Meiosis and Sexual Reproduction

1
MEIOSIS
&
Sexual Reproduction
Unit 6
GSE: SB1b
b. Develop and use models to explain the
role of cellular reproduction (including
binary fission, mitosis, and meiosis) in
maintaining genetic continuity.

Learning Targets
1. I can differentiate between diploid and haploid
cells.
2. I can explain the process of sexual reproduction
at cellular level.
3. I can identify and describe the changes a cell
undergoes during Meiosis.
4. I can differentiate between Mitosis and Meiosis.
5. I can determine types of errors in Meiosis and
how they lead to variations.
6. I can compare and contrast sexual and asexual
reproduction.
2

3
Haploid and Diploid Cells
Sexual Reproduction and Genetics
 Body cells in almost all animals have 2 sets of
chromosomes. One shared by father and the other
shared by mother.
 A cell that contains 2n (2 sets of) chromosomes is
called a diploid cell.
•For sexual reproduction organisms make specialized
cells, called gametes.
Gametes are of two types:
Male gametes, called sperms OR Pollens. (n chromosomes)
Female gametes, called ovum OR egg. (n chromosomes)
 All gametes have only one set of chromosomes. Cells
with n (one set of) chromosomes are called a haploid
cell. All Gametes are haploid.
LT-1: I can differentiate between diploid and haploid cells.

4
• Fusion of male and female gametes is called
fertilization.
• The cell resulted after fertilization is called Zygote.
• Fertilization of male and female gametes restores
the original # of 2n chromosomes.
Haploid (n) and Diploid (2n) Cells
 An organism produces gametes to maintain
the same number of chromosomes in their
body cells from generation to generation.
 Gametes are formed by a process of cell
division called Meiosis.
 In Meiosis a diploid body cell divides to make
4 haploid gametes.

5
 Human body cells have 46 chromosomes
 Therefore, each human gamete has half
the original number, that is 23
chromosomes.
 —one of the two paired chromosomes, one
from each parents is called Homologous
chromosomes. They carry genes of
same trait at the same location.
Example:
 When fertilization takes place, the 2
gametes, each having 23 chromosomes
join together. This restores the # of
chromosomes to 46.

6
These paired human chromosomes separate during
Meiosis. Each gamete will have only one set (23) of
chromosomes.

7
Homologous Chromosomes
 Have same length
 Have same centromere position
 Carry genes that control
the same inherited traits
 Examples of inherited
traits: Hair color, eye color,
skin color, height etc.
 One is shared by father and the other is
shared by mother.

8
Sexual Reproduction
 The sexual life cycle
in animals involves
meiosis.
 Meiosis produces
gametes.
 When gametes
combine in fertilization, the number of
chromosomes is restored.
LT 2:I can explain the process of sexual reproduction at cellular level.

9
23 +23 23 +23
MEIOSIS MEIOSIS
23 23 23 23
23+23
FERTILIZATION
Gametes
Haploid Cell, (n)
One set of
chromosomes
Diploid Cell, (2n)
Two set of
chromosomes
Diploid Cell, (2n)
Two set of
chromosomes
Body Cell
Human Male Human Female
A human body cell
has a total of 46
chromosomes.
Which is actually 2
sets of 23 each.
Each set is
contributed by a
parent

10
___ +___ ___ +___
MEIOSIS MEIOSIS
___ ___ ___ ___
___+___
FERTILIZATION
Gametes
Haploid Cell, (n)
One set of
chromosomes
Diploid Cell, (2n)
Two set of
chromosomes
Diploid Cell, (2n)
Two set of
chromosomes
Body Cell
Father Mother
A dog’s body cell
has a total of 78
chromosomes.
Now complete this
flow chart.

11
___ +___ ___ +___
MEIOSIS MEIOSIS
___ ___ ___ ___
___+___
FERTILIZATION
Gametes
Haploid Cell, (n)
One set of
chromosomes
Diploid Cell, (2n)
Two set of
chromosomes
Diploid Cell, (2n)
Two set of
chromosomes
Body Cell
Father Mother
A cat’s body cell
has a total of 38
chromosomes.
Now complete this
flow chart.

12
MEIOSIS
Cell division that takes place
in specialized cells to produce
gametes (the sex cells)
The end product of Meiosis is
4 daughter cells not identical
to parent cells
[Remember:
The end product of Mitosis is 2 identical daughter cells]
The end product of Meiosis is 4 un-identical daughter
cells]
LT 3: I can identify and describe the changes a cell undergoes during Meiosis.

