Meiosis and Sexual Reproduction

1

• Meiosis and Sexual Reproduction

2
Impacts, Issues Why Sex

• Asexual reproduction is easier and faster
• Sexual reproduction can be an alternative
  adaption in changing environments

3
Why Sex
Fig. 10-1d, p.154
4
Why Sex
Fig. 10-1a, p.154
5
Why Sex
Fig. 10-1b, p.154
6
Why Sex
Fig. 10-1c, p.154
7
Impacts, Issues Why Sex

• Sexual reproduction has advantages when other
  organisms change
• Red Queen hypothesis evolutionary treadmill

8
Impacts, Issues Video
Why Sex
9
Asexual Reproduction

• Single parent produces offspring
• All offspring are genetically identical to one
  another and to parent

10
Sexual Reproduction

• Involves
• Meiosis
• Gamete production
• Fertilization
• Produces genetic variation among offspring

11
Sexual Reproduction

• Chromosomes are duplicated in germ cells

12
Germ cells undergo meiosis and cytoplasmic
division

• Cellular descendents of germ cells become gametes
• Gametes meet at fertilization

13
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

14
Chromosome Number

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

15
Human Karyotype
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
XX (or XY)
Fig. 10-4, p.157
16
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

17
Homologous Chromosomes
Fig. 10-2, p.156
18
Gamete Formation

• Gametes are sex cells (sperm, eggs)
• Arise from germ cells

ovaries
anther
ovary
testes
Figure 10-3Page 156
19
Fig. 10-3, p.156
20
FLOWERING PLANT
anther (where cells that give rise to male
gametes originate)
ovules, inside an ovary (where cells that give
rise to female gametes originate)
Fig. 10-3a, p.156
21
HUMAN MALE
testis (where sperm originate)
Fig. 10-3b, p.156
22
HUMAN FEMALE
ovary (where eggs develop)
Fig. 10-3c, p.156
23
Gamete Formation

• Reproductive organs

24
Meiosis Two Divisions

• Two consecutive nuclear divisions
• Meiosis I
• Meiosis II
• DNA is not duplicated between divisions
• Four haploid nuclei form

25
Meiosis I
Each homologue in the cell pairs with its
partner,
then the partners separate
p. 158
26
Meiosis I - Stages
27
Prophase I

• Each duplicated chromosome pairs with homologue
• Homologues swap segments
• Each chromosome becomes attached to spindle

Fig. 10-5, p. 158
28
Metaphase I

• Chromosomes are pushed and pulled into the middle
  of cell
• The spindle is fully formed

Fig. 10-5, p. 158
29
Anaphase I

• Homologous chromosomes segregate
• The sister chromatids remain attached

Fig. 10-5, p. 158
30
Telophase I

• The chromosomes arrive at opposite poles
• Usually followed by cytoplasmic division

Fig. 10-5, p. 158
31
Prophase II

• Microtubules attach to the kinetochores of the
  duplicated chromosomes

Fig. 10-5, p. 158
32
Metaphase II

• Duplicated chromosomes line up at the spindle
  equator, midway between the poles

Fig. 10-5, p. 158
33
Anaphase II

• Sister chromatids separate to become independent
  chromosomes

Fig. 10-5, p. 158
34
Telophase II

• The chromosomes arrive at opposite ends of the
  cell
• A nuclear envelope forms around each set of
  chromosomes
• Four haploid cells

Fig. 10-5, p. 158
35
MEIOSIS I
newly forming microtubules in the cytoplasm
spindle equator (midway between the two poles)
one pair of homologous chromosomes
plasma membrane
PROPHASE I
METAPHASE I
ANAPHASE I
TELOPHASE I
Fig. 10-5, p.158
36
Telophase I
Meiosis I
Stepped Art
Fig. 10-5a, p.158
37
there is no DNA replication between the two
divisions
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
MEIOSIS II
Fig. 10-5b, p.159
38
Meiosis II
Stepped Art
Fig. 10-5b, p.159
39
Meiosis

• Meiosis I and II

40
Meiosis

• Meiosis step-by-step

41
Crossing Over

• Each chromosome becomes zippered to its homologue
• All four chromatids are closely aligned
• Nonsister chromosomes exchange segments

