2012
Meiosis Notes
Production of
Reproductive Cells
The number of chromosomes per cell depends
on the type of organism
Human 46, reproductive cell ______
Fruit Fly 8, reproductive cell ______
http://morgan.rutgers.edu/morganwebframes/level1/p
age2/ChromNum.html
Diploid Cells: 2 of each chromosome
* Chromosomes vary in size and shape
* Human reproductive cells = 23
chromosomes (sperm, egg)
Homologues: 2 members of each
pair of chromosomes
Gametes (2)/ Sex cells
*Sperm = male repro. Cell = 23 chromosomes
*Egg = female repro. Cell + 23 chromosomes
Fertilization: joining of sperm and egg
Zygote: single cell of fertilized egg =
46 chromosomes
Haploid Cells: one chromosome of each
homologous pair
* Two haploids come together to form one
diploid cell
* Haploids are produced by Meiosis: process
by which haploid nuclei are formed from
diploid nuclei. Correct meiosis ensures
gametes will have the right number and type
of chromosomes.
I. Meiosis in Male Animals
Meiosis I
A. Chromosomes shorten and thicken
B. Chromosomes replicate – two chromatids
joined by centromere
C. Homologous chromosome pairs attract and
come together
D. Tetrads move to the equator
E. One chromosome pair to each pole
F. Cleavage furrow forms 2 new cells with 2
chromosomes each pair
• Short Interphase
• Then Meiosis II begins
Meiosis II (males)
A.
B.
C.
D.
Homologous chromosomes move to equator
Chromatids pull apart
Chromosomes move to poles
Cells divide = 4 new cells with ½ the original
chromosomes
*Formation of 4 haploid cells (immature
sperm)
*a flagella forms from 1 centriole of each cell
as it matures to propel the sperm cells
46
92
46
46
Diagram omits phases of
prophase, metaphase,
anaphase, telophase but
those phases are understood
to be present in order for
cells to divide as they do
23 chromosomes each new immature cell
II. Meiosis in Female Animals
Follows the same basic format as males except…
• Meiosis I in females begins before birth as
early as 12th week of development
• Meiosis then stops until sexual maturity, then
several cells continue meiosis on cyclic basis –
only 1 cell survives per cycle
• The surviving cell divides and the larger of the
2 enters meiosis II
• Meiosis II: the large cell divides again –
Small cell dies and the larger develops into
egg
Just take a look
DRAW THIS!!!!
Oogenesis = making of eggs
Germ cell
Spermatogenesis = making of sperm
Germ cell
Meiosis I
P
M
A
T
Meiosis I
P
M
A
T
Meiosis I
P
M
A
T
Meiosis II
P
M
A
T
Ova
Ovum
Oocyte
(Die off)
sperm
Spermatocyte
spermatozoa
2N – diploid
4N – replicated
N – haploid
Human #’s ????