Chapter 23 Self-test questions

. Which enzymes remove supercoiling in replicating DNA ahead of the replication fork?

Helicases.

DNA polymerases.

Primases.

Topoisomerases.

. Which reaction in DNA replication is catalysed by DNA ligase?

Addition of new nucleotides to the lagging strand.

Addition of new nucleotides to the leading strand.

Base pairing of the template and the newly formed DNA strand.

Formation of a phosphodiester bond between the 3'-OH of one Okazaki fragment and the 5'-phosphate of the next on the lagging strand.

. In which of the following would you find telomeres?

Human mitochondrial DNA.

Human chromosomes.

Bacterial chromosomes.

The influenza virus genome.

. Which of the following reactions is required for proofreading (i.e. correcting replication errors) during DNA replication by DNA polymerase III?

3' – 5' exonuclease activity.

5' – 3' exonuclease activity.

3' – 5' endonuclease activity.

5' – 3' endonuclease activity.

. How does the mismatch repair system distinguish between the parental (i.e. correct) DNA strand and the newly synthesised strand containing the mismatched base?

Thymine in the parental strand of the helix is methylated at GATC.

Thymine in the new strand of the helix is methylated at GATC.

Guanine in the parental strand of the helix is methylated at GATC.

Guanine in the new strand of the helix is methylated at GATC.

. What is the name of the DNA repair system in E. coli in which dual incisions are made in the damaged part of the double helix, and a 12–13 base segment is removed and replaced with new DNA?

Mismatch repair.

Base excision repair.

Nucleotide excision repair.

AP site repair.

. Which of the following is the name of the human genetic disorder resulting from defects in nucleotide excision repair?

Hereditary nonpolyposis colorectal cancer (HNPCC).

Xeroderma pigmentosum (XP).

Lynch syndrome.

Diabetes.

. During which phase of the cell cycle is DNA replicated?

G1 phase.

S phase.

G2 phase.

M phase.

. The supercoils are produced in front of a growing DNA chain during DNA replication are positive supercoils, resulting from overwinding of the helix. True or False?

True.

False.

. During DNA replication, nucleotides are added on to the 5' end of the growing DNA strand. True or False?

True.

False.

. Which of the following statements about the E. coli chromosome is correct? Please select all that apply.

The E. coli chromosome is a single replicon.

Replication begins at oriC.

Replication can start at any point in the chromosome.

A single replication fork moves around the molecule until the chromosome is completely replicated.

. Which of the following statements is correct? Please select all that apply.

Telomeres become shorter as cells age.

Somatic cells have very little telomerase.

Immortal cancer cell lines always have high levels of telomerase.

Telomeres become longer as cells age.

Chapter 23 Self-test questions

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