Think:Think: We have discussed how We have discussed how somatic cells (body cells other somatic cells (body cells other than the gametes) are formed…than the gametes) are formed…what process forms the sex cells what process forms the sex cells (the gametes-sperm/egg)?(the gametes-sperm/egg)?

Take out paper for notes.Take out paper for notes.

This process is called MEIOSIS.This process is called MEIOSIS.

The BasicsThe Basics

Genes are located on the chromosomes.Genes are located on the chromosomes.Each organism must inherit one copy of Each organism must inherit one copy of every gene from both parents.every gene from both parents.Some organisms have 2 complete sets of Some organisms have 2 complete sets of genes.genes.Those two sets must be separated so that Those two sets must be separated so that each gamete produced contains just one each gamete produced contains just one set of genes.set of genes.

Chromosome NumberChromosome NumberHumans have 46 chromosomes.Humans have 46 chromosomes.– 23 came from your mother23 came from your mother– 23 came from your father23 came from your father

Each pair of chromosomes are referred to as Each pair of chromosomes are referred to as homologous chromosomeshomologous chromosomesAll 46 chromosomes are present in every All 46 chromosomes are present in every human body cell (a.k.a. human body cell (a.k.a. somatic cellsomatic cell).).This is the diploid chromosome number (2 This is the diploid chromosome number (2 sets ). sets ). “Di” means 2, so 2 copies of each “Di” means 2, so 2 copies of each chromosome are presentchromosome are present

23 pairs

GametesGametes

GametesGametes are are sex cellssex cells —sperm or egg. —sperm or egg.

Gametes contain only 1 copy of each Gametes contain only 1 copy of each chromosome.chromosome.

These cells are haploid (1 set)These cells are haploid (1 set)– sounds like “half”, so they only have half of sounds like “half”, so they only have half of

the number of chromosomesthe number of chromosomes

Humans have 23 chromosomes in every Humans have 23 chromosomes in every gametegamete. . Not 23 pairs, just 23!Not 23 pairs, just 23!

Diploid & HaploidDiploid & Haploid

DiploidDiploid = a cell with = a cell with two two sets of sets of chromosomes, chromosomes, 2n2n

HaploidHaploid = a cell with = a cell with one one set of set of chromosomes, gametes, chromosomes, gametes, 1n or n1n or n

n = number of chromosomes in the n = number of chromosomes in the organism (23 in humans)organism (23 in humans)

Purpose of MeiosisPurpose of Meiosis

MeiosisMeiosis forms the gametes (sperm and forms the gametes (sperm and eggs) that are necessary for sexual eggs) that are necessary for sexual reproduction.reproduction.Gametes need to beGametes need to be haploid haploid so that when so that when they meet for fertilization, the zygote has 2 they meet for fertilization, the zygote has 2 copies of the genetic materialcopies of the genetic material (diploid) (diploid)Meiosis has 2 stages Meiosis has 2 stages – Meiosis I Meiosis I (different than mitosis)(different than mitosis)– Meiosis IIMeiosis II (almost identical to mitosis) (almost identical to mitosis)

MEIOSIS IMEIOSIS I

Main eventsMain events::Separation of homologous pairsSeparation of homologous pairs

Homologous Pairs: Sets of same Homologous Pairs: Sets of same chromosome from different parentschromosome from different parents

1. Prophase I1. Prophase I

DNA coils into DNA coils into chromosomeschromosomesNuclear envelope & nucleolus break downNuclear envelope & nucleolus break downSynapsisSynapsis = homologous chromosomes = homologous chromosomes pair up into pair up into tetrads tetrads (What does the prefix (What does the prefix “tetra” mean? Think tetrahedron)“tetra” mean? Think tetrahedron)Crossing-overCrossing-over = homologous pairs switch = homologous pairs switch pieces of their chromatidspieces of their chromatidsGenetic RecombinationGenetic Recombination = maternal & = maternal & paternal genetic info get shuffled during paternal genetic info get shuffled during crossing-overcrossing-over

