1. REPRODUCTION II: SEXUAL REPRODUCTION AND MEIOSIS
BIO 2, Lecture 10
REPRODUCTION II:
SEXUAL REPRODUCTION
AND MEIOSIS

2. Many eukaryotic organisms use sexual reproduction to reproduce the entire organism
Some single-celled eukaryotes usually reproduce the organism asexually but can switch to sexual reproduction
Some higher eukaryotes reproduce the organism asexually but most do not
Heredity is the transmission of traits from one generation to the next in sexually-reproducing organisms (where the offspring differ from their parents)

3. It is genes that are actually inherited
In a literal sense, children do not inherit particular physical traits from their parents
It is genes that are actually inherited
Genes are the units of heredity, and are made up of segments of DNA
Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs)
Each gene has a specific location called a locus on a certain chromosome
Figure 12.2 The functions of cell division

4. In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

5. Haploid cells (1n) carry one copy of each chromosome (e. g
Haploid cells (1n) carry one copy of each chromosome (e.g. eggs and sperm)
Diploid cells (2n) carry two copies of each chromosome (e.g. fertilized egg)
Meiosis is the process whereby a diploid cell reproduces to create 4 haploid cells
Each haploid daughter cell is different from the parent cell and from all other daughter cells
Produces much more variation than mitosis (which relies on replication errors to introduce new variation)

6. In animals, the diploid phase is dominant
Most eukaryotic organisms make both haploid and diploid cells in their life cycle
The life cycle of some organisms is primarily haploid, whereas for others it is primarily diploid
In animals, the diploid phase is dominant
In plants, either the haploid (e.g. moss) or diploid (e.g. pine tree) phase may be dominant
In primitive eukaryotes (e.g. yeast), the haploid phase is dominant

7. Key Haploid (n) Diploid (2n) Gametes n Zygote 2n Diploid multicellular
MEIOSIS
FERTILIZATION
Mitosis
Diploid
multicellular
organism
(a) Animals
Figure 13.6a Three types of sexual life cycles—animals

8. Plants and some algae exhibit an alternation of generations
This life cycle includes both a diploid and haploid multi-cellular stage
The diploid organism, called the sporophyte, makes haploid spores by meiosis

9. Each spore grows by mitosis into a haploid organism called a gametophyte
A gametophyte makes haploid gametes by mitosis
Fertilization of gametes results in a diploid sporophyte
Specialized cells in the diploid sporophyte undergo meiosis to produce spores ...

10. Haploid multi-cellular organism (gametophyte)
Key
Haploid (n)
Diploid (2n) n 2n
Mitosis
Zygote
Spores
Gametes
MEIOSIS
FERTILIZATION
Diploid
multicellular
organism
(sporophyte)
Haploid multi-cellular
organism (gametophyte)
(b) Plants and some algae
Figure 13.6b Three types of sexual life cycles—plants and some algae

11. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multi-cellular diploid stage
The zygote produces haploid cells by meiosis
Each haploid cell grows by mitosis into a haploid multicellular organism
The haploid adult produces gametes by mitosis that then fertilize each other to form the zygote ....

12. Haploid unicellular or multicellular organism
Key
Haploid (n)
Diploid (2n)
Mitosis
Gametes
Zygote
Haploid unicellular or
multicellular organism
MEIOSIS
FERTILIZATION n 2n
(c) Most fungi and some protists
Figure 13.6c Three types of sexual life cycles—most fungi and some protists

13. Figure 12.11 Bacterial cell division by binary fission
Key
Haploid (n)
Diploid (2n) n
Gametes
Mitosis
MEIOSIS
FERTILIZATION 2n
Zygote
Diploid
multicellular
organism
(a) Animals
Spores
(sporophyte)
(b) Plants and some algae
(c) Most fungi and some protists
Haploid multi-
cellular organism
(gametophyte)
Haploid unicellular or
multicellular organism
Figure Bacterial cell division by binary fission

14. Human diploid (somatic) cells have 23 pairs of chromosomes
One chromosome in each pair comes from mother, one from father
The two chromosomes in each pair are called homologous chromosomes, or homologs
Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characters
However, they are more dissimilar than sister chromatids (Why?)

