The nucleus of each diploid cell contains 6 x 109 bp of DNA in long molecules called chromosomes. Chromosomes are massive structures containing one molecule of DNA that is wound around histone proteins into small units called nucleosomes and these are further wound to make up the structure of the chromosome itself. Diploid human cells have 46 chromosomes, 23 inherited from each parent; thus there are 23 ‘homologous’ pairs of chromosomes (22 pairs of ‘autosornes’ and two ‘sex chromosomes’). The sex chromosomes, called the X andY chromosomes, are not homologous but are different in size and shape. Males have an X and a Y chromosome, females have two X chromosomes. (Primary male sexual characteristics are determined by the SRY gene on the Y chromosome.) The chromosomes can be classified according to their size and shape, the largest being chromosome 1. The constriction in the chromosome is the centromere, which can be in the middle of the chromosome (metacentric) or at one extreme end (acrocentric). The centromere divides the chromosome into a short armand a long arm, which are referred to as the p arm and the q arm resp ectively . In addition chromosomes can be stained when they are in the metaphase stage of the cell cycle and are very condensed. The stain Structue of a chromosome. The sites at which the chromesome can be split are shown. Nomenclature for the describing loci on chromosome is as follows. Number of chromosome or X or Y plus short arm (p) or long arm (q). The region or subregion is defined by transverse light and dark bunds stained with quinacrine or Giemsa and numbered from the centromere outwards.
chromosome constitution = chromosome number + sex chromoosome + abnormality
46 xx = normal female
47 xx+21 = trisomy 21 (Down’s syndrome)
46 xyt (2;19) (p21; p12) = translocation between
chormosome 2 and 19 with breaking at band p21 and p12 short arm.gives a different pattern of light and dark bands that is diagnostic for each chromosome. Each band is given a number and gene mapping techniques allow genes to be positioned within a band within an arm of a chromosome. For example, the CFTR gene (in which a defect gives rise to cystic fibrosis) maps to 7q21, that is on chromosome 7 in the long arm in band 21.
During cell division (mitosis), each chromosome divides into two so that each daughter nucleus has the same number of chromosomes as its parent cell. During gametogenesis, however, the number of chromosomes is halved by meiosis so that after conception the number of chromosomes remains the same and is not doubled. In the female, each ovum contains one or other X chromosome but, in the male, the sperm bears either an X or a Y chromosome. Chromosomes can only be seen easily in actively dividing cells. Typically lymphocytes from the peripheral blood are stimulated to divide and are processed to allow the chromosomes to be examined. Cells from other tissues can also be used, for example amniotic fluid, placental cells from chorionic villus sampling, bone marrow and skin (Information box 2.1).
The X chromosome and inactivation Although female chromosomes are XX, females do not have two doses of X-linked genes (compared to just one dose for a male XY), because of the phenomenon of X inactivation or Lyonization (after its discoverer, Dr Mary Lyon). In this process one of the two X chromosomes in the cells of females becomes transcriptionally inactive, so the cell has only one dose of the X-linked genes. Inactivation is random and can affect either X chromosome.
THE MITOCHONDRIAL CHROMOSOME
In addition to the 23 pairs of chromosomes in the nucleus of every diploid cell, the mitochondria in the cytoplasm of the cell also have their own chromosomes. The mitochondrial chromosome is a circular DNA molecule of approximately 16500 bp and every base pair makes up part of the coding sequence. These genes encode proteins or RNA molecules involved in mitochondrial function. These proteins are all components of the mitochondrial respiratoy chain involved in oxidative phosphorylation. Every cell contains several hundred mitochondria and therefore several hundred mitochondrial chromosomes. All mitochondria are inherited from the mother as sperm contains no (or very few) mitochondria.