Question

7a). Are mutations good or bad? Explain your answer.

7b). Explain why loss-of-function mutations are frequently recessive, whereas gain-of-function mutations are frequently dominant.

7c). Briefly describe expanding nucleotide repeats. How do they account for the phenomenon of anticipation?

7d). What is the difference between a transition and a transversion? Which type of base substitution is usually more common and why? 3 marks

Answer 1

7a) Mutation occurs when DNA sequence become changed in such a way that alter genetic message which is carried by the gene. Mutations are either good which is helpful for organism or bad which is harmful for that organism. Mutations are also become neutral which means mutation is neither helpful nor harmful. So mutations can be beneficial, harmful or neytral. Now whether mutation become good or bad it depends upon the environment and circumstances where organism lived. Most of the mutations are neutral.

7b) There are 2 allele for each gene which is responsible for a particular trait. When one allele become inactivated, another allele remain activated and shows the trait. But loss of function of a trait occur that means 2 alleles are inactivated. When 2 alleles inactivated, this is called recessive.

On the other hand when gain of function occur that means out of the 2 allele which one is dominant, mutation should occur in that allele. Because in between 2 alleles dominat allele show the trait. So gain of function mutations are dominant.

7c) During DNA replication sometimes increase repeatation of the number of copies of nucleotide sequence. This type of repeation occur due to mutation, unequal recombination. This is called expanding nucleotide repeats. Somtimes this type of nucletide repeatation may increase from one generation to next generation and this is called anticipation where this repeatation can be occur by mutation.

7d) Transition mutation is where one purine base is changed to another purine (A-G) and pyrimidine to another pyrimidine. Where as transversion mutation means where one purine is changed to pyrimidine or vice versa (A/G-C/T).

Although transversion is favoured but about twice as many as possible transition is observed in human genome. Transition is more common because the shape of the molecule. In transition one pyrimidine (single ring structure) is changed to another pyrimidine (single ring structure). This change in similar structure is more favourable than transversion where one single ring structure (pyrimidine) is changed to one double ring structure (purine).

In transition as similar substitution occur (pyrimidine-pyrimidine or purine-purine), which encode same amino acid. So neutral mutation occur.