1

Production of Gametes

MEIOSISMEIOSIS

2

Meiosis Meiosis FactsFacts

3

MeiosisMeiosis• The form of cell divisioncell division by which

gametesgametes, , with halfhalf the number of chromosomeschromosomes, , are produced

• DiploidDiploid (2n) (2n) haploidhaploid (n) (n)• Called a Called a reduction division reduction division

(because # of chromosomes is (because # of chromosomes is reduced)reduced)

• MeiosisMeiosis is sexualsexual reproduction reproduction• Two divisionsTwo divisions (meiosis I meiosis I and

meiosis IImeiosis II)

4

MeiosisMeiosis•Germ Line cells (aka sex

cells) divide to produce gametes (sperm or egg)

•Gametes have half the number of chromosomes

•Meiosis is similar to mitosis with some chromosomal differences

5

MeiosisMeiosis

•Occurs only in gonads (testes or ovaries)

•Male: Spermatogenesis

•Female: Oogenesis

6

SpermatogenesiSpermatogenesiss

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

7

Interphase IInterphase I

• Similar to mitosismitosis interphase• ChromosomesChromosomes replicate (S (S

phase)phase)• Each duplicated

chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres

• CentrioleCentriole pairs also replicate.

8

Interphase IInterphase I

• NucleusNucleus and nucleolusnucleolus visible.

Nuclear membrane

Nucleolus

Cell membrane

Chromatin

9

Meiosis IMeiosis I

10

Stages of Meiosis IStages of Meiosis I

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

• FourFour phases phases:

a.a. Prophase IProphase I

b.b. Metaphase IMetaphase I

c.c. Anaphase IAnaphase I

d.d. Telophase ITelophase I

11

Prophase IProphase I

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

12

Prophase IProphase I - -

SynapsisSynapsis

Homologous chromosomes

Sister chromatids Sister chromatidsTetrad

13

Homologous Homologous ChromosomesChromosomes

• 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

14

Homologous Homologous ChromosomesChromosomes

• Humans have 23 pairs of homologous chromosomeshomologous chromosomes

• First 22 pairs of chromosomes called autosomesautosomes

• Last pair called sex sex chromosomeschromosomes

• XX female or XY maleXX female or XY male

15

Homologous Homologous ChromosomesChromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Gene Loci

16

17

Crossing OverCrossing Over

• Crossing overCrossing over (variation) (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 overcrossing over

18

Crossing Over - Crossing Over - VariationVariation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

19

Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

20

Prophase IProphase I

CentriolesSpindle fiber

Asterfibers

21

Metaphase IMetaphase I

• ShortestShortest phase phase• TetradsTetrads align on the metaphase metaphase

plateplate• Independent assortment occursIndependent assortment occurs• Chromosomes separate Chromosomes separate

randomlyrandomly to the poles of the to the poles of the cellscells

22

Metaphase IMetaphase I• Independent assortment causes

variation in the forming cells• Orientation of homologous pair

to poles is random• Formula for determining

variation: Formula: 2n

ExampleExample: 2n = 4: 2n = 4 then n = 2then n = 2

thus 2thus 22 2 = 4 = 4 combinationscombinations

23

Metaphase IMetaphase I

Metaphase plate

OR

Metaphase plate

24

Question:Question:

• In terms of In terms of Independent Independent Assortment, Assortment, how many how many different combinations different combinations of sperm could a of sperm could a human male produce? human male produce?

25

AnswerAnswer

• Formula: 2Formula: 2nn

• Human chromosomes:Human chromosomes:

2n = 462n = 46

n = 23n = 23

222323 = ~8 million = ~8 million combinationscombinations

26

Anaphase IAnaphase I

• Homologous Homologous chromosomes chromosomes separate and move towards the poles

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres

27

Anaphase IAnaphase I

28

Telophase ITelophase I

•Each pole now has haploidhaploid set of chromosomeschromosomes

• 1n = 23 (human)1n = 23 (human)• CytokinesisCytokinesis occurs and

two haploid daughter cells are formed

29

Telophase ITelophase I

30

31

Meiosis IIMeiosis II

32

Meiosis IIMeiosis II• Short or NoShort or No interphase II interphase II • DNA NOT replicatedDNA NOT replicated

againagain• Remember: Meiosis II Remember: Meiosis II is

similar to mitosismitosis• Prophase I, Metaphase II, Prophase I, Metaphase II,

Anaphase II, and Anaphase II, and Telophase IITelophase II

33

Prophase IIProphase II• Same as prophaseprophase in

mitosismitosis• Nuclear envelope breaks Nuclear envelope breaks

down and the spindle down and the spindle apparatus formsapparatus forms

34

Metaphase IIMetaphase II• Same as metaphasemetaphase in

mitosismitosis• Chromatids lined up at Chromatids lined up at

equatorequator

Metaphase plateMetaphase plate

35

Anaphase IIAnaphase II

• Same as anaphaseanaphase in mitosismitosis

• Sister chromatids Sister chromatids separateseparate

36

Telophase IITelophase II

• Same as telophasetelophase in mitosismitosis

• Nuclei reform• CytokinesisCytokinesis occurs• Remember: Remember: Four haploid Four haploid

daughter cellsdaughter cells produced produced• GametesGametes = sperm or egg = sperm or egg

37

Telophase IITelophase II

38

MeiosisMeiosis

2n=4

sex cell

diploid (2n)

n=2

n=2

Meiosis I

n=2

n=2

n=2

n=2

Sperm

Haploid (n)

Meiosis II

39

40

Genetic Genetic VariationVariation

41

VariationVariation

Important to the Important to the survival of populationssurvival of populations

Aids in natural Aids in natural selectionselection

Strongest individuals Strongest individuals are able to survive are able to survive and reproduceand reproduce

42

QuestionQuestion

•What are the three What are the three sources of genetic sources of genetic variation in sexual variation in sexual reproduction?reproduction?

43

***Answer:***Answer:

1.1. Crossing OverCrossing Over (Prophase I)(Prophase I)

2. 2. Independent Independent AssortmentAssortment (Metaphase (Metaphase I)I)

3. 3. Random FertilizationRandom Fertilization(sperm joins with egg)(sperm joins with egg)

44

Remember: Variation is

helpful to the survival of a

species!

45

Question:Question:

•A diploid cell containing 20 chromosomes20 chromosomes (2n = (2n = 20)20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

46

Answer:Answer:

•10 chromosomes 10 chromosomes (haploid)(haploid)

•1n = 101n = 10

47

KaryotypeKaryotype• A method of organizing the chromosomes

of a cell in relation to number, size, and type.

48

Trisomy• Genetic disorders such as

Down’s Syndrome (Trisomy 21) are the result of nondisjunction

49

50

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

51

Question:Question:

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

52

Answer:Answer:

•Four cells with 10 Four cells with 10 chromosomes chromosomes eacheach