MEIOSIS and Sexual MEIOSIS and Sexual ReproductionReproduction

VocabularyMeoisis Diploid

Gamete fertilization

Somatic cell Crossing Over

Autosomal chromosome Independent

Sex chromosome assortment

Homologous chromosomes

Sperm Meiosis I

Egg Meiosis II

Haploid tetrad

Mitosis Vs. Meiosis

• What can you tell me about both?

Organisms that reproduce Sexually are made up of two different types of cells.

1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc.

2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes.

Meiosis Forms Gametes!!!!!

• The Male Gamete is the Sperm and is produced in the male gonad the Testes.

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

Ovaries.

Meiosis in males is called Meiosis in males is called spermatogenesisspermatogenesis and produces sperm. and produces sperm.

Meiosis in females is called Meiosis in females is called oogenesisoogenesis and produces ova.and produces ova.

Sperm are small because they contain no organelles. Must be

able to move.One egg produced that holds all of

the organelles.

fertilization

Spermatogenesis – Spermatogenesis – Formation of spermFormation of sperm

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

Oogenesis

*** The polar bodies die… only one ovum (egg) is produced from each primary oocyte.

One egg produced.Large holds all organelles

Egg Cell

• Sexual Reproduction: Fusion of two gametes that results in the combo of the genetic material from the two parents.

FertilizationFertilization

• Sperm + Ovum (egg) ---- Zygote• The fusion of a spermsperm and eggegg to form a zygotezygote.

• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

Sex Chromosomes OR Sex cells?

• There are sex cells and then there are sex chromosomes!! Don’t get them confused!

• Sex cells – sperm and egg

• Sex chromosomes XX and XY

Autosomal Chromosomes 1 – 22 Autosomal Chromosomes 1 – 22 ((

In Humans the

“Autosomes” are sets

1 - 22

Homologous ChromosomesHomologous Chromosomes((because a homologous pair consists of 4 chromatids it is called a “Tetrad”)because a homologous pair consists of 4 chromatids it is called a “Tetrad”)

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Sex ChromosomesSex Chromosomes

“Sex Chromosomes” …….the 23rd set

23

This person has 2 “X” chromosomes…

and is a female.

Types of ChromosomesTypes of Chromosomes• Homologous Chromosomes: Pair of chromosomeschromosomes

(maternalmaternal and paternalpaternal) that are similar in shape and size. They carry the genes from parents to offspring.

• Each locuslocus (position of a gene) (position of a gene) is in the same

position on homologues.

• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.

1 - 22 pairs of autosomes – not related autosomes – not related to the sex of the organism.to the sex of the organism.

1 pair of sex chromosomes – control sex chromosomes – control the development of sexual characteristics.the development of sexual characteristics.

Types of chromosomes

Autosomal chromosomes– chromosome 1 -22 set

Sex chromosome – set 23

• Egg – sex chromosome X

• Sperm – sex chromosome X or Y

• Male – XY Female - XX

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

X chromosomeX chromosomeY chromosomeY chromosome

All Organisms have a characteristic number of

chromosomes! • The number has nothing to do with

complexity!

• Fruit fly - 8

• Horse - 64

• Fern - 1200

Chromosome numbersAll are even numbers – diploid (2n) sets of homologous chromosomes!

Ploidy = number of copies of each chromosome. Diploidy

Meiosis

Cell division – ‘reduction division’

Production of sex cells – gametes

Meiosis errors

Mitosis vs. Meiosis

• Most cells in the body produced by mitosis.

• Only gametes are produced by meiosis.

Why do we need meiosis?

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

• Why halve the chromosomes in gametes?

At fertilisation the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

MeiosisMeiosis is Two cell divisionsMeiosis is Two cell divisions

(called meiosis Imeiosis I and meiosis IImeiosis II)

with only one duplication of chromosomes.with only one duplication of chromosomes.Every gamete is different because of the

mixing of genes during metaphase I.Crossing over of genes! Germ Cells - type of cells that for gametes.Females form all eggs before birthMales form sperm whole life

Mitosis Meiosis

Produce identical cells

Unique cells

Diploid cells Haploid cells

Takes place whole life of organism

Takes place at only certain times

Asexual reproduction

Sexual reproduction

Body cells Germ cells

Meiosis – key differences from mitosis

• Meiosis reduces the number of chromosomes by half.• Daughter cells differ from parent, and each other.• Meiosis involves two divisions, Mitosis only one.• Meiosis I involves:

– homologous chromosomes pair up form a tetrad. – crossing over of non-sister chromatids.In Metaphase I,

homologous pairs line up at metaphase plate. This mixes up genes

– In Anaphase I, sister chromatids do NOT separate.– End Product, separation of homologous pairs of

chromosomes, rather than sister chromatids of individual chromosome.

