NEET Biology Molecular Basis of Inheritance MCQs Set 27

Answer: (d) Nucleotides

Explanation:
1. A nucleic acid is a polymer of nucleotides.
2. Phosphoric acid, a base and a pentose sugar are only the parts of one nucleotide.
3. Hence option (d) is correct: Nucleotides

Teacher's Note:
1. Nucleotide = base + pentose sugar + phosphate.
2. Many nucleotides joined by phosphodiester bonds make a polynucleotide.

Answer: (c) Nucleotide

Explanation:
1. A nitrogenous base, a pentose sugar and a phosphate group together make a nucleotide.
2. Base plus sugar alone is a nucleoside.
3. Hence option (c) is correct: Nucleotide

Teacher's Note:
1. Nucleoside has no phosphate.
2. Nucleotide has all three parts.

Answer: (a) H - bonds

Explanation:
1. The bases of the two strands are held by hydrogen bonds: two between A and T and three between G and C.
2. Hence option (a) is correct: H - bonds

Teacher's Note:
1. Hydrogen bonds are weak but many, so they hold the helix together.
2. Strands can be separated by heat.

Answer: (c) Ten

Explanation:
1. A single turn of the DNA helix has 10 base pairs.
2. In each pair one base is a purine, so there are 10 purines in one turn.
3. Hence option (c) is correct: Ten

Teacher's Note:
1. Each base pair has one purine and one pyrimidine.
2. 10 base pairs per turn means 10 purines.

Answer: (b) A + G = T+ C

Explanation:
1. Chargaff's rule says that A = T and G = C, so the total purines equal the total pyrimidines.
2. That is A + G = T + C.
3. Hence option (b) is correct: A + G = T+ C

Teacher's Note:
1. The ratio (A + T)/(G + C) differs from species to species.
2. Only A = T, G = C and purines = pyrimidines hold in all DNA.

Answer: (d) rRNA is less abundant RNA in an animal cell

Explanation:
1. DNA is chemically stable and less reactive than RNA because of the 2'-OH group present in RNA, so (a) and (b) are true.
2. Capping is done by guanyl transferase, so (c) is true.
3. rRNA is the most abundant RNA of a cell, so statement (d) is false.
4. Hence option (d) is correct: rRNA is less abundant RNA in an animal cell

Teacher's Note:
1. rRNA forms about 80 percent of cell RNA.
2. Learn the abundance order rRNA, tRNA, mRNA.

Answer: (c) Specific base pairing between purine and pyrimidine

Explanation:
1. A purine always pairs with a pyrimidine (A with T, G with C).
2. The pair has a constant width, so the diameter of the helix is uniform (about 2 nm).
3. Hence option (c) is correct: Specific base pairing between purine and pyrimidine

Teacher's Note:
1. Purine-purine pairing would be too wide.
2. Pyrimidine-pyrimidine pairing would be too narrow.

Answer: (b) Complementary

Explanation:
1. The two strands are complementary, A pairs with T and G with C.
2. So one strand fixes the sequence of the other.
3. Hence option (b) is correct: Complementary

Teacher's Note:
1. Complementarity is the basis of replication and transcription.

Answer: (a) Hershey and Chase (1952)

Explanation:
1. Hershey and Chase (1952) labelled phage DNA with 32P and protein with 35S.
2. Only DNA entered the bacterium, which proved that DNA is the genetic material.
3. Hence option (a) is correct: Hershey and Chase (1952)

Teacher's Note:
1. Griffith (1928) only showed transformation.
2. Hershey-Chase gave the final proof.

Answer: (b) 50 %

Explanation:
1. Replication is semi-conservative.
2. After one generation both DNA molecules are hybrid (one old strand), and after two generations 2 of the 4 DNA molecules have one old radioactive strand.
3. So 50 percent of the bacteria carry radioactive DNA.
4. Hence option (b) is correct: 50 %

Teacher's Note:
1. Each old strand ends in a different daughter molecule.
2. Meselson and Stahl used this logic.

