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 Combinations1 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