MEIOSIS & &SEXUAL REPRODUCTIONSEXUAL REPRODUCTION

Genetics Lesson 4Genetics Lesson 4

1.1. Somatic CellsSomatic Cells are “body” cells and contain are “body” cells and contain the normal number of chromosomesthe normal number of chromosomes

called the “Diploid” number (the symbol is called the “Diploid” number (the symbol is 2n2n)) e.g. skin cells, brain cells, etc.e.g. skin cells, brain cells, etc.

2.2. GametesGametes are the “sex” cells and contain are the “sex” cells and contain only ½ the normal number of chromosomesonly ½ the normal number of chromosomes

called the “Haploid” number (the symbol is called the “Haploid” number (the symbol is nn)) E.g. sperm and egg cell E.g. sperm and egg cell

Types of CellsTypes of Cells

GametesGametes The The Male Gamete Male Gamete is the is the SpermSperm and is and is

produced in the male gonad the produced in the male gonad the Testes.Testes.

The The Female Gamete Female Gamete is the is the Ovum (ova Ovum (ova = pl.) = pl.) and is produced in the female and is produced in the female gonad the gonad the Ovaries.Ovaries.

The fusion of a The fusion of a spermsperm and and eggegg to form a to form a zygotezygote..

A zygote is a fertilized eggA zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

FertilizationFertilization

Meiosis:Meiosis: involves 2 parents (usually)involves 2 parents (usually)

hereditary material is exchanged (mixed and transferred)hereditary material is exchanged (mixed and transferred)

Each species has a Each species has a specific number of chromosomesspecific number of chromosomes

For example For example humanshumans total number of chromosomes is 46 total number of chromosomes is 46

diploid numberdiploid number is 46 or 2n = 46 is 46 or 2n = 46

the number of chromosomes in the gametes is the the number of chromosomes in the gametes is the haploid haploid numbernumber or n = 23 or n = 23

Other examples:Other examples: crayfish 2n = 126crayfish 2n = 126 fruit flies 2n = 4fruit flies 2n = 4

MeiosisMeiosis

Meiosis animation (1:49 min):Meiosis animation (1:49 min): http://www.youtube.com/watch?v=D1_-mQS_FZ0&NR=1

Each resulting cell still has chromosomes from mom & dadEach resulting cell still has chromosomes from mom & dad

InterphaseInterphase• Meiosis is preceded by interphase

• chromosomes have not yet condensed

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

InterphaseInterphase

• chromosomes have replicated, and the chromatin begins to condense

• 2n=46

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

MeiosisMeiosis Prophase I Prophase I• begins with two complete sets of duplicated chromosomes •homologous chromosomes pair up or synapse to form tetrads

• homologous chromosomes then exchange DNA

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Prophase I – SynapsisProphase I – Synapsis

During Crossing overDuring Crossing over segments of segments of nonsister nonsister chromatidschromatids break and break and reattach to the other reattach to the other chromatidchromatid. .

The The Chiasmata (chiasma) Chiasmata (chiasma) are the are the sites of sites of crossing overcrossing over..

Crossing Over is one of the Two major occurrences of Meiosis

(The other is Non-disjunction)

During Prophase I During Prophase I “Crossing Over” occurs.“Crossing Over” occurs.

non-sister chromatids

chiasmata: site of crossing over

variation

TetradCrossing OverCrossing Over

Crossing Over creates variation (diversity) in the offspring’s traits.

Synapsis in Prophase I of Synapsis in Prophase I of MeiosisMeiosis

Meiosis Meiosis Metaphase IMetaphase I

tetradstetrads line up across line up across the middle of the cellthe middle of the cell

Independent assortmentIndependent assortment

centromerescentromeres of of chromosomes attach to chromosomes attach to spindle fibresspindle fibres

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Meiosis Anaphase IAnaphase I homologous homologous

chromosomeschromosomes separateseparate from the from the tetradtetrad

move along move along spindle spindle fibresfibres towards towards opposite poles of the opposite poles of the cellcell

Sister chromatids remain Sister chromatids remain attached at centromeresattached at centromeres

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Meiosis Telophase I Telophase I && CytokinesisCytokinesis

Nuclear membrane Nuclear membrane reforms around each reforms around each cluster of cluster of chromosomeschromosomes

Chromosomes in Chromosomes in each cell are present each cell are present as duplicateas duplicate

Each cell considered Each cell considered haploidhaploidhttp://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Meiosis Prophase IIProphase II

no DNA replication no DNA replication prior to Prophase IIprior to Prophase II

nuclear membranenuclear membrane dissolves and the dissolves and the spindle fibresspindle fibres begin begin to formto form

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Meiosis Metaphase IIMetaphase II

as in Meiosis I, the as in Meiosis I, the chromosomeschromosomes line line up on the spindle up on the spindle fibersfibers

sister chromatidssister chromatids remain attached at remain attached at their their centromerescentromeres

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Meiosis Meiosis Anaphase IIAnaphase II

centromerescentromeres split split and and sister sister chromatidschromatids separate separate and move to and move to opposite poles of opposite poles of cellcell

chromatidschromatids are now are now called called chromosomeschromosomes

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Telophase II Telophase II && CytokinesisCytokinesis

with the formation of with the formation of four cells, meiosis is four cells, meiosis is over. over.

each of these each of these prospective germ cells prospective germ cells carries half the number carries half the number of chromosomes of of chromosomes of somatic cells. somatic cells.

http://morgan.rutgers.edu/MorganWebFrames/Level1/Page7/meiosis1.html

Comparing Mitosis and Meiosis

Comparing Chromosome Numbers in Mitosis and

Meiosis

GametogenesisGametogenesis: the : the production of production of gametes (sex gametes (sex cells) through cells) through the process of the process of meiosis.meiosis.

GAMETOGENESISGAMETOGENESIS

Meiosis in malesMeiosis in males

occurs in the testesoccurs in the testes Starts with a Starts with a

diploid cell called a diploid cell called a spermatogoniumspermatogonium

produces four produces four non-non-identicalidentical haploidhaploid sperm cellssperm cells

SPERMATOGENESISSPERMATOGENESIS

SPERMATOGENESISSPERMATOGENESIS Occurs all the time from puberty until death. Occurs all the time from puberty until death. It takes approximately 68-74 hours for a sperm to It takes approximately 68-74 hours for a sperm to

be created.be created. Meiosis produces approximately 250 000 000 Meiosis produces approximately 250 000 000

sperm every day in males!sperm every day in males!

SPERMATOGENESISSPERMATOGENESIS

Meiosis in femalesMeiosis in females

Occurs in the ovaries and Occurs in the ovaries and oviductsoviducts

Starts with a diploid cell Starts with a diploid cell called an called an oogoniumoogonium. .

Egg production starts Egg production starts before a female is born, before a female is born, but pauses in Meiosis I but pauses in Meiosis I before the cells (primary before the cells (primary oocyte) divide.oocyte) divide.

The meiotic process The meiotic process resumes at puberty with resumes at puberty with ovulation (and ovulation (and fertilization), for 1 cell fertilization), for 1 cell every month.every month.

OOGENESISOOGENESIS

OOGENESISOOGENESIS After Telophase I and II only one of the cells After Telophase I and II only one of the cells

receives the majority of the cytoplasm, resulting receives the majority of the cytoplasm, resulting in one egg cell and three polar bodies.in one egg cell and three polar bodies.

The purpose of the unequal division is to allow The purpose of the unequal division is to allow the egg cell to have sufficient nutrients to support the egg cell to have sufficient nutrients to support a zygote immediately after fertilization.a zygote immediately after fertilization.