QUESTION 30 Which of the following statements may provide an explanation as to w

Question

QUESTION 30

Which of the following statements may provide an explanation as to why does RNA synthesis need not be as carefully monitored for errors as does DNA synthesis?

An error will affect only one molecule of mRNA out of many synthesized from a gene thus the effect is diluted. Furthermore RNA transcripts are labile (degrade relatively fast), so the effects is only short lived. In addition, the errors do not become a permanent part of the genetic information that is passed on to the cell progeny.

DNA seldom adopts stable tertiary structure folds, whereas RNA does so more readily.

At any given instant, only a fraction of the genome (total DNA) is being transcribed. Consequently, high fidelity is not essential. In contrast, during the S phase of the cell cycle, the whole genome is being replicated requiring a more concerted effort to keep incorporation errors to a minimum.

RNA and DNA polymerases do not share significant levels of homology between them. In particular, their active sites are related by convergent evolution. Therefore it is not surprising that one process has higher fidelity as compared to the other.

Since cytosines can spontaneously mutate into uracils and since RNA is full of uracils, the fidelity of RNA polymerase is not as important.

3.5 points   

QUESTION 31

Write the sequence of the messenger RNA molecule synthesized from a DNA template strand having the sequence:

5'- CGTTACGTAC -3'

5’- Blank 1Blank 2Blank 3Blank 4Blank 5Blank 6Blank 7Blank 8Blank 9Blank 10 -3'.

3.5 points   

An error will affect only one molecule of mRNA out of many synthesized from a gene thus the effect is diluted. Furthermore RNA transcripts are labile (degrade relatively fast), so the effects is only short lived. In addition, the errors do not become a permanent part of the genetic information that is passed on to the cell progeny.

DNA seldom adopts stable tertiary structure folds, whereas RNA does so more readily.

At any given instant, only a fraction of the genome (total DNA) is being transcribed. Consequently, high fidelity is not essential. In contrast, during the S phase of the cell cycle, the whole genome is being replicated requiring a more concerted effort to keep incorporation errors to a minimum.

RNA and DNA polymerases do not share significant levels of homology between them. In particular, their active sites are related by convergent evolution. Therefore it is not surprising that one process has higher fidelity as compared to the other.

Since cytosines can spontaneously mutate into uracils and since RNA is full of uracils, the fidelity of RNA polymerase is not as important.

Explanation / Answer

30- Statement A: correct answer

This is because, once an error is incorporated in DNA & proof reading activities fail to rectify it, it is inherited in the next generations; so it becomes a permanent change in the genome.

On the other hand, if there is a slight error in RNA transcript, the cell will transcribe more RNA correctly than rectify the error as this is more economic to the cell in terms of energy. Also the erroneous RNA will be degraded soon as they are labile.

31- A DNA template strand has the sequence: 5'- CGTTACGTAC -3'.

A DNA template strand is the anti-sense strand; that is the RNA sequence will be complementary to the sequence of the DNA template strand.

Hence, the RNA sequence will be 3'-GCAAUGCAUG-5'.

Hence the RNA sequence will be 5'- GUACGUAACG- 3'

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