Sexual Cell
Reproduction
“Chapter 17”
1
The Importance of
Meiosis
• Meiosis – is a two stage cell division in which the
chromosome number of the parental cell is reduced
by half.
• Meiosis is the process by which sex cells are formed.
• Sex cells are often called gametes (haploid
chromosome number)
• Ex: human cell that has 46 chromosomes (Diploid),
that undergoes meiosis and produce sex cells
(gametes) that have 23 chromosomes (haploid).
• The union of 23 sperm chromosomes and 23
chromosomes from an egg cell will produce a 46
chromosome fertilized egg or zygote.
2
• Organisms that reproduce sexually, show a
greater range of characteristics within a
species.
• Sexual ensures recombination of genes.
(father’s eyes, mother’s hair)
• Paired chromosomes 23 from each parent are
called homologous chromosomes. (similar in
shape, size and gene arrangement)
3
Homologous
Chromosomes
4
Stages of Meiosis
• Meiosis involves two cell divisions
that produce four haploid cells.
• First division (meiosis I) the
homologous chromosomes move to
opposite poles.
• During this division diploid cells
separate into two haploid cell.
• The second division (meiosis II) is
marked by a separation of the two
chromatids.
5
MEIOSIS
Production of Gametes
6
Meiosis
Facts
7
Meiosis
• The form of cell division by
which gametes, with half the
number of chromosomes, are
produced
• Diploid (2n) → haploid (n)
• Meiosis is sexual reproduction
• Two divisions (meiosis I and
meiosis II)
II
8
Meiosis
• Sex cells divide to
produce gametes (sperm
or egg)
• Gametes have half the
number of chromosomes
• Meiosis is similar to
mitosis with some
chromosomal differences
9
Meiosis
• Occurs only in gonads
(testes or ovaries)
• Male: Spermatogenesis
• Female: Oogenesis
10
Spermatogenesis
n=23
Human
sex cell
Sperm
n=23
n=23
2n=46
n=23
Diploid (2n)
Haploid
(n)
n=23
n=23
Meiosis I
Meiosis II
11
Interphase I
• Similar to mitosis interphase
• Chromosomes replicate (S
phase)
• Each duplicated chromosome
consist of two identical sister
chromatids attached at their
centromeres
• Centriole pairs also replicate.
12
Interphase I
• Nucleus and nucleolus visible.
Chromatin
Nuclear
membrane
Cell membrane
Nucleolus
13
Meiosis I
14
Stages of Meiosis I
• Cell division that reduces
the chromosome number
by one-half
• Four phases:
phases
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I
15
Prophase I
• Longest and most complex
phase (90%)
• Chromosomes condense
• Synapsis occurs: homologous
chromosomes come together
to form a tetrad
• Tetrad is two chromosomes or
four chromatids (sister and
nonsister chromatids)
16
Prophase I - Synapsis
Homologous chromosomes
Sister chromatids
Tetrad
Sister chromatids
17
Homologous
Chromosomes
• Pair of chromosomes (maternal
and paternal)
paternal that are similar in
shape and size
• Homologous pairs (tetrads)
carry genes controlling the
same inherited traits
• Each locus (position of a gene)
is in the same position on
homologues
18
Homologous
Chromosomes
• Humans have 23 pairs of
homologous chromosomes
• First 22 pairs of
chromosomes called
autosomes
• Last pair called sex
chromosomes
• XX female or XY male
19
Homologous
Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
20
Crossing Over
• Crossing over (variation) may
occur between nonsister
chromatids at the chiasmata
• Crossing over:
over segments of
nonsister chromatids break and
reattach to the other chromatid
• Chiasmata (chiasma) are the
sites of crossing over
21
Crossing Over - Variation
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
22
variation
Sex Chromosomes
XX chromosome - female
XY chromosome - male
23
Prophase I
Spindle fiber
Centrioles
Aster
fibers
24
“Prophase I Review”
• Prophase I – nuclear membrane dissolves,
centrioles move to opposite poles, spindle fibers
established. Chromosomes come together in pairs
called tetrads.
