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8/15/2013
Chapter 13 – Meiosis and Sexual Life Cycles
Overview
I. Cell Types
II. Meiosis
I. Meiosis I
II. Meiosis II
III. Genetic Variation
IV. Reproduction
Overview: Variations on a Theme
Figure 13.1
 Living organisms are distinguished by their
ability to reproduce their own kind
 Genetics is the scientific study of heredity and
variation
 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
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Offspring acquire genes from parents by
inheriting chromosomes
 In a literal sense, children do not inherit
particular physical traits from their parents
 It is genes that are actually inherited
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)
 Each gene has a specific location called a
locus on a certain chromosome
 Most DNA is packaged into chromosomes
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Comparison of Asexual and Sexual Reproduction
 In asexual reproduction, a single individual
passes genes to its offspring without the fusion
of gametes
Figure 13.2
0.5 mm
 A clone is a group of genetically identical
individuals from the same parent
Parent
Bud
 In sexual reproduction, two parents give rise
to offspring that have unique combinations of
genes inherited from the two parents
(a) Hydra
(b) Redwoods
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Fertilization and meiosis alternate in sexual
life cycles
 A life cycle is the generation-to-generation
sequence of stages in the reproductive
history of an organism
Reproduction
 Asexual reproduction: Parent cell divides into
two daughter cells (Mitosis). The end result is
a two daughter cells identical to parent cell
 Sexual reproduction: The union of two
gametes (sex cells) to form a single zygote
 Eggs and Sperm are gametes
 Fertilized egg is zygote
 Zygote is different from gametes
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Fertilization
 Fertilization is the union between the sperm
and the egg.
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Cell Types
 Mitosis occurs in all the body’s cells except the
cells that are responsible for reproduction
Gametes = reproductive cells
 Sperm and eggs are reproductive cells –
gametes
 Gametes: are the cells that are responsible for
reproduction
 All the rest of the body’s cells are somatic cells
 The cells that divide to produce gametes
undergo meiosis
How do gametes overcome this problem?
 Remember that we have 23 pairs of
chromosomes = 46 chromosomes
 If gametes (sperm and egg) combined with
all these chromosomes then the offspring
will have 92 chromosomes
 Before the gametes come together they
need to reduce their number of
chromosomes in half.
 So instead of 23 pairs (46 chromosomes)
they need to have 23 chromosomes total.
 The answer to their problem is meiosis –
halving their number of chromosomes
Sets of Chromosomes in Human Cells
 Human somatic cells (any cell other than a
gamete) have 23 pairs of chromosomes
 A karyotype is an ordered display of the
pairs of chromosomes from a cell
 The two chromosomes in each pair are
called homologous chromosomes, or
homologs
 Chromosomes in a homologous pair are the
same length and shape and carry genes
controlling the same inherited characters
Figure 13.3
APPLICATION
TECHNIQUE
Pair of homologous
duplicated chromosomes
5 m
Centromere
Sister
chromatids
Metaphase
chromosome
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Sex Chromosomes in Human Cells
 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
 The remaining 22 pairs of chromosomes are
called autosomes
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Sets of Chromosomes in Human Cells
 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
Terminology
 Diploid = Cells that contain two sets of
chromosomes. In humans, cells that have 46
chromosomes or 23 pairs; all somatic cells are
diploid (2n)
 Haploid = Cells that have one set of chromosomes.