Homologous ChromosomesHomologous Chromosomes

• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape and size.

• Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.

• Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.

• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.

a. 22 pairs of autosomesautosomes

b. 01 pair of sex chromosomessex chromosomes

How Does Sexual Reproduction creates genetic variation???

1. Independent Assortment :Meiosis results in genetic variation by shuffling the genes that are placed in the gamete. No daughter cells formed during meiosis are genetically identical to either mother or father 2. Fertilization: During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. 3. Crossing over - During the prophase 1 of meiosis, chromosome segments will switch with homologous chromosomes.

Crossing Over - variation Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

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

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

assortment.assortment.

Interphase I - Like mitosisInterphase I - Like mitosis

• Similar to mitosismitosis interphase.

• ChromosomesChromosomes replicate (S phase).(S phase).

• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.

• CentrioleCentriole pairs also replicate.

Interphase IInterphase I

• NucleusNucleus and nucleolusnucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

Meiosis I (four phases)Meiosis I (four phases)

• Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.

• four phasesfour phases:

a.a. prophase Iprophase I

b.b. metaphase Imetaphase I

c.c. anaphase Ianaphase I

d.d. telophase Itelophase I

Prophase 1: DNA has been replicated.Spindle fibers and centrioles form – Nuclear

membrane and nucleolus disappears. Chromosomes are tetrads!

Prophase IProphase I

• Longest and most complex phase (90%).Longest and most complex phase (90%).

• ChromosomesChromosomes condense.

• SynapsisSynapsis occurs: homologous chromosomes homologous chromosomes come together to form a tetradtetrad.

• TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).

Prophase IProphase I

centriolesspindle fiber

asterfibers

Metaphase 1: Homologous chromosomes align at the equatorial plate. Spindle fibers attach.

• Crossing over occurs with the homologous chromosomes

Crossing OverCrossing Over

• Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata.

• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.

• Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing crossing overover.

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Metaphase IMetaphase I• Shortest phaseShortest phase

• TetradsTetrads align on the metaphase platemetaphase plate.

• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.

(mom and dad’s chromosomes randomly align on each side.)2. Variation

3. Crossing over occurs

Anaphase 1: Homologous pairs separate with sister chromatids

remaining together.

Anaphase IAnaphase I

• Homologous chromosomes Homologous chromosomes separate and move towards the poles.

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Anaphase IAnaphase I

.• Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Telophase ITelophase I

• Each pole now has haploidhaploid set of chromosomeschromosomes.

• CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Telophase ITelophase I

Meiosis IIMeiosis II

• No interphase II No interphase II

(or very short - no more DNA replicationDNA replication)

• Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis

Meiosis 2

Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate. Same as

1• Metaphase 2: Chromosomes align at the

equatorial plate. • Anaphase 2: Centromeres divide and sister

chromatids migrate separately to each pole. • Telophase 2: Cell division is complete. Four

haploid daughter cells are obtained.

Prophase IIProphase II

• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II• same as metaphasemetaphase in mitosis. NO mitosis. NO

CROSSING OVER or HOMOLOGOUS CROSSING OVER or HOMOLOGOUS CHROMOSOMES!CHROMOSOMES!

metaphase platemetaphase plate

Anaphase IIAnaphase II

• same as anaphaseanaphase in mitosismitosis

• sister chromatids separatesister chromatids separate

Telophase IITelophase II

• Same as telophasetelophase in mitosismitosis.

• Nuclei form.

• CytokinesisCytokinesis occurs.

• Remember:Remember: four haploid daughter cells four haploid daughter cells produced.produced.

gametes = sperm or egggametes = sperm or egg

Telophase IITelophase II

Independent Assortment

Process occurring in Metaphase I where it is random which one of the pair is placed in the germ cell.

Mitosis Meiosis

Produce identical cells

Unique cells

Diploid cells Haploid cells

Takes place whole life of organism

Takes place at only certain times

Asexual reproduction

Sexual reproduction

Body cells Germ cells

VariationVariation

• Important to population as the raw Important to population as the raw material for material for natural selectionnatural selection..

• Question:Question:

What are the three sexual sources ofWhat are the three sexual sources of

genetic variation? genetic variation?