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Table 11.1 Characteristics of histone proteins

Note: The sizes of H1, H2A, and H2B histones vary somewhat from species to species. The values given are for bovine histones. Source: Data are from A.L. Lehninger, D. L. Nelson, and M. M. Cox, Principles of Biochemistry, 3d ed. (New York: Worth Publishers, 1993), p. 924.

are associated with chromatin. In spite of these difficulties, we know that some groups of nonhistone proteins are clearly associated with chromatin.

Nonhistone chromosomal proteins may be broadly divided into those that serve structural roles and those that take part in genetic processes such as transcription and replication. Chromosomal scaffold proteins ( FIGURE 11.4) are revealed when chromatin is treated with a concentrated salt solution, which removes histones and most other chromosomal proteins, leaving a chromosomal protein "skeleton" to which the DNA is attached. These scaffold proteins may play a role in the folding and packing of the chromosome. Other structural proteins make up the kine-tochore, cap the chromosome ends by attaching to telomeres, and constitute the molecular motors that move chromosomes in mitosis and meiosis.

11.4 Scaffold proteins play a role in the folding and packing of chromosomes. (Professor U. Laemmli/ Photo Researchers.)

Other types of nonhistone chromosomal proteins play a role in genetic processes. They are components of the replication machinery (DNA polymerases, helicases, primases; see Chapter 12) and proteins that carry out and regulate transcription (RNA polymerases, transcription factors, acetylases; see Chapter 13). High-mobility-group proteins are small, highly charged proteins that vary in amount and composition, depending on tissue type and stage of the cell cycle. Several of these proteins may play an important role in altering the packing of chromatin during transcription.

The highly organized structure of chromatin is best viewed from several levels. In the next sections, we will examine these levels of chromatin organization. __

Concepts B

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.

The nucleosome Chromatin has a highly complex structure with several levels of organization. The simplest level (I Figure 11.5) is the double helical structure of DNA discussed in Chapter 8. At a more complex level, the DNA molecule is associated with proteins and is highly folded to produce a chromosome.

When chromatin is isolated from the nucleus of a cell and viewed with an electron microscope, it frequently looks like beads on a string ( FIGURE 11.6a on page 000), If a small amount of nuclease is added to this structure, the enzyme cleaves the string between the beads, leaving individual beads attached to about 200 bp of DNA ( FIGURE 11.6b). If more nuclease is added, the enzyme chews up all of the DNA between the beads and leaves a core of proteins attached to a fragment of DNA ( FIGURE 11.6c). Such experiments demonstrated that chromatin is not a random association of proteins and DNA but has a fundamental repeating structure.

The repeating core of protein and DNA produced by digestion with nuclease enzymes is the simplest level of chromatin structure, the nucleosome (see Figure 11.5). The nucleosome is a core particle consisting of DNA wrapped about two times around an octamer of eight his-tone proteins (two copies each of H2A, H2B, H3, and H4), much like thread wound around a spool ( FIGURE 11.6d). The DNA in direct contact with the histone octamer is between 145 and 147 bp in length, coils around the histones in a left-handed direction, and is supercoiled. It does not wrap around the octamer smoothly; there are four bends,

Table 11.1 Characteristics of histone proteins

Histone

Molecular

Number of

Protein

Weight

Amino Acids

0 0

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