Essential Cell Biology Chapter 6

The process of DNA replication requires that each of the parental DNA strands be used as a ___________________ to produce a duplicate of the opposing strand. (a) catalyst (b) competitor (c) template (d) copy

(c) template

How does the total number of replication origins in bacterial cells compare with the number of origins in human cells? (a) 1 versus 100 (b) 5 versus 500 (c) 10 versus 1000 (d) 1 versus 10,000

(d) 1 versus 10,000

DNA replication is considered semiconservative because ____________________________. (a) after many rounds of DNA replication, the original DNA double helix is still intact. (b) each daughter DNA molecule consists of two new strands copied from the parent DNA molecule. (c) each daughter DNA molecule consists of one strand from the parent DNA molecule and one new strand. (d) new DNA strands must be copied from a DNA template.

Choice (c) is the correct answer. Choices (a) and (b) are false. Although choice (d) is a correct statement, it is not the reason that DNA replication is called semiconservative.

DNA polymerase catalyzes the joining of a nucleotide to a growing DNA strand. What prevents this enzyme from catalyzing the reverse reaction? (a) hydrolysis of pyrophosphate (PPi) to inorganic phosphate (Pi) + Pi (b) release of PPi from the nucleotide (c) hybridization of the new strand to the template (d) loss of ATP as an energy source

(a) hydrolysis of pyrophosphate (PPi) to inorganic phosphate

How many replication forks are formed when an origin of replication is opened? (a) 1 (b) 2 (c) 3 (d) 4

(b) 2

Initiator proteins bind to replication origins and disrupt hydrogen bonds between the two DNA strands being copied. Which of the factors below does not contribute to the relative ease of strand separation by initiator proteins? (a) replication origins are rich in A-T base pairs (b) the reaction can occur at room temperature (c) they only separate a few base pairs at a time (d) once opened, other proteins of the DNA replication machinery bind to the origin

(b) semiconservative

The classic experiments conducted by Meselson and Stahl demonstrated that DNA replication is accomplished by employing a ________________ mechanism. (a) continuous (b) semiconservative (c) dispersive (d) conservative

(b) semiconservative

The chromatin structure in eukaryotic cells is much more complicated than that observed in prokaryotic cells. This is thought to be the reason that DNA replication occurs much faster in prokaryotes. How much faster is it? (a) 2× (b) 5× (c) 10× (d) 100×

(c) 10×

Which of the following statements correctly explains what it means for DNA replication to be bidirectional? (a) The replication fork can open or close, depending on the conditions. (b) The DNA replication machinery can move in either direction on the template strand. (c) Replication-fork movement can switch directions when the fork converges on another replication fork. (d) The replication forks formed at the origin move in opposite directions.

(d) The replication forks formed at the origin move in opposite directions.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. A. When DNA is being replicated inside a cell, local heating occurs, allowing the two strands to separate. B. DNA replication origins are typically rich in G-C base pairs. C. Meselson and Stahl ruled out the dispersive model for DNA replication. D. DNA replication is a bidirectional process that is initiated at multiple locations along chromosomes in eukaryotic cells.

A. False. The two strands do need to separate for replication to occur, but this is accomplished by the binding of initiator proteins at the origin of replication. B. False. DNA replication origins are typically rich in A-T base pairs, which are held together by only two hydrogen bonds (instead of three for C-G base pairs), making it easier to separate the strands at these sites. C. True. D. True.

Answer the following questions about DNA replication. A. On a DNA strand that is being synthesized, which end is growing—the 3′ end, the 5′ end, or both ends? Explain your answer. B. On a DNA strand that is being used as a template, where is the copying occurring relative to the replication origin—3′ of the origin, 5′, or both?

A. The 3′ end. DNA polymerase can add nucleotides only to the 3′-OH end of a nucleic acid chain. B. Both, as a result of the bidirectional nature of chromosomal replication.

Meselson and Stahl grew cells in media that contained different isotopes of nitrogen (15N and 14N) so that the DNA molecules produced from these different isotopes could be distinguished by mass. A. Explain how "light" DNA was separated from "heavy" DNA in the Meselson and Stahl experiments. B. Describe the three existing models for DNA replication when these studies were begun, and explain how one of them was ruled out definitively by the experiment you described for part A. C. What experimental result eliminated the dispersive model of DNA replication?

A. The DNA samples collected were placed into centrifuge tubes containing cesium chloride. After high-speed centrifugation for 2 days, the heavy and light DNA products were separated by density. B. The three models were conservative, semiconservative, and dispersive. The conservative model suggested a mechanism by which the original parental strands stayed together after replication and the daughter duplex was made entirely of newly synthesized DNA. The semiconservative model proposed that the two DNA duplexes produced during replication were hybrid molecules, each having one of the parental strands and one of the newly synthesized strands. The dispersive model predicted that the new DNA duplexes each contained segments of parental and daughter strands all along the molecule. The conservative model was ruled out by the density-gradient experiments. C. The dispersive model was ruled out by using heat to denature the DNA duplexes and then comparing the densities of the single-stranded DNA. If the dispersive model had been correct, individual strands should have had an intermediate density. However, this was not the case; only heavy strands and light strands were observed, which convincingly supported the semiconservative model for DNA replication.

If the genome of the bacterium E. coli requires about 20 minutes to replicate itself, how can the genome of the fruit fly Drosophila be replicated in only 3 minutes? (a) The Drosophila genome is smaller than the E. coli genome. (b) Eukaryotic DNA polymerase synthesizes DNA at a much faster rate than prokaryotic DNA polymerase. (c) The nuclear membrane keeps the Drosophila DNA concentrated in one place in the cell, which increases the rate of polymerization. (d) Drosophila DNA contains more origins of replication than E. coli DNA.

Choice (d) is the correct answer. Bacteria have one origin of replication, and Drosophila has many. Choice (a) is incorrect because the Drosophila genome is bigger than the E. coli genome. Choice (b) is incorrect, because eukaryotic polymerases are not faster than prokaryotic polymerases.

Use your knowledge of how a new strand of DNA is synthesized to explain why DNA replication must occur in the 5′-to-3′ direction. In other words, what would be the consequences of 3′-to-5′ strand elongation?

There would be several detrimental consequences to 3′-to-5′ strand elongation. One of those most directly linked to the processes of DNA replication involves synthesis of the lagging strand. After the RNA primers are degraded, the DNA segments remaining will have 5′ ends with a single phosphate group. The incoming nucleotide will have a 3′-OH group. Without the energy provided by the release of PPi from the 5′ end, the process of elongation would no longer be energetically favorable.