Download Meiosis II

MEIOSIS NOTES
Chapter 11, section 4
Reproduction
 Asexual (relies on mitosis): Involves 1
parent; offspring are clones
 Advantages:
happens quickly & produces
large #s of offspring
 Disadvantages: no genetic variation
 Sexual (relies on meiosis): Involves 2
parents; offspring are different
 Advantages:
allows for genetic variation
 Disadvantages: have to find mate;
process takes longer; fewer offspring
produced.
2 TYPES OF CELLS
1) SOMATIC CELLS
2) GAMETES
Types of Cells: Somatic Cells
All cells in the body; not sex cells
-Also called Body cells
-Made by mitosis
This scanning electron micrograph (courtesy of Dr. Marion J. Barnhart) shows the characteristic
biconcave shape of red blood cells.
Types of Cells: Somatic Cells
-Contain 2 copies of each
chromosome
-called homologous
chromosomes
-have diploid #, 2n
EX. Humans =46
Types of Cells: Gametes
*Also called sex cells
*Eggs and sperm
*Made by meiosis
*Have haploid # (n)
*EX. Humans =23
Sex Cells (sea urchin egg
being fertilized)
Chromosome #
Egg (haploid #)
Sperm(haploid #)
ZYGOTE (diploid #)
Ex. Humans
23 + 23 =46
Meiosis: produces gametes
Reduction division
-starts with 1 diploid cell and
ends up with 4 haploid cells
Meiosis occurs in 2 steps:
MEIOSIS I & MEIOSIS II
MEIOSIS I
P1, M1, A1, T1)
homologous chromosome
pairs are divided
two new daughter (diploid)
cells are produced
 Prophase I
 *Step 1—The chromosomes (DNA) coil up
becoming visible and a spindle forms. The nuclear
membrane and nucleolus disappear.
 *Step 2— Synapsis occurs




Synapsis is the pairing of homologous chromosomes.
A tetrad is formed.
A tetrad is the pair of homologous chromosomes.
**Crossing over can occur here.
• Crossing over is the exchange of genetic
material (genetic recombination). This
allows for genetic variation among
organisms.
Crossing Over
Adapted from:
Morgan T.H., Sturtevant A.H., Muller H.J., and Bridges C.B., "The Mechansim of Mendelian Heredity", 1915.
 Metaphase I
 Tetrads are moved by spindle fibers to the
equator.
 **They line up in homologous pairs. In
mitosis, they line up independently.
 Anaphase I
 Homologous chromosomes separate; chromosomes
of each pair move to opposite poles.
 **The chromatids do not split, as they do in mitosis.
 Telophase I
 Chromosomes gather at the poles of the cell. The
cytoplasm divides.
 **Each cell has 1 homologous pair. These
chromosomes are still attached by centromeres
forming two daughter cells.

Another cell division is needed because each
chromosome is still doubled, containing two identical
sister chromatids.
MEIOSIS II
 (P2, M2, A2, T2)
 Sister chromatids are split into 4
haploid cells
Meiosis II is similar to mitosis in that the chromatids
separate.
 There are four phases of Meiosis II:
 Prophase II –A new spindle forms around the
chromosomes.
 Metaphase II-Pairs move to the equator of the
cell.
 Anaphase II-Sister chromatids separate and move
to opposite poles.
 Telophase II-Spindle fibers
dissolve, nuclear membrane
forms, and the cytoplasm
divides.
Meiosis Diagram
Gamete Development in
Males &Females
MALES
Meiosis occurs in
testes at puberty
and then occurs
as needed
1 spermatocyte=4 sperm
FEMALES
Meiosis occurs in
the ovaries
before birth
1 oocyte=1 egg and 3
wasted cells
Sperm Formation-Spermatogenesis
Egg Formation - Oogenesis
(unequal cytokinesis)
Oogenesis
Spermatogenesis
Comparing Mitosis
& Meiosis
HOW MANY CELLS ARE MADE?
Mitosis:
2 diploid cells
Meiosis:
4 haploid cells
Comparing Mitosis
& Meiosis
ARE RESULTING CELLS GENETICALLY
THE SAME OR DIFFERENT?
Mitosis:
Genetically the same
Meiosis:
Genetically different
Comparing Mitosis
& Meiosis
WHAT TYPES OF CELLS RESULT IN
THE PROCESS? (SOMATIC CELLS
OR GAMETES)
Mitosis:
Somatic cells
Meiosis:
Gametes
Comparing Mitosis
& Meiosis
WHAT ARE THE ADVANTAGES OF
EACH PROCESS?
Mitosis:
Growth and repair of organism
Meiosis:
Continuation of species and genetic
recombination, which allows for genetic
variation