The human body has 23 pairs of chromosomes (46) total.
- Each parent has 22 pairs of autosomal (body)
chromosomes.
- Each parent also has 1pair of sex chromosomes.
o Females are XX
o Males are XY
- Within the parents, the pair of chromosomes is homologous, meaning each set from
the male parent will match up to those of the female parent.
- A cell that contains both sets of homologous chromosomes is said to be diploid (2N).
- The gametes (reproductive cells) of the parents are
only haploid (1N), containing only one set.
- Example: The fruit fly.
Diploid
Haploid
Meiosis is a reductional division. The number of
chromosomes are reduced to half the number of chromosomes
in the diploid cell.
- If each parent were to contribute a complete set of
chromosomes to their offspring every time we
reproduced, our number of chromosomes would double.
Meiosis involves two stages: Meiosis I and Meiosis II.
Meiosis I
- The cells begin to divide almost identical to the way they
would in mitosis, with one exception, the chromosomes
do not separate.
- Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad.
- Crossing over can occur during Meiosis I as the chromosomes are so close together,
they can exchange alleles and produce new combinations of alleles. Two haploid cells
are produced.
Meiosis II
- In Meiosis II, everything is the same except the chromosomes do not replicate prior to
this division. The two cells from meiosis I divide and produce four haploid cells.
 Males: sperm, through spermatogenesis.
 Females: eggs, through oogenesis. In females, the meiotic divisions tend to be
uneven: only 1 gets the majority of the cytoplasm. The other 3cells are known as
polar bodies dissolve, and tend to be reabsorbed by the body.
Comparing Mitosis and Meiosis
Mitosis
One Division
Two Diploid daughter cells (2N) are
produced.
Cells are identical to parent cells
Associated with growth, replacing cells,
and asexual reproduction.
Meiosis
Two divisions
Four Haploid daughter cells (1N) are
produced.
Daughter cells are not identical to
parent cells.
Associated with sexual reproduction.