responsible is only expressed in the phenotype if there is no dominant allele present

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7
Q

Linkage

A

Genes for different characteristics that are present at different loci on the same chromosome are linked

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8
Q

Crossing Over

A

Where non-sister chromatids exchange alleles during prophase I of meiosis

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9
Q

Epistasis

A

The interaction of genes concerned with the expression of one characteristic. One gene may mask the expression of another gene

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10
Q

Gene

A

A length of DNA that codes for one (or more) polypeptides / proteins. Some genes code for RNA and regulate other genes.

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11
Q

Annealing

A

The term used to describe hydrogen-bond formation between complementary base pairs when sections of single-stranded DNA or RNA join together.

Annealing is seen when complementary sticky-ends join and where DNA probes attach to a complementary DNA section.

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12
Q

Apoptosis

A

Programmed cell death.

An orderly process by which cells die after they have undergone the maximum number of divisions

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13
Q

Chromosome mutation

A

Random change to the structure of a chromosome.

There are different types: - inversion, deletion, translocation, non-disjunction

The shuffling of alleles in prophase I is NOT an example of mutation

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14
Q

Inversion

A

A section of chromosome turns through 180 degrees

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15
Q

Deletion

A

A part of the chromosome is lost

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16
Q

Translocation

A

A piece of one chromosome becomes attached to another

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17
Q

Non-disjunction

A

Homologous chromosomes fail to separate properly at meiosis I or chromatids fail to separate at meiosis II.

If this happens to a whole set of chromosomes, polyploidy results

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18
Q

Cladistics

A

A method of classifying living organisms based on their evolutionary ancestry

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19
Q

Comparative genome mapping

A

The comparison of DNA sequences coding for the production of proteins / polypeptides and regulatory sequences in the genomes of different organisms of different species.

Comparisons include the search for sequences that make some species pathogenic whilst related organisms are not

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20
Q

Continuous variation

A

Genetic variation, also called quantitative variation, where there is a wide range of phenotypic variation within the population.

There are no distinct categories.

It is controlled by many genes.

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21
Q

Diploid

A

Having two sets of chromosomes (Eukaryotes and organisms)

Denoted by 2n

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22
Q

DNA Ligase

A

An enzyme capable of catalysing a condensation reaction between the phosphate group of one nucleotide and the sugar group of another.

This results in the DNA backbone molecules being joined together and is an essential part of recombinant DNA procedures

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23
Q

DNA Mutation

A

A change to DNA structure.

May be substitution of one base pair for another, inversion of a base triplet, deletion of a base pair or triplet of bases (on both strands), addition if a base pair or triplet of bases (on both strands),or a triple nucleotide repeat - a stutter

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24
Q

DRD4

A

Gene that codes for a dopamine receptor molecule

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25
Q

Exergonic

A

Chemical or biochemical reaction that releases energy

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26
Q

Gametes

A

Specialised sex cells

In many organisms the gametes are haploid and are produced by meio

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27
Q

Haploid

A

Eukaryotic cell or organism having only one set of chromosomes.

Denoted by n

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28
Q

Hardy-Weinberg Principle

A

The concept that both genotype frequencies and gene frequencies will stay constant from generation to generation, within a large interbreeding population where mating is random, there is no mutation and no selection or migration

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29
Q

Hayflick constant

A

The number of time that a normal body cell divides before undergoing apoptosis.

The number of divisions is about 50

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30
Q

Hemizygous

A

Cell or individual having only one allele for a particular gene

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31
Q

Heterozygous

A

Eukaryotic cell or organism that has two different alleles for a specific gene

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32
Q

Homeobox genes

A

Genes that control the development of the body plan of an organism

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33
Q

Homeotic selector genes

A

These direct development of individual body segments. They are master genes that control other regulatory genes

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34
Q

Homozygous

A

Eukaryotic cell or organism that has two identical alleles for a specific gene

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35
Q

Hox clusters

A

Groups of homeobox genes.

More complex organisms have more Hox clusters. This is probably due to a mutation that duplicated the Hox clusters

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36
Q

Hypostasis

A

Where two alleles interact to control the expression of one characteristic, one is epistatic and one is hypostatic.

Where a recessive allele at the first locus prevents the expression of another allele at a second locus, the alleles at the first locus are epistatic and the alleles at the second locus are hypostatic

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37
Q

Meiosis

A

Reductive division. The chromosome number is halved. It involves two divisions. It produces cells that are genetically different from each other and from the parent cell

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38
Q

Monogenic

A

Characteristic coded for by one gene

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39
Q

Monophyletic

A

A monophyletic group is one that includes an ancestral organism and all of its descendant species

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40
Q

Mutation

A

Structural change to genetic material - either to a gene or a chromosome

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41
Q

Necrosis

A

Disorderly, often accidental cell death

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42
Q

Operon

A

A unit consisting of genes that work together under the control of an operator gene

An example is the lac operon, which consists of two structural genes and an operator gene

Operon a were first found in prokaryotes but were later found in eukaryotes

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43
Q

Phylogenic group

A

Group of organisms that share evolutionary ancestry

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44
Q

Polygenic

A

Characteristic coded for by many genes.

Polygenic characteristics are more influenced by environmental factors than monogenic characteristics

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45
Q

Polyploid

A

Eukaryotic organisms or cells with more than two sets of chromosomes

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46
Q

Primer

A

Short, single stranded sequences of DNA, around 10 bases in length. They are needed, in sequences reaction and polymerase chain reactions, to bind to a section of DNA because the DNA polymerase enzymes cannot bind directly to single-stranded DNA fragments

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47
Q

Proto-oncogene

A

Gene that can undergo mutations to become an oncogene, which induces tumour formation (cancer)

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48
Q

Allele frequency

A

How common an allele is in a population

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49
Q

Gene pool

A

A measure the amount of variation of all the alleles in the population

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50
Q

Genome

A

Alleles within an individual

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51
Q

Genetic drift

A

Change in allele frequency

Can reduce genetic variation

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52
Q

Antigenic drift

A

Gradual change in antigen structure on a pathogen

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53
Q

Antigenic shift

A

Dramatic change in antigen structure

Can lead to new strains - Pandemic

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54
Q

Artificial selection

A

When humans choose the characteristic to breed for.

