DNA (deoxyribonucleic acid) is the molecule that carries the genetic information in
all cellular forms of life and some viruses. It belongs to a class of molecules called the
nucleic acids, which are polynucleotides - that is, long chains of nucleotides.
Levels of Genome (entirety of an organism’s hereditary information)
Organisation
• Chromosomes are made up of DNA wrapped around proteins (histones at the
most basic level, which form units called nucleosomes)
• The DNA molecule itself is highly condensed
• This is necessary as, for example, all the DNA in one human cell nucleus would
stretch out to about 1 meter in length
Histones are conserved throughout evolution as they have a vital role whereby the
DNA molecules are wrapped around the histones to form nucleosomes. These are
then twisted to form primary chromatin fibers. The fibers form loops on a scaffold.
The loops twist around each other to form what we see in a condensed chromosome.
One of the main functions of chromosomes is to provide a packaging mechanism to
hold/condense all the DNA contained within cells and to ensure that it can go
through the maneuvers of copying DNA and cell division, otherwise it would get
knotted up.
A set of chromosomes in the stage of cell division called metaphase, can be stained
and visualized under a microscope. They can be stained with various chemicals which
produce different banding patterns. This can be used to see if there are
chromosomes missing or extra chromosomes.
Karyotype
• An organism’s karyotype is the description of its compliment of chromosomes:
their number, size, shape and banding patterns.
• Chromosomes are only visible as separate entities at certain stages of the cell’s
growth cycle.
• In humans there are 23 pairs of chromosomes. In 22 pairs, each member of the
pair is homologous to the other. In the 23rd pair (the sex chromosomes), women
have 2 X-chromosomes whilst men have 1 X and 1 Y chromosome.
Chemical Structure of DNA
• A single unit is known as a nucleotide, which has 3 subunits – phosphate, a
deoxyribose sugar and a nitrogenous base
• The nitrogenous bases stick together when in double strand form in a very
particular way (A with T and G with C)
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