Biology - Chromosomes and coding instructions
Chromosomes and coding instructions
Click on each cell to see the type of chromosomes it contains
A nucleosome is the packaging unit of eukaryotic chromosomes. Use your
mouse to explore the structure of a nucleosome.
X and Y chromosomes shown under an electron microscope
Sets of coding instructions are carried by the chromosomes, coils of
molecules of deoxyribonucleic acid (DNA) wrapped at intervals around
protein molecules called histones. Chromosomes form an irregular loop in
the cytoplasm of prokaryotes  and separate structures in the nucleus 
of eukaryotes.  Circular material of DNA,  called plasmids,  are
present in the cytoplasm of many bacterial cells.
Each species of eukaryotic organism has its own particular number or set
of chromosomes  in its body cells usually occurring in matched or
homologous  pairs. For example, humans have 23 pairs = 46 in total. 2n
refers to the total or diploid  number of chromosomes a cell has, and n
refers to the total of one of each pair of chromosomes, the haploid number.
In humans somatic cells  (cells other than gametes)  have the
diploid number (2n = 46), whereas gametes have the haploid  number (n =
23). Twenty-two of the pairs are autosomes  and one is an unmatched
pair, the sex chromosomes (X and Y) which determine the sex of the
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