Only allow the individuals closest to the intended outcome to breed.

Repeat for a number of generations.

Reduces genetic variation. Leads to inbreeding, which reduces fitness (survivability + reproductive success) for most species

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55
Q

What characteristics can be bred for?

A
  • Discontinuous

- Continuous (Only ones where enough of the variation is genetic - heritability is high) rather than environmental

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56
Q

Natural selection

A

Competing selection

  • Selection pressures
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57
Q

Autosome

A

Non-sex linked chromosome

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58
Q

Prophase 1

A

Chromatin condenses and supercoils

Homologous pairs forms a bivalent

Crossing over occurs between non-sister chromatids

Nuclear envelope disintegrates

Nucleolus disappears

A spindle forms

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59
Q

Metaphase I

A

Bivalents line up across spindle equator

They are attached by the centromere to the spindle fibres

Bivalents are randomly assorted with each member of the homologous pair at opposite poles

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60
Q

Anaphase I

A

Homologous chromosomes in bivalents are pulled by the spindle fibres to opposite poles

Centromeres don’t divide

Chiasmata separate and lengths of chromatid that have crossed over remain with the chromatid

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61
Q

Telophase I

A

Two new nuclear envelopes form around each set of chromosomes at each pole

Cytokinesis occurs

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62
Q

Prophase II

A

Nuclear envelope reforms and breaks fown

Nuceolus disappears

Chromosomes condense

Spindles form

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63
Q

Metaphase II

A

Chromosomes move to the spindle equator

They attach to the spindle fibres by their centromeres

Chromatids of each chromosomes are randomly assorted

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64
Q

Anaphase II

A

Centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres

The chromatids randomly segregate

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65
Q

Telophase II

A

Nuclear envelopes reform around the haploid nuclei

The two cells then divide into four haploid cells in animals

In plants a tetrad of four haploid cells is formed

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66
Q

Epistasis: Normal dihybrid cross ratio?

A

9:3:3:1

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67
Q

Recessive epistatic cross ratio?

A

9:3:4

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68
Q

Dominant epistatic cross ratio?

A

12:3:1

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69
Q

What is null hypothesis?

A

There is no significant difference between the observed and the expected result

70
Q

What are the four possible bases in DNA?

A

Adenine, Thymine, Guanine, and Cytosine

71
Q

What are the possible bases for RNA?

A

Adenine, Uracil, Cytosine, and Guanine

72
Q

Which DNA bases pair together in complementary base pairing?

A

Adenine — Thymine

Cytosine — Guanine

73
Q

What determines the order of amino acids in a protein?

A

The order of bases in a gene

74
Q

What are three adjacent bases in mRNA called?

A

A codon

75
Q

Where in the cell is mRNA made?

A

The nucleus

76
Q

Describe the role of mRNA

A

mRNA carries the genetic code from the DNA in the nucleus to the cytoplasm, where it is used to make a protein during translation

77
Q

Describe the role of tRNA

A

tRNA carries the amino acids that are used to make proteins to the ribosomes during translation

78
Q

What are the two stages of protein synthesis called?

A

Transcription and translation

79
Q

Where does the first stage of protein synthesis (transcription) take place?

A

The nucleus

80
Q

Where does the second stage of protein synthesis (translation) take place?

A

At the ribosomes in the cytoplasm

81
Q

When does RNA polymerase stop making mRNA?

A

When it reaches a stop codon

82
Q

Explain why genetic code would not work if nucleotide bases were arranged in pairs, rather than triplets, to make the coding units

A

If the four nucleotide bases were arranged in pairs, the maximum number of different combinations would be 4^2, which is 16.

This is not enough to code for the 20 amino acids involved in protein synthesis, plus some stop codes

83
Q

What is meant by degenerate code?

A

Each amino acid has more than one triplet base code.

This helps reduce the effect of point mutations

84
Q

In which type of cell, prokaryote or eukaryote, would you expect protein synthesis to be faster?

A

Prokaryotes, as the mRNA does not have to exit a nucleus and the DNA in prokaryotes is naked - there are no histone proteins

85
Q

What is the minimum number of different tRNA molecules that are needed for protein synthesis?

A

21 - one for each of the 20 amino acids and a stop

86
Q

What is a mutation?

A

A change in the amount of, or arrangement of, the genetic material in a cell

87
Q

Explain how the degenerate nature of DNA code reduces the effect of point mutations

A

All amino acids, except methionine, have more than one triplet of bases coding for them.some have as many as six.

So some point mutations will not change the amino acid that is added to the polypeptide at that position

88
Q

Vitamin A is stored in the liver of all mammals. Suggest why pregnant women are advised not to eat liver.

A

It contains large amounts of vitamin A.

Retinoic acid is a derivative of vitamin A and too much retanoic acid, particularly in the early stages of pregnancy when the body plan is being organised, can interfere with the normal expression of homeobox genes and causes birth defects.

89
Q

Explain what is meant by a morphagen

A

A morphagen is a substance that governs the pattern of tissue development by activating homeobox genes

90
Q

What are transcription factors?

A

Transcription factors are some of the polypeptides expressed by homeobox genes, which bind to other genes further along the DNA from the homeobox genes, and switch them on.

