Mutations

Question 1

What are mistakes in DNA replication referred to as?

Answer 1

Mutations

Question 2

How does the cell prevent mutations in DNA replication?

Answer 2

It has an active repair system to correct damage in any DNA molecule

Question 3

Give an example in which a mutation in a single nucleotide affects the performance of an organism

Answer 3

Sickle cell anaemia. There is a mutation in the beta-globin chain which cause sickle cells to be produced. Sickle-cell haemoglobin is less soluble than normal haemoglobin and forms fibrous precipitates.

Question 4

A mutation in which cells cause the mutation to be passed on to offspring?

Answer 4

A mutation in the gametes. Any mutation in somatic cells will affect the individual but not the offspring.

Question 5

What is the system which means that the number of DNA mutations is limited?

Answer 5

DNA mismatch repair

Question 6

What is the probability of an error with the DNA mismatch repair system in place?

Answer 6

1 mistake in 10^9 nucleotides copied. Without the system in place, there is 1 mistake in 10^7 nucleotides copied.

Question 7

What is another method of DNA becoming damaged and how?

Answer 7

Ultraviolet radiation. It promotes the covalent linkage of two adjacent pyrimidine bases e.g. thymine dimer

Question 8

The three stages of DNA mismatch repair.

Answer 8

1. Excision
2. Resynthesis
3. Ligation

Question 9

Describe excision

Answer 9

The damage is cut out by a series of nucleases, each specialised for a type of DNA damage

Question 10

Describe resynthesis

Answer 10

The original DNA sequence is restored by repair DNA polymerase which fills in the gap that was created by excision/

Question 11

Describe ligation

Answer 11

DNA ligases seals the nick left in the sugar phosphate backbone of the repaired strand. Nick sealing requires energy from ATP hydrolysis remakes the broken phosphodiester bond between adjacent nucleotides.

Question 12

What is the consequence of this DNA mismatch repair?

Answer 12

Because of the high accuracy, there are very similar sequences for closely related species.

Question 13

How to determine the necessary fundamental sequences between two closely related species?

Answer 13

Compare them as far back as possible. A long time period.

Question 14

What is the effect of homologous recombination?

Answer 14

It causes a change in DNA sequences

Question 15

What is the requirement of the DNA molecules involved in homologous recombination?

Answer 15

Two double stranded DNA will have regions of similar DNA sequences.

Question 16

The steps of homologous recombination

Answer 16

The similar sequences of DNA will align. The DNA molecules then “cross over”, both strands of each double helix are broken and then the broken ends are rejoined to the ends of the opposite DNA molecule to re-form two intact double helices, each made up of two different DNA molecules.

Question 17

Where is the site of exchange in homologous recombination?

Answer 17

The site of exchange can occur anywhere along the homologous nucleotide sequences in the participating DNA molecules

Question 18

Are the nucleotide sequences altered at the site of exchange?

Answer 18

No the nucleotide sequences are not altered at the site of the exchange. The cleavage and rejoining are so precise that a nucleotide is not gained or lost.

Question 19

What are the three forms of homologous recombination in bacterial cells?

Answer 19

• Conjugation - direct cell-to-cell contact
• Transduction - by a virus
• Transformation - uptake, incorporation and expression of foreign genetic material

Question 20

What is a second type of recombination?

Answer 20

Site-specific recombination

Question 21

Describe site-specific recombination.

Answer 21

This involves mobile genetic elements. Short sequences of DNA can move from one position to another in the genome.

Question 22

What is the difference in bacteria between homologous and site-specific recombination?

Answer 22

There is no homology required between the end of the element and the insertion site.

Question 23

How has the diversity of species been affected by mutation?

Answer 23

The diversity has increased. It is a balance of the conservative accuracy of genome replication that enables progeny to develop virtues from their parents and the creative errors which allow them to develop novel features and evolve in capabilities.

Question 24

The five main types of gene mutation

Answer 24

1. Mutation within a gene
2. Gene duplication
3. Gene deletion
4. Exon shuffling
5. Horizontal gene transfer

Question 25

What is mutation within a gene?

Answer 25

Mutations that change a single nucleotide, or delete or duplicate a nucleotide in the DNA sequence. They are ‘point’ mutations caused by failure in DNA repair after damage.

Question 26

What is gene duplication?

Answer 26

A large segment of DNA can be duplicated, creating a set of closely related genes within a single cell. The cell and the progeny of this cell, can use these duplicated genes to undergo additional mutations and assume functions distinct to those of the original gene.

Question 27

What is gene deletion?

Answer 27

Individual genes or whole blocks of genes are lost through chromosome breakage and failure to repair.

Question 28

What is exon shuffling?

Answer 28

Two or more existing genes can be broken and rejoined to make a hybrid gene containing DNA segments belonging to separate genes. Because this often occurs in intron sequences of eukaryotes, it doesn’t have to be precise.

Question 29

What is horizontal gene transfer?

Answer 29

A piece of DNA can be transferred from the genome of that cell to another.