3.4 Genetic information, variation and relationships between organisms

Question 1

Compare and contrast DNA in eukaryotic cells with DNA in prokaryotic cells

Answer 1

Similarities:
- Nucleotide structure is identical - deoxyribose attached to phosphate and a base
- Adjacent nucleotides joined by phosphodiester bonds, complementary bases joined by hydrogen bonds
- DNA in mitochondria / chloroplasts have similar structure to DNA in prokaryotes (Short, circular, not associated with proteins)

Differences:
- Eukaryotic DNA is longer
- Eukaryotic DNA is linear, prokaryotic DNA is circular
- Eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not
- Eukaryotic DNA contain introns, prokaryotic DNA does not

Question 2

What is a chromosome?

Answer 2

• Long, linear DNA + its associated histone proteins
• In the nucleus of eukaryotic cells

Question 3

What is a gene?

Answer 3

A sequence of DNA (nucleotide) bases that codes for:
• The amino acid sequence of a polypeptide
• Or a functional RNA (eg. ribosomal RNA or tRNA)

Question 4

What is a locus?

Answer 4

Fixed position a gene occupies on a particular DNA molecule.

Question 5

Describe the nature of the genetic code

Answer 5

Triplet code - A sequence of 3 DNA bases, called a triplet, codes for a specific amino acid
Universal - The same base triplets code for the same amino acids in all organisms
Non-overlapping - Each base is part of only one triplet so each triplet is read as a discrete unit
Degenerate - An amino acid can be coded for by more than one base triplet

Question 6

What are ‘non-coding base sequences’ and where are they found?

Answer 6

Non-coding base sequence - DNA that does not code for amino acid sequences / polypeptides:
1. Between genes - eg. non-coding multiple repeats
2. Within genes - introns

In eukaryotes, much of the nuclear DNA does not code for polypeptides.

Question 7

What are introns and exons?

Answer 7

Exon - Base sequence of a gene coding for amino acid sequences (in a polypeptide)

Intron - Base sequence of a gene that doesn’t code for amino acids, in eukaryotic cells

Question 8

Define ‘genome’ and ‘proteome’

Answer 8

Genome - The complete set of genes in a cell (including those in mitochondria and /or chloroplasts)

Proteome - The full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)

Question 9

Describe the two stages of protein synthesis

Answer 9

Transcription - Production of messenger RNA (mRNA) from DNA, in the nucleus

Translation - Production of polypeptides from the sequence of codons carried by mRNA, at ribosomes

Question 10

Compare and contrast the structure of tRNA and mRNA

Answer 10

Comparison (similarities)
- Both single polynucleotide strand

Contrast (differences)
- tRNA is folded into a ‘clover leaf shape’, whereas mRNA is linear / straight
- tRNA has hydrogen bonds between paired bases, mRNA doesn’t
- tRNA is a shorter, fixed length, whereas mRNA is a longer, variable length (more nucleotides)
- tRNA has an anticodon, mRNA has codons
- tRNA has an amino acid binding site, mRNA doesn’t

Question 11

Describe how mRNA is formed by transcription in eukaryotic cells

Answer 11

1. Hydrogen bonds between DNA bases break
2. Only one DNA strand acts as a template
3. Free RNA nucleotides align next to their complementary bases on the template strand
   • In RNA, uracil is used in place of thymine (pairing with adenine in DNA)
4. RNA polymerase joins adjacent RNA nucleotides
5. This forms phosphodiester bonds via condensation reactions
6. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

Question 12

Describe how production of messenger RNA (mRNA) in a eukaryotic cell is different from the production of mRNA in a prokaryotic cell

Answer 12

• Pre-mRNA produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells
• Because genes in prokaryotic cells don’t contain introns so no splicing in prokaryotic cells

Question 13

Describe how translation leads to the production of a polypeptide

Answer 13

1. mRNA attaches to a ribosome and the ribosome moves to a start codon (AUG)
2. tRNA brings a specific amino acid
3. tRNA anticodon binds to complementary mRNA codon
4. Ribosome moves along to next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond
   - Using energy from hydrolysis of ATP
5. tRNA released after amino acid joined polypeptide
6. Ribosome moves along mRNA to form the polypeptide, until a stop codon is reached

Question 14

Describe the role of ATP, tRNA and ribosomes in translation

Answer 14

ATP
• Hydrolysis of ATP to ADP + Pi releases energy
• So amino acids join to tRNAs and peptide bonds form between amino acids

tRNA
• Attaches to / transports a specific amino acid, in relation to its anticodon
• tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds
• 2 tRNAs bring amino acids together so peptide bond can form

