CBSE Class 12 Biology: Molecular Basis of Inheritance NCERT Solutions

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

Nitrogenous Bases: These are the fundamental building blocks of nucleic acids, consisting of a nitrogen-containing heterocyclic aromatic ring. From the list, the nitrogenous bases are:

• Adenine
• Thymine
• Uracil
• Cytosine

Nucleosides: A nucleoside is composed of a nitrogenous base covalently bonded to a sugar molecule (either ribose or deoxyribose). From the list, the nucleosides are:

• Cytidine (Cytosine + Ribose sugar)
• Guanosine (Guanine + Ribose sugar)

We can determine the percentage of adenine using Chargaff's rules, which state that in a double-stranded DNA molecule, the amount of adenine is equal to the amount of thymine, and the amount of guanine is equal to the amount of 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 also be 20% (\% G = 20%).

The total percentage of guanine and cytosine is:

$$\backslash \%\; G\; +\; \backslash \%\; C\; =\; 20\backslash \%\; +\; 20\backslash \%\; =\; 40\backslash \%$$

The remaining percentage in the DNA must be adenine and thymine:

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

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

Since the percentage of adenine is equal to the percentage of thymine (\% A = \% T), we can find the percentage of adenine by dividing the combined percentage of A and T by 2:

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

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

DNA strands are antiparallel and complementary to each other. This means that the sequence of bases on one strand dictates the sequence on the other strand, following the base pairing rules: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

The given sequence of one DNA strand is:

5'-ATGCATGCATGCATGCATGCATGC-3'

To find the complementary strand, we apply the base pairing rules and reverse the direction. The base pairing will be:

• A pairs with T
• T pairs with A
• G pairs with C
• C pairs with G

So, the complementary sequence, read in the 3' to 5' direction relative to the given strand, is:

3'-TACGTACGTACGTACGTACGTACG-5'

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

5'-GCATGCATGCATGCATGCATGCAT-3'

Thus, the sequence of the complementary strand in the 5' to 3' direction is 5'-GCATGCATGCATGCATGCATGCAT-3'.

In a transcription unit, the coding strand (also known as the sense strand) has a sequence that is similar to the mRNA sequence, with the difference being that Uracil (U) replaces Thymine (T) in RNA. The template strand (also known as the antisense strand) is complementary to the coding strand and is used as the template for RNA synthesis.

The given sequence of the coding strand is:

5'-ATGCATGCATGCATGCATGCATGC-3'

The mRNA sequence is synthesized using the template strand, but its sequence will be identical to the coding strand, except that Uracil (U) replaces Thymine (T).

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

5'-AUGCAUGCAUGCAUGCAUGCAUGC-3'

The sequence of the mRNA will be 5'-AUGCAUGCAUGCAUGCAUGCAUGC-3'.

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. Complementary Base Pairing: They discovered that the two strands of the DNA double helix are held together by specific hydrogen bonds between complementary bases: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C). This specific pairing ensures that the sequence of bases on one strand precisely determines the sequence on the other strand.
2. Antiparallel Strands: The two polynucleotide strands in the DNA double helix run in opposite directions, referred to as antiparallel (one strand runs 5' to 3', and the other runs 3' to 5').

Explanation of the Hypothesis:

These properties led Watson and Crick to propose that during replication, the two strands of the DNA double helix would separate. Each separated strand would then serve as a template for the synthesis of a new, complementary strand. Because of the strict rules of complementary base pairing (A with T, G with C), the sequence of the new strand would be dictated by the sequence of the template strand.

As a result, each of the two new DNA molecules formed would consist of one strand from the original parental DNA molecule and one newly synthesized strand. This mechanism, where each daughter DNA molecule retains one of the original parental strands, is termed 'semiconservative' replication.

The figure below illustrates this concept:

The term 'semiconservative' signifies that half of the original molecule is conserved in each new molecule.