They regulate the expression of other genes and so influence the development of the embryo

91
Q

Explain the role of maternal-effect genes

A

They determine the embryo’s polarity - which end is the head and which is the tail

92
Q

Explain the role of segmentation genes

A

They specify the polarity of each body segment

93
Q

Explain the effect of making the inner mitochondrial membrane more permeable to hydrogen ion

A

If the proton gradient cannot be built up across the inner membrane then there will be less chemiosmosis and less formation of ATP.

The cell will not be able to carry out active transport or bulk transport and cannot maintain its metabolism - particularly anabolic reactions such as protein synthesis

94
Q

Explain why apoptosis does not damage the nearby cells or tissues

A

Because the hydrolytic enzymes are enclosed in vesicles and these are ingested by phagocytes

95
Q

Suggest how ineffective T lymphocytes would differ from effective lymphocytes

A

Ineffective lymphocytes would have no receptors on their cell surface membranes and could not recognise antigens

96
Q

Suggest how harmful T lymphocytes would differ from effective lymphocytes

A

Harmful T lymphocytes would have receptor on their cell surface membranes, with shapes complementary to self antigens, causing autoimmunity - the immune system attacking your own tissues

97
Q

What is a bivalent?

A

A homologous pair of chromosomes, each consisting of two sister chromatids, paired up for meiosis

98
Q

What is the minimum number of genetically different gametes that can be theoretically be produced by a human?

A

2^23

99
Q

Explain why sexual reproduction has increased the diversity of living organisms on Earth

A

Because gamete production by meiosis and fertilisation increases diversity.

During meiosis there is crossing over, Independant assortment of chromosomes and chromatids.

Fertilisation combines genetic material from two unrelated organisms

100
Q

Which ratio suggests recessive epistasis?

A

9:3:4

101
Q

What ratios suggests dominant epistasis?

A

12:3:1 or 13:3

102
Q

What ratio suggests epistasis by complementary action?

A

9:7

103
Q

What factors can alter the amount of genetic variation within a population?

A

Size of population

Randomness of mating

Mutation

Competition

Selection

104
Q

Explain why the Hardy-Weinberg principle does not need to be used to calculate the frequency of codominant alleles

A

Because both alleles contribute to the phenotype, the genotype of all frequencies of all phenotypes are known

105
Q

What biotic factors limit population growth?

A

Predators

Diseases/Pathogens

Food availability

Mate availability

106
Q

What abiotic factors limit population growth?

A

Temperature

Water availability

Soil type

Habitat availability

Shelter

107
Q

Explain why inbreeding amongst animals, such as dogs or race horses, may be harmful

A

The characteristics selected as useful for humans may make the animal less adapted to its natural environment

108
Q

Explain why marriages between relatives may not be desirable

A

If there is a genetic defect in the family’s gene pool, the chances of two individuals, both carriers, mating and producing an affected individual are increased

109
Q

Explain why, in small populations, where marriage with outsiders is discouraged, there may be high incidences of genetic disorders

A

If one person in the original small group of settlers carried a recessive allele, it could be passed on to his offspring and grandchildren.

Cousins may marry so two carriers may mate. The frequency of this allele will increase within the population.

This is know as the founder affect

110
Q

Suggest why the phylogenetic species concept should be used to classify bacteria

A

Because bacteria do not reproduce sexually, the biological concept is not appropriate.

Their biochemistry, morphology, ecological niche, can all be studied and analysed

111
Q

How is cladistics different from taxonomic classification systems?

A

It focuses on evolution, rather than on similarities between species

It places great importance on using objective and quantitative analysis

It uses DNA and RNA sequencing

It uses computer programmes and the data obtained from nucleic acid sequencing to generate cladograms that represent the evolutionary tree of life

It makes no distinction between extinct and extant species and both may be included in cladograms

112
Q

Explain which classification system should be used to classify a newly discovered fossil organism

A

The phylogenetic (cladistic) system, as we can’t deduce anything about their sexual reproduction and ability to produce fertile offspring, but we can deduce information about morphology, genes, and ecological niche

113
Q

Explain the meaning of the term genetic code

A

The sequence of the bases on a gene is a code with instructions for the construction of proteins. It has a number of characteristics:

     It is a triplet code- three bases code of an amino acid

     It is a degenerate code- All amino acids bar one have more 
     than one code

     Some codes don’t code for amino acids but are ‘stop’ 
     codons- they indicate the end of the polypeptide chain

     It is widespread but not universal- Codons generally always 
     code for the same amino acid in every organism, but this is 
     not always the case.
114
Q

Describe the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide

A
  1. Free RNA nucleotides are activated, two extra phosphoryl groups are added to make ATP, GTP, CTP and UTP.
  2. The gene to be transcribed unwinds and unzips. To do this the length of DNA that makes up the gene dips into the nucleolus & the hydrogen bonds between the nucleotide bases break.
  3. Activated RNA nucleotides binds, using Hydrogen Bonds, with their complementary exposed bases on the template strand. This is catalysed by RNA polymerase
  4. The two extra phosphoryl are released, releasing energy for bonding two adjacent nucleotides The mRNA produced is complementary to the nucleotide base sequence on the template strand of DNA and therefore is a copy of the base sequence on the coding strand of DNA
  5. The mRNA is released from the DNA and passes out of the nucleus through a pore in the nuclear envelope to a ribosome
115
Q

Describe how the sequence of nucleotides within a gene is used to construct a polypeptide, including the roles of messenger RNA, transfer RNA and ribosomes

A
  1. A molecule of mRNA binds to a ribosome. Two codons are attached to the small subunit of the ribosome and exposed to the large subunit.

