Introduction to Cell Division and Genetics
The instructions to build a cell are coded as genetic information or genes.
Cell division in a cell's way of reproducing.
Unicellular - single cell
Multicellular - many cells (many specialized)
Some multicellulars reproduce by "splitting"
Most multicellulars reproduce using special reproductive cells called gametes (sperm or egg cells).
A zygote is a single cell created by the union of a sperm cell and an egg cell.
A zygote undergoes a cell division to create two cells. These undergo further cell divisions and eventually give rise to the multicellular organism.
1 -> 2 -> 4 -> 8 -> 16 -> 32 -> 64
lots more cell divisions 100 trillion (1014) cells = baby
(What would happen if there was one extra cell division before birth?)
[The baby would be twice as big!]
If genetic information is lost or corrupted all descendents (of that cell) will inherit the errors. These genetic errors are called mutations. Some cell mutations occurring after the zygote stage (cancers) while others are inherited from the parents' mutant gametes (cystic fibrosis, Downs syndrome).
(What would happen if cells only divided without growing in size between divisions?)
[They'd get smaller with each division and disappear!]
|The growth phase is an important part of a cell "cycle" because if there were no cell growth between divisions the cells would divide and divide, becoming smaller with each division, until they disappeared!|
Genetic structure (genes) are coded by DNA.
Organized structures of these genes are called chromosomes.
Bacteria (and viruses) - just DNA. ("naked chromosomes")
All other organisms - DNA and proteins.
Eukaryotes = chromosomes in a nucleus.
Prokaryotes = do not have a nucleus.
Eukaryotes are more complicated in how they package, store and
distribute their chromosomes.
Eukaryotic chromosomes are linear (lengths of ds)
Chromosomes are only visible during nuclear division.
During division, chromosomes are wound up ("condensed")
Between division - chromosomes are unwound ("decondensed")
Know - teleomere, centromere, chromatids, sister chromatids.
Cell division in eukaryotes is complicated because they have:
Mitosis and meiosis refer to the division of the NUCLEUS and this is only a part of cell division. M&M increase the number of nuclei, NOT the number of cells.
Mitosis produces identical nuclei,
is the dominant type of nuclear division
and its purpose is to increase the number of nuclei.
Mitosis is also used in asexual reproduction - reproduction without mixing gametes. (Vegetative propagation, cloning, etc.)
Meiosis produces diverse nuclei
is a very specialized type of nuclear division occurring only in cells that will become gametes (sex cells - sperm and egg)
and its purpose is sexual reproduction - reproduction involving the fusion of gametes.
A clear understanding of meiosis is essential in order to understand genetics - the study of heredity.
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