M EIOSIS F ORMATION OF G AMETES 1

F ACTS A BOUT M EIOSIS  Daughter cells contain half the number of chromosomes as the original cell (haploid/monoploid/1n)  Produces gametes  Egg and sperm production  Occurs in the testes in males  spermatogenesis  Occurs in the ovaries in females  Oogenesis 2

W HY D O WE N EED M EIOSIS ?  It is the fundamental basis of sexual reproduction  Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote  Promotes genetic diversity (aka a better gene pool) 3

F ERTILIZATION : P UTTING IT ALL TOGETHER 4 1n =3 2n = 6

T YPES OF C ELLS Somatic Cells Somatic cells are body cells. They comprise your hair, skin, intestines, etc. Germ Cells Will become sperm and egg cells. 5

M EIOSIS F ORMS H APLOID G AMETES  Meiosis must reduce the chromosome number by half  Fertilization then restores the 2n (diploid) number 6 from momfrom dadchild meiosis reduces genetic content too much! The right number!

M EIOSIS : T WO P ART C ELL D IVISION 7 Homologs separate Sister chromatids separate Diploid Meiosis I Meiosis II Diploid Haploid

M EIOSIS I: R EDUCTION D IVISION 8 Nucleus Spindle fibers Nuclear envelope Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Anaphase I Telophase I (diploid)

R EPLICATION OF C HROMOSOMES  Replication is the process of duplicating a chromosome  Occurs prior to division  Replicated copies are called sister chromatids  Held together at centromere 9 Occurs in Interphase

A R EPLICATED C HROMOSOME 10 Homologs Same genes, different alleles Sister Chromatids Same genes, Same alleles Gene X Homologs separate in meiosis I and therefore different alleles separate. Gene: specific region of DNA that codes for a particular protein. Allele: any alternative form of a gene

P ROPHASE I 11 Early prophase Homologs pair. Crossing over occurs Late prophase  Chromosomes condense.  Spindle forms.  Nuclear envelope fragments.

T ETRADS F ORM IN P ROPHASE I 12 Homologous chromosomes (each with sister chromatids) Join to form a TETRAD Synapsis = putting the tetrads together

C ROSSING -O VER  Sister chromatids in a tetrad cross over each other  Pieces of chromosomes or genes are exchanged  Produces genetic recombination in the offspring  These are called homologous chromosomes 13

H OMOLOGOUS C HROMOSOMES D URING C ROSSING -O VER 14

C ROSSING -O VER 15

M ETAPHASE I 16 Homologous pairs of chromosomes align along the equator of the cell

A NAPHASE I 17 Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres.

T ELOPHASE I 18 Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two.

M EIOSIS II Only one homolog of each chromosome is present in the cell. 19 Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information. Gene X

M EIOSIS II: R EDUCING C HROMOSOME N UMBER 20 Prophase II Metaphase II Anaphase II Telophase II 4 haploid cells

P ROPHASE II 21 Nuclear envelope fragments. Spindle forms.

M ETAPHASE II 22 Chromosomes align along equator of cell.

A NAPHASE II 23 Sister chromatids separate and move to opposite poles. Equator Pole

T ELOPHASE II 24 Nuclear envelope assembles. Chromosomes uncoil. Spindle disappears. Cytokinesis divides cell into two.

R ESULTS OF M EIOSIS 25 Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome

26 G AMETOGENESIS

S PERMATOGENESIS  Occurs in the testes  Two divisions produce 4 spermatids  Spermatids mature into sperm  Men produce about 250,000,000 sperm per day 27

S PERMATOGENESIS 28

O OGENESIS  Occurs in the ovaries  Two divisions produce 3 polar bodies that die and 1 egg  Polar bodies die because of unequal division of cytoplasm  Immature egg called oocyte  Starting at puberty, one oocyte matures into an ovum (egg) every 28 days 29

O OGENESIS 30 Oogonium (diploid) Mitosis Primary oocyte (diploid) Meiosis I Secondary oocyte (haploid) Meiosis II (if fertilization occurs) First polar body may divide (haploid) Polar bodies die Ovum (egg) Second polar body (haploid) a A X X a X A X a X a X Mature egg A X A X

C OMPARING M ITOSIS AND M EIOSIS 31

C OMPARISON OF D IVISIONS 32 MitosisMeiosis Number of divisions 1 2 Number of daughter cells 24 Genetically identical? YesNo Chromosome #Same as parentHalf of parent WhereSomatic cellsGerm cells WhenThroughout lifeAt sexual maturity RoleGrowth and repairSexual reproduction

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