Question

dp biology dna structure & replication formative assessment (25 marks)
paper 1: multiple - choice

1. which discovery about dna is watson and crick most credited?

a. dna is the molecule that genes are made of.
b. the amount of adenine equals the amount of thymine in an organism.
c. phosphate - pentose bonding along the nucleotide backbone is covalent.
d. the shape of dna is a double helix.

1. what links the pairs of complementary bases in a dna double helix?

a. covalent bonds
b. hydrogen bonds
c. ionic bonds
d. peptide bonds

1. the percentage of thymine in the dna of an organism is approximately 20%. what is the percentage of guanine?

a. 80%
b. 30%
c. 40%
d. 20%

1. a short sequence of nucleotides reads ggacagagtcgcagacga. in which type of molecule could this sequence be found?

a. dna molecule only
b. rna molecule only
c. both in a dna and an rna molecule
d. in double - stranded dna only

Explanation:

1. Watson and Crick are credited with discovering the double - helix structure of DNA. Option D is correct as it was their key discovery.
2. Complementary base pairs in a DNA double - helix are linked by hydrogen bonds. Adenine pairs with thymine via two hydrogen bonds and guanine pairs with cytosine via three hydrogen bonds. So, option B is correct.
3. In DNA, the amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C). If T = 20%, then A = 20%. The combined percentage of A + T is 40%. So, the combined percentage of G + C is 60%, and the percentage of G is 30%. Option B is correct.
4. The given sequence contains thymine (T), which is characteristic of DNA. RNA has uracil (U) instead of thymine. So, this sequence can be found in a DNA molecule only. Option A is correct.

Answer:

1. D. The shape of DNA is a double helix.
2. B. Hydrogen bonds
3. B. 30%
4. A. DNA molecule only