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DNA and Chromosomes
Chromosomes are thread like strands which are composed of DNA. Chromosomes can only be
seen at distinct times during the cell cycle; this time is when the cell is dividing. Chromosomes appear
as two threads (chromatid) joined at a single point (centromere). The chromosome consist of a protein
coated strand, this strand coils in three ways when the cell is preparing to divide.
There are a series of steps to the packaging of a Chromatin; the structure is based on the
sequence that the DNA is packaged. To pack the DNA into a compact form, the protein Histone is
required, as without this protein the DNA would not be able to fit into the nucleus. There are five
different types of Histone proteins that form a complex with DNA, and it is these nucleosomes which
form the basic unit of DNA packing.
At the simplest level, chromatin is a doublestranded helical structure of DNA, at this level the
size of the chromatin is about 2nm. DNA is then complexed with Histones to form nucleosomes; this
nucleosome bead consists of DNA wrapped 1.65 times around a protein core. This Chromatosome
consists of a nucleosome plus the H1 Histone, which is about 10 11 nm in size. The Histone H1 then
helps the nucleosomes to coil producing a 30 nm fibre that form hoops, these hoops are called looped
domains and they average in 300 nm in length. The looped domains are attached to a scaffold of a
nonhistone protein. The 300 nm looped domains are compressed and folded to produce a 250 nm
wide fibre with a length of 700 nm. The coiling of the 250 nm fibre produces the chromatid of a
chromosome. The coiling of the looped domains is what make the chromatin even more compact and
produce the characteristic metaphase chromosome.
Chromosomes occur in homologous pairs, which is why there is an even number of
chromosomes in the cells of almost all species. The reason that chromosomes occur in these pairs is
that sexually produced organisms are the result of the fusion of the sperm and the egg, which both
contribute a set of chromosomes to the offspring. For that reason a pair is copied from chromosomes of
the mother in the egg (the maternal chromosomes) and the other pair is copied from the chromosomes
of the father from the sperm (the parental chromosomes), these are the homologous pairs; the total
number is known as the diploid number which is 46 in humans.
The homologous pair is the two chromosomes which determine the same genetic
characteristics, but this pair does not mean that the chromosomes are identical. Each homologous pair
of chromosomes posses' information like, eye colour and hair colour, so one chromosome could carry
the alleles for brown eyes and ginger hair, and the other could carry the alleles for green eyes and
black hair. Meiosis causes the number of chromosomes to be halved where each daughter cell receives
one chromosome from each homologous pair this causes the daughter cell to receive one set of
information for each characteristic of the of the organism. When the haploid cells (gametes) combine,
the diploid state, with paired homologous chromosomes is restored.