1. Meiosis
CHAPTER 10

2. What is Meiosis?
The mechanism that halves the number of chromosomes in cells is a form of cell division called meiosis
Meiosis consist of 2 successful nuclear divisions
1. Meiosis I
2. Meiosis II
Each round contains a prophase, metaphase, anaphase and telophase

3. Heredity is the transmission of traits from one generation to the next
Living organisms are distinguished by their ability to reproduce their own kind
Heredity is the transmission of traits from one generation to the next
Variation is demonstrated by the differences in appearance that offspring show from parents and siblings
Genetics is the scientific study of heredity and variation 3

4. What accounts for family resemblance?
Figure 10.1
What accounts for family resemblance? 4

5. Inheritance of Genes
Genes are the units of heredity and are made up of segments of DNA
Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs) 5

6. Most DNA is packaged into chromosomes
For example, humans have 46 chromosomes in their somatic cells, the cells of the body except for gametes and their precursors
Each gene has a specific position, or locus, on a certain chromosome 6

7. The remaining 22 pairs of chromosomes are called autosomes
The sex chromosomes, which determine the sex of the individual, are called X and Y
Human females have a homologous pair of X chromosomes XX
Human males have one X and one Y chromosome XY
The remaining 22 pairs of chromosomes are called autosomes 7

8. A diploid cell (2n) has two sets of chromosomes
Each pair of homologous chromosomes includes one chromosome from each parent
The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father
A diploid cell (2n) has two sets of chromosomes
For humans, the diploid number is 46 (2n  46) 8

9. Each replicated chromosome consists of two identical sister chromatids
In a cell in which DNA synthesis has occurred, each chromosome is replicated (copied)
Each replicated chromosome consists of two identical sister chromatids 9

10. Each set of 23 consists of 22 autosomes and a single sex chromosome
A gamete (sperm or egg) contains a single set of chromosomes and is haploid (n)
For humans, the haploid number is 23 (n = 23)
Each set of 23 consists of 22 autosomes and a single sex chromosome
In an unfertilized egg (ovum), the sex chromosome is X
In a sperm cell, the sex chromosome may be either X or Y 10

11. Behavior of Chromosome Sets in the Human Life Cycle
Fertilization is the union of gametes (the sperm and the egg)
The fertilized egg is called a zygote and has one set of chromosomes from each parent
The zygote produces somatic cells by mitosis and develops into an adult 11

12. Sexual Maturity in Organisms
At sexual maturity, the ovaries and testes produce haploid gametes
Gametes are the only types of human cells produced by meiosis rather than mitosis
Meiosis results in one set of chromosomes in each gamete
Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number 12

13. Multicellular diploid adults (2n  46)
Key
Haploid gametes (n  23)
Haploid (n)
Egg (n)
Diploid (2n)
Sperm (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n  46)
Mitosis and
development
Multicellular diploid
adults (2n  46) 13

14. Meiosis reduces the number of chromosome sets from diploid to haploid

15. Like mitosis, meiosis is preceded by the replication of chromosomes
The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis
Each daughter cell has only half as many chromosomes as the parent cell 15

16. Phases of Meiosis Meiosis takes place in two sets of cell divisions
1. Meiosis I
2. Meiosis II

17. duplicated chromosomes Meiosis II
Figure 10.7
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Chromosomes
duplicate
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Diploid cell with
duplicated
chromosomes
Meiosis I 1
Homologous
chromosomes
separate
Haploid cells with
duplicated chromosomes
Meiosis II 2
Sister chromatids
separate
Haploid cells with unduplicated chromosomes 17

18. Meiosis I Meiosis occurs in diploid reproductive cells
Before meiosis begins, the DNA of the diploid reproductive cells are copied
Meiosis I results in 2 haploid cells

19. MEIOSIS VIDEO SEGMENT

20. Prophase I
DNA coils tightly into chromosomes (as in mitosis), spindle fibers appear
Then the nucleus & nucleolus disassemble
Every chromosome lines up next to its homologous pair
The pairing of homologous chromosomes (which does not happen in mitosis) is called synapsis.
In synapsis, homologous chromosomes pair up, aligned gene by gene

