MEIOSIS

• WHEN CHROMOSOMES OF TWO PARENTS COMBINE TO PRODUCE OFFSPRING, THE PROCESS IS KNOWN AS SEXUAL REPRODUCTION.

• THE CHROMOSOMES ARE CONTAINED IN SPECIAL REPRODUCTIVE CELLS CALLED GAMETES.

• GAMETES CAN BE EITHER EGG OR SPERM

• EGGS are larger than sperm.  An egg is nonmotile.

• SPERM Cells have Flagella, that helps them swim to the nonmotile egg.

• MEIOSIS IS A TYPE OF NUCLEAR DIVISION IN WHICH THE CHROMOSOME NUMBER IS HALVED. 

• MEIOSIS IS FOLLOWED BY CYTOKINESIS.

• IN HUMANS CELLS WITH 46 CHROMOSOMES (2n) (DIPLOID CELL) UNDERGO MEIOSIS AND CYTOKINESIS TO GIVE RISE TO EGG OR SPERM THAT HAVE ONLY 23 CHROMOSOMES (1N) (HAPLOID CELL)

• MEIOSIS ONLY OCCURS IN EUKARYOTIC CELLS

• DAUGHTER CELLS PRODUCED ARE NOT ALL ALIKE.  THEY MAY HAVE DIFFERENT CHROMOSOMES.

• Mitosis - One Parent Cell PRODUCES TWO DIPLOID DAUGHTER CELLS.

• Meiosis - One Parent Cell PRODUCES FOUR HAPLOID DAUGHTER CELLS.

TWO MAIN STAGES:

MEIOSIS I HOMOLOGOUS PAIRS ARE SEPARATED.

MEIOSIS II THE SISTER CHROMATIDS OF EACH CHROMOSOME ARE SEPARATED

• AT THE START OF MEIOSIS I EACH CHROMOSOME CONSIST OF TWO STRANDS OF SISTER CHROMATIDS CONNECTED AT THE CENTROMERE.

• HOMOLOGOUS PAIRS OF CHROMOSOMES COME TOGETHER BEFORE MEIOSIS BEGINS. THIS EVENT IS CALLED SYNAPSIS .

• EACH PAIR OF HOMOLOGOUS CHROMOSOMES IS CALLED A TETRAD .

PROPHASE I

• Chromosomes become thick and visible, the chromosomes of each homologous pair are tangled together.

• Portions of Chromatids may Break Off and attach to Adjacent Chromatids on the homologous Chromosome - a process called CROSSING-OVER

Crossing-Over results in Genetic Recombination by producing a New Mixture of Genetic Material.

EACH PARTS CONSISTS OF FOUR CHROMATIDS, BECAUSE EACH CHROMOSOME IN THE PAIR HAD REPLICATED BEFORE MEIOSIS BEGAN

The Nucleoli and the Nuclear Envelope disappear and the spindle fibers form.

• CROSSING-OVER IS THE EXCHANGE OF GENES BETWEEN PAIR OF HOMOLOGOUS CHROMOSOMES.

• CROSSING-OVER OCCURS ONLY DURING PROPHASE I.

• METAPHASE IHomologous pairs (Tetrads) are still together and arrange in the middle of the cell.

• ANAPHASE IThe homologous pairs of chromosomes separate from each other, spindle fibers pull one member from each pair to opposite ends of the cell.The Random separation of the Homologous Chromosomes is called INDEPENDENT ASSORTMENT.

• TELOPHASE I

Cytokinesis takes place. Each new cell is Diploid

MEIOSIS II 

Each Diploid Cell from Meiosis I will go through a second division, forming the FOUR GAMETES HAPLOID CELL (gametes).

• The production of Sperm Cells is called SPERMATOGENESIS

• OOGENESIS is the production of Mature Egg Cells or OVA.

• Female only produces ONE EGG

• The other Three products of meiosis are called POLAR BODIES .

• B/c of unequal dividing of the cytoplasm.

• Meiosis and sexual reproduction results in new characteristics.

• Organisms produced are not alike.

• Differences are called variation.

• Variations results from crossing-over.

• Environment determines which characteristics are beneficial.

• Traits that help an organism to survive will be passed on to their offspring.

• This leads to changes in populations. Why?

• This is the Basis for Evolution.

• Asexual offspring have the same traits.

• A change in the environment that can destroy one individual could destroy the whole population.