MEIOSIS

1

Fertilization occurs when the nucleus of a male reproductive cell combines with the nucleus of a female reproductive cell

The reproductive cells are called gametes

In animals, the male gamete is the sperm cell and the female gamete is the ovum

In flowering plants, the male gamete is a cell in thepollen grain and the female gamete is an egg cellin the ovule

When the male and female gametes combine, the resulting cell is called a zygote

2Fertilization

pollen nucleus

egg cell

nuclei combine cell division (mitosis)embryo formed

PLANT

sperms

ovumnuclei combine cell division (mitosis) embryo

formed

ANIMAL

3

As a result of fertilization, the chromosomes from the male and female cells are combined in the same nuclear membrane

Do you see a problem with this?

5

The problem is that if the full complement of male andfemale chromosomes combine, the zygote and embryo would have twice as many chromosomes as its parents

If 46 male chromosomes combined with 46 femalechromosomes, the offspring would have 92 chromosomesin their cells

And the next generation would have 184 chromosomes,and so on

In fact, when the gametes are formed, the number of chromosomes is halved so that the zygote ends up with the same number of chromosomes as its parents.

6

The type of cell division which gives rise to gametesis called

Reduction division in which the chromosome number is halved from diploid to haploid.

The following slides describe this process.Only two pairs of chromosomes are shown

7

The chromosomes appear,shorten and thicken just asin mitosis

The ‘blue’ chromosomesare from the male parent; the ‘red’ chromosomesare from the female parent

The two long chromosomes and the two short chromosomes are called homologous pairs

8Meiosis 1

Homologous chromosomes come to lie closely alongsideeach other and behave like a single chromosome

9Meiosis 2

The nuclear membrane disappears

A spindle forms

At this stage it is thechromosomes (and notthe chromatids) which areseparated

The cell starts to constrict

10Meiosis 3

One long chromosomeand one short chromosomego to either end of the dividing cell

So the cells now containonly two, rather than fourchromosomes

By this time the chromatidshave become clear

11Meiosis 4

Two new spindles form, at right angles to the originalspindle

At this stage, it is the chromatids which separate and pass to opposite ends of thecells

The cell constricts in the planeat right angles to the first constriction

12Meiosis 5

Cell division is completed,forming four gameteseach with half the numberof chromosomes of the parent cell

gametes

13Meiosis 6

The number of chromosomes in the body cells is called the diploid number

The number of chromosomes in the gametes is calledthe haploid number (half the diploid number)

The diploid number in human cells is 46. The haploidnumber in the gametes is 23

14

23

46

4623

23

23

23

23

23

46

sperm mothercell

ovummothercell

sperms produced by meiosis

fertilizationzygote

ova produced by meiosis but only one develops tomaturity

15

46 46

46 46

46 46 4646

46 46 46 46

4646

46 46

4646

Cell division continues by mitosis, so all the cells willcontain 46 chromosomes early embryo

16