Biology Class 12 NCERT Solutions: Chapter 6 Molecular Basis of Inheritance Part 1

Download PDF of This Page (Size: 197K)

Q.1 Group the following as nitrogenous bases and nucleosides: Adenine, Cytidine, Thymine, Guanosine, Uracil and Cytosine.

Answer:

  • Nitrogenous bases present in the list are adenine, thymine, uracil, and cytosine.

  • Nucleosides present in the list are Cytidine and Guanosine.

Question 2: If a double stranded DNA has per cent of cytosine, calculate the per cent of adenine in the DNA.

Answer:

  • According to Chargaff’s rule, the DNA molecule should have an equal ratio of pyrimidine (cytosine and thymine) and purine (adenine and guanine).

  • It means that the number of adenine molecules is equal to thymine molecules and the number of guanine molecules is equal to cytosine molecules.

  • If dsDNA has of cytosine, then according to the law, it would have of guanine.

  • Thus, percentage of

  • The remaining represents both molecules.

  • Since adenine and guanine are always present in equal numbers, the percentage of adenine molecule is .

Q.3 If the sequence of one strand of DNA is written as follows:

Write down the sequence of complementary strand in direction

Answer:

  • The DNA strands are complementary to each other with respect to base sequence. Hence, if the sequence of one strand of DNA is

  • Then, the sequence of complementary strand in direction will be

  • Therefore, the sequence of nucleotides on DNA polypeptide in direction is

Q.4 If the sequence of the coding strand in a transcription unit is written as follows:

. Write down the sequence of mRNA.

Answer:

  • If the coding strand in a transcription unit is

  • Then, the template strand in 3’ to 5’ direction would be

  • It is known that the sequence of mRNA is same as the coding strand of DNA. However, in RNA, thymine is replaced by uracil.

  • Hence, the sequence of mRNA will be

Q.5: Which property of DNA double helix led Watson and Crick to hypothesise semiconservative mode of DNA replication? Explain.

Answer:

  • Watson and Crick observed that the two strands of DNA are anti-parallel and complementary to each other with respect to their base sequences.

  • This type of arrangement in DNA molecule led to the hypothesis that DNA replication is semiconservative.

  • It means that the double stranded DNA molecule separates and then, each of the separated strand acts as a template for the synthesis of a new complementary strand.

  • As a result, each DNA molecule would have one parental strand and a newly synthesized daughter strand.

  • Since only one parental strand is conserved in each daughter molecule, it is known as semi-conservative mode of replication.

Image of Causes of Mutations

Image of Causes of Mutations

Image of Causes of Mutations