Question

Please no long explanations, just short and clear answers. Explanations will be valued only if answer is clear and separate from explanation, thank you so much!

Question 3: Mutations, 5 points Since we expect one SNP difference to occur once every thousand base pairs between any two human genomes, we expect that on average each person is heterozygous at 3 million SNP positions. Below is a diagram showing a person heterozygous for a specific SNP allele. From mom GGGGAAGACTGTAACTA HETEeey灬 ou are rom dad GGGGAAGACTATAS A. 3 points. Since Exons comprise 1% of the human genome, how many of the 3 million SNP differences do you think would exist in exons if SNPs occur randomly and are not selected against. You must provide an equation to show how you derived your answer for any credit. B. 1 point. The number of SNPs in exons are considerably less than you might expect by chance alone. Provide a hypothesis for this observation. C. 1 point. Explain what a "nonsense" mutation is. What kind of mutation in DNA causes a Nonsense mutation and what is the effect does this have on the protein made from an RNA transcript?

Answer 1

A.
The total human genome contains 3 X 10^6 SNPs.
Percentage of exons in the genome = 1%
100% SNPs ------> 3 X 10^6
1% SNPs --------> ?

= 3 X 10^4
= 30000 SNPs

B.
Exons are coding sequences. So, if there is a mutation in exon sequences, they will affect protein function. Hence exons are under selection pressure. If a mutation in exon results in reduced fitness of the allele, it will be soon eliminated from the population. There will be a strong negative selection against such deleterious alleles.
On the other hand, mutations in introns are not harmful (except for locations that affect splicing and gene expression.) There is no selection pressure on them for their sequence conservation.

C.
Nonsense mutation = A mutation that generates a translation non-sense codon.
Translation non-sense codons = UAA, UAG and UGA

Nonsense mutation can arise by,
1. Insertion
Ex: ATG CGA CGU GAU UAG -----> ATG CGA ACG UGA UUA G
2. Deletion
Ex: ATG CGA CGU GAU UAG -----> ATG CGA UGA UUA G
3. Substitution
Ex: ATG CGA CGU GAU UAG -----> ATG UGA CGU GAU UAG

Non-sense mutations cause premature termination of translation and lead to the production of truncated non-functional or dominant negative proteins.