Ribosomes
• mRNA binds to ribosome, with space for 2 codons
• Allows tRNA with anticodons to bind
• Catalyses formation of peptide bond between amino acids (held by tRNA molecules)
• Moves along (mRNA to the next codon) / translocation

Question 15

Describe how the base sequence of nucleic acids can be related to the amino acid sequence of polypeptides when provided with suitable data

Answer 15

You may be provided with a genetic code to identify which triplets / codons produce which amino acids

• tRNA anticodons are complementary to mRNA codons
- Eg. mRNA codon = ACG → tRNA anticodon = UGC
• Sequence of codons on mRNA are complementary to sequence of triplets on DNA template strand
- Eg. mRNA base sequence = ACG UAG AAC
→ DNA base sequence = TGC ATC TTG
• In RNA, uracil replaces thymine

Question 16

What is a gene mutation?

Answer 16

• A change in the base sequence of DNA (on chromosomes)
• Can arise spontaneously during DNA replication (interphase)

Question 17

What is a mutagenic agent?

Answer 17

A factor that increases rate of gene mutation, eg. ultraviolet (UV) light or alpha particles.

Question 18

Explain how a mutation can lead to the production of a non-functional protein or enzyme

Answer 18

1. Changes sequence of base triplets in DNA (in a gene) so changes sequence of codons on mRNA
2. So changes sequence of amino acids in the polypeptide
3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
4. So changes protein tertiary structure (shape) of protein
5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form

Question 19

Explain the possible effects of a substitution mutation

Answer 19

1. Base / nucleotide in DNA replaced by a different base / nucleotide
2. This changes one triplet so changes one mRNA codon
3. So one amino acid in polypeptide changes
   - Tertiary structure may change if position of hydrogen / ionic / disulphide bonds change
   OR amino acid doesn’t change
   - Due to degenerate nature of genetic code (triplet could code for same amino acid) OR if mutation is in an intron

Question 20

Explain the possible effects of a deletion mutation

Answer 20

1. One nucleotide / base removed from DNA sequence
2. Changes sequence of DNA triplets from point of mutation (frameshift)
3. Changes sequence of mRNA codons after point of mutation
4. Changes sequence of amino acids in primary structure of polypeptide
5. Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein
6. Changes tertiary structure / shape of protein

Question 21

Describe features of homologous chromosomes

Answer 21

Same length, same genes at same loci, but may have different alleles.

Question 22

Describe the difference between diploid and haploid cells

Answer 22

• Diploid - has 2 complete sets of chromosomes, represented as 2n
• Haploid - has a single set of unpaired chromosomes, represented as n

Question 23

Describe how a cell divides by meiosis

Answer 23

In interphase, DNA replicates → 2 copies of each chromosome (sister chromatids), joined by a centromere.

1. Meiosis I (first nuclear division) separates homologous chromosomes
   - Chromosomes arrange into homologous pairs
   - Crossing over between homologous chromosomes
   - Independent segregation of homologous chromosomes
2. Meiosis II (second nuclear division) separates chromatids

• Outcome = 4 genetically varied daughter cells
• Daughter cells are normally haploid (if diploid parent cell)

Question 24

Explain why the number of chromosomes is halved during meiosis

Answer 24

Homologous chromosomes are separated during meiosis I (first division).

Question 25

Explain how crossing over creates genetic variation

Answer 25

- Homologous pairs of chromosomes associate / form a bivalent - Chiasmata form (point of contact between (non-sister) chromatids) - Alleles / (equal) lengths of (non-sister) chromatids exchanged between chromosomes - Creating new combinations of (maternal & paternal) alleles on chromosomes

Question 26

Explain how independent segregation creates genetic variation

Answer 26

• Homologous pairs randomly align at equator → so random which chromosome from each pair goes into each daughter cell • Creating different combinations of maternal & paternal chromosomes / alleles in daughter cells

Question 27

Other than mutation and meiosis, explain how genetic variation within a species is increased

Answer 27

• Random fertilisation / fusion of gametes • Creating new allele combinations / new maternal and paternal chromosome combinations

Question 28

Explain the different outcomes of mitosis and meiosis

Answer 28

1. Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells • As 1 division in mitosis, whereas 2 divisions in meiosis 2. Mitosis maintains the chromosome number (eg. diploid → diploid or haploid → haploid) whereas meiosis halves the chromosome number (eg. diploid - haploid) • As homologous chromosomes separate in meiosis but not mitosis 3. Mitosis produces genetically identical daughter cells, whereas meiosis produces genetically varied daughter cells • As crossing over and independent segregation happen in meiosis but not mitosis

Question 29

Explain the importance of meiosis

Answer 29

• Two divisions creates haploid gametes (halves number of chromosomes) • So diploid number is restored at fertilisation → chromosome number maintained between generations • Independent segregation and crossing over creates genetic variation

Question 30

How can you recognise where meiosis and mitosis occur in a life cycle?