The first exposed mRNA codon is always AUG. Using ATP energy and an enzyme, a tRNA molecule with the amino acid methionine and the anticodon UAC forms hydrogen bonds with this codon

  1. A seconds tRNA molecule, bearing a different amino acid, binds to the second exposed condon with its complementary anticodon
  2. A peptide bonds forms between the two adjacent amino acids. This is catalysed by an enzyme in the small ribosomal sub unit
  3. The ribosome now moves along the mRNA reading the next codon. A third tRNA brings another amino acid and a peptide bonds forms between it and the dipeptide. The first tRNA leaves and is able to collect and bring another of its amino acids.
  4. The polypeptide chain grows until a stop codon is reached, for which there are no corresponding tRNAs and the polypeptide chain is complete
116
Q

Mutations cause changes to

A

The sequence of nucleotides in DNA molecules

117
Q

Explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions

A

Beneficial

The mutation changes the sequence of amino acids and therefore the phenotype, but this gives the organism an advantageous characteristic
E.g. Paler skin in more temperate climates absorbs more vitamin D

Neutral

It is a mutation in a non-coding region of the DNA

It is a silent mutation- although the base triplet has changed, it still codes for the same amino acid and so the protein is unchanged.

Harmful

The mutation changes the sequence of amino acids and therefore the phenotype, and the resulting characteristic is harmful

E.g. paler skin in a hotter climate burns more easily

118
Q

Cyclic AMP activates proteins by…

A

Altering their three-dimensional structure

119
Q

When lactose is absent in lac operon…

A
  1. The regulator gene is expressed and the repressor protein is synthesised. It has two binding sites. One binds to lactose and one that binds to the operator region
  2. In binding to the operator region, it covers part of the promoter region where RNA polymerase normally attaches
  3. RNA polymerase cannot bind to the promoter region so the structural genes cannot be transcribed into mRNA
  4. Without mRNA the genes cannot be translated and the enzymes cannot be synthesised
120
Q

When lactose is present in lac operon…

A
  1. Lactose binds to the other site on the repressor protein, causing the molecule to change shape. This prevents the other binding site from binding to the operator region. The repressor dissociates from the operator region
  2. The leaves the promoter region unblocked. RNA polymerase can now bind to it and initiate the transcription of mRNA.
  3. The operator- repressor- inducer system acts as a molecular switch. It allows synthesis of the structural genes
  4. As a result, the bacteria can now use the lactose permease enzyme to take up lactose from the medium into their cells. They can then convert it to glucose and galactose using the β-galactosidase enzyme. These sugars can then be used for respiration
121
Q

Explain genetic control of protein production in a prokaryote using the lac operon

A

E. coli grown in a culture medium with no lactose can be placed in a growth medium with lactose.

At first they cannot metabolise the lactose because they only have tiny amounts of the enzymes needed to catalyse the reaction.

A few minutes after the lactose is added, E. coli increases the rate of synthesis of these enzymes by about 1000 times, so lactose must trigger the production of them - it is the inducer.

122
Q

Explain that the genes that control development of body plans are similar in plants, animals and fungi, with reference to homeobox sequences

A

Homeotic genes are similar in plants, animals and fungi.

These genes control the development of body plans and are expressed in specific patterns and in particular stages of development depending on when they are activated.

The homeobox is a sequence of DNA that codes for a region of 60 amino acids, and the resulting protein is found in most, if not all, eukaryotes.

The region binds to DNA so that they can regulate transcription.

In animals the homeobox is common in genes concerned with the control of developmental events, such as segmentation, the establishment of the anterior-posterior axis and the activation of genes coding for body parts such as limbs

123
Q

Outline how apoptosis (programmed cell death) can act as a mechanism to change body plans.

A

Apoptosis is an integral part of plant and animal tissue development.

It is a series of biochemical events that leads to an orderly and tidy cell death, in contrast to cell necrosis, which leads to the release of harmful hydrolytic enzymes.

Apoptosis ensures that the rate of cells produced by mitosis is the same as the rate of cells dying, so the number of cells remains constant. Not enough apoptosis leads to cancer.

Apoptosis causes the digits (toes and fingers) to separate from each other during development.

  1. Enzymes break down the cell cytoskeleton
  2. The cytoplasm becomes dense with organelles tightly packed
  3. The cell surface membrane changes and blebs form
  4. The Chromatin condenses and the nuclear envelope breaks. DNA breaks into fragments
  5. The cell breaks down into vesicles that are taken up by phagocytosis. The cellular debris is disposed of so that it does not damage other cells or tissue.
124
Q

Describe the behaviour of chromosomes during prophase I

A
  1. The chromatin condenses and supercoils
  2. The chromosomes come together in their homologous pairs to form a bivalent.

Each member of the pair has the same genes at the same loci.

Each pair consists of one maternal and one paternal chromosome

  1. The non sister chromatids wrap around each other and attach at points called chiasmata
  2. They may cross over and swap sections of chromatids with each other
  3. The nucleolus disappears and the nuclear envelope breaks down
  4. A spindle forms
125
Q

Describe the behaviour of chromosomes during metaphase I

A
  1. Bivalents line up across the equator of the spindle, attached to spindle fibres at the centromeres
  2. The bivalents are arranged randomly (random assortment) with each member of the homologous pair facing opposite poles
126
Q

Describe the behaviour of chromosomes during anaphase I

A
  1. The homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles
  2. The centromeres do not divide
  3. The chiasmata separate and the lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached
127
Q

Describe the behaviour of chromosomes during telophase I

A

Telophase I
1. In most animal cells two new nuclear envelopes form- one around each set of chromosomes at each pole and the cell divides by cytokenesis.

