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

Why halve the chromosomes in Why halve the chromosomes in gametes?gametes?

At fertilization the male and female At fertilization the male and female sex cells will provide sex cells will provide ½ of the ½ of the chromosomeschromosomes each – so the each – so the offspring has genes from both offspring has genes from both parentsparents

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

InterphaseInterphaseEach of the chromosomes Each of the chromosomes

replicatereplicateThe result is two genetically The result is two genetically

identical sister chromatids identical sister chromatids which remain attached at their which remain attached at their centromerescentromeres

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

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

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

Metaphase IMetaphase IThe chromosomes line up at the The chromosomes line up at the

equator attached by their equator attached by their centromeres to spindle fibers centromeres to spindle fibers from centrioles.from centrioles.Still in homologous pairsStill in homologous pairs

Anaphase IAnaphase IThe spindle guides the The spindle guides the

movement of the chromosomes movement of the chromosomes toward the polestoward the polesSister chromatids remain attachedSister chromatids remain attachedMove as a unit towards the same Move as a unit towards the same

polepoleThe homologous chromosome The homologous chromosome

moves toward the opposite polemoves toward the opposite poleContrasts mitosis – chromosomes Contrasts mitosis – chromosomes

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

Telophase ITelophase IThis is the end of the first This is the end of the first

meiotic cell division. meiotic cell division. The cytoplasm divides, forming The cytoplasm divides, forming

two new daughter cells. two new daughter cells. Each of the newly formed cells Each of the newly formed cells

has half the number of the has half the number of the parent cell’s chromosomes, but parent cell’s chromosomes, but each chromosome is already each chromosome is already replicated ready for the second replicated ready for the second meiotic cell divisionmeiotic cell division

CytokinesisCytokinesisOccurs simultaneously with Occurs simultaneously with

telophase Itelophase IForms 2 daughter cellsForms 2 daughter cells

Plant cells – cell platePlant cells – cell plateAnimal cells – cleavage Animal cells – cleavage

furrowsfurrowsNO FURTHER REPLICATION NO FURTHER REPLICATION

OF GENETIC MATERIAL OF GENETIC MATERIAL PRIOR TO THE SECOND PRIOR TO THE SECOND DIVISION OF MEIOSISDIVISION OF MEIOSIS

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 mitosisTHERE IS NO INTERPHASE II !THERE IS NO INTERPHASE II !

Prophase IIProphase II

Each of the daughter cells Each of the daughter cells forms a spindle, and the double forms a spindle, and the double stranded chromosomes move stranded chromosomes move toward the equatortoward the equator

Metaphase IIMetaphase II

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

Anaphase IIAnaphase II

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

The sister chromatids of each The sister chromatids of each pair move toward opposite pair move toward opposite polespolesNow individual chromosomesNow individual chromosomes

Telophase II and CytokinesisTelophase II and Cytokinesis

Nuclei form at opposite poles of Nuclei form at opposite poles of the cell and cytokinesis occursthe cell and cytokinesis occurs

After completion of cytokinesis After completion of cytokinesis there are four daughter cells there are 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?

REFERENCES Mauri , S. (2010). Meiosis.

http://www.slideshare.net/SergioMauri98/presentacin-mitosis-y-meiosis-31090980?qid=4359ea49-4b39-4171-b4b4-65faee17ab36&v=qf1&b=&from_search=1

Accessed: on 2014 March 07

Swan, J.(2011). Meiosis and sexual life cycle. http://www.slideshare.net/jayswan/chapter-13-presentation-6504581

Accessed: on 2014 March 07