Answer 30

• Mitosis occurs between stages where chromosome number is maintained (eg. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n)) • Meiosis occurs between stages where chromosome number halves (eg. diploid (2n) - haploid (n))

Question 31

Describe how mutations in the number of chromosomes arise

Answer 31

- Spontaneously by chromosome non-disjunction during meiosis - Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis - So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)

Question 32

Suggest how the number of possible combinations of chromosomes in daughter cells following meiosis can be calculated

Answer 32

2^n where n = number of pairs of homologous chromosomes (half the diploid number)

Question 33

Suggest how the number of possible combinations of chromosomes following random fertilisation of two gametes can be calculated

Answer 33

(2^n)^2 where n = number of pairs of homologous chromosomes (half the diploid number)

Question 34

What is genetic diversity?

Answer 34

Number of different alleles of genes in a population

Question 35

What are alleles and how do they arise?

Answer 35

• Variations of a particular gene (same locus) → different DNA base sequence • Arise by mutation

Question 36

What is a population?

Answer 36

A group of interbreeding individuals of the same species.

Question 37

Explain the importance of genetic diversity

Answer 37

• Enables natural selection to occur • As in certain environments, a new allele of a gene might benefit its possessor • By resulting in a change in the polypeptide (protein) coded for that positively changes its properties • Giving possessor a selective advantage (increased chances of survival and reproductive success)

Question 38

What is evolution?

Answer 38

- Change in allele frequency (how common an allele is) over many generations in a population - Occurring through the process of natural selection Adaptation and selection are major factors in evolution and contribute to the diversity of living organisms.

Question 39

Explain the principles of natural selection in the evolution of populations

Answer 39

1. Mutation - Random gene mutations can result in [named] new alleles of a gene 2. Advantage - In certain [named] environments, the new allele might benefit its possessor [explain why] → organism has a selective advantage 3. Reproductive success - Possessors are more likely to survive and have increased reproductive success 4. Inheritance - Advantageous allele is inherited by members of the next generation (offspring) 5. Allele frequency - Over many generations, [named] allele increases in frequency in the population Natural selection results in species that are better adapted to their environment. You should be able to use unfamiliar information to explain how selection produces changes within a population of a species and interpret data relating to the effect of selection in producing change within populations.

Question 40

Describe 3 types of adaptations

Answer 40

• Anatomical - structural / physical features that increase chance of survival • Physiological - processes / chemical reactions that increase chance of survival • Behavioural - ways in which an organism acts that increase chance of survival

Question 41

Explain two types of selection, with examples

Answer 41

Directional selection Example: - Antibiotic resistance in bacteria Key feature - who has a selective advantage? - Organisms with an extreme variation of a trait e.g. bacteria with high level of resistance to a particular antibiotic Change in environment? - Yes, usually eg. antibiotic introduced Effect on population over many generations: • Increased frequency of organisms with / alleles for extreme trait • Normal distribution curve shifts towards extreme trait Stabilising selection Example: - Human birth weight Key feature - who has a selective advantage? - Organisms with an average / modal variation of a trait eg. babies with an average weight Change in environment? - No, usually stable Effect on population over many generations: • Increased frequency of organisms with / alleles for average trait • Normal distribution curve similar, less variation around the mean

Question 42

What is a species?

Answer 42

A group of organisms that can (interbreed to) produce fertile offspring

Question 43

Suggest why 2 different species are unable to produce fertile offspring

Answer 43

• Different species have different chromosome numbers → offspring may have odd chromosome number • So homologous pairs cannot form → meiosis cannot occur to produce gametes

Question 44

Explain why courtship behaviour is a necessary precursor to successful mating

Answer 44

• Allows recognition of members of same species → so fertile offspring produced • Allows recognition / attraction of opposite sex • Stimulates / synchronises mating / production / release of gametes • Indicates sexual maturity / fertility • Establishes a pair bond to raise young

Question 45

Describe a phylogenetic classification system

Answer 45

• Species (attempted to be) arranged into groups, called taxa, based on their evolutionary origins (common ancestors) and relationships • Uses a hierarchy: - Smaller groups are placed within larger groups - No overlap between groups

Question 46

Name the taxa in the hierarchy of classification

Answer 46

Domain (largest / broadest) → kingdom → phylum → class → order → family → genus → species (smallest)

Question 47

How is each species universally identified?

