M E I O SI S 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 I S
M E I O SI S ??
M E I O SI S M EI OSI S I S THE PROCESS OF CUTTI NG BY HALF THE NUM BER OF CHROM OSOM ES TO FORM GAM ETES and
M E I O SI S
M E I O SI S causes a change in the genetic information to increase diversity in the offspring.
M E I O SI S
Every normal body cell is DI PLOI D ‌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
OVARI AN FOLLI CLES M ATURE THEN BURST, RELEASI NG 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 (SPERMATOGONI A) in the wall of each tubule. 2. These are the young sperm just starting to swim.
M EI OSI S I S THE PROCESS OF CUTTI NG BY HALF THE NUM BER OF CHROM OSOM ES FOUND I N GAM ETES.
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?