Gene Mutations - 20.1

Question 1

What is the substitution of bases?

Answer 1

The type of gene mutation in which a nucleotide in a section of a DNA molecules is replaced by another nucleotide that has a different base.

Question 2

What are the three possible consequences of the substitution of base?

Answer 2

• Formation of one of the three stop codons
• Formation of a codon for a different amino acid.
• Formation of a different codon but one that produces a codon for the same amino acid as before.

Question 3

What would happen to the structure of a protein if a substitution resulted in a stop codon?

Answer 3

The production of the polypeptide coded for by the section of DNA would be stopped prematurely. The final protein would almost certainly be significantly different and the protein could not perform its normal function.

Question 4

What would happen to the structure of a protein if a substitution resulted in a different amino acid?

Answer 4

The structure would differ in a single amino acid. The protein may differ in shape and not function properly. For example, if it is an enzyme, its active site may no longer fit the substrate and it will not catalysed the reaction.

Question 5

What condition can be caused if substitution results in the formation of a different amino acid?

Answer 5

Sickle Cell Anaemia

Question 6

What would happen to the structure of a protein if a substitution resulted in the same amino acid?

Answer 6

The mutation would have no effect on the polypeptide produce and so the mutation would have no overall effect.

Question 7

What is the deletion of bases?

Answer 7

The loss of a nucleotide base from a DNA sequence.

Question 8

What does the deletion of a base cause?

Answer 8

Frame Shift

Question 9

What would happen to the structure of a protein if a deletion of bases occurs?

Answer 9

The gene is now read In the wrong three-base groups and the coded information is altered. Most triplets will then be different. The resulting polypeptide will be different and lead to the production of a non-functional protein that could considerably alter the phenotype.

Question 10

What is the addition of bases?

Answer 10

When an extra base is inserted into the sequence of amino acids.

Question 11

What would happen to the structure of a protein if addition of bases occurs?

Answer 11

This has a similar effect to a base deletion in that there is normally frame shift and the whole sequence of triplets becomes altered. If three extra bases are added, or any multiple of three, there will not be a frame shift.

Question 12

What is the duplication of bases and what does it cause?

Answer 12

When one or more bases are repeated. This produces a frame shift to the right.

Question 13

What is the inversion of bases and what does it cause?

Answer 13

When a group of bases become separated from the DNA sequence and rejoin at the same position but in the inverse order. The base sequence of this portion is therefore reversed and effects the amino acid sequence that results.

Question 14

What is the translocation of bases and what does it cause?

Answer 14

When a group of bases becomes separated from the DNA sequence on one chromosome and become inserted into the DNA sequence of a different chromosome. Translocations often have significant effects on gene expression leading to an abnormal phenotype. These effects include the development of certain forms of cancer and also reduced fertility.

Question 15

What causes gene mutations?

Answer 15

Gene mutations can arise spontaneously during DNA replication. Spontaneous mutations are permanent changes in DNA that occur without any outside influence.
The basic mutation rate can be increased by outside factors known as mutagenic agents or mutagens.

Question 16

What are some examples of mutagenic agents?

Answer 16

• High energy ionising radiation, for example, alpha and beta particles as well as short wavelength radiation such as X-rays and ultra violet light. These forms of radiation can disrupt the structure of DNA.
• Chemical, such as nitrogen dioxide may directly alter the structure of DNA or interfere with transcription. Benzopyrene, a constituent of tobacco smoke, is a powerful mutagen that inactivates a tumour-suppressor gene TP53 leading to a cancer.

Question 17

What are the costs of gene mutations?

Answer 17

• Almost always harmful
• Produce an organism that is less well suited for its environment
• Mutations can occur in body cells rather than gametes leading to disruption of normal cellular activities, such as cell division, for example cancer.

Question 18

What is the benefit of gene mutations?

Answer 18

They produce the genetic diversity necessary for natural selection and speciation.

Question 19

What is a gene mutations?

Answer 19

A mutation is a sudden inheritable change in the genetic material. Gene mutation is sometimes called point mutation because the mutation may only involve change of one nucleotide (base) in the codon sequence of the gene.

Question 20

What mutation occurs in sickle cell anaemia and what does this result in?

Answer 20

In sickle cell anaemia the mutation effects the amino acid sequence of part of the beta-globin protein chain of the haemoglobin. This results in the formation of abnormal haemoglobin-S which in conditions of low oxygen tension causes the red blood cells to collapse into sickle shapes.

Question 21

What is a ‘missense’ mutation?

Answer 21

A base alteration in a codon that leads to a different amino acid being coded for.

Question 22

What is a ‘samesense’ mutation?

Answer 22

A base alteration in a codon that does not alter the amino acid being coded for.

Question 23

What is a ‘nonsense’ mutation?

Answer 23

A base alteration in a codon that changes an amino-acid-specifying codon into a chain-terminating (stop) codon.

Question 24

What is ‘frame shift’?

Answer 24

A mutation arising from the insertion or deletion of one or more nucleotides that causes the gene to be misread during translation.

Question 25

What causes spontaneous mutations?

Answer 25

Spontaneous mutations result from errors in the replication of DNA. Replication of DNA is a complicated process involving DNA polymerase and a large number of other enzymes. These enzymes make rare mistakes and these causes base changes in the composition of DNA.

Question 26

What are induced mutations?

Answer 26

Changes In the DNA caused by the effects of mutagens. These may be radiation or chemical in nature and cause genes to mutate at rates that are much higher than their spontaneous rate.

Question 27

What are some examples of gene mutations in nature that have been beneficial to the mutated organism?

Answer 27

• The development of antibiotic resistance in certain strains of bacteria.
• The development of warfarin (rat poison) resistance in rats.
• The development of DDT resistance in mosquitos and many other species of insect.
• The development of camouflaging shell colours and banding on the shells of the snail. this reduces predation by thrushes.
• The development of resistance to the myxomatosis virus by many rabbits.