42
Crossing Over
Stepped Art
Fig. 10-6, p.160
43
Crossing Over
a Both chromosomes shown here were duplicated
during interphase, before meiosis. When prophase
I is under way, sister chromatids of each
chromosome are positioned so close together that
they look like a single thread.
Fig. 10-6a, p.160
44
Crossing Over
b Each chromosome becomes zippered to its
homologue, so all four chromatids are tightly
aligned. If the two sex chromosomes have
different forms, such as X paired with Y, they
still get zippered together, but only in a tiny
region at their ends.
Fig. 10-6b, p.160
45
Crossing Over
c We show the pair of chromosomes as if they
already condensed only to give you an idea of
what goes on. They really are in a tightly
aligned, threadlike form during prophase I.
d The intimate contact encourages one crossover
(and usually more) to happen at various intervals
along the length of nonsister chromatids.
e Nonsister chromatids exchange segments at the
crossover sites. They continue to condense into
thicker, rodlike forms. By the start of metaphase
I, they will be unzippered from each other.
f Crossing over breaks up old combinations of
alleles and puts new ones together in the cells
pairs of homologous chromosomes.
Fig. 10-6c, p.160
46
Effect of Crossing Over

• After crossing over, each chromosome contains
  both maternal and paternal segments
• Creates new allele combinations in offspring

47
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

48
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

49
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)

50
Possible ChromosomeCombinations
1
2
3
combinations possible
or
or
or
Fig. 10-7, p.161
51
Alignment at metaphase I
Stepped Art
Fig. 10-7, p.161
52
Crossing Over

• Crossing over

53
Plant Life Cycle
sporophyte
zygote
diploid
fertilization
meiosis
haploid
spores
gametes
gametophytes
Fig. 10-8a, p.162
54
Animal Life Cycle
multicelled body
zygote
diploid
meiosis
fertilization
haploid
gametes
Fig. 10-8b, p.162
55
Animal Life Cycle

• Random alignment

56
Oogenesis
three polar bodies (haploid)
first polar body (haploid)
primary oocyte (diploid)
oogonium (diploid)
secondary oocyte (haploid)
ovum (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
Figure 10-10Page 163
57
Oogenesis

• Generalized life cycles

58
Spermatogenesis
primary spermatocyte (diploid)
spermato-gonium (diploid )
sperm (mature, haploid male gametes)
secondary spermatocytes (haploid)
spermatids (haploid)
Meiosis I, Cytoplasmic Division
Meiosis II, Cytoplasmic Division
Growth
cell differentiation, sperm formation
Figure 10-9Page 163
59
Animal Egg Formation

• Sperm formation

60
Animal Egg Formation

• Egg formation

61
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

62
Fig. 10-10, p.163
63
Factors Contributing to Variation among Offspring

• Crossing over during prophase I
• Random alignment of chromosomes at metaphase I
• Random combination of gametes at fertilization

64
Mitosis Meiosis Compared

• Mitosis
• Functions
• Asexual reproduction
• Growth, repair
• Occurs in somatic cells
• Produces clones

• Meiosis
• Function
• Sexual reproduction
• Occurs in germ cells
• Produces variable offspring

65
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

66
Meiosis II

• The two sister chromatids of each duplicated
  chromosome are separated from each other

two chromosomes (unduplicated)
one chromosome (duplicated)
p. 158
67
Anaphase, Anaphase I, and Anaphase II

• Anaphase I (Meiosis)
• Homologous chromosomes separate from each other
• Anaphase/Anaphase II (Mitosis/Meiosis)
• Sister chromatids of a chromosome separate from
  each other

68
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

69
Meiosis I
Telophase I
Prophase I
Anaphase I
Metaphase I
Crossing over occurs between homologues.
Homologous pairs align randomly.
Cytoplasm may divide before meiosis II.
Homologues separate from their partner.
Fig. 10-11a, p.164
70
Meiosis II
no interphase and no DNA replication between the
two nuclear divisions
Prophase II
Telophase II
Anaphase II
Metaphase II
New spindle forms in each nucleus.
All chromosomes aligned at the equator.
Sister chromatids moved to opposite spindle poles.
Haploid cells function as gametes or spores.
Fig. 10-11b, p.164
71
Mitosis
Telophase
Prophase
Anaphase
Metaphase
A spindle forms tethers chromosomes to spindle
poles.
All chromosomes aligned at the spindle equator.
Sister chromatids moved to opposite spindle poles.
Two diploid (2n) nuclei form.
Fig. 10-11c, p.164
72
Results of Mitosis and Meiosis

• Comparing mitosis and meiosis

73
An Ancestral Connection

• Was sexual reproduction a giant evolutionary step
  from aseuxal reproduction?
• Giardia intestinalis
• Chlamydomonas

74
2n
germ cell
germ cell
each chromosome duplicated during interphase
n
MEIOSIS I separation of homologues
MEIOSIS II separation of sister chromatids
gametes
gametes
2n
diploid number restored at fertilization
zygote
Fig. 10-12, p.166