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Meiosis Chapter 13.

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Presentation on theme: "Meiosis Chapter 13."— Presentation transcript:

1 Meiosis Chapter 13

2 Octopus Sex Male reaches under his mantle with tentacle, removes packet of sperm, and insert it into female’s egg chamber Eggs are fertilized and give rise to new octopuses

3 Limpet Sex Larva can become adult of either sex
Depends on whether other limpets are present and what their sex is Adults can change sex in response to new arrivals

4 Aphid Sex Females are produced from unfertilized eggs in summer
Males are produced as autumn approaches Females produced by sexual reproduction can overwinter and begin producing new females in spring

5 Sexual Reproduction Chromosomes are duplicated in germ cells
Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes Gametes meet at fertilization

6 Asexual Reproduction Single parent produces offspring
All offspring are genetically identical to one another and to parent

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

8 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

9 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

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

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

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

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

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

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

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

17 Prophase I Each duplicated, condensed chromosome pairs with its homologue Homologues swap segments; called crossing over Each chromosome becomes attached to microtubules of newly forming spindle

18 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

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

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

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

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

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

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

25 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

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

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

28 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

29 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

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

31 Possible Chromosome Combinations
1 2 3 Possible Chromosome Combinations or or or

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

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

34 Spermatogenesis secondary spermatocytes (haploid) spermato-
gonium (diploid male reproductive cell) primary spermatocyte (diploid) spermatids (haploid) Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division Growth

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

36 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

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

38 Mitosis & Meiosis Compared
Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones Meiosis Function Sexual reproduction Occurs in germ cells Produces variable offspring

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

40 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

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

42 Film of Meiosis

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