Word(s) of the Day

• Diploid : Is a Cell that contains two identical sets of DNA

• Haploid: Cell that contains Half the total number of chromosomes

Goals for the day !

Identify the order in which different cellular organelles produce haploid cells

Use new vocabulary such as haploid and diploid to describe meiosis

Compare and Contrast Mitosis and Meiosis

MEIOSISMEIOSISNotNot

Meiosis

• The form of cell divisioncell division by which gametes, gametes, with halfhalf the number of chromosomes,chromosomes, are produced.

• Diploid (2n) Diploid (2n) haploid (n)haploid (n)

• MeiosisMeiosis is sexual reproduction.sexual reproduction.

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

MeiosisMeiosis

• Sex cells Sex cells divide to produce gametesgametes (sperm (sperm or egg)or egg).

• GametesGametes have halfhalf the # of chromosomeschromosomes.

• Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).

Male: spermatogenesisMale: spermatogenesis

Female: oogenesisFemale: oogenesis

• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.

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

“Sperm” “Formation”

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

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

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.

Interphase IInterphase I

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

• 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 - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

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.

Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Crossing OverCrossing Over• Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids

• 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.

Crossing Over - variation Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

And once again because sex cells are weird ….

Sex Chromosomes Sex Chromosomes DO NOT DO NOT crossover !crossover !

XX chromosome - female XY chromosome - male

Prophase IProphase I

centrioles

spindle fiber or “Fishing Line”

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.2. Variation3. Formula: 2n

Example:Example: 2n = 42n = 4then then n = 2 n = 2

thusthus 2 22 2 = 4 combinations= 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

AnswerAnswer

• Formula: 2Formula: 2nn

• Human chromosomes:Human chromosomes: 2n = 462n = 46

n = 23n = 23

• 222323 = ~8 million combinations = ~8 million combinations

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

metaphase plate

Telophase ITelophase I

• Each pole now has 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

Prophase IIProphase II

• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II

• same as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate

Anaphase IIAnaphase II

• same as anaphaseanaphase in mitosismitosis

• sister chromatids separatesister chromatids separate

Telophase IITelophase II

• Same as telophasetelophase in mitosismitosis.

• Nucleus reforms .

• CytokinesisCytokinesis occurs.

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

gametes = sperm or egggametes = sperm or egg

Telophase IITelophase II

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?