There is a brief interphase and the chromosomes uncoil

  1. In most plant cells the cell goes straight from Anaphase I to Meiosis II
128
Q

Describe the behaviour of chromosomes during meiosis II

A

This occurs at right angles to Meiosis I

Prophase II
1. If a nuclear envelope has reformed, it breaks down again
2. The nucleolus disappears, chromosomes condense and
spindles form

Metaphase II
1.The chromosomes arrange themselves on the equator of
the spindle. They are attached to spindle fibres at the
centromeres

      2. The chromatids of each chromosome are randomly 
          assorted

Anaphase II
1. The centromeres divide and the chromatids are pulled to
opposite poles by the spindle fibres. The chromatids
randomly segregate

Telophase II
1. Nuclear envelopes reform around the haploid daughter
nuclei
2. In animals, the two cells now divide to give four daughter
cells
3. In plants, a tetrad of four haploid cells if formed

129
Q

Explain how meiosis and fertilisation can lead to variation through independent assortment

A

Meiosis:
Crossing over ‘shuffles’ alleles

      Random distribution and subsequent segregation of 
      maternal and paternal chromosomes in the homologous  
      pairs during meiosis I leads to genetic reassortment

      Random distribution and segregation of the chromatids at 
      meiosis II leads to genetic reassortment

      Random mutations

Fertilisation
Randomly combining two sets of chromosomes, one from
each of two genetically unrelated individuals

130
Q

Describe the interactions between loci (epistasis)

A

Epistasis is the interaction of different gene loci so that one gene locus makes or suppresses the expression of another gene locus.

Recessive Epistasis

      The homozygous presence of a recessive allele prevents 
      the expression of another allele at a second locus

E.g. flower colour in Salvia:
The alleles for purple (B) and pink (b) can only be expressed in the presence of the allele A. When the genotype is aa– the phenotype is white

Dominant Epistasis

      A dominant allele at one gene locus masks the expression      
      of alleles at the second gene locus

E.g. feather colour in poultry:
If the dominant allele A is present, the chickens will be white; even if the dominant allele of the second gene, B/b is present. The genotype must be aaB- for any colour to be expressed

131
Q

Describe the differences between continuous and discontinuous variation

A

Discontinuous variation describes qualitative differences between phenotypes - they fall into clearly distinguishable categories with no intermediates.
E.g. blood type is either A, B, AB or O

Continuous variation is quantitative differences between phenotypes - there is a wide range of variation within the population with no distinct catagories
E.g. height

132
Q

Explain the basis of continuous and discontinuous variation by reference to the number of genes which influence the variation

A

Discontinuous variation
Different alleles at a single gene locus have large effects on
the phenotype.
Different gene loci have different effects on the trait

Continuous variation
Different alleles at the same gene locus have small effects
Different gene loci have the same, often additive effect on
the trait
A large number of gene loci may have a combined effect on
the trait

133
Q

Explain that both genotype and environment contribute to phenotypic variation.

A

While an organism may have the genetic potential to achieve a certain characteristic, e.g. length of corn cob, the environment also has an influence. The corn cob may have the genetic potential to be 12cm long, but the plant may be short of water, light or certain minerals, meaning that the cob is shorter, as the environmental factors have limited the expression of the genes.

134
Q

Explain why variation is essential in selection

A

So that when the environment changes, there will be individuals that are better adapted, so they will survive and reproduce passing on the advantageous alleles to their offspring and allowing the species to continue

135
Q

Explain,with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection

A

In unchanging conditions, stabilising selection maintains existing adaptations and so maintains existing allele frequencies.

In changing conditions, directional selection alters allele frequencies.

A mutation may be disadvantageous in existing conditions, and so is removed in stabilising selection, but if the conditions change, the mutation might be advantageous and selected for, meaning that selection becomes an evolutionary force

136
Q

Explain how genetic drift can cause large changes in small populations

A

Genetic drift is a change in allele frequency that occurs by chance because only some of the organisms in each generation reproduce.

It is particularly noticeable when a small number of individuals are separated from the rest of the large population.

They form a small sample of the original population and so are unlikely to be representative of the large population’s gene pool.

Genetic drift will alter the allele frequency still further.

137
Q

Explain the role of isolating mechanism in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms

A

If two sub-populations are separated from each other, they will evolve differently as they have different selection pressures, so different alleles will be eliminated or increased within each sub population.

Eventually the sub populations will not be able to interbreed and so will be different species.

The sub populations may be split by various isolating mechanisms
Geographical barriers e.g. rivers or mountains

Seasonal barriers e.g. climate change throughout the year

Reproductive mechanisms e.g. their genitals, breeding seasons or courtship rituals may be different

138
Q

Explain the significance of the various concepts of the species, with reference to the biological species concept and the phylogenetic (cladistic/evolutionary) species concept

A

The biological species concept

A species is ‘a group of similar organisms that can interbreed and produce fertile offspring and it reproductively isolated from such other groups’

But

Not all organisms reproduce sexually
Members of the same species can look very different to each other Males can look different to females
Isolated populations may appear to be very different from each other

The phylogenetic species concept

A species is ‘a group of organisms that have similar morphology, physiology, embryology and behaviour, and occupy the same ecological niche’.

