09人类染色体eng v.ppt

09 人类染色体 human chromosome
A chromosome is the visible state of ic material during a phase of the division of the cell (metaphase).
Humans have 23 pairs of chromosomes, which makes the diploid number 46. The diploid number is the number of chromosomes of a normal cell.
The haploid number is the number of chromosomes in a gamete. Usually, the diploid number is twice the haploid number.
The number of chromosomes varies depending on the species.
1. chromatin
Chromatin is that portion of the cell nucleus which contains all of the DNA of the nucleus in animal cells. (A small amount of special DNA is also found in the mitochondria of  the cell cytoplasm outside of the cell nucleus.)
A. heterchromatin and euchromatin
The degree of condensation of the nuclear DNA is variable.
Strongly condensed DNA is called heterochromatin,
Less condensed one euchromatin.
Heterochromatin is stained more intense than euchromatin by certain nuclear dyes.
Heterochromatin is the condensed form of anization. It is seen as dense patches of chromatin.
Abundant heterochromatin is seen in resting, or reserve cells such as small lymphocytes (memory cells) waiting for exposure to a foreign antigen. Heterochromatin is considered transcriptionally inactive.