Biology Genetics

Question 1

State Mendel’s 1st law

Answer 1

The law of segregation - The characteristics of an organism are determined by factors (genes) which occur in pairs. Only one member of a pair of factors (genes) can be represented in a single gamete

Question 2

State Mendel’s Second Law

Answer 2

The Law of Independent Assortment. Each of a pair of contrasted characters may be combined with either of another pair.

Question 3

When do most mutations occur in meiosis

Answer 3

Most mutations occur during crossing over in prophase-I and non-disjunction in anaphase-I and anaphase-II.

Question 4

What do gene (point) mutations affect

Answer 4

Single bases in a gene

Question 5

How can a mutation cause cancer

Answer 5

Some genes called proto-oncogenes can mutate to become oncogenes which are involved causing uncontrolled cell division to form a cancer.

Question 6

Give an example of a Gene point mutation

Answer 6

Sickle cell anaemia • One mutated base in the DNA code for haemoglobin. • So incorrect amino acid added into a polypeptide chain. • Red blood cell forms a sickle shape and is less efficient at carrying oxygen

Question 7

Give an example of a Chromosomal mutation

Answer 7

Down’s syndrome Failure of chromosome 21 to separate from its homologous pair (nondisjunction) during anaphase I in meiosis forms gametes with 2 copies and at fertilisation the zygote then has 3 copies of chromosome 21.

Question 8

Define epigenetics

Answer 8

Epigenetics refers to the control of gene expression by factors other than changes in the DNA sequence.

Question 9

Epigenetic modifications - DNA methylation

Answer 9

The addition of methyl groups to bases prevents those bases being recognised and hence transcripted and reduces the ability of that gene to be expressed.

Question 10

Epigenetic modifications - Histone modification and acetylation

Answer 10

Addition of acetyl groups to histones means the chromatin is coiled more loosely, encouraging gene expression, allowing transcription and protein synthesis.

Question 11

What are selective agencies

Answer 11

Environmental factors that can alter the frequency of alleles in a population, when they are limiting. Selective agencies increase the chances of some phenotypes and therefore some alleles being
passed on to the next generation.

Question 12

Give 3 examples of selective agencies

Answer 12

Supply of food • Breeding sites • Climate

Question 13

How does selection pressure work in natural selection

Answer 13

If a phenotype gives a selective advantage, the alleles responsible for that phenotype will be selected for and it is more likely that they will be passed on to the next generation.

Question 14

What 5 conditions are required for The Hardy-Weinberg principle (frequency of different alleles within a
population will remain constant from one generation to the next)

Answer 14

• the population is large, • there is no selection pressure • Mating is random. • No mutations occur. • No emigration or immigration

Question 15

Define genetic drift

Answer 15

A change in allele frequency because of chance, more likely in small populations

Question 16

Define speciation

Answer 16

Allele changes can significantly alter the phenotype of the isolated population so that they can no longer breed with the original population to give fertile young. A new species has been formed.

Question 17

Define the founder effect

Answer 17

When a small population has become separated from the original population and so an allele in the small population becomes more frequent in the succeeding generations.

Question 18

Define demes

Answer 18

A sub-group of a population that interbreed more frequently, reducing gene f low with the rest of the population.

Question 19

In the Hardy-Weinburg equation, what does p stand for compared to p^2

Answer 19

p is the frequency of the dominant allele, p^2 is the frequency of homozygous dominant

Question 20

3 reasons speciation can occur

Answer 20

genetic drift in isolated populations, the founder effect of disproportionate allele frequencies in small populations, natural selection

Question 21

What is the isolating mechanism of allopatric speciation

Answer 21

Geographical isolation - geographically separating demes e.g. species isolated by mountain ranges, deserts, oceans, rivers

Question 22

What is the isolating mechanism of sympatric speciation

Answer 22

Reproductive isolation e.g differences in courtship behaviour (behavioural isolation), breeding seasons are not compatible (seasonal isolation)

Question 23

What has been obtained from genome projects

Answer 23

Complete DNA sequence

Question 24

How is the 100k Genome project different to the Human genome project

Answer 24

The 100k project uses new techniques e.g. Next Generation Sequencers (NGS) can sequence an entire genome in just a
few hours, compared to ‘Sanger Sequencing’ which sequences relatively small sections of
DNA at a time

Question 25

What does a DNA profile represent

Answer 25

Only non-coding portions of DNA (introns - STRs)

Question 26

What produces variation in individuals

Answer 26

Introns contain blocks of repeated nucleotides (Short Tandem Repeats). It is the number of times that these blocks are repeated that produces the variation in individuals.

