Meiosis and Sexual Reproduction

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Oscar Fowler
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1 Meiosis and Sexual Reproduction

2 Asexual Reproduction Single parent produces offspring All offspring are genetically identical to one another and to parent Produces identical somatic (body) cells

3 Sexual Reproduction Chromosomes are duplicated in germ cells (sex cells) Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes (sperm and egg) Gametes meet at fertilization

4 Sperm and Egg Gametes

5 Sexual Reproduction Involves: Meiosis Gamete production Fertilization Produces genetic variation among offspring

6 Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change

7 Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles

8 Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells ovaries testes anther ovary

9 Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

10 Meiosis: Two Divisions Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Four haploid nuclei are formed

11 Meiosis I Each homologue in the cell pairs with its partner, then the partners separate

12 Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated)

13 Stages of Meiosis Meiosis I Meiosis II Prophase I Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

14 Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I

15 Prophase I Each duplicated, condensed chromosome pairs with its homologue Homologues swap segments Each chromosome becomes attached to microtubules of newly forming spindle

16 Metaphase I Chromosomes are pushed and pulled into the middle of cell Sister chromatids of one homologue orient toward one pole, and those of other homologue toward opposite pole The spindle is now fully formed

17 Anaphase I Homologous chromosomes segregate from each other The sister chromatids of each chromosome remain attached

18 Telophase I The chromosomes arrive at opposite poles The cytoplasm divides There are now two haploid cells This completes Meiosis I

19 Prophase II Metaphase II Anaphase II Telophase II Meiosis II - Stages

20 Anaphase II

21 Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes Motor proteins drive the movement of chromosomes toward the spindle s equator

22 Metaphase II All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles

23 Anaphase II Sister chromatids separate to become independent chromosomes Motor proteins interact with microtubules to move the separated chromosomes to opposite poles

24 Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes The cytoplasm divides There are now four haploid cells

25 Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Non-sister chromosomes exchange segments

26 Effect of Crossing Over After crossing over, each chromosome contains both maternal and parental segments Creates new allele combinations in offspring

27 Random Alignment During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes Initial contacts between microtubules and chromosomes are random

28 Random Alignment Either the maternal or paternal member of a homologous pair can end up at either pole The chromosomes in a gamete are a mix of chromosomes from the two parents

29 Possible Chromosome Combinations As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2 n (n is number of chromosome types)

30 Possible Chromosome Combinations or or or

31 Plant Life Cycle mitosis multicelled sporophyte zygote fertilization Diploid Haploid meiosis gametes multicelled gametophytes spores mitosis

32 Animal Life Cycle mitosis zygote multicelled body fertilization Diploid Haploid meiosis gametes

33 Spermatogenesis secondary spermatocytes (haploid) spermatogonium (diploid male germ cell) primary spermatocyte (diploid) Growth Mitosis I, Cytoplasmic division spermatids (haploid) Meiosis II, Cytoplasmic division

34 Oogenesis first polar body (haploid) three polar bodies (haploid) oogonium (diploid reproductive cell) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) Growth Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division

35 Fertilization Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Which two gametes unite is random Adds to variation among offspring

36 Factors Contributing to Variation among Offspring Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization

37 Mitosis & Meiosis Compared Mitosis Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones 1 nuclear division Function Meiosis Sexual reproduction Occurs in germ cells Produces variable offspring Two nuclear divisions

38 Prophase vs. Prophase I Prophase (Mitosis) Homologous pairs do not interact with each other Prophase I (Meiosis) Homologous pairs become zippered together and crossing over occurs

39 Anaphase, Anaphase I, and Anaphase II Anaphase I (Meiosis) Homologous chromosomes are separated from each other Anaphase/Anaphase II (Mitosis/Meiosis) Sister chromatids of a chromosome are separated from each other

40 Results of Mitosis and Meiosis Mitosis Two diploid cells produced Each identical to parent Meiosis Four haploid cells produced Differ from parent and one another

Intitial Question: How can the mathematically impossible become the biologically possiblenamely,

Intitial Question: How can the mathematically impossible become the biologically possiblenamely, a cell with 46 chromosomes splits to form tow cells each with 46 chromosomes/ This means 46 divided by 2