MEIOSIS and MEIOSIS and Sexual Sexual

ReproductionReproduction

Cell Division Cell Division Simplified…Simplified…

Types of CellsTypes of Cells

Body cellsBody cells Sex cellsSex cells All cells contain chromosomes!All cells contain chromosomes!

Number of ChromosomesNumber of Chromosomes Any cell with the Any cell with the total number of chromosome pairs is called of chromosome pairs is called diploiddiploid– 2n2n– HumansHumans 2n=46 chromosomes 2n=46 chromosomes

All BODY cells are diploid!All BODY cells are diploid!

The chromosomes of diploid cells The chromosomes of diploid cells come in pairscome in pairs

Two members of a pair are Two members of a pair are considered considered homologoushomologous (similar) (similar)

– Are called Are called homologshomologs. .

Each homolog carries the Each homolog carries the message for the same trait, message for the same trait, although although notnot necessarily the necessarily the

same message.same message.

Diploid CellsDiploid Cells

Homologous Pair ExampleHomologous Pair Example

One homologous pair has a One homologous pair has a (gene) code for eye color.(gene) code for eye color.

One homolog may code for One homolog may code for brown and the other for brown and the other for blue.blue.

It is the It is the combinationcombination of the of the two homologs that two homologs that determines eye color. determines eye color.

Number of ChromosomesNumber of Chromosomes

Any cell with Any cell with half the number of chromosomes of chromosomes pairs (or only 1 set) pairs (or only 1 set) is called is called haploidhaploid– nn– HumansHumans n=23 chromosomes n=23 chromosomes

Includes SEX cellsIncludes SEX cells– Also known as Also known as gametesgametes

Making Haploid CellsMaking Haploid Cells

We already learned mitosis is We already learned mitosis is used to make diploid (body) used to make diploid (body) cellscells

Another process is used to make Another process is used to make cells with only half of the cells with only half of the genetic information (haploid genetic information (haploid

cells)--cells)--MEIOSIS!!MEIOSIS!!

MEIOSISMEIOSISMeiosis converts a Meiosis converts a diploiddiploid cell cell

into into 4 haploid gametes4 haploid gametes. . Benefit of meiosis:Benefit of meiosis:

–It increases genetic diversity in It increases genetic diversity in the offspring.the offspring.

Stages of MeiosisStages of Meiosis

Most stages are very Most stages are very similar to mitosis!similar to mitosis!– Also begins after the Also begins after the chromosomes have chromosomes have replicated (Interphase)replicated (Interphase)

Differs because it is Differs because it is broken down into 2 broken down into 2 cells divisions:cells divisions:– Meiosis IMeiosis I– Meiosis IIMeiosis II

MEIOSIS IMEIOSIS I PROPHASE IPROPHASE I

–The nuclear membrane The nuclear membrane disappears. disappears.

–Spindle fibers begin to Spindle fibers begin to appearappear

MEIOSIS IMEIOSIS I PROPHASE IPROPHASE I

–Homologous Homologous chromosomes chromosomes pairpair up up (a step unique to (a step unique to meiosis) meiosis)

–The two pairs of The two pairs of chromosomes (or four chromosomes (or four chromatids) are called chromatids) are called a a tetradtetrad. .

Tetrad

METAPHASE IMETAPHASE I The The homologouhomologous s pairspairs line up line up in the in the middle of middle of the cell. the cell.

MEIOSIS IMEIOSIS I

ANAPHASE IANAPHASE I –The pairs The pairs

separate and the separate and the homologs move homologs move to separate poles.to separate poles.

MEIOSIS IMEIOSIS I

TELOPHASE ITELOPHASE I

–Each of the daughter Each of the daughter cells is now haploid, cells is now haploid, but each chromosome but each chromosome still has two still has two chromatids. chromatids.

MEIOSIS IMEIOSIS I

MEIOSIS IMEIOSIS I

CYTOKINESIS ICYTOKINESIS I– Cytokinesis occurs and forms two new cellsCytokinesis occurs and forms two new cells

MEIOSIS IIMEIOSIS II

Meiosis II is similar to mitosis. Meiosis II is similar to mitosis. However, there is no However, there is no

interphase.interphase.

MEIOSIS IIMEIOSIS II

PROPHASE IIPROPHASE II–A new spindle fiber A new spindle fiber formsforms

METAPHASE IIMETAPHASE II

–The chromosomes line up The chromosomes line up in the middle of the two in the middle of the two new cells. new cells.

MEIOSIS IIMEIOSIS II

ANAPHASE IIANAPHASE II –The chromatids separate The chromatids separate

and move to opposite ends and move to opposite ends of the two cells.of the two cells.

MEIOSIS IIMEIOSIS II

TELOPHASE IITELOPHASE II

–Both new cells complete division Both new cells complete division making making fourfour new cells. new cells.

–Cytokinesis occurs Cytokinesis occurs –The cells formed are The cells formed are gametesgametes!!

4 Haploid daughter cells are formed, each having only one chromosome of each homologous pair

MEIOSIS IIMEIOSIS II

GametesGametes

Female: Egg or ovumFemale: Egg or ovum– Occurs in the ovariesOccurs in the ovaries

Male: SpermMale: Sperm– Occurs in the testesOccurs in the testes

MEIOSIS END RESULTMEIOSIS END RESULT

Meiosis makes Meiosis makes 44 sex cells. sex cells. Each cell has Each cell has halfhalf the number of the number of

chromosomes as the parent cell chromosomes as the parent cell (haploid). (haploid).

Each cell has Each cell has one chromatidone chromatid from the original tetrad (one of from the original tetrad (one of each kind or size).each kind or size).

SEXUAL REPRODUCTIONSEXUAL REPRODUCTION

FertilizationFertilization- the combination of - the combination of chromosomes (genetic information) chromosomes (genetic information) from two sex cellsfrom two sex cells

Each sex cell gives half of the total Each sex cell gives half of the total chromosomes.chromosomes.

After fertilization, the developing After fertilization, the developing organism is called a organism is called a zygotezygote and is and is

diploid.diploid. –Haploid + Haploid= DiploidHaploid + Haploid= Diploid

Independent AssortmentIndependent Assortment How do you get genetic diversity?How do you get genetic diversity?

– By producing varied sex chromosomesBy producing varied sex chromosomes– Notice your siblings are not identical Notice your siblings are not identical even though the same two parents produced even though the same two parents produced them!them!

The arrangement of the chromosomes in The arrangement of the chromosomes in Metaphase IMetaphase I is the key to this is the key to this variation!variation!

How many different sex How many different sex cells?cells?

Because there are many ways to Because there are many ways to arrange the chromosomes during arrange the chromosomes during Metaphase, the following math Metaphase, the following math problem is used to calculate problem is used to calculate the number of DIFFERENT sex the number of DIFFERENT sex cells made:cells made:– 22nn

– n=# of PAIRS (haploid #)n=# of PAIRS (haploid #)– So….So….

Human ExampleHuman Example

Our haploid number is 23!Our haploid number is 23!– 222323=8 million possibilities!=8 million possibilities!

What about the chances of a What about the chances of a possible zygote during possible zygote during fertilization?fertilization?– 8 million x 8 million = 64 8 million x 8 million = 64 trilliontrillion

More VariationMore Variation

Other causes of varied Other causes of varied offspring results from mom offspring results from mom giving ½ the chromosomes and giving ½ the chromosomes and dad giving ½dad giving ½

Though each parent gives the Though each parent gives the instructions for the same instructions for the same genes, the version is often genes, the version is often DIFFERENTDIFFERENT– Example: brown hair vs blonde hair Example: brown hair vs blonde hair