The Molecular Biology Notebook OnlineYou are here: Gene to Function > Chromosomes
The chromosomes
Definition and description
A chromosome is the visible state of genetic 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.
Just before mitosis, DNA, which contains the
genetic information, is replicated. Now there are two identical copies
of all the DNA in this cell. The cell is ready to divide.
chromosome fibre... |
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| The DNA molecule is twisted onto itself, and the supercoiled molecule
is wrapped around proteins which maintain its shape. These proteins have
a special name - histone proteins. The fibre has the structure of a spring
(a solenoid, to be technical), as shown in the illustration on the right.
The DNA is wrapped around the protein, and then, twisted together again
in the shape of a spring. The complex DNA protein is called chromatin, while the structure formed by two turns of DNA around one histone is called a nucleosome. |
 |
 The LOOP, drawn on the right, is a well packed chromosome fibre.
The chromosome fibre is coiled around a protein scaffold. The string of
little packets is then wrapped around another protein structure to form
a chromatid |
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A model of a chromosome during metaphase is displayed on the left:
A chromosome like this one is made of two identical molecules of DNA The efficiency of this packing is pretty impressive! Human chromosome 1 is hardly 10 micrometers long. But it contains 7 centimetres of DNA! DNA, DNA... what is DNA? Its structure? Its role? |
chromosomes carry genes... and some concepts can be a bit complex. Chromosomes, genes, genotypes and other concepts are explained elsewhere...
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Chromosomes on Wikipedia
Chromosomes at Answers. com
Related Links
-Chromosome at the National Human Genome Research InstituteOne of the threadlike packages of genes and other DNA in the nucleus of a cell.
-Human Chromosome Launchpad
Links to Research Resources for Each Chromosome
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