Answer: (d) CTA, ATC, GTA, CTG

Explanation:
1. The complement of GAT is CTA, of TAG is ATC, of CAT is GTA and of GAC is CTG.
2. So the sequence is CTA, ATC, GTA, CTG.
3. Hence option (d) is correct: CTA, ATC, GTA, CTG

Teacher's Note:
1. Pair A with T and G with C in each triplet.
2. Write the new strand in the same left to right order as asked.

Answer: (a) From 5' end towards 3' end

Explanation:
1. DNA polymerase adds nucleotides only at the 3' end.
2. So a new strand grows in the 5' to 3' direction.
3. Hence option (a) is correct: From 5' end towards 3' end

Teacher's Note:
1. The template is read 3' to 5'.
2. The new strand is made 5' to 3'.

Answer: (d) DNA Polymerase

Explanation:
1. DNA polymerase adds nucleotides to a growing DNA strand during replication.
2. RNA polymerase makes RNA.
3. Hence option (d) is correct: DNA Polymerase

Teacher's Note:
1. DNA polymerase needs a primer.
2. It cannot start a new chain by itself.

Answer: (d) Semi conservative and bidirectional

Explanation:
1. E. coli DNA has one origin and two replication forks moving in opposite directions, so replication is bidirectional.
2. Each new DNA has one old and one new strand, so it is semi-conservative.
3. Hence option (d) is correct: Semi conservative and bidirectional

Teacher's Note:
1. Meselson-Stahl proved semi-conservative replication in E. coli.
2. The circular DNA is copied bidirectionally.

Answer: (b) Lagging strands of DNA only

Explanation:
1. The lagging strand is made in short pieces called Okazaki fragments.
2. These are later joined by DNA ligase.
3. The leading strand is made continuously.
4. Hence option (b) is correct: Lagging strands of DNA only

Teacher's Note:
1. Lagging strand = discontinuous, leading strand = continuous.
2. DNA ligase seals the nicks.

Answer: (b) Small opening of the DNA helix

Explanation:
1. The replication fork is the Y-shaped region where the helix has been opened.
2. It is a small opening that moves along the DNA.
3. Hence option (b) is correct: Small opening of the DNA helix

Teacher's Note:
1. Helicase opens the fork.
2. Single-strand binding proteins keep it open.

Answer: (b) UCGA

Explanation:
1. The RNA formed on the DNA template AGCT is complementary to it: A gives U, G gives C, C gives G and T gives A.
2. So the RNA is UCGA.
3. Hence option (b) is correct: UCGA

Teacher's Note:
1. RNA has uracil in place of thymine.
2. Write complement base by base.

Answer: (c) tRNA

Explanation:
1. tRNA carries an amino acid at one end and reads the codon with its anticodon at the other.
2. Crick called it the adaptor molecule.
3. Hence option (c) is correct: tRNA

Teacher's Note:
1. Adaptor = links codon to amino acid.
2. Crick proposed the adaptor hypothesis.

Answer: (c) Uracil

Explanation:
1. In RNA adenine pairs with uracil, since there is no thymine in RNA.
2. Cytosine pairs with guanine.
3. Hence option (c) is correct: Uracil

Teacher's Note:
1. A-U and G-C are the pairs in RNA.
2. A-T is in DNA.

Answer: (a) Transcription

Explanation:
1. Copying of information from the DNA template into mRNA is transcription.
2. Translation is mRNA to protein.
3. Hence option (a) is correct: Transcription

Teacher's Note:
1. Transcription happens in the nucleus.
2. Translation happens on ribosomes.

Answer: (c) N- formyl methionine

Explanation:
1. In bacteria, AUG as the starting codon codes for N-formyl methionine.
2. In eukaryotes it codes for methionine.
3. Hence option (c) is correct: N- formyl methionine

Teacher's Note:
1. fMet starts bacterial proteins.
2. It is removed later.