CHROMOSOME STRUCTURE

Having mentioned earlier that genes are segments of DNA of varying lengths, it is now time to consider the relationship of genes to chromosomes. Every human somatic or gametic cell has within its nucleus a mesh or network of threads called chromatin, chemically made up of DNA - protein complexes. Fig. 11. Each of these threads stain strongly and is called a chromosome. A somatic cell which is a cell from a system, for example the the urinary system will have 46 or the diploid number (2n) of chromosomes, while a gametic cell like a spermatozoon or an ovum will have, half that number, 23 or the haploid number (n).

Each of these chromosomes is composed of a single molecule of DNA organised and packaged to fit into the shape and size of the chromosome. The total length of DNA molecules in each somatic cell is approximately 1.74 metres, the average height of a man while the total length of the 46 chromosomes placed end to end does not even reach a length of 0.5 millimetres. This highlights the need for an efficient system of coiling and packaging to allow such a long length of DNA to be accommodated within such a short length of chromosome.

Fig. 12, shows the organisation of DNA needed to fit into a chromosome. A structure called the nucleosome is the fundamental packaging unit made up of eight proteins called histones. The first step in the packaging is the tight coiling of 146 base pairs of the DNA strand twice round the surface of the histone disc. This is followed by 60 base pairs of loose DNA thread followed again by a 146 base pair histone disc and so on, giving a beaded appearance 11nm thick as shown. The nucleosomes are packed further together with the aid of another histone to form a 30nm wide fibre. This fibre goes into further coiling to give a thickness of 300nm and loops further to reach a thickness of 700nm, the bindings for the loops provided by nonhistone proteins.

In a metaphase chromosome the chromatin is believed to be more condensed compared to that at interphase. Euchromatin which forms the main body of the chromosome is active and composed of a high proportion of coding genes, while heterochromatin is made up of inactive DNA with either inactive genes or absent genes.

The normal human somatic chromosome complement consists of 46 chromosomes or 23 pairs. Of the 23 pairs, 22 pairs are called autosomes, made up of homologous pairs each one of a pair coming from a parent. The 23rd pair is the pair of sex chromosomes. In the female it consists of two X chromosomes and in the male an X and a small Y. Fig. 13. Males are thus 46,XY and females 46,XX. Unlike the thread like form of chromosomes seen in interphase, the metaphase chromosome consists of two long identical threads called chromatids, held together by a primary constriction or centromere. The centromeric proteins which form this structure participate with the spindle fibre microtubules in the separation of sister chromatids or chromosomes in anaphase of mitosis or meiosis.

Depending on the position of the centromere, the chromosomes may have short arms (p) or long arms (q). Depending on the position of the centromere, human chromosomes are divided into three types. Fig. 14.

Metacentric - where the centromere is in the middle (p = q)
Submetacentric - where the centromere is displaced from the centre (p < q)
Acrocentric - where the centromere is at one end (p << q)

The ends of the chromatids are referred to as telomeres. The DNA sequences at these ends are responsible for maintaining the stability of the chromosomes. Without them the chromosomes tend to recombine with other chromatin segments and produce aberrations.