Meiosis – A Source of DistinctionMeiosis – A Source of Distinction

Why do you share some but not all characters of each parent?

What are the rules of this sharing game?

At one level, the answers lie in meiosis.

Meiosis does two things -Meiosis does two things -

1) Meiosis takes a cell with 1) Meiosis takes a cell with two copiestwo copies of every of every chromosome (diploid) and makes cells with a chromosome (diploid) and makes cells with a single copysingle copy of every chromosome (haploid). of every chromosome (haploid).

This is a good idea if you’re going to combine This is a good idea if you’re going to combine two cells to make a new organism. This trick two cells to make a new organism. This trick is accomplished by is accomplished by halvinghalving chromosome chromosome number. number.

In meiosis, In meiosis, one diploid cells produces four one diploid cells produces four haploid cells.haploid cells.

Why do we need meiosis?Why do we need meiosis?

• Meiosis is necessary to halve the number Meiosis is necessary to halve the number of chromosomes going into the sex cellsof chromosomes going into the sex cells

Why halve the chromosomes in gametes?Why halve the chromosomes in gametes?• At fertilization the male and female sex At fertilization the male and female sex

cells (gametes) will provide cells (gametes) will provide ½ of the ½ of the chromosomeschromosomes each – so the offspring each – so the offspring has genes from both parentshas genes from both parents

2) Meiosis 2) Meiosis scramblesscrambles the specific forms of the specific forms of each gene that each sex cell (egg or sperm) each gene that each sex cell (egg or sperm) receives. receives.

This makes for a lot of This makes for a lot of genetic diversitygenetic diversity. This . This trick is accomplished through trick is accomplished through independent independent assortmentassortment and and crossing-overcrossing-over..

Genetic diversity is important for the Genetic diversity is important for the evolution evolution of populations and speciesof populations and species..

MeiosisMeiosis

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Meiosis – mouse testes Meiosis – mouse testes Parent cell

4 gametes

1st division

2nd division

The Stages of Meiosis:

• aka: Reduction Division

Meiosis I : Separates Meiosis I : Separates Homologous ChromosomesHomologous Chromosomes

• InterphaseInterphase– Each of the Each of the chromosomes replicatechromosomes replicate– The result is The result is two genetically identical two genetically identical

sister chromatidssister chromatids which remain which remain attached at their centromeresattached at their centromeres

Prophase IProphase I• This is a crucial phase for mitosis.This is a crucial phase for mitosis.• During this phase each pair of During this phase each pair of

chromatids don’t move to the equator chromatids don’t move to the equator alone, they match up with their alone, they match up with their homologous pair and fasten together homologous pair and fasten together (synapsis) in a group of four called a (synapsis) in a group of four called a tetrad.tetrad.

• Extremely IMPORTANT!!! It is during this Extremely IMPORTANT!!! It is during this phase that crossing over can occur. phase that crossing over can occur.

• Crossing Over is the exchange of Crossing Over is the exchange of segments during synapsis.segments during synapsis.

Metaphase IMetaphase I

• The The chromosomes line up at the chromosomes line up at the equator attachedequator attached by their by their centromeres to spindle fibers from centromeres to spindle fibers from centrioles.centrioles.– Still in Still in homologous pairshomologous pairs

Anaphase IAnaphase I

• The spindle guides the movement of the The spindle guides the movement of the chromosomes toward the poleschromosomes toward the poles– Sister chromatids remain attachedSister chromatids remain attached– Move as a unit towards the same poleMove as a unit towards the same pole

• The The homologous chromosome moves homologous chromosome moves toward the opposite poletoward the opposite pole– Contrasts mitosis – chromosomes Contrasts mitosis – chromosomes

appear as individuals instead of pairs appear as individuals instead of pairs (meiosis)(meiosis)

Telophase ITelophase I

• This is the This is the end of the first meiotic cell end of the first meiotic cell division. division.

