Molecular Biology

Step 1: Understanding nitrogen-containing bases.
Both DNA and RNA share two nitrogenous bases: adenine (A) and cytosine (C). However, DNA contains thymine (T), while RNA contains uracil (U) instead.
Step 2: Analyzing the options.
(A) Four: Incorrect, there are four nitrogenous bases in each, but only two are common to both DNA and RNA.
(B) Two: Correct, adenine (A) and cytosine (C) are common to both DNA and RNA.
(C) Three: Incorrect, only two nitrogenous bases are common to both DNA and RNA.
(D) One: Incorrect, there are two common bases, not one.
Step 3: Conclusion.
The correct answer is (B) Two, as adenine and cytosine are common to both DNA and RNA.

Step 1: Understanding the role of telomeres.
Telomeres are repetitive sequences at the ends of chromosomes that protect the chromosome from degradation and prevent them from sticking together during cell division.
Step 2: Analyzing the options.
(A) condensation of chromatin network: Incorrect, this is not a property of telomeres.
(B) prevention of ends of the chromosomes from sticking together: Correct, this is one of the key roles of telomeres, ensuring chromosomes remain intact during division.
(C) attaching to spindle fibres: Incorrect, telomeres do not play a role in attaching chromosomes to the spindle fibres.
(D) regulation of protein synthesis: Incorrect, telomeres are not involved in protein synthesis.
Step 3: Conclusion.
The correct answer is (B) prevention of ends of the chromosomes from sticking together.

Step 1: Understanding the structure of DNA.
DNA has two strands that coil around each other to form a double helix. The number of complete spirals (turns of the helix) in a given segment depends on the number of nucleotides. Each complete spiral in the DNA helix corresponds to about 10 base pairs (20 nucleotides).
Step 2: Analyzing the options.
(A) 5: Correct. For 100 nucleotides, there will be 5 complete turns, as each turn of the helix contains approximately 20 nucleotides.
(B) 6: Incorrect. 6 turns would require 120 nucleotides.
(C) 3: Incorrect. 3 turns would require 60 nucleotides.
(D) 4: Incorrect. 4 turns would require 80 nucleotides.
Step 3: Conclusion.
The correct answer is (A) 5, as 100 nucleotides correspond to 5 complete spirals.