2. Metaphase I2. Metaphase I

TetradsTetrads line up in the middle of the cellline up in the middle of the cell

Spindle fibers attach at the Spindle fibers attach at the centromerescentromeres of of each homologous chromosome (one each homologous chromosome (one member of the pair)member of the pair)

3. Anaphase I3. Anaphase I

Each homologous chromosome moves to Each homologous chromosome moves to the opposite end of the cell (one member the opposite end of the cell (one member of the pair)of the pair)

Independent AssortmentIndependent Assortment = random = random separation of chromosomes, allows for separation of chromosomes, allows for genetic variationgenetic variation

4. Telophase I & Cytokinesis I4. Telophase I & Cytokinesis I

Two newly formed cells are Two newly formed cells are haploidhaploid

Each chromosome is still made up of Each chromosome is still made up of sister chromatids (two copies of sister chromatids (two copies of chromosome)chromosome)

MEIOSIS IIMEIOSIS IISeparation of Separation of sister chromatidssister chromatids

Sister ChromatidsSister Chromatids = identical halves of a = identical halves of a chromosome that has been duplicatedchromosome that has been duplicated

5. Prophase II5. Prophase II

2 newly created cells from 2 newly created cells from Meiosis I Meiosis I start start to divideto divide

6. Metaphase II6. Metaphase II

Chromosomes line up in the middleChromosomes line up in the middle

7. Anaphase II7. Anaphase II

ChromatidsChromatids separate separate

8. Telophase II & Cytokinesis II8. Telophase II & Cytokinesis II

4 new cells, each with half the number of 4 new cells, each with half the number of chromosomes of the original cellchromosomes of the original cell

Note Quiz 2/22Note Quiz 2/22

Put your name and period on the small sheet Put your name and period on the small sheet of paper, answer the following questions:of paper, answer the following questions:

1.1. Are gametes produced through Are gametes produced through mitosismitosis or or meiosismeiosis??

2.2. What is separated during meiosis II?What is separated during meiosis II?

3.3. What is separated during meiosis I?What is separated during meiosis I?

4.4. What is one way that genetic diversity of What is one way that genetic diversity of daughter cells is daughter cells is increasedincreased during meiosis? during meiosis?

Agenda for today:Agenda for today:

Finish notes (3 or less slides)Finish notes (3 or less slides)

Using available resources, complete Using available resources, complete worksheet. If you are seen not working, worksheet. If you are seen not working, points will be deducted.points will be deducted.– Due tomorrow at the beginning of class. Will Due tomorrow at the beginning of class. Will

be looking at CONTENT, be looking at CONTENT, notnot COMPLETION COMPLETION

Development of GametesDevelopment of Gametes

SpermatogenesisSpermatogenesis = = production of sperm production of sperm cellscells

OogenesisOogenesis = = production of mature production of mature egg cellsegg cells

NondisjunctionNondisjunction

NondisjunctionNondisjunction = = failure of homologous failure of homologous pairs to separate in pairs to separate in Meiosis IMeiosis I

If an organism If an organism survives, it may have survives, it may have a genetic disordera genetic disorder

PolyploidyPolyploidy

PolyploidPolyploid = = organisms with more organisms with more than the usual than the usual number of number of chromosome setschromosome sets

Occurs frequently in Occurs frequently in plants, results in plants, results in plants that are larger plants that are larger than normalthan normal

http://cellsalive.com/meiosis.htmhttp://cellsalive.com/meiosis.htm

Mitosis vs. MeiosisMitosis vs. Meiosis

MitosisMitosis

Body cellsBody cells

2 cells produced2 cells produced

Diploid (2 of each Diploid (2 of each chromosome)chromosome)

Genetically identical Genetically identical to each other AND to to each other AND to the original cellthe original cell

MeiosisMeiosis

Sex cells (sperm/egg)Sex cells (sperm/egg)

4 cells produced4 cells produced

Haploid (1 of each Haploid (1 of each chromosome)chromosome)

Genetically differentGenetically different