Concepts Summary

Chromosomes contain very long DNA molecules that are tightly packed. Packing is accomplished through tertiary structures and the binding of DNA to proteins.

Supercoiling results from strain produced when rotations are added or removed from a relaxed DNA molecule. Overrotation produces positive supercoiling; underrotation produces negative supercoiling.

Topoisomerases control the degree of supercoiling by adding or removing rotations to DNA.

A bacterial chromosome consists of a single, circular DNA molecule that is bound to proteins and exists as a series of large loops. It usually appears in the cell as a distinct clump known as the nucleoid.

Each eukaryotic chromosome contains a single, very long linear DNA molecule that is bound to histone and nonhistone chromosomal proteins. Euchromatin undergoes the normal cycle of decondensation and condensation in the cell cycle. Hetero-chromatin remains highly condensed throughout the cell cycle.

The nucleosome is a core of eight histone proteins (two each of H2A, H2B, H3, and H4) and DNA (145 -147 bp) that wraps around it. The H1 protein holds DNA onto the histone core.

Nucleosomes are folded into a 30-nm fiber that forms a series of 300-nm-long loops; these loops are anchored at their bases by proteins associated with the nuclear scaffold. The 300-nm loops are condensed to form a fiber that is 250 nm in diameter, which is itself tightly coiled to produce a 700-nm-wide chromatid.

• Chromosomal puffs are regions of localized unpacking of the DNA that are associated with regions of active transcription. Chromosome regions that are undergoing active transcription are relatively sensitive to digestion by DNase I, indicating that DNA unfolds during transcription.

• Centromeres are chromosomal regions where spindle fibers attach; chromosomes without centromeres are usually lost in the course of cell division. Centromeres play an important role in the regulation of the cell cycle.

• Telomeres stabilize the ends of chromosomes. Telomeric sequences consist of many copies of short sequences, which usually consist of a series of cytosine nucleotides followed by several adenine nucleotides. Longer telomere-associated sequences are found adjacent to the telomeric sequences.

• The C value is the amount of DNA in an organism's genome. Eukaryotic organisms exhibit much variation in C value owing to differences in sequence complexity, which can be measured by observing the time required for denatured DNA to reanneal in a hybridization reaction, as plotted by a C0t curve.

• Eukaryotic DNA exhibits three classes of sequences. Unique-sequence DNA exists in very few copies. Moderately repetitive DNA consists of moderately long sequences that are repeated from hundreds to thousands of times. Highly repetitive DNA consists of very short sequences that are repeated in tandem from many thousands to millions of times.

• Transposable elements are mobile DNA sequences that insert into many locations within a genome and often cause mutations and DNA rearrangements.

• Most transposable elements have two common characteristics: terminal inverted repeats and the generation of short direct repeats in DNA at the point of insertion.

• Transposition may take place through a DNA molecule or through the production of an RNA molecule that is then reverse transcribed into DNA. Transposition may be replicative, in which the transposable element is copied and the copy moves to a new site, or nonreplicative, in which the transposable element excises from the old site and moves to a new site.

• Retrotransposons transpose through RNA molecules that undergo reverse transcription to produce DNA.

• In many transposable elements, transposition is tightly regulated.

• Insertion sequences are small bacterial transposable elements that carry only the information needed for their own movement. Composite transposons in bacteria are more complex elements that consist of DNA between two insertion sequences. Some complex transposable elements in bacteria do not contain insertion sequences.

• Some transposable elements in eukaryotic cells are similar to those found in bacteria, ending in short inverted repeats and producing flanking direct repeats at the point of insertion. Others are retrotransposons, similar in structure to retroviruses and transposing through RNA intermediates.

• Hybrid dysgenesis is the appearance of numerous mutations, chromosome rearrangements, and sterility when transposable P elements undergo a burst of transposition in Drosophila.

• The evolutionary significance of transposable elements is unknown, but three hypotheses have been proposed to explain their common occurrence. The cellular function hypothesis suggests that transposable elements provide some important function for the cell; the genetic variation hypothesis proposes that transposable elements provide evolutionary flexibility by inducing mutations; and the selfish DNA hypothesis suggests that transposable elements do not benefit the cell but are widespread because they can replicate and spread.

(important terms_

transgenic mouse (p. 000) transposable element (p. 000) supercoiling (p. 000) relaxed state of DNA (p. 000) positive supercoiling (p. 000) negative supercoiling (p. 000) topoisomerase (p. 000) nucleoid (p. 000) euchromatin (p. 000) heterochromatin (p. 000) nonhistone chromosomal proteins (p. 000) chromosomal scaffold protein (p. 000)

high-mobility-group proteins (p. 000)

nucleosome (p. 000) chromatosome (p. 000) linker DNA (p. 000) polytene chromosome (p. 000) chromosomal puff (p. 000) centromeric sequence (p. 000) telomeric sequence (p. 000) telomere-associated sequence (p. 000) C value (p. 000) denaturation (melting) (p. 000) melting temperature (Tm) (p. 000) renaturation (reannealing) (p. 000) hybridization (p. 000)

unique-sequence DNA (p. 000) repetitive DNA (p. 000) moderately repetitive DNA (p. 000)

tandem repeat sequence(p. 000) interspersed repeat sequences (p. 000) short interspersed element

(SINE) (p. 000) long interspersed element

(LINE) (p. 000) highly repetitive DNA (p. 000) flanking direct repeat (p. 000) terminal inverted repeats (p. 000)

transposition (p. 000) replicative transposition (p. 000) nonreplicative transposition (p. 000) cointegrate structure (p. 000) transposase (p. 000) resolvase (p. 000) retrotransposon (p. 000) insertion sequence (p. 000) composite transposon (p. 000) delta sequence (p. 000) hybrid dysgenesis (p. 000)

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