Answer 47

A binomial consisting of the name of its genus and species, e.g. Homo sapiens

Question 48

Suggest an advantage of binomial naming

Answer 48

Universal so no confusion as many organisms have more than one common name.

Question 49

How can phylogenetic trees be interpreted?

Answer 49

• Branch point = common ancestor • Branch = evolutionary path • If two species have a more recent common ancestor, they are more closely related

Question 50

Describe two advances that have helped to clarity evolutionary relationships between organisms

Answer 50

1. Advances in genome sequencing → allowing comparison of DNA base sequences • More differences in DNA base sequences → more distantly related / earlier common ancestor • As mutations (change in DNA base sequences) build up over time 2. Advances in immunology → allowing comparison of protein tertiary structure (eg. albumin) • Higher amount of protein from one species binds to antibody against the same protein from another species → more closely related / more recent common ancestor • As indicates a similar amino acid sequence and tertiary structure • So less time for mutations to build up

Question 51

What is biodiversity?

Answer 51

• Variety of living organisms (species, genetic and ecosystem diversity) • Can relate to a range of habitats, from a small local habitat to the Earth

Question 52

What is a community?

Answer 52

All populations of different species that live in an area.

Question 53

What is species richness?

Answer 53

A measure of the number of different species in a community

Question 54

What does an index of diversity do?

Answer 54

Describes the relationship between: 1. The number of species in a community (species richness) 2. The number of individuals in each species (population size)

Question 55

Suggest why index of diversity is more useful than species richness

Answer 55

- Also takes into account number of individuals in each species - So takes into account that some species may be present in small or high numbers

Question 56

What is the formula for index of diversity?

Answer 56

d = N(N-1)/Σn(n-1) N = total number of organisms of all species n = total number of organisms of each species

Question 57

Describe how index of diversity values can be interpreted

Answer 57

• High → many species present (high species richness) and species evenly represented • Low → habitat dominated by one / a few species

Question 58

Explain how some farming techniques reduce biodiversity

Answer 58

• Removal of woodland and hedgerows • Monoculture (growing one type of crop) • Use of herbicides to kill weeds = Reduces variety of plants = So fewer habitats and niches = And less variety of food sources • Use of pesticides to kill pests = Predator population of pest decreases

Question 59

Explain the balance between conservation and farming

Answer 59

• Conservation required to increase biodiversity • But when implemented on farms, yields can be reduced, reducing profit / income for farmers - Eg. by reducing land area for crop growth, increasing competition, increasing pest population • To offset loss, financial incentives / grants are offered

Question 60

Give examples of how biodiversity can be increased in areas of agriculture

Answer 60

• Reintroduction of field margins and hedgerows (where farmers only grow one type of crop) • Reduce use of pesticides • Growing different crops in the same area (intercropping) • Using crop rotation of nitrogen fixing crops instead of fertilisers

Question 61

How can genetic diversity within or between species be measured?

Answer 61

• Comparing frequency of measurable or observable characteristics • Comparing base sequence of DNA • Comparing base sequence of mRNA • Comparing amino acid sequence of a specific protein encoded by DNA and mRNA

Question 62

Explain how comparing DNA, mRNA and amino acid sequences can indicate relationships between organisms within a species and between species

Answer 62

• More differences in sequences → more distantly related / earlier common ancestor • As mutations (change in DNA base sequences) build up over time • More mutations cause more changes in amino acid sequences

Question 63

Explain the change in methods of investigating genetic diversity over time

Answer 63

• Early estimates made by inferring DNA differences from measurable or observable characteristics - Many coded for by more than one gene → difficult to distinguish one from another - Many influenced by environment → differences due to environment not genes • Gene technologies allowed this to be replaced by direct investigation of DNA sequences

Question 64

Explain the key considerations in quantitative investigations of variation within a species

Answer 64

• Collect data from random samples (use a random number generator) → removes bias • Use large sample size (or sample until stable running mean) - representative of whole population • Ethical sampling (where applicable) → must not harm organism / allow release unchanged • Calculate a mean value of collected data and standard deviation of that mean • Interpret mean values and their standard deviations (S.D) - S.D shows spread of values about the mean → higher S.D = higher variation - If standard deviations overlap, causing values of two sets of data to be shared, any difference between the two may be due to chance / not significant • Use [named] statistical test → analyse whether there is a significant difference between populations