15. A karyotype is an ordered display of the pairs of chromosomes from a cell

16. TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere
Figure 13.3 Preparing a karyotype
Sister
chromatids
Metaphase
chromosome

17. The sex chromosomes are called X and Y
Human females have a homologous pair of X chromosomes (XX)
Human males have one X and one Y chromosome
The 22 pairs of chromosomes that do not determine sex are called autosomes

18. Other organisms have different numbers of chromosomes
The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father
For humans, the diploid number is 46 (2n = 46) and the haploid number is 23
Other organisms have different numbers of chromosomes
Cats: 38 diploid, 19 haploid
Dogs: 78 diploid, 39 haploid
Chimpanzee: 48 diploid, 24 haploid
Arabidopsis: 10 diploid, 5 haploid
Figure 12.4 Chromosome duplication and distribution during cell division

19. In a diploid cell in which DNA synthesis has occurred, each chromosome is replicated
The chromosomes look like this at the start of meiosis:
Key
Maternal set of chromosomes (n = 3)
Paternal set of chromosomes (n = 3)
2n = 6
Centromere
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

20. In females, meiosis produces
In humans, the primary spermatocytes (in the testes) and primary oocytes (in the ovaries) are the only cells that undergo meiosis
In females, meiosis produces
An egg (ovum), carrying the sex chromosome is X and one copy of each autosome (23 chromosomes in all)
In males, meiosis produces
A sperm, carrying either X or Y and one copy of each autosome (23 chromosomes in all)

21. Figure 12.5 The cell cycle

22. Multicellular diploid adults (2n = 46)
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Egg (n)
Sperm (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
Figure 13.5 The human life cycle

23. Meiosis involves two cell divisions
In the first cell division (meiosis I), homologous chromosomes separate
Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division
In the second cell division (meiosis II), sister chromatids separate
Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division

24. Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number

25. Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Reductional division
Meiosis I
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number 1
Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes

26. Reductional division Equational division
Interphase
Homologous pair of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
Homologous
separate 1
Haploid cells with
replicated chromosomes
Meiosis II 2
Sister chromatids
Haploid cells with unreplicated chromosomes
Reductional division
Equational division
Figure 13.7 Overview of meiosis: how meiosis reduces chromosome number

27. The single centrosome replicates, forming two centrosomes
Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids
The sister chromatids are genetically identical and joined at the centromere
The single centrosome replicates, forming two centrosomes
These early processes are identical to those preceding mitosis
Figure 12.6 The mitotic division of an animal cell

28. Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
Figure 12.6 The mitotic division of an animal cell

29. Metaphase I Prophase I Anaphase I Centrosome (with centriole pair)
Telophase I and
Cytokinesis
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Homologous
chromosomes
Fragments
of nuclear
envelope
Centromere
(with kinetochore)
Metaphase
plate
Microtubule
attached to
kinetochore
Sister chromatids
remain attached
separate
Cleavage
furrow
Figure 13.8 The meiotic division of an animal cell

30. Chromosomes begin to condense
Prophase I
Prophase I typically occupies more than 90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
This is very different from mitosis, where the homologous chromosomes ignore one another

31. Metaphase I
In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole
Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad
Microtubules from the other pole are attached to the kinetochore of the other chromosome

32. Prophase I Metaphase I Centrosome (with centriole pair) Sister
chromatids
Chiasmata
Spindle
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Microtubule
attached to
kinetochore
Figure 13.8 The meiotic division of an animal cell

33. Anaphase I
In anaphase I, pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the spindle apparatus
Sister chromatids remain attached at the centromere and move as one unit toward the pole

34. Telophase I and Cytokinesis
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

35. Telophase I and Cytokinesis
Anaphase I
Sister chromatids
remain attached
Figure 13.8 The meiotic division of an animal cell
Homologous
chromosomes
separate
Cleavage
furrow

36. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already in the replicated state

37. Division in meiosis II also occurs in four phases:
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II and cytokinesis
Meiosis II is essentially a mitotic division of a haploid cell

38. Prophase II
In prophase II, a spindle apparatus forms
In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate

39. Metaphase II
In metaphase II, the sister chromatids are arranged at the metaphase plate
Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
The kinetochores of sister chromatids attach to microtubules extending from opposite poles

40. Prophase II Metaphase II
Figure 13.8 The meiotic division of an animal cell

41. Anaphase II
In anaphase II, the sister chromatids separate
The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles

42. Telophase II and Cytokinesis
In telophase II, the chromosomes arrive at opposite poles
Nuclei form, and the chromosomes begin decondensing

43. Cytokinesis separates the cytoplasm
At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

44. Telephase II and Cytokinesis
Anaphase II
Telephase II and
Cytokinesis
Sister chromatids
separate
Haploid daughter cells
forming
Figure 13.8 The meiotic division of an animal cell

45. Chromosome replication Daughter cells of mitosis
MEIOSIS
MEIOSIS I
Prophase I
Chiasma
Chromosome
replication
Homologous
chromosome
pair
2n = 6
Parent cell
Prophase
Replicated chromosome
Metaphase
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
Daughter
cells of
meiosis I
MEIOSIS II
Daughter cells of meiosis II n 2n
Daughter cells
of mitosis
Anaphase
Telophase
Figure 13.9 A comparison of mitosis and meiosis in diploid cells

46. Figure 13.9 A comparison of mitosis and meiosis in diploid cells
SUMMARY
Meiosis
Mitosis
Property
DNA
replication
Number of
divisions
Occurs during interphase before
mitosis begins
One, including prophase, metaphase,
anaphase, and telophase
Synapsis of
homologous
chromosomes
Does not occur
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase, and
telophase
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Four, each haploid (n), containing half as many chromosomes
as the parent cell; genetically different from the parent
cell and from each other
Produces gametes; reduces number of chromosomes by half
and introduces genetic variability among the gametes
Figure 13.9 A comparison of mitosis and meiosis in diploid cells

47. 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
– In metaphase I, paired homologous chromosomes (tetrads), line up on either side of the plate (instead of individual replicated chromosomes)
– At anaphase I, homologous chromosomes, separate (not sister chromatids)
Figure 12.7 The mitotic spindle at metaphase

48. Meiosis reshuffles variation that already exists
Mutations (changes in an organism’s DNA) are the original source of genetic diversity
Mutations create different versions of genes called alleles
Reshuffling of alleles during sexual reproduction produces genetic variation in gametes
It brings together alleles that were not together before

49. Three mechanisms contribute to genetic variation:
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation
Three mechanisms contribute to genetic variation:
Independent assortment of chromosomes
Crossing over
Random fertilization

50. Independent Assortment of Chromosomes:
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs
Figure 12.9 Cytokinesis in animal and plant cells

51. The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
For humans (n = 23), there are more than million (223) possible combinations of chromosomes
Figure Mitosis in a plant cell

52. The frequency of cell division varies with the type of cell
These cell cycle differences result from regulation at the molecular level
The cell cycle appears to be driven by specific chemical signals present in the cytoplasm
Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei
Figure The structure of transfer RNA (tRNA)

53. Possibility 1 Possibility 2 Two equally probable arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Figure The independent assortment of homologous chromosomes in meiosis

54. Crossing Over:
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent
Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
Figure Do molecular signals in the cytoplasm regulate the cell cycle?

55. Recombinant chromosomes
Prophase I
of meiosis
Pair of
homologs
Nonsister
chromatids
held together
during synapsis
Chiasma
Centromere
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
TEM
Figure The results of crossing over during meiosis

56. Random Fertilization:
Random fertilization adds to genetic variation because any sperm can fuse with any egg
The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations
And that’s not counting variation introduced into gametes by crossing over!!
Figure Translation: the basic concept