Deck 10: Chromosome Structure and Transposable Elements

Question 1

Overrotation or underrotation of a DNA double helix places strain on the molecule, causing it to supercoil. Supercoiling is controlled by topoisomerase enzymes. Most cellular DNA is negatively supercoiled, which eases the separation of nucleotide strands during replication and transcription and allows DNA to be packed into small spaces.

-A DNA molecule 300 bp long has 20 complete rotations. This DNA molecule is

A) positively supercoiled.
B) negatively supercoiled
C) relaxed.

Answer 1

negatively supercoiled

Question 2

A typical bacterial chromosome consists of a large, circular molecule of DNA that is a series of twisted loops. Bacterial DNA appears as a distinct clump, the nucleoid, within the bacterial cell.
-How does bacterial DNA differ from eukaryotic DNA?

Answer 2

Bacterial DNA is not complexed to histone proteins and is circular.

Question 3

Chromatin, which consists of DNA complexed to proteins, is the material that makes up eukaryotic chromosomes. The most abundant of these proteins are the five types of positively charged histone proteins: H1, H2A, H2B, H3, and H4. Variant histones may at times be incorporated into chromatin in place of the normal histones

-Neutralizing their positive charges would have which effect on the histone proteins?

A) They would bind the DNA tighter.
B) They would separate from the DNA.
C) They would no longer be attracted to each other.
D) They would cause supercoiling of the DNA.

Answer 3

They would separate from the DNA.

Question 4

The nucleosome consists of a core particle of eight histone proteins and DNA that wraps around the core. Chromatosomes, which are nucleosomes bound to an H1 histone, are separated by linker DNA. 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.

-How many copies of the H2B histone would be found in chromatin containing 50 nucleosomes?

A) 5
B) 10
C) 50
D) 100

Question 5

The centromere is a region of the chromosome to which spindle fibers attach. Centromeres display considerable variation in structure and are distinguished by epigenetic alterations to chromatin stucture, including the use of a variant H3 histone in the nucleosome. In addition to their role in chromosome movement, centromeres help control the cell cycle by inhibiting anaphase until chromosomes are attached to spindle fibers from both poles.
-What happens to a chromosome that loses its centromere?

Question 6

A telomere is the stabilizing end of a chromosome. At the end of each telomere are many short telomeric sequences. Longer, more complex telomere-associated sequences are found adjacent to the telomeric sequences

-Which is a characteristic of DNA sequences at the telomeres?

A) They consist of guanine and adenine (or thymine) nucleotides.
B) They consist of repeated sequences.
C) One strand protrudes beyond the other, creating some singlestanded DNA at the end.
D) All of the above.

Question 7

Eukaryotic DNA comprises three major classes: unique-sequence DNA, moderately repetitive DNA, and highly repetitive DNA. Unique-sequence DNA consists of sequences that exist in one or only a few copies; moderately repetitive DNA consists of sequences that may be several hundred base pairs in length and is present in thousands to hundreds of thousands of copies. Highly repetitive DNA consists of very short sequences repeated in tandem and is present in hundreds of thousands to millions of copies. The density of genes varies greatly among and even within chromosomes.

-Most of the genes that encode proteins are found in

A) unique-sequence DNA.
B) moderately repetitive DNA.
C) highly repetitive DNA.
D) all of the above

Question 8

Transposable elements are mobile DNA sequences that often cause mutations. There are many different types of transposable elements; most generate short flanking direct repeats at the target sites as they insert. Many transposable elements also possess short terminal inverted repeats.
-How are flanking direct repeats created in transposition?

Question 9

Transposable elements frequently cause mutations and DNA rearrangements. Many cells regulate transposition by altering DNA or chromatin structure, by controlling the amount of transposase produced, or by direct inhibition of the transposition event.
-Briefly explain how transposition causes mutations and chromosome rearrangements.

Question 10

Insertion sequences are prokaryotic transposable elements that carry only the information needed for transposition. A composite transposon consists of two insertion sequences plus intervening DNA. Noncomposite transposons in bacteria lack insertion sequences but have terminal inverted repeats and carry information not related to transposition. All of these transposable elements generate flanking direct repeats at their points of insertion

-Which type of transposable element possesses terminal inverted repeats?

A) Insertion sequence
B) Composite transposons
C) Noncomposite transposon Tn3
D) All the above

Question 11

A great variety of transposable elements exist in eukaryotes. Some resemble transposable elements in prokaryotes, having terminal inverted repeats, and transpose as DNA. Others are retrotransposons with long direct repeats at their ends and transpose through an RNA intermediate

-Hybrid dysgenesis results when

A) a male fly with P elements (P⁺) mates with a female fly that lacks P elements (P^-).
B) a P^- male mates with a P⁺ female.
C) a P⁺ male mates with a P⁺ female.
D) a P^- male mates with a P^- female

Question 12

Many transposable elements appear to be genomic parasites, existing in large numbers because of their ability to efficiently increase in copy number. Increases in copy number of transposable elements have contributed to the large size of may eukaryotic genomes. In several cases, transposable elements and their ability to transpose have been adopted for specific cellular functions.
-What evidence suggests that the ability to replicate telomeres may have evolved from a retrotransposon?