21. Each pair of homologous chromosomes is called a tetrad
In each tetrad, chromatids of the homologous chromosomes are aligned lengthwise so that the genes on one chromosome are adjacent of the gene on the other chromosomes

22. Crossing Over
During synapsis, the chromatids within a homologous pair twist around one another
Portions of the chromatids may break off and attach to adjacent chromatids in a process called crossing over
This permits the exchange of genetic material between maternal and paternal chromosomes
Crossing over results in genetic recombination by producing a mixture of genetic material

23. Crossing over
Crossing over occurs when chromosomes that make up a tetrad exchange portions of their chromatids.
Crossing over results in an exchange of genes and in new combinations of genes

24. Prophase I video segment

25. Metaphase I
The tetrads line up randomly along the midline of the dividing cell
Spindle fibers from one pole attach to the Centromere of one homologous chromosome
Spindle fibers from the opposite pole attach to the other homologous chromosome of the pair

26. Metaphase I video segment

27. Anaphase I
Each homologous chromosome (consisting of 2 chromatids attached by a Centromere) moves to an opposite pole of the dividing cell
The random separation is called independent assortment (results in a random separation of the maternal and paternal chromosome resulting in genetic variation)

28. Anaphase I video segment

29. Telophase I & Cytokinesis I
The final phase of meiosis I
The chromosomes reach the opposite ends of the cell and cytokinesis begins
The new cells contain a haploid number of chromosomes

30. Telophase I video segment

31. Points to remember about meiosis I
The original cell produces 2 new cells
Each new cell contains 1 chromosome from each homologous pair
New cells contain half the number of chromosomes of the original cell
Each new cell contains 2 copies (as chromatids) because the original cell copied its DNA before meiosis I

32. MEIOSIS I: Separates homologous chromosomes
Figure 10.8a
MEIOSIS I: Separates homologous chromosomes
Telophase I and
Cytokinesis
Prophase I
Metaphase I
Anaphase I
Sister
chromatids
Centromere
(with kinetochore)
Sister chromatids
remain attached
Centrosome
(with centriole
pair)
Cleavage
furrow
Chiasmata
Metaphase
plate
Spindle
Homologous
chromosomes
separate
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
Homologous
chromosomes 32

34. Division in meiosis II also occurs in four phases
Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
Meiosis II is very similar to mitosis 34

35. Meiosis II
Occurs in each cell formed during meiosis I and is not preceded by the copying of DNA
In some species, Meiosis II begins after the nuclear membrane re-forms in the new cells
In other species, Meiosis II begins immediately following meiosis I

36. Prophase II Nuclear membrane & nucleolus disappear
Spindle fibers form and begin to move the chromosomes toward the midline of the dividing cell
Chromosomes are visible

37. Prophase II video segment

38. Metaphase II
The chromosomes move to the midline of the dividing cell, facing opposite poles of the dividing cells

39. Metaphase II video segment

40. Anaphase II
The sister chromatids separate and move toward opposite poles of the cell

41. Anaphase II video segment

42. Telophase II & Cytokinesis II
A nuclear membrane forms around the chromosomes in each of the 4 new cells
Cytokinesis II happens resulting in 4 new cells (each containing half of the original cells number of chromosomes)
Spindle disappears

43. Telophase II video segment

44. MEIOSIS II: Separates sister chromatids
Figure 10.8b
MEIOSIS II: Separates sister chromatids
Telophase II and
Cytokinesis
Prophase II
Metaphase II
Anaphase II
Sister chromatids
separate
Haploid
daughter
cells forming 44

46. At the end of meiosis, there are four daughter cells, each with a haploid set of unduplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell 46

47. Mitosis
Meiosis
Reduces the number of chromosome sets from 2 (diploid) to 1 (haploid)
Produce cells that differ from each other and the parent cell
Includes 2 divisions after replication, each with specific stages
Conserves the number of chromosomes sets 2 (diploid)
Produce cells that are identical to the parent cell

