Meiosis

Cell Division Mitosis is the division of one cell into two IDENTICAL DIPLOID cells Identical to each other Identical to the original cell Same number of chromosomes

Cell Division MEIOSIS is the division of one cell into four genetically DIFFERENT HAPLOID cells (known as gametes or sex cells) Different from each other Different from the original cell HALF the number of chromosomes

Sexual Reproduction When a sperm fertilizes an egg… Does the fertilized egg have twice as many chromosomes? Why not? Because the sperm and the egg have HALF the chromosomes of a normal cell

Chromosome Pairs Humans have 46 chromosomes Each of our 46 chromosomes has a “twin” 23 sets of “twins”

Chromosome Pairs HOMOLOGOUS PAIRS Two chromosomes with same genetic info Mom’s gene for hair color match up with dad’s gene for hair color to determine your hair color 23 chromosomes come from mom 23 chromosomes come from dad 23 + 23 = 46 23 HOMOLOGOUS PAIRS

Sexual Reproduction Gamete – any reproductive cell Egg or sperm Gametes are haploid (23) HAPLOID = 1 set of unpaired chromosomes HA = Half PLOID = Chromosomes

Sexual Reproduction Haploid + Haploid = Diploid Egg + Sperm = Fertilized Egg Meiosis is the cell division in which the resulting cells (gametes) have half the number of chromosomes as the original cell

Meiosis Meiosis has two parts: Meiosis I Meiosis II

PMAT PMAT Meiosis I & II Meiosis I Meiosis II Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II PMAT PMAT

Meiosis Overview Very similar to mitosis Here are the differences… Starts as diploid (2n) Ends as haploid (1n)

Meiosis Overview Very similar to mitosis Here are the differences… Starts as diploid (2n) Ends as haploid (1n)

Meiosis I: Homologous Pairs Separate Prophase I DNA condenses Nuclear envelope breaks down Spindle fibers form Difference: Homologous pairs begin to pair up

Crossing Over In Prophase I, the homologous pairs, pair up with each other During this time, crossing over can occur The chromatids are so close to each other, they can literally cross each other and swap genetic information Always in equal amounts Crossing over creates genetic variation

Genetic Variation: Crossing Over

Meiosis I: Homologous Pairs Separate Metaphase I Homologous pairs Line up at the middle DIFFERENT FROM METAPHASE Metaphase each one lines up Metaphase 1 pairs line up SAME – spindle fibers attach

Meiosis I: Homologous Pairs Separate Anaphase I Homologous pairs separate and move to opposite poles SIMILAR TO ANAPHASE BUT Sister chromatids are still together

Meiosis I: Homologous Pairs Separate Telophase I Chromosomes gather Cytokinesis begins SAME AS TELOPHASE

Meiosis II: SISTER Chromatids Separate Prophase II DNA stays condensed Nuclear envelope breaks down Spindle fibers form SAME AS PROPHASE AND PROPHASE 1

Meiosis II: SISTER Chromatids Separate Metaphase II Chromosomes line up at equator SAME AS METAPHASE Metaphase I – pairs separate Metaphase II – chromatids separate

Meiosis II: SISTER Chromatids Separate Anaphase II Chromatids separate and move to opposite poles SAME AS ANAPHASE

Meiosis II: SISTER Chromatids Separate Telophase II Chromosomes gather and nuclear envelope forms Cytokinesis begins SAME AS TELOPHASE

Oogenesis- formation of ovum (egg cells) Oo = egg Genesis = beginning or make Oogenesis is meiosis for females Female mammals are believed to be born with finite number of potential eggs 7 million at 20 weeks of gestation 1-2 million at birth 400,000 at puberty ~500 will be released in their lifetime Few if any complete meiosis

Spermatogenesis- formation of sperm cells Spermatogenesis is meiosis for males Begins at puberty, ends at death One spermatocyte yields four spermatozoa Process takes about 64 days 170 million sperm made a day

Meiosis Video

Meiosis