This classification shows the evolutionary relationships, or phylogeny. The phylogenetic linage is called a clade

139
Q

Compare and contrast natural selection and artificial selection

A

Natural selection

The organisms best adapted for their environment are more likely to survive and pass on the favourable characteristics to their offspring

Artificial selection

Humans select the organisms with the useful characteristics

Humans allow those with useful characteristics to breed and prevent the ones without the characteristics from breeding

Thus, humans have a significant impact on the evolution of these populations or species

140
Q

Describe how artificial selection has been used to produce the modern dairy cow

A

Each cow’s milk yield is measured and recorded

The progeny of bulls is tested to find out which bulls have produced daughters with high milk yields

Only a few good-quality bulls need to be kept are the semen from one bull can be used to artificially inseminate many cows

Some elite cows are given hormones so they produce many eggs

The eggs are fertilized in vitro and the embryos are implanted into surrogate mothers

These embryos could also be clones and divided into many more identical embryos

141
Q

Describe how artificial selection has been used to produce bread wheat

A

Wheat can undergo polyplody- the nuclei can contain more than one diploid set of chromosomes.

Modern bread wheat is hexaploid, having 42 chromosomes in the nucleus of each cell, meaning that the cells are bigger.

142
Q

Outline the differences between reproductive and non-reproductive cloning

A

Reproductive cloning is the production of offspring which are genetically identical to either the mother (nuclear transfer), or the other offspring (splitting embryos)

Non-reproductive cloning is the use of stem cells in order to generate replacement cells, tissues or organs which may be used to treat particular diseases or conditions of humans

143
Q

Describe the production of natural clones in plants using the example of vegetative propagation in elm trees

A

The English Elm is adapted to reproduce asexually following damage to the parent plant.

New growth in the form of basal sprouts appears within two months of the destruction of the main trunk

These suckers grow from meristem tissue in the trunk close to the ground where the least damage is likely to have occured

144
Q

Describe the production of artificial clones of plants from tissue culture

A

A small piece of tissue is taken from the plant to be cloned, usually from the shoot tip- this is called the explant

The explant is placed on a nutrient growth medium

Cells in the tissue divide but do not differentiate. Instead they form a mass of undifferentiated cells called a callus

After a few weeks, single callus cells can be removed from the mass and placed on a growing medium containing plant hormones that encourage shoot growth

After a further few weeks, the growing shoots are transferred onto a different growing medium that encourages root growth

The growing plants are then transferred to a greenhouse to be acclimatised and grown further before they are planted outside

145
Q

Discuss the advantages and disadvantages of plant cloning in agriculture

A

Advantages

Very many genetically identical plants can be produced from one original plant

Plants can be produced at any time of the year and air-freighted around the world

Callus can be genetically engineered

Disadvantages

Because all of the plants are genetically identical, they are all susceptible to a newly mutated pathogen or pest, or to changing environmental conditions

The process is labour intensive- it is more difficult to plant plantlets than to sow seed

146
Q

Describe how artificial clones of animals can be produced

A

Nuclear transfer

A nucleus from an adult differentiated cell is placed in an enucleated egg cell.

The egg then goes through the stages of development using the genetic information from the inserted nucleus

Splitting embryos

Cells from a developing embryo are separated out, with each one going on to produce a separate, genetically identical organism

147
Q

Discuss the advantages and disadvantages of cloning animals

A

Advantages

High value animals, e.g. cows giving a high milk yield, can be cloned in high numbers

Rare animals can be cloned to preserve the species

Genetically modified animals- e.g. sheep that produce pharmaceutical chemicals in their milk- can be quickly reproduced

Disadvantages

High value animals are not necessarily produced with animal welfare in mind. Some strains of meat producing chickens jave been developed that are unable to walk

As with plants, excessive genetic uniformity in a species makes it unlikely to be able to cope with, or adapt to, changes in the environment

It is still unclear whether animals cloned using the nuclear material of adult cells will remain healthy in the long term

148
Q

Explain why microorganisms are often used in biotechnological processes

A

Grow rapidly in favourable conditions, with a generation time of as little as 30 minutes

Often produce proteins or chemicals that are given out into the surrounding medium and can be harvested

Can be genetically engineered to produce specific products

Grow well are relatively low temperatures, much lower than those required in the chemical engineering of similar processes

Can be grown anywhere in the world and are not dependent on climate

Tend to generate products that are in a more pure form than those generated via chemical processes

Can often be grown using nutrient materials that would otherwise be useless or even toxic to humans

149
Q

Describe how enzymes can be immobilised

A

Adsorption
Enzyme molecules are mixed with the immobilising support
and bind to it due to a combination of hydrophobic
interactions and ionic links.

Covalent bonding
Enzyme molecules are covalently bonded to a support,
often by covalently linking enzymes together and to an
insoluble material using a cross-linking agent

Entrapment
Enzymes are trapped, for example in a gel bead or network
of cellulose fibres.
Substrate and product molecules can pass through the
material to the enzyme, but the enzyme cannot pass
through to the solution

Membrane separation
Enzymes are physically separated from the substrate
mixture by a partially permeable membrane. The substrate
and product molecules can pass through the membrane,
but the enzymes cannot

150
Q

Explain why immobilised enzymes are used in large-scale production

A

Enzymes can be recovered easily and used many times

The product is not contaminated by the enzyme

Protection by the immobilising material means the enzyme is more stable in changing temperatures or pH

Enzyme activity can be controlled more easily

151
Q

Compare and contrast the processes of continuous culture and batch culture

A

Batch Culture
Growth rate is slower because nutrient level declines with
time
Easy to set up and maintain
If contamination occurs, only one batch is lost
Less efficient- fermenter is not in operation all of the time
Very useful for processes involving the production of
secondary metabolites

Continuous Culture
Growth rate is higher as nutrients are continuously added
to the fermentation tank
Set up is more difficult, maintenance of required growing
conditions can be difficult to achieve
If contamination occurs, huge volumes of product may be
lost
More efficient- fermenter operates continuously
Very useful for processes involving the production of
primary metabolites

152
Q

Describe the differences between primary and secondary metabolites

A

Primary metabolites are substances produced by an organism as part of its normal growth.

