Chapter 16

In a nucleosome, the DNA is wrapped around

histones

In analyzing the number of different bases in a DNA sample, which result would be consistent with the base-pairing rules?

A + G = C + T

What are chromosomes made of?

DNA and proteins Long DNA molecules are associated with proteins that allow DNA to coil and pack.

A biochemist isolates, purifies, and combines in a test tube a variety of molecules needed for DNA replication. When she adds some DNA to the mixture, replication occurs, but each DNA molecule consists of a normal strand paired with numerous segments of DNA a few hundred nucleotides ling. What has she probably left out of the mixture?

DNA ligase

What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized?

DNA polymerase can join new nucleotides only to the 3' end of a growing strand

Select the most accurate statement describing DNA replication complexes.

DNA replication complexes are grouped into factories, which are anchored to the nuclear matrix

Meselson and Stahl cultured E. coli for several generations in a medium with a heavy isotope of nitrogen, N-15. They transferred the bacteria to a medium with a light isotope of nitrogen, N-14. After two rounds of DNA replication, half the DNA molecules were light (both strands had N-14) and half were hybrids (N-15 - N-14). What did the researchers conclude from these results?

DNA replication is semiconservative

During DNA replication, the leading strand is synthesized continuously, whereas the lagging strand is synthesized as Okazaki fragments. Why is this so?

DNA synthesis can take place only in the 5' to 3' direction

DNA replication is said to be semiconservative. What does this mean?

Each new double helix consists of one old and one new strand

Who conducted the X-ray diffraction studies that were key to the discovery of the structure of DNA?

Franklin

Nucleotides are added to a growing DNA strand as nucleoside triphosphates. What is the significance of this fact?

Hydrolysis of the two phosphate groups (P-Pi) and DNA polymerization are a coupled exergonic reaction

Which of the following is true of DNA during interphase?

It exists as chromatin and is less condensed than mitotic chromosomes Chromatin is the term for the loosely coiled DNA observed in cells during interphase. Even during interphase, however, parts of the chromosome may still be rather tightly coiled and packed.

What is the function of helicase in DNA replication?

It untwists the double helix and separates the two DNA strands By pulling apart and untwisting the DNA strands, helicase makes them available for replication.

In the Hershey and Chase experiment that helped confirm that DNA, not protein, was the hereditary material, what was the key finding?

Radioactively labeled phosphorus was present inside the infected bacteria When the bacteria had been infected with T2 phage whose DNA was tagged with radioactive phosphorus, the pellet of mainly bacterial material contained most of the radioactivity, indicating that the labeled phage DNA had entered the cells.

DNA is a self-replicating molecule. What accounts for this important property of DNA

The nitrogenous bases of the double helix are paired in specific combinations: A/T and G/C Because of the specificity of the pairing of the nitrogenous bases, each strand of the double helix specifies the matching sequence of bases on the other strand. During replication, when the strands separate, each strand serves as a template for the replication of other strand.

Griffith's experiments with S. pneumonia were significant because they showed that traits could be transferred from one organism to another. What else did he find that was significant?

The transferred traits were heritable The fact that offspring of transformed bacteria also showed the pathogenic trait meant that the transforming agent had to be involved with the genetic material

The elongation of the leading strand during DNA synthesis

depends on the action of DNA polymerase

In nucleotide excision repair, damaged DNA is excised by what enzyme(s)?

nuclease Nucleases are enzymes that cut DNA.

The spontaneous loss of amino groups from adenine in DNA results in hypoxanthine, an uncommon base, opposite thymine. What combination of proteins could repair such damage?

nuclease, DNA polymerase, DNA ligase

What process repairs damage to a preexisting double helix?

nucleotide excision repair In nucleotide excision repair, a section of DNA containing the damage is removed, and the gap is then filled in and completed by DNA polymerase and DNA ligase.

E. coli cells grown on N-15 medium are transferred to N-14 and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?

one low-density and one intermediate-density band

In his work with pneumonia-causing bacteria and mice, Griffith found that

some substance from pathogenic cells was transferred to nonpathogenic cells making them pathogenic

What are the repetitive DNA sequence present at the ends of eukaryotic chromosomes called?

telomeres Telomeres consist of many copies of a short DNA sequence that is bound by specific proteins.