Chromosome Structure Help
The Union of Cytology with Genetics
Perhaps one reason that Mendel's discoveries (1865) were not appreciated by the scientific community of his day was that the mechanics of mitosis and meiosis had not yet been discovered. During the years 1870–1900 rapid advances were made in the study of cells (cytology). At the turn of the 20th century, when Mendel's laws were rediscovered, the cytological basis was available to render the statistical laws of genetics intelligible in terms of physical units. Cytogenetics is the hybrid science that attempts to correlate cellular events, especially those of the chromosomes, with genetic phenomena.
Eukaryotic DNA is primarily associated with a basic class of proteins known as histones. Together, the DNA, histones, and nonhistone chromosomal proteins form nucleoprotein, or chromatin. In the first level of packaging (Fig. 7.1), about 150 bp of the double-stranded DNA molecule are wrapped around an octomer of four pairs of different histone proteins to form a nucleosome. A fifth kind of histone protein occupies the linker DNA that connects one core particle with another (analogous to beads on a string). This "string" first coils up into a solenoid of 30-nmdiameter and then into a filament of 300-nm diameter. Even higher levels of compaction occur during cell division when the chromatin material appears to condense from an amorphous chromatin mass into distinctive chromosomes. Nonhistone proteins (including various DNA and RNA polymerases, regulatory proteins, etc.) can also be found associated with chromatin, but they are not responsible for the basic structure of chromatin.
The highly compacted chromatin that can be seen in the light microscope during interphase is called heterochromatin; the much more open form of chromatin that is difficult to see is called euchromatin. Eukaryotic genes cannot be expressed (i.e., cannot serve as templates for RNA sythesis) if they are tightly bound to histones. So the first step in gene activation requires a dissociation of the DNA from the histones. Thus, euchromatic regions contain active genes whereas heterochromatic regions contain silenced genes or highly repetitive DNA sequences. DNA sequences surrounding centromeres are one example of a chromosomal region that is generally heterochromatic. Various mechanisms exist to regulate which regions become heterochromatic.
EXAMPLE 7.1 In female mammals, one of the two X chromosomes in each somatic cell appears highly condensed (heterochromatic), indicating that it is genetically inactive. In some cells one of the X chromosomes is inactive, and in other cells the other X chromosome is inactive. Thus, the female is a mosaic consisting of a mixture of two cell types with regard to the activity of sex-linked genes.
EXAMPLE 7.2 The stained giant polytene chromosomes of Drosophila (see Fig. 7.2, page 217) consist of alternating dark and light bands. The dark bands are heterochromatin; the light bands are euchromatin. At various stages during larval development, specific regions appear to decondense and form fluffy puffs of open chromatin material. At least some of the genes in these puffs are actively synthesizing RNA. Furthermore, the pattern of these puffed regions changes during larval development, indicating that different groups of genes are being activated and silenced as cellular differentiation proceeds.
The DNA in a metaphase chromosome attached to a protein scaffold constructed from nonhistone proteins. These proteins not only help organize the chromosomal DNA, but help the metaphase chromosome more along the Spindle during cell division and meiosis. This association persists in metaphase chromosomes that have had their histones and most of their nonhistones removed. How this elegent scaffold becomes assembled and its role in the induction of super helicity of the DNA is not known.
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