What forms the backbone of a DNA strand?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Sugar and Base
• Phosphate and Base
• Sugar and Phosphate
• Base pairs
Correct Option: C  [ Sugar and Phosphate ]

Remark: The backbone of the DNA strand is formed by the alternating sugar (deoxyribose) and phosphate groups. The nitrogenous bases project inwards from this backbone.

The reading frame of mRNA is shifted by?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Point mutation (substitution)
• Silent mutation
• Frameshift mutation (insertion/deletion)
• Inversion
Correct Option: C  [ Frameshift mutation (insertion/deletion) ]

Remark: Insertions or deletions of nucleotides (not in multiples of 3) shift the reading frame of the ribosome, altering the amino acid sequence downstream of the mutation.

If the ratio of (A+T)/(G+C) in one strand is 0.7, what is it in the complementary strand?

[Molecular-Basis-of-Inheritance] [class-xii ]

• 0.3
• 0.7
• 1.0
• 1.4
Correct Option: B  [ 0.7 ]

Remark: Since A pairs with T and G pairs with C, the amount of A+T on one strand equals the amount of T+A on the complementary strand. The same applies to G+C. Therefore, the ratio (A+T)/(G+C) remains the same for both strands.

Which enzyme is used to synthesize DNA from an RNA template (Reverse Transcription)?

[Molecular-Basis-of-Inheritance] [class-xii ]

• RNA Polymerase
• Reverse Transcriptase
• DNA Polymerase
• Primase
Correct Option: B  [ Reverse Transcriptase ]

Remark: Reverse Transcriptase is an enzyme found in retroviruses that synthesizes a complementary DNA (cDNA) strand using an RNA template, reversing the central dogma flow.

H2A, H2B, H3, and H4 occur in the nucleosome as?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Dimers
• Tetramers
• Pairs (Octamer)
• Monomers
Correct Option: C  [ Pairs (Octamer) ]

Remark: The histone core consists of two molecules each of H2A, H2B, H3, and H4, forming an octamer (8 units) around which DNA wraps.

The process of translation occurs in the?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Nucleus
• Nucleolus
• Cytoplasm
• Mitochondria only
Correct Option: C  [ Cytoplasm ]

Remark: Translation, the synthesis of proteins from mRNA, takes place on ribosomes located in the cytoplasm (or on the rough ER) of the cell.

The start codon AUG codes for which amino acid?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Valine
• Methionine
• Tryptophan
• Leucine
Correct Option: B  [ Methionine ]

Remark: AUG is the universal start codon and it specifically codes for the amino acid Methionine. In prokaryotes, it codes for formylated-methionine.

How many nitrogen atoms are present in a Purine ring?

[Molecular-Basis-of-Inheritance] [class-xii ]

• 2
• 4
• 6
• 9
Correct Option: B  [ 4 ]

Remark: A purine ring consists of a nine-membered double-ring structure containing 4 nitrogen atoms at positions 1, 3, 7, and 9.

Which bonds hold the two DNA strands together?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Covalent bonds
• Ionic bonds
• Hydrogen bonds
• Van der Waals forces
Correct Option: C  [ Hydrogen bonds ]

Remark: The two strands of the double helix are held together by hydrogen bonds between complementary nitrogenous bases (A-T and G-C).

In a nucleoside, the nitrogen base is attached to which carbon of the sugar?

[Molecular-Basis-of-Inheritance] [class-xii ]

• 1-prime
• 2-prime
• 3-prime
• 5-prime
Correct Option: A  [ 1-prime ]

Remark: The nitrogenous base is always attached to the C1 (1-prime) carbon of the pentose sugar via the N-glycosidic bond.