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.
|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.
|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 efficiently packed into chromatids. As can be seen in the illustration, a chromatid is made of a protein frame, and loops around it.
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...