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What's in a chromosome?
A chromosome consists of hundreds or thousands of genes (a gene is the basic unit of inheritance), and specialised parts that are thought to be important to the chromosomes stability and function. The deoxyribonucleic acid (DNA) that makes up the genes is packaged with the aid of proteins to form a complex structure. Chromosomes also contain small amounts of ribonucleic acid (RNA).
DNA is packaged in chromosomes
Each human chromosome contains one very long DNA molecule which unravelled would measure about 4.8 cm in length. The total length of DNA in the nucleus of a human cell has been estimated to be about 2.2 m. This poses a packaging problem: how does a chromosome measuring оn average 6 µm long contain about 8000 times its length of DNA? The answer is that chromosomal DNA is intricately folded and is tightly bound to protein molecules called histones. Histones are small proteins that are rich in the amino acids lysine and/or arginine.
The complex formed between DNA and histones is called chromatin. Chromatin takes up stain and is visible in non-dividing nuclei. Individual chromosomes can be seen under the light microscope only during cell division (mitosis or meiosis).
Nucleosomes - the basic structural unit
Each DNA molecule is wound around histones arranged in groups of eight known as octamers.
The DNA and octamers form bead-like structures known as nucleosomes. Positively charged groups on the side-chains of the histones form strong ionic bonds with negatively charged phosphate groups in the backbone of the DNA.
In each nucleosome, a length of DNA containing about 150 base pairs is wrapped around the octamer.
Another histone molecule attached to the outside of the nucleosome binds DNA to the octamer.
The nucleosome is regarded as the basic unit of the structure. The linker region, the stretch of DNA between the nucleosomes, varies in length from 14 to over 100 base pairs.
Nucleosomes fold to form solenoid fibres
More histones in the linker region help to fold the thread of DNA and nucleosomes (the nucleosome fibre) into a tightly coiled structure called a solenoid. The solenoids are thought to be further looped and coiled around non-histone proteins called scaffolding proteins. The precise details of this higher level of folding are not known.
Each chromosome has acentromere which usually appears as a constriction when the chromosomes condense during mitosis and meiosis. The position of the centromere can be used to distinguish between different chromosomes.
Centromeres do not contain any genes. However they do contain large segments of highly repetitive DNA, called alpha satellite DNA. This is thought to play a significant role in centromere function. The centromere contains the kinetochore. This is a densely staining structure that attaches the chromosome to the spindle apparatus during nuclear division. Centromeres control the distribution of chromosomes during cell division. Chromosomes that do not have centromeres cannot divide.
Telomeres are located at the ends of chromosomes. They consist of DNA and protein. The telomeres appear to play a vital role in maintaining the stability of the chromosomes, 'sealing' the ends of linear DNA. They have been likened to the tips of shoelaces, and have a similar function: to stop the DNA fraying. They also seem to play an important role in regulating cell division. Under normal circumstances, telomeres become shorter and shorter with each cell division. When the telomeres have shortened to a certain critical length, the cell stops dividing.
If the telomeres are removed, the chromosome disintegrates. It is thought that the ageing process may be linked to telomere damage.
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