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Notes E 4.3
Meiosis is a special form of cell
division.
Meiosis is necessary for sexual
reproduction.
Organisms that reproduce sexually produce
gametes, sex cells that contain ½ as many
chromosomes as body cells.
• Eggs—female gametes
• Sperm—male gametes
• 1n—gametes, also called haploid cells,
produced by the process of meiosis
In fertilization, 2 gametes join. The resulting cell
is 2n, or diploid. I has pairs of chromosomes,
with a homolog from each parent.
1n+1n=2n
Cells divide twice during meiosis.
Before meiosis, the DNA in each homolog is copied .
The copies are referred to as chromatids and held
together by a centromere. Meiosis differs from
mitosis because:
• The genetic material is divided not once, but
twice
• Four 1n (haploid) daughter cells are formed.
In meiosis I, the first 2 daughter cells are produced.
They receive one of the homologs.
In meiosis II, these 2 daughter cells divide again,
splitting the homolog so that each cell gets
one of the copies.
Each of the 4 resulting daughter cells has half
the number of chromosomes needed for
offspring.
Meiosis and mitosis differ in important
ways.
• Cells that become gametes go through
meiosis; all other cells divide by mitosis.
• In meiosis, DNA is copied; then the cells divide
twice.
• Each cell has only one homolog from one
parent.
• In mitosis, DNA is copied, and the cells divide
just once. Each cell has 2 homologs, one from
each parent.
• Meiosis produces haploid (1n) daughter cells
that have half of the genetic material of the
parents.
• Mitosis produces diploid (2n) daughter cells
that have the same genetic material as the
parents.
• Meiosis produces gametes, cells specialized
for reproduction.
• During fertilization, 2 gametes combine and 2
haploid cells form into one diploid cells.
• The cell then divides by mitosis, developing
into a multicellular organism.