• Synapsis – the pairing of homologous
chromosomes.
• Crossing over – the exchange of genetic material
between to homologous chromosomes.
• Often as tetrads come closer together the
chromatids intertwine. The pairing of these
homologous chromosomes is called synapsis and
they sometimes exchange genetic material.
25
Crossing Over
26
Metaphase I
• Shortest phase
• Tetrads align on the
metaphase plate
• Independent assortment
occurs
• Chromosomes separate
randomly to the poles of the
cells
27
Metaphase I
• Independent assortment causes
variation in the forming cells
• Orientation of homologous pair to
poles is random
• Formula for determining variation:
Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
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Metaphase I
OR
Metaphase plate
Metaphase plate
29
Question:
• In terms of Independent
Assortment, how many
different combinations of
sperm could a human male
produce?
30
Answer
•
•
Formula: 2n
Human chromosomes:
2n = 46
n = 23
223 = ~8 million combinations
31
“Metaphase I Review”
• Metaphase I – homologous chromosomes
attach themselves to spindle fibers and
line up along the equatorial plate.
32
Anaphase I
• Homologous chromosomes
separate and move
towards the poles
• Sister chromatids remain
attached at their
centromeres
33
Anaphase I
34
“Anaphase I Review”
• Anaphase I – homologous chromosomes
move toward opposite poles. This process
is known as segregation.
• At this point one member of each
homologous pair will be found in each of
the new cells. The diploid mother cell
becomes two haploid daughter cells.
• Each chromosome remains double
stranded.
35
Telophase I
• Each pole now has haploid
set of chromosomes
• 1n = 23 (human)
• Cytokinesis occurs and two
haploid daughter cells are
formed
36
Telophase I
37
“Telphase I Review”
• Telophase I – the cytoplasm
divides forming two cells.
Nuclear membrane begins to
form around the chromosomes
within each of the two cells.
38
Meiosis I
39
40
Meiosis II
41
Meiosis II
•
•
Short or No interphase II
DNA NOT replicated
again
• Remember: Meiosis II is
similar to mitosis
• Prophase I, Metaphase
II, Anaphase II, and
Telophase II
42
Prophase II
• Same as prophase in mitosis
• Nuclear envelope breaks down and
the spindle apparatus forms
43
Metaphase II
• Same as metaphase in mitosis
• Chromatids lined up at equator
Metaphase plate
Metaphase plate
44
Anaphase II
• Same as anaphase in mitosis
• Sister chromatids separate
45
Telophase II
• Same as telophase in
mitosis
• Nuclei reform
• Cytokinesis occurs
• Remember: Four haploid
daughter cells produced
• Gametes = sperm or egg
46
Telophase II
47
Meiosis
sex cell
n=2
Sperm
n=2
n=2
2n=4
Haploid (n)
n=2
diploid (2n)
n=2
n=2
Meiosis I
Meiosis II
48
49
“Meiosis II Review”
• Occurs at approx. same time in each of
the haploid daughter cells.
• Pairs of chromatids separate and move to
opposite poles.
• Prophase II – nuclear membrane disolve
and spindle fibers begin to form.
• Metaphase II – signalled by the
arrangement of the chromosomes, each
with two chromatids, along equatorial
plate.
50
• Anaphase II – movement of
sister chromatids to opposite
poles. Nuclear membrane
begind to form around the
chromosomes.
• Telophase II – second division
of cytoplasm, four daughter
51
cells are produced.
Meiosis II
52
Genetic
Variation
53
Variation
• Important to the survival
of populations
• Aids in natural selection
• Strongest individuals are
able to survive and
reproduce
54
Question
• What are the three
sources of genetic
variation in sexual
reproduction?
