NCERT Class 12 Biology: Chapter –6 Molecular Basis of Inheritance Part 7

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Question 9:

What would happen if histones were to be mutated and made rich in acidic amino acids such as aspartic acid and glutamic acid in place of basic amino acids such as lysine and arginine?

Answer:

If histones were to be mutated and made rich in acidic amino acids such as aspartic acid and glutamic acid in place of basic amino acids such as lysine and arginine. This mutation would make the histones acidic and negatively charged in nature and histones would not be able to bind to DNA because both would be negatively charged. Hence, the DNA would not be packed and chromatin would not be formed.

Question 10:

Recall the experiments done by Frederick Griffith, Avery, MacLeod and McCarty, where DNA was speculated to be the genetic material. If RNA, instead of DNA was the genetic material, would the heat killed strain of Pneumococcus have transformed the R-strain into virulent strain? Explain.

Answer:

If RNA, instead of DNA was the genetic material, the heat killed strain of Pneumococcus would not have transformed the R-strain into virulent strain. This is because RNA is not thermo stable. Which means that RNA is more prone to degradation by heat because of the presence of 2’OH group in its ribose. As a result of which it would have lost the capability to change the R-strain to virulent strain.

Question 11:

You are repeating the Hershey-Chase experiment and are provided with two isotopes: 32P and 15N (in place of in the original experiment). How do you expect your results to be different?

Answer:

Use of 15N in place of 35S in the original experiment would not give any conclusive result because:

1. 32P is radioactive and 15N is non-radioactive. So, it’s presence won’t be detected.

2. If hypothetically 15N would be radioactive, then it’s presence would be seen both inside the cell as it gets incorporated in the DNA as nitrogenous base and also as a supernatant as it gets incorporated in the amino group of amino acids.

Question 12:

There is only one possible sequence of amino acids when deduced from a given nucleotides. But multiple nucleotides sequence can be deduced from a single amino acid sequence. Explain this phenomena.

Answer:

Some amino acids are coded by more than one codon. This phenomenon is called as degeneracy of codon. Hence, from such amino acids multiple nucleotide sequence would be obtained. For example, Isoleucine has three codons AUU, AUG and AUA.The following nucleotide sequences are obtained:

a. AUG-AUU

b. AUG-AUG

c. AUG-AUA

All the three sequences code for Met-Ile.

Question 13:

A single base mutation in a gene may not ‘always’ result in loss or gain of function. Do you think the statement is correct? Defend your answer.

Answer:

The above statement is correct. It is because if mutation takes place at the third base pair, it does not lead to a phenotypic change. These mutations are known as silent mutations. If the mutations take place at any other base than the third base phenotypic changes may take place. For Example: Substitution of Valine instead of Glutamic acid in the beta haemoglobin chain of the RBC’S leads to sickle cell Anaemia.

Question 14:

A low level of expression of lac operon occurs at all the time. Can you explain the logic behind this phenomena?

Answer:

Lactose present in the external medium can only enter the bacterium when the bacterium has permease within it. Low level of expression of lac operon is required for permease to form within the bacteria.

Question 15:

How has the sequencing of human genome opened new windows for treatment of various genetic disorders? Discuss amongst your classmates.

Answer:

The sequencing of human genome opened new windows for treatment of various genetic disorders because:

1. It led to better understanding of genetic disorders.

2. Diagnosis, treatment and prevention of genetic disorders are better understood.

3. Which genes is for which disease is identified.

Question 16:

The total number of genes in humans is far less than the previous estimate. Comment.

Answer:

The total number of genes in humans is far less than the previous estimate (up to gene) because of the presence of large portions of repetitive sequencing regions in the human genome. These are the stretches of the DNA sequence that are repeated numerous times hence the non-repetitive gene count is less, if the repetitive portions are not added. These stretches have no coding functions but make it possible to understand chromosome structure, dynamics and evolution.

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