In
eukaryotes, chromosomes consist of a
single molecule of DNA [
Link to visual proof] associated with:
- many copies of 5 kinds of histones. Histones are proteins rich in lysine and arginine residues and thus positively-charged. For this reason they bind tightly to the negatively-charged phosphates in DNA.
- a small number of copies of many different kinds of non-histone proteins. Most of these are transcription factors that regulate which parts of the DNA will be transcribed into RNA.
- For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.
- Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).
- As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope.
- These duplicated chromosomes are called dyads.
- When first seen, the duplicates are held together at their centromeres. In humans, the centromere contains ~1 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.
- While they are still attached, it is common to call the duplicated chromosomes sister chromatids, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.
- The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fibers that will separate the sister chromatids as mitosis proceeds into anaphase.
- The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.
- Staining with the trypsin-giemsa method reveals a series of alternating light and dark bands called G bands.
- G bands are numbered and provide "addresses" for the assignment of gene loci.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html
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