48. Three events are unique to meiosis, and all three occur in meiosis l
Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
Homologous pairs at the metaphase plate: Homologous pairs of chromosomes are positioned there in metaphase I
Separation of homologs during anaphase I 48

49. Figure 10.9 A comparison of mitosis and meiosis in diploid cells
Parent cell
Chiasma
MEIOSIS I
Prophase
Prophase I
Chromosome
duplication
Chromosome
duplication
Homologous
chromosome
pair
Duplicated
chromosome
2n = 6
Individual
chromosomes
line up.
Pairs of
chromosomes
line up.
Metaphase
Metaphase I
Anaphase
Sister chromatids
separate.
Homologs
separate.
Anaphase I
Telophase
Telophase I
Sister
chromatids
separate.
Daughter
cells of
meiosis I 2n 2n
MEIOSIS II
Daughter cells
of mitosis n n n n
Figure 10.9 A comparison of mitosis and meiosis in diploid cells
Daughter cells of meiosis II
SUMMARY
Property
Mitosis
Meiosis
DNA replication
Occurs during interphase before mitosis begins
Occurs during interphase before meiosis I begins
Number of divisions
One, including prophase, prometaphase,
metaphase, anaphase, and telophase
Two, each including prophase, metaphase, anaphase, and telophase
Synapsis of homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over between nonsister chromatids;
resulting chiasmata hold pairs together due to sister chromatid cohesion
Number of daughter cells
and genetic composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many chromosomes as the parent cell;
genetically different from the parent cell and from each other
Role in the animal body
Enables multicellular adult to arise from zygote;
produces cells for growth, repair, and, in some
species, asexual reproduction
Produces gametes; reduces number of chromosome sets by half and introduces
genetic variability among the gametes 49

50. SUMMARY Property Mitosis Meiosis DNA replication
Figure 10.9b
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase
before mitosis begins
Occurs during interphase before meiosis I
begins
Number of
divisions
One, including prophase,
prometaphase, metaphase,
anaphase, and telophase
Two, each including prophase, metaphase,
anaphase, and telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing
over between nonsister chromatids; resulting
chiasmata hold pairs together due to sister
chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and
genetically identical to the
parent cell
Four, each haploid (n), containing half as
many chromosomes as the parent cell;
genetically different from the parent cell and
from each other
Role in the
animal body
Enables multicellular adult to
arise from zygote; produces
cells for growth, repair, and,
in some species, asexual
reproduction
Produces gametes; reduces number of
chromosome sets by half and introduces
genetic variability among the gametes 50

51. Formation of Gametes
In animals, meiosis produces haploid reproductive cells called gametes
Because only those cells involved in the production of gametes divided by meiosis in animals, meiosis occurs only within their reproductive organs
In humans, meiosis occurs in the testes and in the ovaries

52. Testes
In testes, meiosis is involved in the production of male gametes known as sperm cells or spermatozoa
In the development of sperm cells, a diploid reproductive cell divides meiotically to form 4 haploid cells called spermatids
Each spermatids then develops into a mature sperm cell called spermatogenesis

53. Gametogenesis is the production of gametes
It differs between females and males
Eggs contribute DNA, cytoplasm, and organelles to an embryo.
During meiosis, the egg gets most of the contents; the other cells from polar bodies.
Sperm become streamlined and motile.
Sperm primarily contribute DNA to an embryo.

55. Egg Cells Oogenesis is the production of mature egg cells or ova
A diploid reproductive cell divides meiotically to produce 1 mature egg cell (ovum)
During cytokinesis II of oogenesis, the cytoplasm of the original cell is divided unequally between new cells
1 cell which develops into a mature egg cell, receives most of the cytoplasm of the original cell
As a result, 1 egg cell is produced by meiosis, the other 3 are called polar bodies & will eventually degenerate.