In the Hershey-Chase experiment, radioactive Phosphorus-32 (P-32) was used to label?

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

• Viral Protein
• Viral DNA
• Bacterial Cell Wall
• Viral Capsule
Correct Option: B  [ Viral DNA ]

Remark: Phosphorus is a key component of the DNA backbone (phosphate groups) but is not found in amino acids (proteins). Therefore, Hershey and Chase used radioactive Phosphorus-32 to specifically label the viral DNA, distinguishing it from the protein coat which contains sulfur but no phosphorus.

The transforming principle in Griffith’s experiment was identified as DNA by?

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

• Hershey and Chase
• Watson and Crick
• Avery, MacLeod, and McCarty
• Meselson and Stahl
Correct Option: C  [ Avery, MacLeod, and McCarty ]

Remark: Oswald Avery, Colin MacLeod, and Maclyn McCarty purified biochemicals (proteins, DNA, RNA) from the S-strain bacteria. They discovered that digestion with DNase inhibited transformation, while proteases and RNases did not, proving that the transforming principle described by Griffith was DNA.

Which of the following is NOT a reason for DNA being more stable than RNA?

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

• Presence of Thymine instead of Uracil
• Double-stranded structure
• Absence of 2-prime OH group in sugar
• Catalytic activity
Correct Option: D  [ Catalytic activity ]

Remark: Catalytic activity (acting as a ribozyme) is a property of RNA that makes it reactive and unstable, not stable. DNA is more stable due to the absence of the reactive 2-prime hydroxyl group, the presence of thymine (which is more stable than uracil), and its double-stranded helical structure providing protection.

Which isotope was used by Meselson and Stahl to prove the semi-conservative nature of DNA replication?

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

• Radioactive N-14
• Radioactive N-15
• Heavy isotope N-15
• Radioactive P-32
Correct Option: C  [ Heavy isotope N-15 ]

Remark: Meselson and Stahl used Nitrogen-15 (N-15), which is a heavy isotope of nitrogen, not a radioactive one. They distinguished it from the normal Nitrogen-14 (N-14) based on density differences using Cesium Chloride density gradient centrifugation.

Who used Vicia faba (Faba beans) to prove that DNA replication is semi-conservative in eukaryotes?

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

• Meselson and Stahl
• Taylor and colleagues
• Hershey and Chase
• Watson and Crick
Correct Option: B  [ Taylor and colleagues ]

Remark: While Meselson and Stahl worked on E. coli, Taylor and his colleagues used radioactive thymidine on Vicia faba (faba beans) to demonstrate that the semi-conservative mode of DNA replication also applies to eukaryotic chromosomes.

The enzyme responsible for unwinding the DNA helix during replication is?

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

• DNA Polymerase
• Ligase
• Helicase
• Primase
Correct Option: C  [ Helicase ]

Remark: Helicase is the enzyme responsible for breaking the hydrogen bonds between the nitrogenous bases, thereby unwinding the double helix and creating a replication fork to allow the machinery to access the template strands.

Which enzyme joins the Okazaki fragments on the lagging strand?

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

• DNA Polymerase III
• DNA Ligase
• Primase
• Topoisomerase
Correct Option: B  [ DNA Ligase ]

Remark: On the lagging strand, DNA is synthesized in short, discontinuous segments called Okazaki fragments. The enzyme DNA Ligase acts as a molecular glue to join these fragments together by forming phosphodiester bonds, creating a continuous DNA strand.

DNA replication occurs in which phase of the cell cycle?

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

• G1 phase
• G2 phase
• S phase
• M phase
Correct Option: C  [ S phase ]

Remark: DNA replication takes place during the Synthesis (S) phase of the cell cycle. During this phase, the amount of DNA in the cell doubles to ensure that each daughter cell receives a complete set of genetic material after division.

The synthesis of the leading strand during DNA replication is?

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

• Discontinuous
• Continuous
• Conservative
• Random
Correct Option: B  [ Continuous ]

Remark: The leading strand is synthesized continuously in the 5-prime to 3-prime direction, which is the same direction as the movement of the replication fork. This allows DNA polymerase to add nucleotides continuously without interruption.

Which strand of DNA acts as the template for transcription?

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

• Coding strand (5-3)
• Template strand (3-5)
• Both strands
• None of the above
Correct Option: B  [ Template strand (3-5) ]

Remark: The enzyme RNA polymerase catalyzes polymerization in the 5-prime to 3-prime direction. Therefore, it reads the DNA template in the 3-prime to 5-prime direction. The strand with 3-prime to 5-prime polarity acts as the template strand.