The Eukaryotic Chromosome

Individual eukaryotic chromosomes contain enormous amounts of DNA. Like bacterial chromosomes, each eukaryotic chromosome consists of a single, extremely long molecule of DNA. For all of this DNA to fit into the nucleus, tremendous packing and folding are required, the extent of which must change through time. The chromosomes are in an elongated, relatively uncondensed state during interphase of the cell cycle (see p. 000 in Chapter 2), but the term relatively is an important qualification here.

Although the DNA of interphase chromosomes is less tightly packed than DNA in mitotic chromosomes, it is still highly condensed; it's just less condensed. In the course of the cell cycle, the level of DNA packaging changes — chromosomes progress from a highly packed state to a state of extreme condensation. DNA packaging also changes locally in replication and transcription, when the two nucleotide strands must unwind so that particular base sequences are exposed. Thus, the packaging of eukaryotic DNA (its tertiary, chromosomal structure) is not static but changes regularly in response to cellular processes.

Chromatin Structure

As mentioned in Chapter 2, eukaryotic DNA is closely associated with proteins, creating chromatin. The two basic types of chromatin are: euchromatin, which undergoes the normal process of condensation and decondensation in the cell cycle, and heterochromatin, which remains in a highly condensed state throughout the cell cycle, even during interphase. Euchromatin constitutes the majority of the chromosomal material, whereas heterochromatin is found at the centromeres and telomeres of all chromosomes, at other specific places on some chromosomes, and along the entire inactive X chromosome in female mammals (see p. 000 in Chapter 4).

The most abundant proteins in chromatin are the his-tones, which are relatively small, positively charged proteins of five major types: H1, H2A, H2B, H3, and H4 (Table 11.1). All histones have a high percentage of arginine and lysine, positively charged amino acids that give them a net positive charge. The positive charges attract the negative charges on the phosphates of DNA and holds the DNA in contact with the histones.

A heterogeneous assortment of nonhistone chromosomal proteins make up about half of the protein mass of the chromosome. A fundamental problem in the study of these proteins is that the nucleus is full of all sorts of proteins; so, whenever chromatin is isolated from the nucleus, it may be contaminated by nonchromatin proteins. On the other hand, isolation procedures may also remove proteins that

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