The production of primary metabolites matches the growth in population of the organism

Secondary metabolites are substances produced by an organism that are not part of its normal growth.

The production of secondary metabolites usually begins after the main growth period of the organism and so does not match the growth in population of the organism

153
Q

Explain the importance of manipulating the growing conditions in a fermentation vessel in order to maximise the yield of product required

A

The growing conditions can be manipulated and controlled in order to ensure that the microorganism is growing in its optimum conditions, and so the yield can be maximised

Temperature
Too hot and enzymes will be denatured, too cold and growth
will be slowed

Type and addition of nutrient
This depends on whether the product is a primary or a
secondary metabolite

Oxygen concentration
Most organisms are grown under aerobic conditions so
there must be a sufficient supply of oxygen to prevent the
unwanted products of anaerobic respiration and a reduction
in growth rate

pH
Changes in pH can reduce the activity of enzymes and
therefore reduce growth rates

154
Q

Explain the importance of asepsis in the manipulation of microorganisms.

A

Asepsis is the absence of unwanted microorganisms which could:

    Compete with the culture microorganisms for nutrients and 
    space 
    Reduce the yield of useful products from the culture.   
    microorganisms 
    Cause spoilage of the product
    Produce toxic chemicals
    Destroy the culture microorganisms and their products
155
Q

Outline the steps involved in sequencing the genome of an organism

A

Genomes are mapped to identify which part of the genome that they come from. Information that is already known is used, such as the location of microsatellites

Samples of the genome are mechanically sheared into smaller sections of around 100,000 base pairs

These sections are places in separate Bacterial Artificial Chromosomes (BACs) and transferred to E. coli cells. As the cells are grown in culture, many copies of the sections are produced- referred to as Clone Libraries

Cells containing specific BACs are taking and cultured. The DNA is extracted from the cells and restriction enzymes used to cut it into smaller fragments. The use of different restriction enzymes on a number of samples gives different fragment types

The fragments are separated using electrophoresis

The many copies of the fragments are put in a reaction mixture containing DNA polymerase, free DNA nucleotides and primers, with some of the nucleotides containing a florescent marker.

The primer anneals to the 3’ end of the template strand, allowing DNA polymerase to attach

DNA polymerase adds free nucleotides

If a modified nucleotide is added, the polymerase enzyme is thrown off and the reaction stops on that template strand

As the reaction proceeds, many molecules of DNA are made. The fragments generally differ in size, as different numbers of nucleotides will have been added

As the strands run through a machine, a laser reads the colour sequence. The sequence of colours, and therefore the sequence of bases can then be displayed

156
Q

Outline how gene sequencing allows for genome-wide comparisons between individuals and between species

A

The identification of genes for proteins found in all or many living organisms gives clues to the relative importance of such genes to life

Comparing the DNA of different species shows evolutionary relationships

Modelling the effects of changing DNA can be carried out

Comparing genomes from pathogenic and similar but non-pathogenic organisms can be used to identify the genes or base-pair sequences that are more important in causing the disease, so more effective drugs can be developed

The DNA of individuals can be analysed to reveal the presence of alleles associated with particular diseases

157
Q

Define the term recombinant DNA

A

A section of DNA, often in the form of a plasmid, which is formed by joining DNA sections from two different sources

158
Q

Explain that genetic engineering involves the extraction of genes from one organism, or the manufacture of genes, in order to place them in another organism (often of a different species) such that the receiving organism expresses the gene product

A

The required gene is obtained

A copy of the gene is placed in a vector

The vector carries the gene to the recipient cell

The recipient expressed the gene through protein synthesis

159
Q

Describe how sections of DNA containing a desired gene can be extracted from a donor organism using restriction enzymes

A

A DNA probe can be used to locate the gene on DNA fragments and the gene can be cut from a DNA fragment using a restriction enzyme

Restriction enzymes cut through DNA at specific points, only where a specific base sequence occurs, normally about 10 base pairs long.

The enzyme catalyses a hydrolysis reaction which breaks the sugar phosphate backbone of the DNA at different places. The gives a staggered cut which leaving some exposed bases called sticky ends

160
Q

Outline how DNA fragments can be separated by size using electrophoresis

A

DNA samples are treated with restriction enzymes to cut them into fragments

The DNA samples are placed into cells cut into the negative electrode end of the gel

The gel is immersed in a tank of buffer solution and an electric current is passed through the solution for a fixed period of time, usually around two hours

DNA is negatively charged because of its phosphoryl groups. It is attracted to the positive electrode.

Shorter lengths of DNA move faster than longer lengths, so move further in the fixed time that current is passed through the gel

The position of the fragments can be shown by using a dye that stains DNA molecules

161
Q

Describe how DNA probes can be used to identify fragments containing specific sequences

A

A DNA probe is a short single-stranded section of DNA that is complementary to the section of DNA being investigated.

The probe is labelled in one of two ways:
Using a radioactive marker so that the location can be
revealed by exposure to photographic film
Using a fluorescent marker that emits a colour on exposure
to UV light

Copies of the probe are added to a sample of DNA fragments and will anneal to any fragment where a complementary base strand is present

162
Q

Outline how the polymerase chain reaction (PCR) can be used to make multiple copies of DNA fragments

A

The DNA sample is mixed with a supply of DNA nucleotides and DNA polymerase

The mixture is heated to 95°C. This breaks the hydrogen bonds holding the strands together, so the samples are now single stranded

Primers (short lengths of single stranded DNA) are added

The temperature is reduced to 55°C to allow the primers to bind and form small sections of doubl stranded sections

DNA polymerase can bind to these double-stranded sections

The temperature is raised to 72°C. The enzyme extends the double stranded section by adding nucleotides to the unwound DNA

