Exam 10: DNA Replication and Recombination

Meselson and Stahl determined that DNA replication in E. coli is semiconservative. What additive did they initially supply to the medium in order to distinguish "new" from "old" DNA?

Given the diagram below, assume that a G1 chromosome (left) underwent one round of replication in 3H -thymidine and the metaphase chromosome (right) had both chromatids labeled. Which of the following replicative models (conservative, dispersive, semiconservative) could be eliminated by this observation?

Which structural circumstance in DNA sets up the requirement for the semidiscontinuous nature of its replication?

Telomerase is an RNA -containing enzyme that adds telomeric DNA sequences onto the ends of linear chromosomes.

Each of the following terms refers to the replication of chromosomes. Describe the role (relationship) of each in (to) chromosome replication. a() Okazaki fragment b() lagging strand c() bidirectional

Given that the nature of DNA replication in eukaryotes is not as well understood as in prokaryotes, (a) present a description of DNA (chromosome) replication as presently viewed in eukaryotes and (b) state the differences known to exist between prokaryotic and eukaryotic DNA replication.

DNA replication occurs in the 5' to 3' direction; that is, new nucleoside triphosphates are added to the 3' end.

DNA polymerase III adds nucleotides _.

List four enzymes known to be involved in the replication of DNA in bacteria.

Assume that a culture of E. coli was grown for approximately 50 generations in 15N (provided in the medium in the ammonium ion), which is a heavy isotope of nitrogen (14N). You extract the DNA from the culture, and it has a density of 1.723 gm/cm3 (water = 1.00 gm/cm3). From the literature, you determine that DNA containing only the common form of nitrogen, 14N, has a density of 1.700 gm/cm3. Bacteria from the 15N culture were washed in buffer and transferred to 14N medium for one generation immediately after which the DNA was extracted and its density determined. a() What would be the expected density of the extracted DNA? b() After you heat the extracted DNA until it completely denatures (95°C for 15 minutes), what would you expect the density of the DNA in the denatured extract to be? For the purposes of this question, assume that DNA has the same density regardless of whether it is single - or double -stranded. c() Assuming that the molar percentage of adenine in the extracted DNA was 20 percent, what would be the expected molar percentages of the other nitrogenous bases in this DNA? d() Assume that a fraction of the extracted DNA was digested to completion with the enzyme snake venom diesterase. This enzyme cleaves between the phosphate and the 3' carbon. Present a "simplified" diagram that would illustrate the structure of the predominant resulting molecule.

In the Meselson and Stahl (1958) experiment, bean plants (Vicia faba) were radioactively labeled so that autoradiographs could be made of chromosomes.

Assume that you grew a culture of E. coli for many generations in medium containing 15N (from the ammonium ion), a heavy isotope of nitrogen. You extract DNA from a portion of the culture and determine its density to be 1.723 gm/cm3 (call this sample A). You then wash the remaining E. coli cells, grow them for one generation in 14N, and extract the DNA from a portion of the culture (call this sample B). You let the culture grow for one more generation in 14N and extract the DNA (call this sample C) is then subjected to ultracentrifugation. Present the centrifugation profiles that you would expect under (a) semiconservative replication and (b) conservative replication. (Note: Assume that unlabeled [14N] DNA has a density of 1.700 gm/cm3.) C). Each sample of DNA (A, B, and

Describe a somewhat extraordinary finding related to the Tetrahymena telomerase enzyme.

Refer to the following diagram of a generalized tetranucleotide to answer questions (a) through (e). a() Is this a DNA or an RNA molecule? ________ b() Place an "X" (in one of the circles in the diagram) at the 3' end of this tetranucleotide. c() Given that the DNA strand, which served as a template for the synthesis of this tetranucleotide, was composed of the bases 5' - A C A G - 3', fill in the parentheses (in the diagram) with the expected bases. d() Suppose that one of the precursors for this tetranucleotide was a 32P -labeled guanine nucleoside triphosphate (the innermost phosphate containing the radioactive phosphorus). Circle the radioactive phosphorus atom as it exists in the tetranucleotide. e() Given that spleen diesterase (breaks between the phosphate and the 5' carbon) digests the pictured tetranucleotide, which base(s) among the breakdown products would be expected to be attached to the 32P?

What primary ingredients, coupled with DNA polymerase I, are needed for the in vitro synthesis of DNA?

During replication, primase adds a DNA primer to RNA.

Describe the DNA base sequence arrangement at the end of the Tetrahymena chromosome and the resolution of DNA replication at the end of a linear DNA strand.

A nucleosome is a structure associated with the nuclear membrane. It helps maintain a stable relationship between the extracellular matrix and the membrane itself.

DNA strand replication begins with an RNA primer.

Assume that you were growing cells in culture and had determined the cell -cycle time to be 24 hours. You introduce 3H thymidine and prepare autoradiographs of metaphase chromosomes after 48 hours. Of the chromosomes that are labeled, you expect two classes: one class that had completed one S phase in the label, and a second class that had completed a cellular division and an additional S phase in the label. Draw the DNA (double -stranded) labeling pattern for each chromosome that you would expect to find in these two types of metaphase chromosomes. (Use a broken line { - - -} for labeled single strands of DNA and a solid line for unlabeled single strands of DNA.) a() metaphase chromosome having replicated once in label b() metaphase chromosome having gone through two S phases in label