MEIOSIS
by R.B. Perry
The Marine Research Facility
Malibu High School
ASEXUAL
vs
SEXUAL
REPRODUCTION
Asexual (vegetative) reproduction
A form of duplication using only mitosis.
Example, a new plant grows out of the root or a
shoot from an existing plant.
Produces only genetically identical offspring since
all divisions are by mitosis.
Offspring called clones meaning that each is an
exact copy of the original organism
This method of reproduction is rapid and effective
allowing the spread of an organism
Since the offspring are identical, there is no
mechanism for introducing diversity
Sexual reproduction
Formation of new individual by a combination of
two haploid sex cells (gametes).
Fertilization- combination of genetic information
from two separate cells that have one half the
original genetic information
Gametes for fertilization usually come from
separate parents
Female- produces an egg
Male - produces sperm
Both gametes are haploid, with a single set of
chromosomes
The new individual is called a zygote, with two sets
of chromosomes (diploid).
WHAT IS
MEIOSIS
??
MEIOSIS
MEIOSIS IS THE PROCESS
OF CUTTING BY HALF THE
NUMBER OF CHROMOSOMES
TO FORM GAMETES and
MEIOSIS
MEIOSIS
causes a change in the
genetic information to
increase diversity in the
offspring.
MEIOSIS
Every normal
body cell is
DIPLOID
…2 sets of
chromosomes
(except
gametes).
GAMETES:
OVA ARE
HAPLOID
…one set of
chromosomes
SPERM ARE
HAPLOID
…one set of
chromosomes
OFFSPRING ARE
DIPOLID
…two sets of chromosomes
One member of
each pair comes from
your mother.
One member of
each pair comes from
your father.
How does every cell end up with a pair of
chromosomes, when one comes from each parent ?
ovum
sperm
The answer is carried
in the gametes.
1N
ovum
sperm
The ovum carries
one set of chromosomes.
Symbol for 1 set of chromosomes = 1 N
OVARIAN FOLLICLES MATURE
THEN BURST, RELEASING AN OVUM.
1. This is a cross section
through one ovarian
follicle which are found
in each ovary.
1.
2.
2. This is the young
developing ovum
inside the follicle.
1N
ovum
1N
sperm
The sperm carry
one set of chromosomes.
Symbol for 1 set of chromosomes = 1 N
1.
2.
This is a cross section
through one seminferous
tubule which makes up
the testis.
1. These are the normal
sperm producing cells
(SPERMATOGONIA)
in the wall of each
tubule.
2. These are the young
sperm just starting
to swim.
MEIOSIS IS THE PROCESS
OF CUTTING BY HALF THE
NUMBER OF
CHROMOSOMES
FOUND IN GAMETES.
This is the precursor
cell found in the ovarian
follicles or seminferous
tubules.
These are the
steps and
stages
for cutting by
half the
chromosome
number.
The end product is gametes
with only ONE set of
chromosomes.
OR
CROSSING-OVER:
(during the first meiotic division process)
--Genetic material from the homologous chromosomes is
randomly swapped. This creates four unique chromatids.
--Since each chromatid is unique, the overall genetic
diversity of the gametes is greatly increased !
1N
ovum
1N
sperm
MEIOSIS produces gametes
that are HAPLOID.
Symbol for 1 set of chromosomes = 1 N
Sperm cell = 1N (one set of chromosomes)
Egg cell = 1N (one set of chromosomes)
1N
ovum
1N
sperm
fertilization
1n
sperm
ovum
1n
Fertilization
1n + 1n = 2n
haploid + haploid = diploid
1n + 1n = 2n
Fertilization:
restores the normal
DIPLOID number
for the offspring.
1N
sperm
1N
ovum
2N
4 week old
human embryo
1N
sperm
1N
ovum
2N
6 week old
human embryo
1N
sperm
1N
ovum
2N
human fetus
1N
ovum
1N
sperm
2N
ANIMATION LINK:
http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
REVIEW CHECKPOINT:
1. What is meiosis ?
2. Explain why meiosis is necessary.
3. Name the only 2 kinds of human cells that
do NOT have a full 2N set.
4. What is meiosis in females called ? males ?
5. Where does meiosis in females occur ?
6. Where does meiosis in males occur ?
7. Give the mathematical formula for
fertilization using chromosome numbers.
8. What would happen to chromosome numbers
after a few generations without meiosis ?
9. How does meiosis contribute to genetic diversity?
10. How does sexual reproduction contribute to
genetic diversity?