• The cytoplasm divides, forming The cytoplasm divides, forming two two new daughter cells. new daughter cells.

• Each of the newly formed cells has Each of the newly formed cells has half the number of the parent cell’s half the number of the parent cell’s chromosomeschromosomes, but each chromosome , but each chromosome is already replicated is already replicated ready for the ready for the second meiotic cell divisionsecond meiotic cell division

CytokinesisCytokinesis

• Occurs simultaneously with Occurs simultaneously with telophase Itelophase I– Forms 2 daughter cellsForms 2 daughter cells

• Plant cells – cell platePlant cells – cell plate• Animal cells – cleavage furrowsAnimal cells – cleavage furrows• NO FURTHER REPLICATION OF NO FURTHER REPLICATION OF

GENETIC MATERIAL PRIOR TO GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF THE SECOND DIVISION OF MEIOSISMEIOSIS

Figure 13.7 The stages of meiotic cell division: Meiosis I

Meiosis II : Meiosis II :

Separates sister chromatidsSeparates sister chromatids

• Proceeds similar to mitosisProceeds similar to mitosis• THERE IS NO INTERPHASE II !THERE IS NO INTERPHASE II !

Prophase IIProphase II

• Each of the daughter cells forms a Each of the daughter cells forms a spindle, and the double stranded spindle, and the double stranded chromosomes move toward the chromosomes move toward the equatorequator

Metaphase IIMetaphase II

• The chromosomes are positioned The chromosomes are positioned on the metaphase plate in a mitosis-on the metaphase plate in a mitosis-like fashionlike fashion

Anaphase IIAnaphase II

• The centromeres of sister The centromeres of sister chromatids finally separatechromatids finally separate

• The sister chromatids of each pair The sister chromatids of each pair move toward opposite polesmove toward opposite poles– Now individual chromosomesNow individual chromosomes

Telophase II and CytokinesisTelophase II and Cytokinesis

• Nuclei form at opposite poles of the Nuclei form at opposite poles of the cell and cytokinesis occurscell and cytokinesis occurs

• After completion of cytokinesis there After completion of cytokinesis there are are four daughter cells four daughter cells – All are haploid (n)All are haploid (n)

Figure 13.7 The stages of meiotic cell division: Meiosis II

One Way Meiosis Makes Lots of One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Different Sex Cells (Gametes) – Independent AssortmentIndependent Assortment

Independent assortment produces 2Independent assortment produces 2nn distinct gametes, where n = the number distinct gametes, where n = the number of unique chromosomes. of unique chromosomes.

That’s a lot of diversity by this That’s a lot of diversity by this mechanism alone.mechanism alone.

In humans, n = 23 and 2In humans, n = 23 and 22323 = 6,000,0000. = 6,000,0000.

Another Way Meiosis Makes Lots of Different Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-OverSex Cells – Crossing-Over

Crossing-over multiplies the already huge number of different gamete Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.types produced by independent assortment.

The Key Difference Between Mitosis and Meiosis is The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in the Way Chromosomes Uniquely Pair and Align in

Meiosis Meiosis

MitosisMitosis The first (and The first (and distinguishing) distinguishing) division of meiosisdivision of meiosis

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

X chromosomeX chromosomeY chromosomeY chromosome

Boy or Girl? The Y Chromosome “Decides”Boy or Girl? The Y Chromosome “Decides”

Meiosis – division errorMeiosis – division error

Chromosome pair

Meiosis error - fertilizationMeiosis error - fertilization

Should the gamete with the Should the gamete with the chromosome pair be fertilized chromosome pair be fertilized then the offspring will not be then the offspring will not be ‘normal’.‘normal’.

In humans this often occurs In humans this often occurs with the 21with the 21stst pair – producing pair – producing a child with Downs Syndromea child with Downs Syndrome

21 trisomy – Downs Syndrome21 trisomy – Downs Syndrome

Can you see the extra 21st chromosome?

Is this person male or female?