Mutations and Variation 19.1

Question 1

What is a gene?

Answer 1

A gene is a section of DNA that contains a complete sequence of bases that codes for a specific protein

Question 2

What is a gene mutation?

Answer 2

A gene mutation is a change in the sequence of bases in a gene.

Question 3

What process is disrupted by mutations?

Answer 3

Protein synthesis

Question 4

How do mutations occur?

Answer 4

They can occur because of :

• a base insertion
• a base deletion
• a base substitution

Question 5

Point mutation

Answer 5

When only one nucleotide is affected by the mutation.

Question 6

What happens in a base insertion or deletion?

Answer 6

A frameshift happens and all the bases after the mutation will be shifted over so the codon sequence changes which can change the amino acid chain and therefore produce a different protein.

Question 7

What happens in a base substitution?

Answer 7

One base is replaced by another which could result in the change of one codon only so might not have as big of an impact.

Question 8

What could mutations do to a protein?

Answer 8

A mutation could cause a big change in the nucleotide sequence which means that when the gene is transcribed, the codons could be very different to what the original gene would’ve been and therefore could code for completely different amino acids. Their R-groups could then interact in very different ways and the proteins would have a completely different shape. This could be important if it’s an enzyme as it won’t have the correct complimentary active site.

Question 9

Why might silent mutations not have any effect?

Answer 9

A mutation might not lead to a change in the amino acid sequence because even though the base sequence is different and there may be different codons, DNA code is degenerate so the same amino acid could code for both the original and mutated codons which means the same amino acid sequence would be present, therefore there would be no change in the protein. These are called silent mutations

Question 10

Even if there is a change in the amino acid sequence, explain why this might not lead to a change in phenotype?

Answer 10

• The allele may be recessive
• There may be several other genes that express that phenotype, so one mutated gene won’t make a difference
• Epistasis could occur which is where a gene prevents the expression of another

Question 11

What is epistasis?

Answer 11

One gene prevents the expression of another.

Question 12

Neutral mutations

Answer 12

Mutations where protein structures are affected, but the phenotype doesn’t change. This doesn’t give any advantages or disadvantages. An example is the mutation that allows you to taste the bitter chemical in Brussel Sprouts

Question 13

Beneficial Mutations

Answer 13

Mutations that cause a change in the protein structure and in the phenotype for the better. An example is a protein present on the cell surface membrane that prevents HIV from binding to the cell which means they are immune to HIV.

Question 14

Harmful mutation

Answer 14

Mutations that cause a change in the protein structure for the worse. Proteins may no longer be synthesised or they can be non-functional and this can result in many genetic disorders.

Question 15

Causes of mutations?

Answer 15

• Can be spontaneous during cell replication
• Mutagens
• The loss of a base can occur so an incorrect base will be inserted
• Free radicals can disrupt base pairing and affect the structure of nucleotides.

Question 16

Types of mutagens

Answer 16

Ionising radiations, Deaminating agents, Biological agents.

Question 17

Chromosome mutations

Answer 17

This is when the entire chromosome is affected by a mutation which normally happens during meiosis. This can cause:

• Deletion where a section of the chromosome breaks off
• Translocation - a section of one chromosome breaks off and joins another non-homologous chromosome.

Question 18

What are protooncogenes?

Answer 18

They are genes that promote growth and they code for growth factors or their receptors. There are enzymes that can switch them off once the required division has taken place.

Question 19

How can uncontrolled cell division occur?

Answer 19

A single mutation in one of the protooncogenes can result in oncogenes that cannot be switched off which means the cells will divide uncontrollably.

Question 20

Tumour suppressor genes

Answer 20

These genes are involved in halting cell division. If these are mutated, cell division won’t be stopped which can lead to cancer and this is more likely to happen if you smoke.