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

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Question 2:

Who revealed biochemical nature of the transforming principle? How was it done?


1. When Griffith performed the experiment. He was unable to conclude the biochemical nature of the inherited genetic material.

2. After which Oswald Avery, Colin McLeod and Maclyn McCarty in 1933-34 performed an experiment to find this genetic material.

3. They purified the biochemical (DNA, RNA, & Protein) from the heat killed S-strain cells.

4. They noticed that the RNA digesting enzyme (RNases) , Protein digesting enzymes (Proteases) did not affect the transformation of the non-virulent R-strain to the virulent S-strain.

5. This led to the conclusion that DNA was the transforming principle.

Question 3:

Discuss the significance of heavy isotope of nitrogen in the Meselson and Stahl’s experiment.


Heavy isotope of nitrogen in the Meselson and Stahl’s experiment showed that the DNA is of intermediate density. Which was between N14 and N15.This showed that the hybrid DNA has one strand of N14 and one strand of N15 showing DNA was semi-conservative in nature.

Question 4:

Define a cistron. Giving examples differentiate between monocistronic and polyeistronic transcription unit.


A segment of DNA coding for polypeptide is called cistron. A cistron is basically a gene. If a stretch of replicating DNA contains a single cistron (or gene), it is called monocistronic e.g. eukaryotes. If a stretch of replicating DNA contains more than one cistron, it is called polyeistronic, e.g. bacteria and prokaryotes.

Question 5:

Give any six features of the human genome.


Human Genome is the genome of the Homo Sapiens. It consists of the coding regions of the DNA which encodes all the genes. It also consists of all the non-coding regions of the DNA which does not encode for any genes.

Six features of the human genome are:

1. The genome has around 3164.7 million nucleotide bases.

2. 99.9 % of the nucleotide bases are same in all humans.

3. The largest gene is Dystrophin having 2.4 million bases.

4. Chromosome 1 has most genes (2968) and Y has the least genes (231).

5. Less than 2% of the genome has the coding sequence for proteins.

6. Only 50% of the total discovered genes have known functions.

Question 6:

During DNA replication, why is it that the entire molecule does not open in one go? Explain replication fork. What are the two functions that the monomers (d NTPs) play?


During DNA replication, why is it that the entire molecule does not open in one go otherwise the whole molecule would destabilise as it consumes a lot of energy.

2. This is because the opening or unwinding of the DNA creates tension in the molecule. Uncoiled parts start forming supercoils due to the interaction of the exposed nucleotides.

3. To avoid this scenario the helicase enzyme acts on the site of origin (ori site) of the double stranded DNA and opens only a small stretch.

4. With the help of enzymes these exposed strands are copied.

5. After this the site of origin moves in both the directions forming a ‘Y’ shaped structure called Replication Fork.

DNTP’S are Deoxyribonucleotides triphosphate. The two functions that the monomers (d NTPs) play are:

1. They provide energy for polymerisation reaction.

2. They provide deoxyribonucleotides for DNA replication.

Question 7:

Retroviruses do not follow central Dogma. Comment.


Central Dogma is the process of transfer of information within the genes to DNA than RNA and finally to the proteins. Retroviruses do not follow central Dogma because their genetic material is RNA instead of DNA. This RNA is than converted to DNA by the process called Reverse Transcription with the help of enzyme reverse transcriptase. Hence central dogma is not followed.

Question 8:

In an experiment, DNA is treated with a compound which tends to place itself amongst the stacks of nitrogenous base pairs. As a result of this, the distance between two consecutive base increases. From to calculate the length of DNA double helix (which has) in the presence of saturating amount of this compound.


Distance between two consecutive base pairs: .

Length of the DNA:

Length of