Mitosis. making identical copies of diploid cells

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1 Meiosis

2 Mitosis + making identical copies of diploid cells

3 Meiosis + making haploid gametes from a diploid precursor

4 Some terminology Diploid containing two copies of the genome per cell Haploid - containing one copy of the genome per cell Gametes cells that are the haploid cells combined during fertilization

5 Meiosis Haploid gametes (n = 23) n Egg cell n Sperm cell Key Haploid stage (n) Diploid stage (2n) Fertilization Ovary Testis 2n Multicellular diploid adults (2n = 46) Mitosis and development Diploid zygote (2n = 46)

6 Learning Outcomes Describe the steps in meiosis Compare and contrast mitosis and meiosis Distinguish between haploid and diploid, sister chromatids and nonsister chromatids Describe how genetic variability is generated through meiosis (independent assortment of homologous chromosomes and crossing over)

7 Function of Meiosis Create haploid cells from diploid precursors Ensure that each gamete has one complete copy of the genome Generate genetic variation

8 How does meiosis produce genetic variation? Genes occur in pairs on homologous chromosomes. The members of each pair of genes may be identical, or they may differ slightly, as alleles. Many genes have multiple alleles in the population, so new combinations of alleles are produced when gametes combine to form a new individual.

9 Karyotypes show Replicated Chromosomes Centromere Sister chromatids Pair of homologous chromosomes Sex chromosomes

10 Overview of Meiosis

11 Overview of Meiosis Meiosis 1 homologous chromosomes separate Meiosis 2 sister chromatids separate

12 Meiosis I -- Homologous chromosomes separate PROPHASE 1 Tetrads form and crossing over takes place

13 Meiosis I -- Homologous chromosomes separate METAPHASE 1 Tetrads line up on the metaphase plate

14 Meiosis I -- Homologous chromosomes separate ANAPHASE 1 Homologous chromosomes move to opposite ends of the spindle.

15 Meiosis I -- Homologous chromosomes separate TELOPHASE 1 (and cytokinesis) Two haploid cells with replicated chromosomes are formed.

16 Meiosis II -- Sister chromatids separate PROPHASE 2 Replicated chromosomes condense and the spindle starts to form.

17 Meiosis II -- Sister chromatids separate METAPHASE 2- Replicated chromosomes line up independently at the metaphase plate

18 Meiosis II -- Sister chromatids separate ANAPHASE 2 Sister chromatids are separated and move to opposite poles of the spindle.

19 Meiosis II -- Sister chromatids separate TELOPHASE 2 chromosomes decondense and cytokinesis occurs.

20 Meiosis Meiosis 1 homologous chromosomes separate Meiosis 2 sister chromatids separate

21 Meiosis leads to genetic variation

22 Meiosis leads to genetic variation 1) homologous chromosomes assort independently in humans, with 23 pairs 2 23 = 8,000,000 combinations 2) chromosomes can undergo crossing over generates more new combinations of genes

23 Crossing Over Occurs during Prophase I When tetrads form, non-sister chromatids can exchange corresponding pieces of DNA. Crossing over generates new combinations of genes

24 Crossing Over Occurs during Prophase I multiple cross overs can occur between non-sister chromatids in a given tetrad centromere crossover crossover

25 Non-disjunction in Meiosis Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II Non-disjunction events can occur either in Meiosis 1 or Meiosis 2 from Chapter 14

26 Human Trisomy Conditions result in severe developmental problems Only trisomy 21 produces viable offspring, all the others are lethal. Trisomy 21 (Down syndrome) Trisomy 18 (Edwards syndrome) Trisomy 13 (Patau syndrome) Trisomy 9 Trisomy 8 (Warkany syndrome 2) Trisomy 22 In contrast, extra X or Y chromosomes are usually well tolerated. XX = normal female, but XXX and XO are normal also XY = normal male, but XXY and XYY are viable also. YO is not viable.

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29 Polyploidy