In humans, cells that have 23 chromosomes;
gametes are haploid (1n)
 Polyploidy = three sets of chromosomes; rare in
animals, common in plants
 For humans, the diploid number is 46 (2n = 46)
 Meiosis is when a diploid cell divides to produce
haploid reproductive cells
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Meiosis
DNA Replication
 First the chromosomes (DNA) are
duplicated during Interphase
 In a cell in which DNA synthesis has
occurred, each chromosome is replicated
 Then there are two cell divisions
 Each replicated chromosome consists of
two identical sister chromatids
 Remember that mitosis had chromosome
(DNA) duplication followed by one cell
division
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Figure 13.4
 Remember that there are pairs
chromosomes, each chromosome has two
chromatids just after DNA replication
Key
2n  6
Maternal set of
chromosomes (n  3)
Paternal set of
chromosomes (n  3)
Sister chromatids
of one duplicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Figure 13.7-1
Meiosis
Pair of homologous
chromosomes
(one from each set)
Interphase
Pair of homologous
chromosomes in
diploid parent cell
 The DNA has already replicated during interphase
– the chromosomes have become duplicated
Duplicated pair
of homologous
chromosomes
 In Meiosis the chromosome homologous pairs
separate and the cell divides = 1st cell division
Chromosomes
duplicate
Sister
chromatids
Diploid cell with
duplicated
chromosomes
 Then the chromatids separate and cell divide = 2 cd
cell division
 The figures are going to show only one pair of
chromosomes – but there are 23 pairs at the start
Figure 13.7-2
Figure 13.7-3
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
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
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Meiosis I
1 Homologous
chromosomes separate
1 Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Haploid cells with
duplicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unduplicated chromosomes
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Meiosis Overview
 Remember Meiosis happens to form gametes
– the reproductive cells (sperm and eggs)
 The cells that produce the gametes start out
diploid before meiosis, and will end up haploid
 There are two stages of Meiosis: Meiosis I
and II
 Each Stage of Meiosis has Prophase,
Metaphase, Anaphase, and Telophase
1. DNA replicates – chromosomes become
duplicated (two chromatids), the cell is diploid
(2n). This happens in Interphase.
2. Meiosis 1: homologous chromosomes separate
and the cell divides resulting in two haploid
cells (1n)
3. Meiosis 2: The chromatids separate and then
the cell divides resulting in four haploid cells
(1n)
Interphase
 Meiosis I is preceded by interphase, when
the chromosomes are duplicated to form
sister chromatids
 The sister chromatids are genetically
identical and joined at the centromere
 The single centrosome replicates, forming
two centrosomes
BioFlix: Meiosis
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Homologous Chromosomes in Prophase I
 During Prophase I the homologous
chromosomes are attracted to each other and
become associated with each other forming a
tetrad.
 The process of homologous chromosomes
pairing up during prophase I is called
synapsis.
 A tetrad contains two chromosomes, both are
duplicated so there are four chromatids.
Crossing Over
 Prophase I: Duplicated homologous
chromosomes condense and intertwine – this
produces genetic variation
 Crossing over: genetic material is exchanged
between the homologous chromosomes
 The sites of crossing over are called
chiasmata (singular, chiasma)
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Crossing Over
Prophase I
37
Metaphase I
38
Independent Assortment
 During Metaphase I homologous pairs of
chromosomes line up the at the center of the
cell (the equator)
 The tetrads arrange themselves randomly – this
also gives genetic variation = independent
assortment (alignment)
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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
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Independent Assortment
Anaphase I
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Telophase I
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
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End of Telophase I - Cytokinesis
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
 We now have two haploid cells (1n) which
means there are 23 chromosomes total in each
cell
 The chromosomes are still in the duplicated
form – two chromatids
 Note: not all species have cytokinesis after
telophase I
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Interphase
Meiosis II
 Interphase between Meiosis I and II is brief.
 The S phase does not take place
 Preparation for Meiosis II: centrosome
replicates
 Prophase II: The 23 chromosomes are already
condensed. The Nuclear membrane dissolves.
 Metaphase II: Chromosomes line up at the
equator
 Anaphase II: Chromatids separate
 Telophase II and cytokinesis: Cells separate
 Now there are four haploid cells: each has 23
chromosomes (not in the duplicated state)
Figure 13.8b
Prophase II
Metaphase II
Prophase II
Anaphase II
Telophase II and
Cytokinesis
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Haploid daughter
cells forming
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Metaphase II
Anaphase II
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Figure 13.8
Telophase II
MEIOSIS I: Separates sister chromatids
MEIOSIS I: Separates homologous chromosomes
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Fragments
of nuclear
envelope
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Metaphase II
Prophase II
Telophase II and
Cytokinesis
Anaphase II
Sister chromatids
remain attached
Centromere
(with kinetochore)
Spindle
Homologous
chromosomes
Telophase I and
Cytokinesis
Anaphase I
Metaphase
plate
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Chromosomes line up
by homologous pairs.