55
Answer:
1. Crossing Over
(Prophase
I)
2. Independent Assortment
(Metaphase I)
3. Random Fertilization
(sperm joins with egg)
56
Remember:
Variation is helpful
to the survival of a
species!
57
Question:
• A diploid cell containing 20
chromosomes (2n = 20) at
the beginning of meiosis
would, at its completion,
produce cells containing
how many chromosomes?
chromosomes
58
Answer:
• 10 chromosomes
(haploid)
• 1n = 10
59
Karyotype
• A method of organizing the chromosomes of a
cell in relation to number, size, and type.
60
61
Fertilization
• The fusion of a sperm and egg to form
a zygote.
zygote
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
62
Question:
• A cell containing 40
chromatids at the beginning
of meiosis would, at its
completion, produce cells
containing how many
chromosomes?
chromosomes
63
Answer:
• Four cells with 10
chromosomes each
64
Comparing Mitosis and
Meiosis
• Mitosis – Asexual reproduction.
Results in two daughter cells that
are identical to each other. Same
genetic info and number of
chromosomes as parent.
65
• Meiosis – Sexual reproduction.
Results in four daughter cells
that are different from each
other and from the parent cell.
Carry only half the number of
chromosomes from the parent.
66
Comparing Mitosis and
Meiosis
67
Comparing Mitosis and
Meiosis
68
Comparing Mitosis and
Meiosis
69
Comparing Mitosis and Meiosis
70
Development of Male and
Female Gametes
• Gametogenesis – the formation of
gametes (sex cells) in animals.
• Cytoplasm of female gametes do not
divide equally after each nuclear division.
As shown in figure 9, one of the daughter
cells called the ootid receives most of
the cytoplasm. The other cells the polar
bodies die.
71
• Only one ovum (egg) is
produced from meiosis.
• In contrast with sperm there
is an equal division of the
cytoplasm.
72
Development of Male and
Female Gametes
73
• Sex chromosomes – are pairs
of chromosomes that
determine the sex of an
individual.
• Autosomes – are chromosomes
not involved with sex
determination.
74
Sex Chromosomes
75
Summary
76
Homework
• Review Questions
• Page 453 Questions 1 - 9
77
Activity
78
Abnormal Meiosis
• Nondisjunction- failure of a pair of
homologous chromosomes to separate
properly during meiosis.
• Polyploidy – a condition in which an
organism has more than two complete
sets of chromosomes.
• Trisomy – there are three homologous
chromosomes in place of a pair of
homologous.
• Monosomy – there is a single chromosome
79
in place of a homologous pair.
Nondisjunction
Disorders
• Down Syndrome – a trisomic
condition, three copies of
chromosome 21.
• Older women having babies are at
higher risk.
• Traits include: round full face,
enlarged and creased tongue, short
height, large forehead. Associated
80
with mental retardation
Down Syndrome
81
Karyotype of Normal Male
vs. Down Syndrome
Female
82
Nondisjunction
Disorders
83
Turner Syndrome
• Monosmis disorder produces a
female with a single X
chromosome. In the egg both
X chromosomes move to the
same pole.
• Individuals appear female, but
do not developed sexually and
tend to be short and have
84
thick necks.
Klinefelter Syndrome
• Child inherits two X
chromosomes and asingle Y.
Child appears to be male at
birth, however as he enters
maturity, he begins producing
high levels of female
hormones.
• Males are sterile.
85
Karotype Charts
• Detects abnormal meiosis.
• Mix a small sample of tissue with a solution that simulates
mitotic division. A different solution is added to stop the
division at metaphase.
• Since chromosomes are in their most condensed form, their
size, length, and centromere location is best to see.
• The metaphase cells are placed on a slide and then stained.
• A photograph is taken, the image enlarged and each chromosome
is cut out and paired up with its homologue.
• They are all aligned in decreasing order. Sex chromosomes
always placed last.
86
Activity
87
Summary
88
Homework
• Review Questions
• Page 462 Questions 10-17
89
Lab
90
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