Читать книгу Cell Biology - Stephen R. Bolsover - Страница 68

It is hard to find drugs that will inhibit replication of the HIV virus, which causes the disease AIDS, without damaging the host cell because the HIV virus uses the host cell's synthetic machinery. However, the first act of the virus is to make DNA from its RNA genome using an enzyme contained within its viral envelope, called reverse transcriptase. Azidothymidine (AZT), the most widely used anti‐AIDS drug, inhibits this enzyme.

Figure 3.5. How DNA is packaged into chromosomes.

In a normal interphase cell about 10% of the chromatin is in the highly compacted form and is visible in the light microscope as darkly staining heterochromatin (Figure 2.3 on page 23). Heterochromatin is the portion of the genome where there is no RNA synthesis taking place. Euchromatin, which is chromatin that is being transcribed into RNA, is wholly or partly unpacked from the histones to allow it to be read and has a less dense appearance in the microscope. Chromatin is in its most compacted form when the cell is preparing for mitosis, as shown at the top left of Figure 3.5. The chromatin folds and condenses further to form the 1400 nm‐wide chromosomes we see under the light microscope. Because the cell is to divide, the DNA has been replicated, so that each chromosome is now formed by two chromatids, each one a DNA double helix. This means each daughter cell will receive a full set of 46 chromosomes. Figure 3.6 is a photograph of human chromosomes as they appear at cell division.