Meiosis

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?