Download Meiosis

Meiosis
Meiosis – Type of cell division that happens only in sex
cells (gametes)
The resulting cells receive only half the number of
chromosomes present in somatic (body) cells.
Meiosis Video
How many chromosomes do human
somatic cells have?

Karyotypes show that human somatic cells have 46
chromosomes and these chromosomes come in pairs.
Human somatic cells are said to be diploid.
Diploid – cells having pairs of chromosomes
These pairs are said to be homologous.
Homologous – similar in structure; the two members
of each pair of chromosomes carry DNA with
information for corresponding traits.
Why Pairs?
Mitosis
Fertilization
Mitosis
92
46
Mitosis
92
92
92
92
Mitosis
92
92
46
Fertilization

This means that the chromosome number has to be 23
to produce gametes (sperm and egg).
How is the chromosome number reduced?
Meiosis!
Mitosis vs. Meiosis

Prophase
• Homologous chromosomes
pair to form tetrad
Mitosis vs. Meiosis

Metaphase
• Homologous pairs line
up on equatorial plane

Tetrads can be in any order as long as homologous
chromosomes are paired
Random Assortment
Mitosis vs. Meiosis

Anaphase
• Homologous chromosomes
of each tetrad separate and
move to opposite poles
Mitosis vs. Meiosis

Telophase
Cytoplasm divides
to form two cells.
•
Mitosis vs. Meiosis
Second Stage of Meiosis
 Prophase


Chromosomes skip
replication
Begin to move toward
middle
Mitosis vs. Meiosis

Metaphase


Chromosomes line up
on equatorial plate
Spindle fibers attach
at centromeres
Mitosis vs. Meiosis

Anaphase

Sister chromatids
separate at
centromere
Mitosis vs. Meiosis

Telophase


Nuclear membrane
reforms.
Chromosome number
is cut in half. Now
haploid.
Haploid – cells having only
one chromosome from
each homologous pair.
Differences between Mitosis
and Meiosis
1.
Meiosis makes sex cells (gametes)
2.
Meiosis cuts chromosome number in half.
No more pairs.
3.
Meiosis has two rounds of cell division while
mitosis only has one.
4.
Chromosome pairs (tetrads) meet in the
middle during first metaphase.

How many sperm are produced from one cell
in meiosis?
Four

How many eggs are produced from one cell in
meiosis?
One
Timing of Meiosis

Females


Begins before birth.
Release one egg a month from puberty until
menopause.
Timing of Meiosis

Males


Begins at puberty
Continues until death. Slows down – sperm count
lowers.
Terms to be familiar with:



Diploid – cells having two sets of
chromosomes; in humans this number is 46
Haploid – cells having only one chromosome
fromm each homologous pair; in humans this
number is 23.
Homologous – similar in structure; homologues
in diploid cells are the two members of each
pair of chromosomes that carry DNA with
information for corresponding traits.