Mitosis & Meiosis

Mitosis Review

Cell makes a copy of itself

1) Copy its DNA (part of a chromosome).

2) Copies are separated & sorted into two sides of the cell.

3) The cell then splits in two.

4) Part of each parent is carried to the two new cells.

5) Two EXACT daughter cells (same number of chromosomes)

Examples: skin, bones, internal organs

Meiosis – Sexual Reproduction• Cell divisionCell division that forms gametes (egg and sperm

cells)

• GametesGametes have halfhalf the # of chromosomeschromosomes.

• Two divisionsTwo divisions (meiosis I meiosis I and meiosis IImeiosis II).

• Part of each parent is carried to the four new cells.

• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.

Mitosis: Meiosis:

Each resulting cell still has chromosomes from Mom & DadEach resulting cell still has chromosomes from Mom & Dad

Tips

• MITosis takes one cell and Makes It Two

• Meiosis has to do with sex

• From the cell’s point of view:– mITosis results in Identical Twins

CELLS– mEioSis results in Egg and Sperm

Homologous ChromosomesHomologous Chromosomes

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

• Homologous pairs carry genes controlling the same inherited traits.

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

a. 22 pairs of autosomesautosomes

b. 1 pair of sex chromosomessex chromosomes

Homologous chromosomes

sister chromatids paternal

sister chromatids maternalTetrad

Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.

Homologue

Homologue

AutosomesAutosomes(The Autosomes code for most of the offspring’s traits)(The Autosomes code for most of the offspring’s traits)

In Humans the “Autosomes” are sets 1 - 22

Sex ChromosomesSex ChromosomesThe Sex Chromosomes code for the sex of the offspring.The Sex Chromosomes code for the sex of the offspring.

Two “X” chromosomes = Two “X” chromosomes = femalefemale.. One “X” chromosome and one “Y” chromosome = One “X” chromosome and one “Y” chromosome = malemale..

XX chromosome - female XY chromosome - male

In Humans the “Sex

Chromosomes” are the 23rd set

Cell Division in Sex Cells

• Interphase I

• Meiosis I and Meiosis 2

• Similar to Mitosis EXCEPT changes in chromosomes

Interphase IInterphase I

• Similar to mitosismitosis interphase.

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

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

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 IProphase I

centriolesspindle fiber

TETRAD

Prophase IProphase I

• HHomologous chromosomesomologous chromosomes come together to form a tetradtetrad.

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

Homologous chromosomes

sister chromatids paternal

sister chromatids maternalTetrad

Prophase 1 – Crossing OverProphase 1 – Crossing Over

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

• Crossing over Crossing over causescauses variation variation

Crossing Over - variation Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Metaphase IMetaphase I• TetradsTetrads align on the in center of cell on spindle

• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation

OR

Anaphase IAnaphase I• Homologous chromosomes Homologous chromosomes separate and

move towards the poles.

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Telophase ITelophase I• Each pole now has haploidhaploid set of

chromosomeschromosomes.• CytokinesisCytokinesis occurs and two haploid daughter

cells are formed.

Meiosis IIMeiosis II

• No interphase II No interphase II

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

Prophase IIProphase II

• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II

• same as metaphasemetaphase in mitosismitosis

Anaphase IIAnaphase II

• same as anaphaseanaphase in mitosismitosis

• sister chromatids separatesister chromatids separate

Telophase IITelophase II• Four haploid daughter cells producedFour haploid daughter cells produced

gametes = sperm or egggametes = sperm or egg

2n diploid

n haploid

n haploid

SpermatogenesisSpermatogenesis

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

FertilizationFertilization

• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

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?

Answer:Answer:

1. crossing over (prophase I)1. crossing over (prophase I)

2. independent assortment (metaphase I)2. independent assortment (metaphase I)

3. random fertilization3. random fertilization

Remember: variation is good!

Question:Question:

• A cell containing 20 chromosomes20 chromosomes (diploid)(diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

Answer:Answer:

• 10 chromosomes (haploid)10 chromosomes (haploid)

Question:Question:

• A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

Answer:Answer:

• 10 chromosomes10 chromosomes

• Meiosis animation: http://www.youtube.com/watch?v=D1_-mQS_FZ0&NR=1