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3 Lesson #1.3 Meiosis

Mitosis/Meiosis Overview Difference between Mitosis and Meiosis –Mitosis Reproduction –Asexual, required 1 parent cell –Division increases population Growth –Organism develops from a single cell –100 trillion cells in a human body  from single cell –RBC reproduce  1 million/second Repair 4

Mitosis/Meiosis Overview –Meiosis Reproduction –Sexual, requires TWO parents –Produces gametes: egg and sperm cells –Leads to genetic variability and diversity within species 5

6 Meiosis Formation of Gametes (Eggs & Sperm)

7 Facts About Meiosis Preceded by interphase which includes chromosome replication Preceded by interphase which includes chromosome replication Goes through Two meiotic divisions Goes through Two meiotic divisions Meiosis I and Meiosis II Meiosis I and Meiosis II Original cell is diploid (2n) Original cell is diploid (2n) Four daughter cells produced, are haploid (1n) Four daughter cells produced, are haploid (1n)

8 Facts About Meiosis Daughter cells contain half the number of chromosomes as the original cell Daughter cells contain half the number of chromosomes as the original cell Produces gametes (eggs & sperm) Produces gametes (eggs & sperm) Occurs in the testes in males Occurs in the testes in males Occurs in the ovaries in females Occurs in the ovaries in females

9 Start with 46 double stranded chromosomes After 1 division - 23 double stranded chromosomes After 1 division - 23 double stranded chromosomes After 2nd division - 23 single stranded chromosomes After 2nd division - 23 single stranded chromosomes Occurs in our germ cells that produce gametes More Meiosis Facts

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

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

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

A Replicated Chromosome Homologous Homologouschromosomes (same genes, different alleles) (same genes, different alleles) Sister Chromatids (same genes, same alleles) Gene X

14 Meiosis Forms Haploid Gametes Meiosis must reduce the chromosome number by half Meiosis must reduce the chromosome number by half Fertilization then restores the 2n number Fertilization then restores the 2n number from momfrom dadchild meiosis reduces genetic content too much! The right number!

15 Meiosis: Two Part Cell Division Homologsseparate Sisterchromatidsseparate Haploid Meiosis I Meiosis II Diploid Haploid

16 Meiosis I: Reduction Division Nucleus Spindlefibers Nuclearenvelope Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Anaphase I Telophase I

17 Prophase I Early prophase Homologs pair. Homologs pair. Crossing over occurs Crossing over occurs. Late prophase Chromosomes condense. Chromosomes condense. Spindle forms. Spindle forms. Nuclear envelope fragments. Nuclear envelope fragments.

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

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

20 Homologous Chromosomes During Crossing-Over

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

22 Metaphase I Homologous pairs of chromosomes align along the equator of the cell  Microtubules from one pole are attached to the centromere of one chromosome of each tetrad, while those from the other pole are attached to the other.

23 Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres attached at their centromeres.

24 Telophase I -Homologous chromosomes move until there is a haploid set at each pole. -Each chromosome consists of linked sister chromatids. -Cytokinesis by the same mechanisms as mitosis usually occurs simultaneously Nucleus reforms, but there is no further replication of chromosomes.

Meiosis II NO interphase between Meiosis I and Meiosis II 25

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

27 Prophase II During prophase II a spindle apparatus forms, attaches to centromeres of each sister chromatids, and moves them around. Spindle fibers from one pole attach to the centromere of one sister chromatid and those of the other pole to the other sister chromatid.

28 Metaphase II Chromosomes align along equator of cell.

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

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

Results of Meiosis -Gametes (egg & sperm) form -Four haploid cells with one copy of each chromosome -One allele of each gene is inherited from each parent -Different combinations of alleles for different genes along the chromosome

32 Lesson#1.3 Gametogenesis

33 Gametogenesis Oogenesis or Spermatogenesis

34 Spermatogenesis Occurs in the testes…begins at puberty Occurs in the testes…begins at puberty Two divisions produce 4 spermatids Two divisions produce 4 spermatids Spermatids mature into sperm Spermatids mature into sperm Men produce about 250,000,000 sperm per day Men produce about 250,000,000 sperm per day

Spermatogenesis

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

37 Oogenesis Oogonium(diploid) Mitosis Primaryoocyte(diploid) Meiosis I Secondaryoocyte(haploid) Meiosis II (if fertilization occurs) First polar body may divide (haploid)Polarbodiesdie Ovum (egg) Second polar body (haploid) a A X X a X A X a X a X Matureegg A X A X

38 Putting it all together All your body cells (somatic cells) go through mitosis for repair and growth 46 chromosomes, 2n (diploid) At puberty, your sex cells (gametes) go through meiosis to prepare for fertilization 23 chromosomes, n (haploid) FERTILIZATION 1 haploid egg + 1 haploid sperm = 1 diploid zygote embryo babyAdult mitosis Mitosis for repair

39 Comparing Mitosis and Meiosis

40 MitosisMeiosis Number of divisions 12 Number of daughter cells 24 Genetically identical? YesNo Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction Comparison of Divisions