Chromatin

DNA is composed of four different nucleotide subunits that contain the nitrogenous bases adenine, guanine, cytosine, and thymine. These nucleotides form two polynucleotide chains, joined by complementary base pairing and twisted to form a double helix. This structure is discussed in chapter 2 and illustrated in figures 2.30 and 2.31.

The DNA within the cell nucleus is combined with protein to form chromatin, the threadlike material that makes up the chromosomes. Much of the protein content of chromatin is of a type known as histones. Histone proteins are positively charged and organized to form spools, about which the negatively charged strands of DNA are wound. Each spool consists of two turns of DNA, comprising 146 base pairs, wound around a core of histone proteins. This spooling creates particles known as nucleosomes (fig. 3.16).

Chromatin that is active in genetic transcription (RNA synthesis) is in a relatively extended form known as euchromatin. Chromatin regions called heterochromatin, in contrast, are highly condensed and form blotchy-looking areas in the nucleus. The condensed heterochromatin contains genes that are said to be "silenced," which means that they are permanently inactivated.

In the euchromatin, genes may be activated or repressed at different times. This is believed to be accomplished by chemical changes in the histones. Such changes include acetylation (the addition of two-carbon-long chemical groups), which turns on genetic transcription, and deacetylation (the removal of those groups), which stops the gene from being transcribed. The acety-lation of histone proteins produces a less condensed, more open configuration of the chromatin in specific locations (fig. 3.17), allowing the DNA to be "read" by transcription factors (those that promote RNA synthesis, described in the next section).

membrane Pore complex

Figure 3.15 The nuclear pores. (a) An electron micrograph of a freeze-fractured nuclear membrane showing the nuclear pores. (b) A diagram showing the nuclear pore complexes.

membrane Pore complex

Figure 3.15 The nuclear pores. (a) An electron micrograph of a freeze-fractured nuclear membrane showing the nuclear pores. (b) A diagram showing the nuclear pore complexes.

Cell Structure and Genetic Control

Chromosome

Cell Structure and Genetic Control

Chromosome

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