When the DNA polymerase reaches the other end of the DNA, a new, double stranded DNA molecule is generated

The whole process can be repeated many times so the amount of DNA increase exponentially

163
Q

Explain how isolated DNA fragments can be placed in plasmids, with reference to the role of ligase

A

The plasmids and the fragments are both cut with the same restriction enzyme so they have complementary sticky ends

The base pairs anneal and DNA ligase joins together the phosphate sugar backbones to form recombinant DNA

164
Q

State other vectors into which fragments of DNA may be incorporated

A

Liposomes

Viral DNA e.g. bacteriophages

Hybrid vectors with the propertied of both plasmids and bacteriophages

165
Q

Explain how plasmids may be taken up by bacterial cells in order to produce a transgenic microorganism that can express a desired gene product

A

Large quantities of the plasmid are mixed with bacterial cells

Calcium salts are added, and the temperature of the culture is lowered to freezing before the being quickly raised to 40°C.

This increases the rate at which plasmids are taken up by bacterial cells to around 0.25%

166
Q

Describe the advantage to microorganisms of the capacity to take up plasmid DNA from the environment

A

Genetic variation

In the case of antibiotic resistance genes, survival in the presence of these chemicals

167
Q

Outline how genetic markers in plasmids can be used to identify the bacteria that have taken up a recombinant plasmid

A

Not all bacteria take up the plasmid.
Some of the bacteria take up a plasmid that has not seals
with a copy of the gene, but just sealed up on itself to reform
the original plasmid

A plasmid is used which carries genes which make any bacteria receiving them resistance to two different antibiotics. The resistance genes are known as genetic markers

The plasmids are cut by an enzyme which has its resistance site in the middle of one of the resistance genes (G1), so that if the required gene is taken up, the resistance gene for one of the antibiotics does not work, but it other (G2) does

The DNA is placed in the plasmids, and the plasmids in bacteria cells

The bacteria are grown on an agar plate to produce a colony

Some cells from the colonies are transferred onto agar that has been made from the antibiotic that remains intact, meaning that all bacteria that have taken up a plasmid will grow.

Some cells are transferred onto agar that has been made from the second antibiotic. Only the bacteria which have taken up a plasmid that is not recombinant will grow.

By keeping track of which colonies are which, we now know that any bacteria which grow on the agar containing the first antibiotic, but not on the agar containing the second antibiotic must have taken up the recombinant plasmid

The required colonies can now be identified and be grown on a large scale

168
Q

Outline the process involved in the genetic engineering of bacteria to produce human insulin

A
  1. mRNA from human insulin is extracted from pancreas cells
  2. Reverse transcriptase uses mRNA as a template to make single stranded cDNA, and this is made double stranded by DNA polymerase
  3. A single sequence of nucleotides (GGG) is added to each end of the DNA to make sticky ends
  4. Plasmids are cut open when a restriction enzyme
  5. Cut plasmids have a single sequence of nucleotides (CCC) asses to each to make sticky ends
  6. Plasmids and the insulin gene are mixed so that sticky ends form base pairs
  7. DNA ligase links sugar-phosphate backbones of plasmid and insulin gene
  8. Plasmids are mixed with bacteria in the presence of calcium ion
  9. Bacteria take up plasmids and multiply to form a clone
  10. Genetically engineered bacteria transcribe and translate the human gene to make insulin
169
Q

Outline the process involved in the genetic engineering of ‘Golden Rice

A

Two genes from the Daffodil and one from the bacterium Erwina urefovora were inserted into TI plasmids and taken up by the bacterium Agrobacterium tumefaciens.

This introduced the genes into rice embryos

The resulting rice plants produced seeds with β-carotene in the endosperm, which is yellow.

Vitamin A is produced in our bodies from β-carotene

170
Q

Outline how animals can be genetically engineered for xenotransplantation

A

Pigs have been engineered to lack the enzyme α-1,3-transferase, which is a key trigger for rejection of organs in humans

The human nucleotidase enzyme has been grafted into pig cells in culture.

It reduces the number of immune cell activities involved in xenotransplant rejection

171
Q

Explain the term gene therapy

A

Any therapeutic technique where the functioning allele of a particular gene is placed in the cells of an individual lacking the functioning alleles of that particular gene.

Can be used to treat some recessive conditions, but not dominant conditions

172
Q

Explain the differences between somatic cell gene therapy and germ line cell gene therapy

A

Somatic cell gene therapy

The functioning allele of the gene is introduced into target cells - ∴ techniques are needed to get the gene to the target location, or the specific cells must be removed, treated and then replaced

Introduction into somatic cells means that any treatment is short-lived and has to be repeated regularly. The specialised cells containing the gene will not divide to pass on the allele

There are difficulties in getting the allele into the genome in a functioning state. Genetically modified viruses have been tried, but the host becomes immune to them so cells will not accept the vector on second and subsequent treatments. Liposomes are used but these may be inefficient

Genetic manipulations are restricted to the actual patient

Germline cell gene therapy

The functioning allele of the gene is introduced into germline cells- delivery techniques are more straightforward

Introduction into germline cells means that all cells derived from the germline cells will contain a copy of the functioning allele. The offspring may also contain the allele

Although more straightforward, it is considered unethical to engineer human embryos. It is not possible to know whether the allele has been successfully introduced without any unintentional changes to it which may damage the embryo

Genetic manipulations could be passed on to the patient’s children

173
Q

Discuss the ethical concerns raised by the genetic manipulation of animals (including humans), plants and microorganisms

A

Religious objections

Objections to tampering with an organism’s natural genotype

Fears of unforeseen effects of the gene

Fears of the consequences of escape into the wild

Growing GM plants might damage the environment

Eating GM plants might be bad for health

Biology (13 decks)

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