Eukaryotic and Prokaryotic Chromosomes
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Eukaryotic and Prokaryotic Chromosomes Eukaryotic and Prokaryotic Chromosomes Eukaryotic and Prokaryotic Chromosomes Eukaryotic and Prokaryotic Chromosomes Eukaryotic and Prokaryotic Chromosomes Eukaryotic and Prokaryotic Chromosomes Eukaryotic and Prokaryotic Chromosomes
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Eukaryotic and Prokaryotic Chromosomes
- 1. Eukaryotic chromosomes The label eukaryote is taken from the Greek for 'true nucleus', and eukaryotes (all organisms except viruses, Eubacteria and Archaea) are defined by the possession of a nucleus and other membrane-bound cell organelles. The nucleus of each cell in our bodies contains approximately 1.8 metres of DNA in total, although each strand is less than one millionth of a centimetre thick. This DNA is tightly packed into structures called chromosomes, which consist of long chains of DNA and associated proteins. In eukaryotes, DNA molecules are tightly wound around proteins - called histone proteins - which provide structural support and play a role in controlling the activities of the genes. A strand 150 to 200 nucleotides long is wrapped twice around a core of eight histone proteins to form a structure called a nucleosome. The histone octamer at the centre of the nucleosome is formed from two units each of histones H2A, H2B, H3, and H4. The chains of histones are coiled in turn to form a solenoid, which is stabilised by the histone H1. Further coiling of the solenoids forms the structure of the chromosome proper. Each chromosome has a p arm and a q arm. The p arm (from the French word 'petit', meaning small) is the short arm, and the q arm (the next letter in the alphabet) is the long arm. In their replicated form, each chromosome consists of two chromatids. The chromosomes - and the DNA they contain - are copied as part of the cell cycle, and passed to daughter cells through the processes of mitosis and meiosis.
- 2. Figure 1: Chromosome unraveling to show the base pairings of the DNA Human beings have 46 chromosomes, consisting of 22 pairs of autosomes and a pair of sex chromosomes: two X sex chromosomes for females (XX) and an X and Y sex chromosome for males (XY). One member of each pair of chromosomes comes from the mother (through the egg cell); one member of each pair comes from the father (through the sperm cell). A photograph of the chromosomes in a cell is known as a karyotype. The autosomes are numbered 1-22 in decreasing size order. Prokaryotic chromosomes The prokaryotes (Greek for 'before nucleus' - including Eubacteria and Archaea) lack a discrete nucleus, and the chromosomes of prokaryotic cells are not enclosed by a separate membrane.
- 3. Most bacteria contain a single, circular chromosome. (There are exceptions: some bacteria - for example, the genus Streptomyces - possess linear chromosomes, and Vibrio cholerae, the causative agent of cholera, has two circular chromosomes.) The chromosome - together with ribosomes and proteins associated with gene expression - is located in a region of the cell cytoplasm known as the nucleoid. The genomes of prokaryotes are compact compared with those of eukaryotes, as they lack introns, and the genes tend to be expressed in groups known as operons. The circular chromosome of the bacterium Escherichia coli consists of a DNA molecule approximately 4.6 million nucleotides long. In addition to the main chromosome, bacteria are also characterised by the presence of extra-chromosomal genetic elements called plasmids. These relatively small circular DNA molecules usually contain genes that are not essential to growth or reproduction.