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The humanbody contains100 trillioncells.

There is anucleus insideeach humancell (except redblood cells).

Each nucleuscontains 46chromosomes,arranged in 23 pairs.

Onechromosomeof every pair isfrom eachparent.

Thechromosomesare filled withtightly coiledstrands ofDNA.

Genes are segmentsof DNA that containinstructions to makeproteins— thebuilding blocksof life.

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

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

2Fertilization

sperms

ovumnuclei combine cell division (mitosis)

embryoformed

ANIMAL

3

This sperm will fertilize the ovum

4

Gamete Formation

7

Meiosis reduces the number of chromosome sets from diploid to haploid.

STAGES OF MEIOSIS

•Meiosis is preceded by the replication of chromosomes.

•Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

•One replication followed by the two cell divisions result in four daughter cells.

•Each daughter cell has only half as many chromosomes as the parent cell.

• In the first cell division (meiosis I), homologous chromosomes (each with two sister chromatids) separate from each other and go to daughter cells

• In the second cell division (meiosis II), sister chromatids separate.

• Each division has prophase, metaphase anaphase and telophase. The interphase between Meiosis I and II is usually very short or even nonexistent.

egg

polarbody

spermatogonium

primaryspermatocyte

secondaryspermatocyte

oogonium

primaryoocyte

secondaryoocyte

polar bodies(will be degraded)

spermatids

meiosis ll

meiosis l

SPERMATOGENESIS OOGENESISa b

•Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids

•The sister chromatids are genetically identical and joined at the centromere

•The single centrosome replicates, forming two centrosomes.

Prophase I

• Occupies more than 90% of the time required for meiosis

• Chromosomes begin to condense

• In synapsis, homologous chromosomes loosely pair up, aligned gene by gene

• Each pair of chromosomes forms a tetrad, a group of four chromatids joined together

• In crossing over or recombination , nonsister chromatids exchange DNA segments

• Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

•Crossing over allows exchange of genetic information between homologous chromosomes

Tetrad

Metaphase I

• In metaphase I, tetrads line up at the metaphase plate, with one homologous chromosome facing each pole

•Microtubules attach homologous chromosomes to opposite spindle poles

• The alignment of homologous chromosomes is random. There is a 50:50 chance they will align in any one direction

CartoonVersionOfProphase IAndMetaphase I

Prophase I Metaphase I

Centrosome(with centriole pair)

Sisterchromatids

Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomes

Fragmentsof nuclearenvelope

Microtubuleattached tokinetochore

Anaphase I

• In anaphase I, pairs of homologous chromosomes separate-synapsis is over.

•Spindle fibers pull the tetrads apart towards the opposite side of the cell.

•Cytokinesis begins.

Telophase I• In the beginning of telophase I, each half of

the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

•Cytokinesis usually occurs simultaneously, forming 2 haploid daughter cells

•There is no S phase between Meiosis I and II

•New nuclear and nucleolus form.

Cartoon version of Anaphase I and Telophase I

Anaphase ITelophase I and

Cytokinesis

Sister chromatidsremain attached

Homologouschromosomesseparate

Cleavagefurrow

2 haploid daughter cell

END OF FIRST MEIOTIC STAGE

MEIOSIS II

• No chromosomes replication between meiosis I and meiosis II.

• Similar to the events of mitosis.

Prophase II• In prophase II, a spindle apparatus forms• In late prophase II, chromosomes (each still

composed of two chromatids) move toward the metaphase plate

Metaphase II• In metaphase II, the sister chromatids are arranged

at the metaphase plate• Because of crossing over in meiosis I, the two

sister chromatids of each chromosome are no longer genetically identical.

Cartoon Version of Prophase II and Metaphase II

Prophase II Metaphase II

Anaphase II• In anaphase II, the sister chromatids separate• The sister chromatids of each chromosome now

move as two newly individual chromosomes toward opposite poles

Telophase II and Cytokinesis•In telophase II, the chromosomes arrive at opposite poles•Nuclei form, and the chromosomes begin decondensing

Cartoon Version of Anaphase II and Telophase II

Anaphase IITelephase II and

Cytokinesis

Sister chromatidsseparate Haploid daughter cells

forming

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

13Meiosis 6

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

Genes for any one characteristicoccupy corresponding positions on homologous chromosomes

But they do not necessarily controlthe characteristic in the same way

For example, one of the gene pairresponsible for eye colour mightdetermine brown eyes and its partner determine blue eyes*

gene for brown eyesgene forblue eyes

gene for curly hair

gene forstraight hair

17Genes

Usually only one of a gene pair will be expressed in anindividual

A person inheriting the gene for brown eyes and the gene for blue eyes will have brown eyes

The gene for brown eyes is said to be dominant tothe gene for blue eyes. The gene for blue eyes is notexpressed in this individual

The gene for blue eyes is said to be recessive to thegene for brown eyes

18

In the first stage of meiosis, the illustration (slide 10) showed one ‘red’ and one ’blue’ chromosome going to each daughter cell

B

One gamete will receive the gene combination for brown eyes and curly hair. The other will receive the genes for blue eyes and straight hair B

b

C

c

B = gene for brown eyes

b = gene for blue eyesC = gene for curly hairc = gene for straight hair

19Gene combinations

It is just as likely that both ‘blue’ chromosomeswill go to one daughter cell and both ‘red’ chromosomes go to the other

B

b

c

C

One gamete will receive thegenes B and c (brown eyes and straight hair)

The other gamete will receive genes b and C (blue eyes andcurly hair)

20

So, there could be 4 types of gamete with different combinations of the genes

BC brown eyes, curly hair

bc blue eyes, straight hair

Bc brown eyes, straight hair

bC blue eyes, curly hair

21

Meiosis not only halves the number of chromosomes but can also rearrange the genes

This is one cause of the variations that occur in members of the same species

22Variation

Rearrangement of genes can also take place at fertilization

A sperm may carry a gene for brown eyes (B) or a gene for blue eyes (b)

An ovum may carry a gene for brown eyes (B) or a gene for blue eyes (b)

At fertilization, four possible combinations can occur

23

Bb

BB

bB

bb

bb

sperm ovum 4 Possible combinations

BB, Bb and bB have the same effect of producing brown eyes

Only bb gives rise to blue eyes

Although there are 4 possible combinations of genes

B B

24

fertilization

Question 1

Which of the following are gametes ?

(a) sperms

(b) dividing cells

(c) ova

(d) nuclei

Question 3

What is the correct sequence of events in meiosis ?

(a) (b) (c) (d) (e) (f)

(a) a, b, d, c, e, f

(b) b, a, d, c, e, f

(c) b, d, a, c, e, f

(d) a, b, d, c, e, f

Question 4

Which of the following represent variation within a species ?

(a) black cats and tabby cats

(b) collie dogs and dachshunds

(c) goldfinch and greenfinch

(d) shire horses and race horses

Answer

Correct

Answer

Incorrect