Chapter 9 Flashcards
What are the 2 different types of nucleic acids?
Deoxyribonucleic acid (DNA)
Ribonucleic acid (RNA)
Explain Griffiths Discovery.
Griffith injected heat-killed Type IIIS bacteria (virulent when alive) and living Type IIR bacteria (avirulent) into mice.
Many of the mice developed pneumonia and died, and living Type IIIS cells were recovered from their carcasses.
Something from the heat-killed cells—the “transforming principle”—had converted the living Type IIR cells into Type IIIS—that is, it had changed their hereditary material
Explain Avery, MacLeod, and McCartys experiment.
DNA purified from heat killed IIIS cells was treated with the enzymes DNase, which degrades DNA, RNase, which degrades RNA, and protease, which degrades proteins.
Only DNase eliminated the transforming activity entirely. (no IIIS colonies grew since DNA was not present)
Thus, DNA was the essential ingredient in the transformation of Type IIR cells into Type IIIS cells.
It was the “transforming principle.”
Explain Alfred Hershey and Martha Chases experiment.
DNA contains phosphorus but no sulfur, whereas proteins contain sulfur but virtually no phosphorus.
- T2 phage particles labeled with 35S were mixed with E. coli cells for a few minutes and the phage-infected cells were then subjected to shearing forces in a blender, most of the radioactivity (and thus the proteins) could be removed from the cells without affecting progeny phage production. Radioactivity was seen in the supernatant.
- When T2 particles in which the DNA was labeled with 32P were used, however, essentially all the radioactivity was found inside the cells; that is, the DNA was not subject to removal by shearing in a blender. The sheared-off phage coats were separated from the infected cells by low-speed centrifugation, which pellets (sediments) cells while leaving phage particles suspended. These results indicated that the DNA of the virus enters the host cell, whereas the protein coat remains outside the cell. Radioactivity was seen in the pellet.
What are supercoils and how are they formed?
Supercoils are introduced into a DNA
molecule when one or both strands are cleaved and when the complementary strands at one end are rotated or twisted around each other with the other end held fixed in space—and thus not allowed to spin.
This supercoiling causes a DNA molecule to collapse into a tightly coiled structure similar to a coiled electrical cord or twisted rubber band.
What type of coiling do all organisms display?
negative supercoiling
What does the chemical analysis of isolated chromatin show? (What does chromatin consist of?)
Chromatin consists primarily of:
1. DNA
2. Proteins
a. histones
b. nonhistone chromosomal proteins
3. RNA.
What are the five different types of histones found in humans? What are their ratios?
H1
H2a
H2b
H3
H4
Ratio: 1:2:2:2:2
Why are histones considered to be basic?
They contain 20 to 30 percent arginine and lysine, two positively charged amino acids.
What are DNA linkers?
The unprotected DNA double helix that connects adjacent nucleosomes.
It is suseptible to nuclease attacks.
What is a nucleosome core? What does it do?
The segment of DNA is associated with two molecules of histones H2a, H2b,
H3, and H4.
This octamer of histones protects the DNA from degradation by nucleases.
What does a complete chromatin subunit consist of?
- The nucleosome core
- The linker DNA
- The associated nonhistone chromosomal proteins
- All stabilized by the binding of one molecule of histone H1 to the outside of the structure
At what stage in mitosis are eukaryotic chromosomes maximally condensed?
metaphase of mitosis or meiosis
What does the tight packaging of chromosomes facilitate in mitosis? What does it help to prevent?
It facilitates their segregation into daughter nuclei during anaphase
It helps to prevent different chromosomes from becoming entangled and minimizing the possibility of breakage
What are the 3 levels of condensation needed to package eukaryotic DNA?
- Packaging DNA as a negative supercoil into nucleosomes, to produce the 11-nm diameter interphase chromatin fiber. This involves an octamer of histone molecules.
- An additional folding or supercoiling of the 11-nm nucleosome fiber, to produce the 30-nm chromatin fiber by using Histone H1.
- Nonhistone chromosomal proteins form a scaffold that is involved in condensing the 30-nm chromatin fiber into the tightly packed metaphase chromosomes. It involves the separation of segments of the DNA molecules into independently supercoiled domains or loops. The mechanism by which this third
level of condensation occurs is not known.
