Biochemistry Chapter 8

Which of the following is NOT a pyrimidine base?

• cytosine

• uracil

• adenosine

• thymine

Answer explanation

Adenosine is not a pyrimidine base; it is a purine. Pyrimidine bases include cytosine, uracil, and thymine. Therefore, the correct answer is adenosine.

Which of the following is NOT part of a deoxyribonucleotide?

• a deoxyribonucleoside

• phosphate

• a nitrogenous base

• ribose

Answer explanation

A deoxyribonucleotide consists of a deoxyribonucleoside (which includes a nitrogenous base and deoxyribose) and a phosphate group. Ribose is not part of a deoxyribonucleotide; it is found in ribonucleotides.

Why is DNA more stable than RNA?

• RNA can never be double-stranded; therefore, DNA is more stable.

• The 2’-hydroxyl groups in RNA (absent in DNA) are directly involved in the hydrolysis of oligonucleotides.

• Thymine forms more stable hydrogen bonds than uracil.

• RNA does not have secondary structure.

Answer explanation

The correct choice highlights that the 2’-hydroxyl groups in RNA make it more prone to hydrolysis, leading to less stability compared to DNA, which lacks these groups.

A strand of DNA or RNA:

• contains 5′ - 2′ phosphodiester bonds.

• might be an oligonucleotide.

• has a C-terminus and an N-terminus.

• has no charge.

Answer explanation

A strand of DNA or RNA can be an oligonucleotide, which is a short sequence of nucleotides. The other options are incorrect: phosphodiester bonds are 5′ to 3′, they do not have C-terminus and N-terminus, and they carry a negative charge.

Which statement is true about the three-dimensional structure of DNA?

• Native DNA consists of two parallel chains in a right-handed double-helical arrangement.

• Complementary base pairs, A≡C and G=T, are formed by hydrogen bonding between chains in the helix.

• The base pairs are stacked parallel to the long axis of the double helix.

• The offset pairing of the two strands creates a major groove and a minor groove on the surface of the duplex.

Answer explanation

The correct statement highlights that the offset pairing of DNA strands results in a major groove and a minor groove, which are important for protein binding and recognition, distinguishing it from the other incorrect options.

Which statement regarding bases in DNA is false?

• They absorb UV light.

• Pyrimidines are aromatic; purines are not.

• They have vertical stacking interactions.

• They hydrogen bond between two or more complementary DNA strands.

Answer explanation

The statement 'Pyrimidines are aromatic; purines are not' is false. Both pyrimidines (cytosine, thymine, uracil) and purines (adenine, guanine) are aromatic compounds, as they contain conjugated pi electron systems.

Some effort has been made to extract DNA from fossilized dinosaur bones that have been buried for many millions of years in arid climates. In what form is this DNA MOST likely to be found?

• A

• B

• Z

• completely denatured

Answer explanation

The DNA extracted from fossilized dinosaur bones is most likely to be found in a completely denatured form due to the extensive time and environmental conditions that would have degraded the original structure.

Telomeres are noncoding sequences on the ends of chromosomes. They contain a high concentration of guanosine. Given this information, what other structure may be in telomeres?

• cyclobutane dimers

• a high concentration of triplexes

• tetraplexes

• A-form DNA

Answer explanation

Telomeres, rich in guanosine, can form tetraplex structures due to the ability of guanine to pair in four-stranded configurations. This characteristic distinguishes them from other structures like cyclobutane dimers or triplexes.

Which of the following is NOT associated with DNA structure?

• A. Z-DNA

• B. G tetraplexes

• C. three hydrogen bonds between A and T

• D. pyrimidines in the anti conformation

Answer explanation

The correct answer is C. Adenine (A) and Thymine (T) form two hydrogen bonds, not three. Z-DNA, G tetraplexes, and pyrimidines in the anti conformation are all associated with DNA structure.

Which statement regarding mRNA is false?

• It is always monocistronic.

• It carries genetic information to the ribosomes.

• It is formed by transcription.

• It can be of varying lengths.

Answer explanation

The statement "It is always monocistronic" is false because mRNA can be polycistronic in prokaryotes, meaning it can encode multiple proteins. The other statements about mRNA are true.