DNA enzymes

26) What is the function of the enzyme topoisomerase in DNA replication?

• A) relieving strain in the DNA ahead of the replication fork caused by the untwisting of the double helix

• B) elongating new DNA at a replication fork by adding nucleotides to the existing chain

• C) reattaching the hydrogen bonds between the base pairs in the double helix

• D) building RNA primers using the parental DNA strand as a template

27) What is the role of DNA ligase in the elongation of the lagging strand during DNA replication?

• D) It stabilizes the unwound parental DNA

• A) It synthesizes RNA nucleotides to make a primer

• B) It joins Okazaki fragments together.

• C) It unwinds the parental double helix

28) Which of the following types of molecules help to hold the DNA strands apart while they are being replicated?

• A) primase

• B) ligase

• C) DNA polymerase

• D) single-strand DNA binding proteins

29) Individuals with the disorder xeroderma pigmentosum are hypersensitive to sunlight, and mutations to the DNA in their skin cells are left uncorrected. Why are the mutations not corrected in individuals with this disorder?

• A) The disorder makes cells unable to replicate DNA.

• B) The disorder causes mitosis to stop during metaphase

• C) The disorder makes cells unable to form chromosomes

• D) The disorder causes cells to be unable to repair thymine dimers

30) Which of the following characteristics would you expect of a eukaryotic organism that lacks the enzyme telomerase?

• A) a high probability of somatic cells becoming cancerous

• B) an inability to produce Okazaki fragments

• C) an inability to repair thymine dimers

• D) a reduction in chromosome length in gametes

31) Use the figure to answer the following question.

In the late 1950s, Meselson and Stahl grew bacteria in a medium containing "heavy" (radioactive) nitrogen (13N) and then transferred them to a medium containing 14N (nonradioactive). Which of the results in the figure would be expected after one round of DNA replication in the presence of 14N?

• A) A

• B) B

• C) C

• D) D

32) Use the figure to answer the following question

A space probe returns with a culture of a microorganism found on a distant planet. Analysis shows that it is a carbon-based life-form that has DNA. You grow the cells in 15N medium for several generations and then transfer them to 14N medium. Which pattern in the figure would you expect if the DNA was replicated in a conservative manner?

• A) A

• B) B

• C) C

• D) D

33) After the first replication was observed in their experiments testing the nature of DNA replication, Meselson and Stahl could be confident of which of the following conclusions?

• A) Replication is semi-conservative

• B) Replication is not dispersive.

• C) Replication is not conservative

• D) Replication is neither dispersive nor conservative.

34) You briefly expose bacteria undergoing DNA replication to radioactively labeled nucleotides. When you centrifuge the DNA isolated from the bacteria, the DNA separates into two classes. One class of labeled DNA includes very large molecules (thousands or even millions of nucleotides long), and the other includes short stretches of DNA (several hundred to a few thousand nucleotides in length). Which two classes of DNA do these different samples represent?

• A) leading strands and Okazaki fragments

• B) lagging strands and Okazaki fragments

• C) Okazaki fragments and RNA primers

• D) leading strands and RNA primers

35) Within a double-stranded DNA molecule, adenine forms hydrogen bonds with thymine, and cytosine forms hydrogen bonds with guanine. What is the significance of the structural arrangement?

• A) It allows variable width of the double helix.

• B) It permits complementary base pairing

• C) It determines the tertiary structure of a DNA molecule.

• D) It determines the type of protein produced.