Meiosis Formation of Gametes Eggs Sperm 1 Definition

Meiosis Formation of Gametes (Eggs & Sperm) 1

Definition of Meiosis is the process of reduction division occurring only in reproductive cells, in which one diploid (2 n) cell produces four haploid (n) cells that are not genetically identical. 2

Facts About Meiosis üPreceded by interphase which includes chromosome replication üTwo meiotic divisions --- Meiosis I and Meiosis II üCalled Reduction- division üOriginal cell is diploid (2 n) üFour daughter cells produced that are haploid (1 n) 3

Facts About Meiosis üDaughter cells contain half the number of chromosomes as the original cell üProduces gametes (eggs & sperm) üOccurs in the testes in males (Spermatogenesis) üOccurs in the ovaries in females (Oogenesis) 4

More Meiosis Facts ü Start with 46 double stranded chromosomes (2 n) üAfter 1 division - 23 double stranded chromosomes (n) üAfter 2 nd division - 23 single stranded chromosomes (n) ü Occurs in our germ cells that produce gametes 5

Why Do we Need Meiosis? üIt is the fundamental basis of sexual reproduction üTwo haploid (1 n) gametes are brought together through fertilization to form a diploid (2 n) zygote 6

Fertilization – “Putting it all together” 2 n = 6 1 n =3 7

Replication of Chromosomes ü Replication is the process of duplicating a chromosome ü Occurs prior to division ü Replicated copies are called sister chromatids ü Held together at centromere Occurs in Interphase S Phase 8

Chromosomes in Dividing Cells üDuplicated chromosomes are called chromatids & are held together by the centromere Called Sister Chromatids 9

A Replicated Chromosome Gene X Homologs (same genes, different alleles) Sister Chromatids (same genes, same alleles) Homologs separate in meiosis I and therefore different alleles separate. 10

Meiosis Forms Haploid Gametes ü Meiosis must reduce the chromosome number by half ü Fertilization then restores the 2 n number from mom from dad child too much! meiosis reduces genetic content The right number! 11

Eukaryotic Chromosomes üAll eukaryotic information in cells store genetic chromosomes ü Most eukaryotes have between 10 and 50 chromosomes in their body cells ü Human body cells have 46 chromosomes or 23 identical pairs 12

Karyotype ü A picture of the chromosomes from a human cell arranged in pairs by size ü First 22 pairs are called autosomes (non-sex chromosomes) ü Last pair are the sex chromosomes ü XX female or XY male 13

Boy or Girl? The Y Chromosome Decides Y - Chromosome X - Chromosome 14

Meiosis: Two Part Cell Division Sister chromatids separate Homologs separate Meiosis II Diploid Haploid 15

Meiosis I: Reduction Division Nucleus Early Late Prophase I (Chromosome Prophase I number doubled) Spindle fibers Nuclear envelope Metaphase Anaphase Telophase I I I (diploid) 16

Prophase I Early prophase üHomologs pair. üCrossing over occurs. Late prophase üChromosomes condense. üSpindle forms. üNuclear envelope fragments. 17

Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Join to form a TETRAD Called Synapsis 18

Crossing-Over ü Homologous chromosomes in a tetrad cross over each other ü Pieces of chromosomes or genes are exchanged ü Produces Genetic recombination in the offspring 19

Homologous Chromosomes During Crossing-Over http: //highered. mheducation. com/sites/9834092339/student_view 0/chapter 11/meiosis_with_crossing_over. html 20

Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment 21

Metaphase I Homologous pairs of chromosomes align along the equator of the cell 22

Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres. This random separation of homologs is called Independent Assortment. 23

Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. 24

Meiosis II Gene X Only one homolog of each chromosome is present in the cell Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. 25

Meiosis II: Reducing Chromosome Number Prophase Metaphase II Telophase II Anaphase 4 Identical II II haploid cells 26

Prophase II Nuclear envelope fragments. Spindle forms. 27

Metaphase II Chromosomes align along equator of cell. 28

Anaphase II Equator Pole Sister chromatids separate and move to opposite poles. 29

Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. 30

Results of Meiosis 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 31

Gametogenesis Oogenesis or Spermatogenesis 32

Spermatogenesis ü Occurs in the testes ü Two divisions produce 4 spermatids ü Spermatids mature into sperm ü Men produce about 250, 000 sperm per day 33

Spermatogenesis in the Testes Spermatid 34

Spermatogenesis 35

Oogenesis üOccurs in the ovaries üTwo divisions produce 3 polar bodies that die and 1 egg üPolar bodies die because of unequal division of cytoplasm üImmature egg called oocyte üStarting at puberty, one oocyte matures into an ovum (egg) every 28 days 36

Oogenesis in the Ovaries 37

Oogenesis First polar body may divide (haploid) a Mitosis Oogonium (diploid) X A X Primary oocyte (diploid) X a a Polar bodies die X Meiosis II (if fertilization A occurs) X A X Secondary oocyte (haploid) Ovum (egg) Mature egg A X Second polar body (haploid) 38

Comparing Mitosis and Meiosis 39

Comparison of Divisions Mitosis Number of divisions Number of daughter cells Genetically identical? 1 Meiosis 2 2 4 Yes No Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Growth and repair Sexual reproduction Chromosome # Role 40

Chromosomal Structure Mutations when pieces of chromosomes are "broken" off, have other pieces added on or moved around; sometimes can be very harmful, but depends on the number and type of genes that are affected 41

NONDISJUCTION of Chromosomes When the chromosomes do not split during the process of meiosis, it is called NONDISJUNCTION. In this case, gametes can have more or less than the n number of chromosomes. In trisomy there is an extra chromosome. In monosomy the is a single chromosome without a pair 42

NONDISJUCTION of Chromosomes If an entire set of chromosomes fail to separate, you have a polyploid organism (where the chromosome number is 3 n or 4 n instead of the diploid 2 n condition Polyploidy is common in plants, an example is the plump strawberries. In animals, polyploidy is usually lethal: this is because so many genes are affected, and animals are much more complex than plants 43