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Cleavage
furrow
Each pair of homologous
chromosomes separates.
Sister chromatids
separate
Haploid daughter
cells forming
Two haploid cells
form; each chromosome
still consists of two
sister chromatids.
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Figure 13.8a
1.
2.
3.
4.
Metaphase
plate
At the end of Meiosis I are these cells haploid or diploid?
50%
50%
At the end of Meiosis I, how many chromosomes
are there in each cell?
50%
1. 23 chromosomes
2. 46 chromosomes
50%
ch
ro
m
23
oi
d
ip
l
D
H
ap
lo
id
os
o
m
es
1. Haploid
2. Diploid
ur
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
Fo
Chromosomes line up
by homologous pairs.
25%
m
es
Each pair of homologous
chromosomes separates.
25%
re
e
Microtubule
attached to
kinetochore
Cleavage
furrow
O
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Homologous
chromosomes
separate
25%
Th
Fragments
of nuclear
envelope
25%
ne
Homologous
chromosomes
One
Two
Three
Four
os
o
Spindle
Sister chromatids
remain attached
Centromere
(with kinetochore)
ch
ro
m
Chiasmata
At the end of Meiosis I how many cells are there?
46
Centrosome
(with centriole pair)
Sister
chromatids
Telophase I and
Cytokinesis
Anaphase I
o
Metaphase I
Tw
Prophase I
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ur
At the end of Meiosis II, how many
chromosomes are there in each cell?
50%
1. 23 chromosomes
2. 46 chromosomes
50%
os
o
ch
ro
m
46
23
ip
l
oi
d
ch
ro
m
os
o
m
es
50%
D
lo
id
ap
H
25%
Fo
O
25%
re
e
o
25%
m
es
50%
25%
Th
One
Two
Three
Four
N
Ye
At the end of Meiosis II are these cells
haploid or diploid?
1. Haploid
2. Diploid
1.
2.
3.
4.
o
s
50%
Tw
50%
1. Yes
2. No
At the end of Meiosis II how many cells are
there?
ne
At the end of Meiosis I, are the chomosomes
in the duplicated state?
At the end of Meiosis II, are the
chromosomes in the duplicated state?
50%
 Three events are unique to meiosis, and all
three occur in meiosis l
o
N
s
 Synapsis and crossing over in prophase I:
Homologous chromosomes physically
connect and exchange genetic information
 At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead
of individual replicated chromosomes
 At anaphase I, it is homologous
chromosomes, instead of sister chromatids,
that separate
Ye
1. Yes
2. No
50%
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Genetic diversity through meiosis
Independent Assortment
 There are three places in this process that
contribute to the genetic diversity of the
offspring.
 Prophase I: The pairs of chromosomes
crossing over.
 Metaphase I: The way the chromosomes
line up on the equator is random =
independent assortment
 Random fertilization
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Figure 13.10-1
Figure 13.10-2
Possibility 2
Possibility 1
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Figure 13.10-3
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
 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 8
million (223) possible combinations of
chromosomes
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
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Figure 13.11-1
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Figure 13.11-2
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Figure 13.11-3
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Figure 13.11-4
Prophase I
of meiosis
Pair of homologs
Chiasma
Nonsister chromatids
held together
during synapsis
Chiasma
Centromere
Centromere
TEM
TEM
Anaphase I
Anaphase I
Anaphase II
Figure 13.11a
Random Fertilization
Chiasma
Centromere
TEM
 Random fertilization adds to genetic variation
because any sperm can fuse with any ovum
(unfertilized 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
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Meoisis and Gender
 The gametes now contain 23 chromosomes,
haploid, and are not in the duplicated form
 One of these chromosomes will be a sex
chromosome
 Eggs will contain a X chromosome
 Sperms will contain either a X or a Y
chromosome
 X and Y are non-homologous chromosomes
Animation: Genetic Variation Rightclick slide / select “Play”
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Spermatogenesis
 In the male testes sperm are produced. One
cell produces 4 sperm.