Question 27

What techniques are required to produce a DNA profile

Answer 27

1. The polymerase chain reaction (PCR) 2. Gel electrophoresis.

Question 28

Describe the process of PCR

Answer 28

PCR is used to amplify small sections of DNA (specifically STRs) rapidly. Heat the DNA to 95C to separate the two strands. Then cool to 50-60C to allow the primers (single stranded DNA complimentary to the start of the sequence) to bind to the DNA strands (annealing). Heat to 70C allows a thermally stable DNA polymerase (Taq) to add complementary nucleotides (extension - forms phosphodiester bonds in the sugar-phosphate backbone). This cycle is repeated.

Question 29

Describe the process of gel electrophoresis

Answer 29

The DNA is cut into small fragments using restriction endonucleases then the samples are loaded into wells at one end of an agarose gel (porous) and a voltage is applied. DNA is attracted to the positive electrode (phosphate group negative charge). Smaller fragments travel further than large fragments in the same time. Fragment size can be estimated by running a DNA ladder (fragments of known size) alongside.

Question 30

What does gel electrophoresis do

Answer 30

Gel electrophoresis separates DNA fragments according to size.

Question 31

Aims of the Human Genome project

Answer 31

Identify the position of genes in the human genome, improve understanding, diagnosis and treatment of genetic disorders

Question 32

Function of restriction endonucleases

Answer 32

Cuts DNA at a specific base sequence

Question 33

How can a gene be identified and then isolated for genetic engineering

Answer 33

A gene can be identified using a gene probe (specific segment of single-strand DNA that is complementary to a section of the gene). The gene can be isolated with either: a) Reverse transcriptase or b) Restriction endonuclease

Question 34

How is reverse transcriptase used to isolate a gene

Answer 34

By extracting mRNA from a cell actively synthesising the required protein/ polypeptide and using reverse transcriptase and then DNA polymerase to produce a double stranded cDNA fragment

Question 35

2 advantages of using reverse transcriptase

Answer 35

• This method avoids the need to locate the gene. • The DNA produced does not include introns because the cDNA is copied from functional mRNA.

Question 36

2 disadvantages of using restriction endonuclease

Answer 36

• If the recognition site occurs within the gene of interest, the gene will be broken into fragments that have no function. • Eukaryotic genes contain introns, prokaryotic genes do not. If a eukaryotic gene was transferred into a bacterium the introns would not be removed creating a non-functional protein

Question 37

Cells are unlikely to take in a gene spontaneously, so the gene must be carried into the cell by a _____

Answer 37

vector e.g viruses or bacterial plasmids

Question 38

Limitations of PCR

Answer 38

DNA polymerase make errors and corrects them, Taq polymerase cannot do this so errors can accumulate. Primers can only be used when we know the structure of the target gene.

Question 39

Define sticky ends

Answer 39

Partial sequences of DNA which contain complimentary bases.

Question 40

What is the use of antibiotic resistance genes in recombinant DNA

Answer 40

Used as markers to show genetic engineering has been successful, adding antibiotic will destroy any bacteria that hasn't taken up the recombinant DNA

Question 41

Function of DNA ligase

Answer 41

An enzyme that joins DNA fragments by forming phosphodiester bonds

Question 42

Describe the process of creating recombinant DNA

Answer 42

Remove bacterial plasmid and cut with restriction enzymes, cut out useful gene from human cell using the same restriction enzyme (create complimentary sticky ends), combine gene to plasmid using DNA ligase, insert plasmid back into bacteria

Question 43

Exon skipping treatment for DMD example

Answer 43

Drug introduces a 'molecular patch' over the exon with the mutation making the gene readable again. A shorter from of dystrophin is produced, but one thought to be more functional than the untreated version.

Question 44

Define stem cell

Answer 44

A stem cell is an undifferentiated cell capable of dividing to give rise to cells which can develop into different types of specialized cells.

Question 45

Why are embryonic stem cells described as totipotent

Answer 45

Cells that are totipotent have the ability to differentiate into all possible cell types for that organism.