Читать книгу Cell Biology - Stephen R. Bolsover - Страница 67

A human cell contains 46 chromosomes (23 pairs), each of which is a single DNA molecule bundled up with various proteins. On average, each human chromosome contains about 1.3 × 10⁸ base pairs (bp) of DNA. If the DNA in a human chromosome were stretched as far as it would go without breaking it would be about 5 cm long, so the 46 chromosomes in all represent about 2 m of DNA. The nucleus in which this DNA must be contained has a diameter of only about 10 μm, so large amounts of DNA must be packaged into a small space. This represents a formidable problem that is dealt with by binding the DNA to proteins to form chromatin. As shown in Figure 3.5, the DNA double helix is packaged at both small and larger scales. In the first stage, shown on the right of the figure, the DNA double helix with a diameter of 2 nm is bound to proteins known as histones. Histones are positively charged because they contain large amounts of the amino acids arginine and lysine (page 104) and bind tightly to the negatively charged phosphates on DNA. A 146 bp length of DNA is wound around a protein complex composed of two molecules each of four different histones – H2A, H2B, H3, and H4 – to form a nucleosome. Because each nucleosome is separated from its neighbor by about 50 bp of linker DNA, this unfolded chromatin state looks like beads on a string when viewed in an electron microscope. Nucleosomes undergo further packaging. A fifth type of histone, H1, binds to the linker DNA and pulls the nucleosomes together, helping to further coil the DNA into chromatin fibers 30 nm in diameter, which are referred to as 30‐nm solenoids. The fibers then form loops with the help of a class of proteins known as nonhistones and this further condenses the DNA (panels on left‐hand side of Figure 3.5) into a higher order set of coils in a process of supercoiling.

TABLE 3.1. Numbers of predicted protein‐coding genes in various organisms.

Organism	Number of predicted genes
Bacterium – Escherichia coli	4288
Yeast – Saccharomyces cerevisiae	6091
Fruit fly – Drosophila melanogaster	14 133
Worm – Caenorhabdites elegans	19 735
Plant – Arabidopsis thaliana	27 029
Human – Homo sapiens	19 116