Haploid vs. Diploid
A cell in which the chromosomes are present as single chromosomes is
called haploid.
A cell in which the chromosomes are paired is called diploid.
Number of chromosomes
Some organisms contain a single chromosome, and others contain multiple
chromosomes. For example:
- the bacterium E. coli contains a single, circular chromosome
- a yeast (S. cerevisiae) cell contains 16 chromosomes
- a mouse cell contains 20 pairs of chromosomes (19 autosomes plus the X/Y
sex chromosome)
- as noted above, a human cell contains 23 pairs of chromosomes (22 autosomes plus the X/Y sex chromosome).
Humans do not necessarily have the largest number of chromosomes, however. The other great apes, for example, have 48 chromosomes, and some plants have hundreds of chromosomes. Members of the genus Ophioglossum (adder's tongue fern, an apparently rather ancient lineage) have 100's of chromosomes. Ophioglossum reticulatum, for example, is estimated to have from 240 to 510 chromosomes in the haploid state, or up to 1020 chromosomes in the diploid state!
If you are interested in the genome sizes (in base pairs) of various organisms,
one helpful resource is DOGS: Database of Genome Sizes.
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