CBSE Class 12 Biology Chapter 6: Molecular Basis of Inheritance - NCERT Solutions

The given list contains both nitrogenous bases and nucleosides. Let's categorize them:

Nitrogenous Bases: These are heterocyclic aromatic organic compounds containing nitrogen atoms. From the list, the nitrogenous bases are:

• Adenine (A)
• Thymine (T)
• Uracil (U)
• Cytosine (C)

Nucleosides: These are molecules formed by the glycosidic linkage of a nitrogenous base with a pentose sugar (ribose or deoxyribose). From the list, the nucleosides are:

• Cytidine (Cytosine + Ribose/Deoxyribose)
• Guanosine (Guanine + Ribose/Deoxyribose)

Note: Adenosine and Guanosine are also nucleosides, formed from Adenine and Guanine respectively, linked to a sugar.

We can use Chargaff's rules for double-stranded DNA (dsDNA), which state that the amount of adenine is equal to thymine, and the amount of guanine is equal to cytosine.

Mathematically, this is represented as:

$$\backslash \%\; A\; =\; \backslash \%\; T$$

$$\backslash \%\; G\; =\; \backslash \%\; C$$

Given that the double-stranded DNA has 20% cytosine (%C = 20%).

According to Chargaff's rule, the percentage of guanine will be equal to the percentage of cytosine:

$$\backslash \%\; G\; =\; \backslash \%\; C\; =\; 20\backslash \%$$

The total percentage of purines (Adenine + Guanine) must equal the total percentage of pyrimidines (Cytosine + Thymine). Also, the sum of all bases must be 100%:

$$\backslash \%\; A\; +\; \backslash \%\; T\; +\; \backslash \%\; G\; +\; \backslash \%\; C\; =\; 100\backslash \%$$

We know %G and %C, so:

$$\backslash \%\; A\; +\; \backslash \%\; T\; +\; 20\backslash \%\; +\; 20\backslash \%\; =\; 100\backslash \%$$

$$\backslash \%\; A\; +\; \backslash \%\; T\; +\; 40\backslash \%\; =\; 100\backslash \%$$

This means the combined percentage of Adenine and Thymine is:

$$\backslash \%\; A\; +\; \backslash \%\; T\; =\; 100\backslash \%\; -\; 40\backslash \%\; =\; 60\backslash \%$$

Since %A = %T in dsDNA, we can divide the remaining percentage equally between Adenine and Thymine:

$$\backslash \%\; A\; =\; \backslash \%\; T\; =\; \backslash frac\{60\backslash \%\}\{2\}\; =\; 30\backslash \%$$

Therefore, the percentage of adenine in the DNA is 30%.

DNA strands are antiparallel and complementary. This means that the bases on one strand pair specifically with bases on the other strand (Adenine with Thymine, and Guanine with Cytosine), and the strands run in opposite directions (5' to 3' and 3' to 5').

The given sequence of one DNA strand is:

5'-ATGCATGCATGCATGCATGCATGC-3'

To find the complementary strand, we apply the base pairing rules (A pairs with T, G pairs with C) and reverse the directionality.

The complementary pairing will be:

A pairs with T

T pairs with A

G pairs with C

C pairs with G

So, the complementary strand sequence, read from 3' to 5', is:

3'-TACGTACGTACGTACGTACGTACG-5'

The question asks for the sequence of the complementary strand in the 5' → 3' direction. To get this, we simply reverse the 3' to 5' sequence:

5'-GCATGCATGCATGCATGCATGCAT-3'

Therefore, the sequence of the complementary strand in the 5' → 3' direction is 5'-GCATGCATGCATGCATGCATGCAT-3'.

In a transcription unit, the coding strand (also known as the sense strand) has a sequence similar to the mRNA that will be produced. The template strand (also known as the antisense strand) is the one that is actually read by RNA polymerase to synthesize mRNA.

The given sequence of the coding strand is:

5'-ATGCATGCATGCATGCATGCATGC-3'

The sequence of the mRNA is identical to the coding strand, with one key difference: Uracil (U) replaces Thymine (T) in RNA.

Therefore, to find the mRNA sequence, we replace every 'T' in the coding strand sequence with 'U'.

Coding strand: 5'-ATGCATGCATGCATGCATGCATGC-3'

Replacing T with U:

mRNA sequence: 5'-AUGCAUGCAUGCAUGCAUGCAUGC-3'

The mRNA sequence is synthesized in the 5' to 3' direction.

Watson and Crick's hypothesis of the semiconservative mode of DNA replication was primarily based on two key properties of the DNA double helix they elucidated:

1. Complementarity of Strands: The two polynucleotide strands of the DNA double helix are held together by hydrogen bonds between specific base pairs: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This strict base pairing rule means that the sequence of bases on one strand dictates the sequence of bases on the other strand.
2. Antiparallel Nature of Strands: The two strands run in opposite directions, with one strand oriented 5' to 3' and the other 3' to 5'.

Explanation of the Hypothesis:

Given these properties, Watson and Crick proposed that during replication, the hydrogen bonds holding the two strands together would break, allowing the double helix to 'unzip' or unwind. Each of the separated parental strands would then serve as a template for the synthesis of a new complementary strand. Because of the specific base pairing rules, the new strand synthesized would be exactly complementary to the template strand.

As a result, each of the two new DNA molecules formed would consist of one original (parental) strand and one newly synthesized strand. This mechanism, where each daughter DNA molecule retains one of the parent strands, is termed 'semiconservative' replication (semi = half, conservative = retaining).

This model elegantly explained how genetic information could be accurately passed down from one generation of DNA molecules to the next, ensuring the fidelity of inheritance.

Parental Strand Daughter strand