Download Meiosis- Cell Division in Sex Cells

Meiosis- Cell Division in Sex Cells
I.
Meiosis is the process of cell division that reduces the # of
chromosomes to ½ the original number.
 In humans, this occurs in egg and sperm cells (called gametes).
A. Process- much like mitosis- but the cells divide twice (see. Pp.
263-273). In humans, the number of chromosomes is reduced
from 46 (diploid) to 23 (haploid).
1. Egg Formation- Oogenesis-- (pp. 1001) Occurs in ovaries
of females. The first meiotic division occurs before birth,
these eggs are immature and cannot be fertilized. The second
meiotic division occurs approximately monthly with the
maturation of the female. In this process, one egg is produced
from one original cell (3 small cells, called polar bodies,
degenerate).
2. Sperm formation – Spermatogenesis- (pp. 1001) Occurs in
testes of males. Also involves 2 meiotic divisions that begin
during puberty. Unlike females, the number of sperm
produced in a lifetime are not pre-determined. Four haploid
sperm cells are produced from each original cell.
B. Importance of Meiosis- Allows for genetic diversity because
gametes (egg and sperm) fuse to form a zygote (fertilized egg)
that is genetically different than the parents.
Genetic diversity can also occur when chromosomes pair up
(called synapsis) during the 1st stage of meiosis. The
chromatids of chromosome pairs can come in contact with each
other, break off, then reattach to the other chromosome. This is
called crossing over. (pg. 269).
C. Genetic Diseases resulting from Nondisjunction (improper
splitting of chromosomes during meiosis p. 271).
1. Sometimes chromosomes do not split properly. A cell that has
only 45 chromosomes, thus missing a copy of a particular
chromosome, is called a monosomy. If a cell has 1 too many
chromosomes, the condition is called trisomy. These
conditions occur at fertilization of egg and sperm.
#of daughter cells
Mitosis
All but egg and
sperm
2
Diploid or haploid?
Diploid to diploid
Meiosis
Egg and sperm
(gametes)
4 sperm
1 egg (3 polar
bodies)
Diploid to haploid
# of cell divisions
1
2
# of chromosomes in
human cells
46
23
Type of Cells