Chapter 11 -chromosome structure

Question 1

What is meant by “higher order DNA structure”?

Answer 1

low order ( DNA strand) to high order (chromosome)
-nucleosome, solonoid, 30 nm fiber, 250nm fiber

Question 2

What is supercoiling?

Answer 2

supercoiling takes place when the DNA helix is subjected to strain by being overwound or under wound.

Question 3

What are two kinds of supercoiling?

Answer 3

Positive supercoiling: over rotated

Negative supercoiling: under rotated

Question 4

What are topoisomerases?

How do these enzymes work?

Answer 4

enzymes that add or remove rotations from the DNA helix by temporarily breaking the nucleotide strands, rotating the ends around each other, and then rejoining the broken ends.

Question 5

What is chromatin?

Answer 5

The combination of DNA and protein

Question 6

What are the differences between heterochromatin and euchromatin?

Answer 6

Eurochromatin: undergoes the normal process of condensation and decondensation in the cell cycle.

Heterochromatin: remains in highly condensed state throughout the cell cycle, even during interphase.

Question 7

Describe the major steps involved in packaging DNA into chromosomes (Figure 11.5).

Answer 7

A small amount of nuclease cleaves the “string” between the beads… releasing individual beads attached to about 200bp of DNA. More nuclease destroys all of the unprotected DNA between the beads…. leaving a core of proteins attached to 145-147 bp of DNA.

Question 8

What are histones?

Answer 8

small, positively charged proteins.

Question 9

Why do histones associate strongly with DNA?

Answer 9

They contain positively-charged lysine residues in N-terminal tail. The positive charge attracts negative charges on phosphates of DNA.

Question 10

What is are nucleosomes?

Answer 10

is a core particle consisting of DNA wrapped about two times around an octamer of eight histone proteins.

Question 11

Why is the association of nucleosomes with DNA arguably the most important step in DNA condensation

Answer 11

nucleosomes fold to form a 30 nm chromatin fiber, which appears as a series of loops that pack to create a 250 nm wide fiber. Helical coiling of the 250 nm fiber produces a chromatid.

Question 12

What are DNAases?

Answer 12

enzymes that digest DNA

Question 13

What are nucleases?

Answer 13

enzyme that cuts nucleic acids at phosodiester bond

Question 14

What is the difference between exo- and endonuclease?

Answer 14

exonuclease- cuts at the end of polynucleotide chain.

endonuclease- cuts within polynucleotide chain

Question 15

What are centromeres?

Answer 15

are heterochromatin, consist of particular, highly repetitive sequences. connect sister chromatids

Question 16

What happens to chromosomes lacking centromeres?

Answer 16

chromosomes lacking centromeres are lost during mitosis

Question 17

What are telemeres?

Answer 17

are the natural ends to eukaryotic chromosomes

Question 18

Why do telomeres need protection from degradation?

Answer 18

are the natural ends to eukaryotic chromosomes

Question 19

How are they protected?

Answer 19

Telomeres enable the chromosome to be degraded. The telomere serves as a cap that stabilizes the chromosome. Telomeres also provide a means of replicating the ends of the chromosome.

Question 20

How efficient is this protection?

Answer 20

This process is not always efficient

Question 21

What is a transposon?

Answer 21

are mobile genetic elements

Question 22

What is their basic structure?

Answer 22

short flanking direct repeats and terminal inverted repeats

Question 23

What enzyme(s) are required for transposition?

Answer 23

transposase

Question 24

Compare and contrast replicative, non replicative, and retro-transposition

Answer 24

replicative- the transposable element is introduced at a new site while old copy remains behind at the original site, and so the number of copies of the transposable element increases.
Non replicative- the transposable element excises from old site and inserts at a new site without any increase in the number of copies.
retro-transposition- like replicative but uses RNA as an intermediate.

Question 25

How can transposons act as mutagens? Give a general example

Answer 25

by inserting into other genes and disrupting their function. An example is in the color of grapes.

Question 26

Compare and contrast three major types of transposons in bacteria.

Answer 26

insertion sequences- carries only the genetic information necessary for movement.
Composite transposons: any segment if DNA that becomes flanked by two copies of an insertion sequence may itself transpose
noncomposite transposons- lack insertion sequences

Question 27

Compare and contrast the two types of transposons in corn? How does activity of these transposons lead to variegation in corn kernels?

Answer 27

Ac element: has basic structure of transposon encodes for transposes gene.
Ds element- deletion of transposon itself; cannot transpose.
Ac element productes transposase…stimulates transposition of Ds element into C allele…disrupting production of pigment… resulting in a yellow kernel.

Ac element produces transposase…stimulates transposition of Ds out of C allele in some cells… restores C allele function…resulting in a yellow kernel with purple spots.

Question 28

Understand the three major hypothesis describing the evolutionary significance of transposon?

Answer 28

Cellular Function hypothesis- transposons serve a valuable function within the cell. Regulation of gene expression or development.
Genetic variation hypothesis- transposons exist because of their mutagenic activity. Mechanism to introduce genetic variability.
Selfish Gene Hypothesis- transposons serve no purpose in the cell. They exist because they can replicate. “Selfish parasites” of DNA with potentially deter mental effects.