B2. Mutations

Question 1

Gene mutations

Gene mutations involve a change in the DNA base sequence of chromosomes. The types of errors that can occur include:

Why is this important?

Answer 1

Substitution - one base is substituted with another,
e.g. ATGCCT becomes ATTCCT (G is swapped for T).

Deletion - one base is deleted
e.g. ATGCCT becomes ATCCT (G is removed).

The order of DNA bases in a gene determines the order of amino acids in a particular protein. If a mutation occurs in a gene, the sequence of amino acids it codes for (and the protein formed) could be altered

Question 2

Effects of mutations

What can mean no effect for which tyoe of mutation and why?

What will always make an effect and why?

Answer 2

Effects of mutations
The degenerate nature of the genetic code means that some amino acids are coded for by more than one DNA triplet.

This means that not all substitution mutations will result in a change to the amino acid sequence of the protein some substitutions will still code for the same amino acid.

Substitution mutations won’t always lead to changes in the amino acid sequence, but deletions will the deletion of a base will change the number of bases present, which will cause a shift in all the base triplets after it.

Question 3

Mutagenic agents
Mutations occur ____________, e.g. when DNA is misread during replication. But some things can cause an increase in the rate of mutations - these are called mutagenic agents. Ultraviolet radiation, ionising radiation, some chemicals and some viruses are examples of mutagenic agents. Mutagenic agents increase the ____________of a mutation occurring.

Answer 3

Mutagenic agents
Mutations occur spontaneously, e.g. when DNA is misread during replication. But some things can cause an increase in the rate of mutations - these are called mutagenic agents. Ultraviolet radiation, ionising radiation, some chemicals and some viruses are examples of mutagenic agents. Mutagenic agents increase the probability of a mutation occurring.

Question 4

Tip: Mutations are always _________-only the ____of mutation is affected by _________ ______.

Answer 4

Tip: Mutations are always random-only the rate of mutation is affected by mutagenic agents.

Question 5

Chromosome mutations
In humans, when meiosis works properly, all four daughter cells will end up with 23 whole chromosomes - one from each homologous pair (1 to 23). But sometimes ________goes wrong and the cells produced contain __________in the numbers of whole chromosomes or parts of chromosomes. E.g. two cells might have 23 whole chromosomes, one each of 1 to 23, but the other two might get a bit muddled up, one having two chromosome 6’s and the other no chromosome 6. This is called chromosome mutation and is caused by errors during _________. Chromosome mutations lead to ___________conditions because the errors are present in the gametes (the hereditary cells). One type of chromosome mutation is called chromosome ___-____________—it’s a failure of the chromosomes to separate properly.

Answer 5

Chromosome mutations
In humans, when meiosis works properly, all four daughter cells will end up with 23 whole chromosomes - one from each homologous pair (1 to 23). But sometimes meiosis goes wrong and the cells produced contain variations in the numbers of whole chromosomes or parts of chromosomes. E.g. two cells might have 23 whole chromosomes, one each of 1 to 23, but the other two might get a bit muddled up, one having two chromosome 6’s and the other no chromosome 6. This is called chromosome mutation and is caused by errors during meiosis. Chromosome mutations lead to inherited conditions because the errors are present in the gametes (the hereditary cells). One type of chromosome mutation is called chromosome non-disjunction—it’s a failure of the chromosomes to separate properly.