Chromatin
The DNA of a cell forms a complex with proteins called chromatin. The DNA wraps itself around the proteins which makes it more compact.
Chromatin looks like long, unravelled string, which can condense when the cell is about to divide. It is not one long piece of string and when condensed it forms linear structures called chromosomes.
Chromosomes
Chromosome – a structure in which a single molecule of DNA is tightly packaged.
Chromosomes are thread like structures and contain hundred to thousands of genes – the sequence of which is specific. The position of a gene is called its locus.
Human cells, excluding sperm and egg cells, contain 46 chromosomes. These chromosomes make up 23 pairs.
The first 22 pairs are called autosomes. The 23rd pair of chromosomes are the sex chromosomes and are called allosomes.
The autosomes are all homologous pairs with one chromosome in the pair inherited from each parent. Each chromosome in the pair has the same genes (although these can be different versions of the same gene) at the same positions.
The allosomes consist of two X chromosomes in females or an X and Y chromosome in males. As females have two X chromosomes the pair is homologous. The X and Y combination in males, however, means the allosomes are not homologous.
Chromatids and Centromeres
During interphase (the process in which a cell prepares to divide) the cell creates an identical copy of each of its chromosomes.
The copies of a chromosome are called sister chromatids and are joined to each other at a point called the centromere. Whist joined the sister chromatids are still classed as one chromosome.
The diagram below shows a chromosome with two sister chromatids joined by a centromere.
During mitosis the sister chromatids are separated at the centromere, which leaves two singles chromosomes (each a single chromatid).
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