 Each sperm has 23 chromosomes, they are
not in the duplicated form
 The sperm can have either an X or a Y sex
chromosome
 The sperm have a small head and a long tail
= flagellum for locomotion
 The sperm need to contain the genetic
material and deliver it to the egg.
 The heads contain the chromosomes and
lots of mitochondria to power the flagella
 About 400 million sperm are produced
each day
Egg Formation cont
 All of the cells that produce the eggs are
made before the female mother is even born.
 So when a girl is born, her ovaries contain all
the cells that produce her eggs
 Each month one of these cells will leave the
ovary and go on to mature – and produce the
egg and polar bodies
Egg Formation
 The ovaries in females produce eggs
 One cell will produce one egg and three
non-functioning “polar bodies”
 The one egg gets most of the cytoplasm,
leaving the other three cell not able to
survive
 The one egg has 23 chromosomes, with a X
sex chromosome
 The one egg is large enough to support the
embryo
Fertilization
 Fertilization is the union between the
sperm and the egg. Results in a diploid
zygote.
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Reproduction in Plants
 Mosses and Ferns – use spores and egg
and sperm, have alternation of generation
 Angiosperms – flowing plants use sperm
and egg
Reproduction using spores
Fig. 11.2c
 Gametophytes produce gametes (eggs
and sperm)
 Sporophyte:
 Produce spores
 Are dependent on the gametophytes
(they grow out of the gametophytes)
Sexual Reproduction in Angiosperms
 Sperm – in pollen produced in anther of
stamen
 Egg – in ovary of carpel
 Both these cells are haploid
 Fruit – mature ovary
 Pollination – transfer of pollen to stigma of
female carpel
 Fertilization – when the pollen grain fuses with
the egg – producing the diploid zygote
 Embryo develops inside seed
 Seed germinates into plant
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Review of Mitosis vs Meiosis
Mitosis
 Mitosis and Meiosis both start with a diploid
cell (46 chromosomes, 23 pairs)
 Before both Mitosis and Meiosis the DNA
replicates during interphase, forming
duplicated chromosomes, each containing
two chromatids
 Mitosis occurs in somatic cells (cells other
than those that produce the gametes),
Meiosis produces gametes
 During Mitosis:
 The chromatids are separated to
produce two cells, each with 46
chromosomes, 23 pairs of non
duplicated chromosomes
 These cells are diploid (2n) cells
 There is no exchanging of genetic
material
Meiosis – Two stages
 Meiosis I: the pairs of chromosomes line up
and the chromosomes are separated,
resulting in 2 cells, each with 23
chromosomes, in the duplicated state =
haploid cells
 Meiosis II: The chromatids are separated
producing two haploid cells that contain 23
non duplicated chromosomes.
 One original cell produces four haploid
cells
Important concepts
 Know all the vocabulary presented in the
lecture
 Know which cells undergo mitosis vs meiosis
 How is genetic diversity introduced into
meiosis? What events contribute to genetic
diversity and when (what stage of meiosis) do
these events take place
 How is the gender of the offspring determined.
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Important Concepts
 How many functioning sperm are produced
from one spermatocyte. What sex
chromosomes can a sperm have? How many
functioning eggs are produced from one
oocyte? What sex chromosomes do eggs
have?
 Know what pollen and eggs are in plants,
know the reproductive parts of plants, know
what pollination and fertilization are in plants,
what is the seed
Important Concepts for Lab Exam
 For Meiosis: Know each stage, the order of
the stages, and what happens in each stage.
 Know what the end result is of meiosis I and II
 Know what state the cell and the
chromosomes are in at the beginning and end
of mitosis, meiosis I and at the end of meiosis
II. For example: Are the cells haploid or
diploid? Are the chromosomes duplicated, or
not duplicated? How many chromosomes are
there in the cell? Are they in pairs?
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