13
Phases of Meiosis
Meiosis consists of 8 phases
Reduces the Chromosome # by half.
Prophase – I
Metaphase – I
Anaphase – I
Telophase – I
Prophase – II
Metaphase – II
Anaphase – II
Telophase – II
Meiosis - I Meiosis - II
Cytokinesis – 1 may
or may not occur.
Cytokinesis results
in 4 un- identical
daughter cells.

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Meiosis I
10.1 Meiosis
 Interphase
 DNA replicates.
 Chromatin condenses.
Chapter
10
Interphase

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Meiosis I
 Prophase I
 Pairing of homologous
chromosomes occurs.
 Each individual chromosome consists of 2
chromatids, (therefore called a diad.)
 After pairing the bunch has 4 chromatids,
(therefore called a tetrad.)
 The nuclear envelope breaks down.
 Spindles form.
Chapter
10
Prophase I

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Tetrad:
Consists of 2
duplicated
chromosomes
or 4 chromatids.
Diad:
Consists of 1
duplicated
chromosome or
2 chromatids.
Single
Chromatid or
Chromosome
A pair of
homologous
Chromosomes

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Meiosis I
 Prophase I
 Crossing over produces exchange of genetic
information.
 After pairing, the homologous chromosomes
exchange a segment of their chromatids. This is
called Crossing over. (or genetic recombination)
 This happens between non sister chromatids

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Sister & Non-sister chromatids
Sister chromatids Sister chromatids
Non – sister chromatids (Homologous)

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Meiosis I
10.1 Meiosis
 Metaphase I
 Chromosome
centromeres attach
to spindle fibers.
 Homologous chromosomes (Tetrad) line
up at the equator.
Chapter
10
Metaphase I

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Meiosis I
10.1 Meiosis
 Anaphase I
 Centromere doesn’t
split.
Chapter
10
Anaphase I
Homologous chromosomes
(Tetrad) separate and move
to opposite poles of the cell as
Diads
This reduces the # of chromosomes
from 2n to n. Therefore this phase
is also called reduction division.

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Meiosis I
10.1 Meiosis
 Telophase I
 The spindle
fibers break
down.
 Chromosomes uncoil and
form two nuclei. (Diad)
 The cell divides.
Chapter
10
Telophase I
 Note: Cytokinesis may not occur after
Meiosis -1 .

22
Meiosis II
 Prophase II
10.1 Meiosis
Chapter
10
A second set of phases begins as the spindle
apparatus forms and the chromosomes
condense. Nucleus begins to disappear.
Prophase II

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Meiosis II
 Metaphase II
10.1 Meiosis
Chapter
10
A haploid number
of chromosomes
(Diads) line up at
the equator.
Metaphase II

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Meiosis II
10.1 Meiosis
 Anaphase II
 Centromere splits.
Chapter
10
Anaphase II
 The sister
chromatids are
pulled apart at the centromere by spindle
fibers and move toward the opposite poles
of the cell.

25
10.1 Meiosis
Meiosis II
Chapter
10
 Telophase II
 The chromosomes
reach the poles, and
the nuclear membrane and
nuclei reform.
4 un-identical nuclei are formed. Telophase II

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Cytokinesis
Cytokinesis results
in four haploid
cells, each with n
number of
chromosomes.
10.1 Meiosis
Chapter
10
Cytokinesis

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Interphase
Prophase-1 Prophase-2
Metaphase-1 Metaphase-2
Anaphase-1 Anaphase-2
Telophase-1 Telophase-2
Cytokinesis
1. Come get the Meiosis Picture
cards from me.
Arrange the picture cards in the
Following way (picture down)
2. Flip the cards to see the
pictures. Compare and
remember the differences.
3. Mix the cards. Arrange like the
table but this time pictures up.
4. Now flip the cards to check if
you were right.
5. Repeat step 3 and 4 until you
get it right several times.
6. Now mix the cards.
7. Pick cards randomly and see if
your partner can identify the
phase correctly.
8. Repeat # 7 several times.
ACTIVITY-1
Rearrange
Have you successful completed activity-
1at least a couple of times?
Return your cards and come see me for
“Phases of Meiosis” and Meiosis Table.
(Lab Grade)

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Formal Assessment
Have you successfully completed activity-1
at least a couple of times?
Return your cards and come see me for
“Phases of Meiosis” and Meiosis Table.
(Lab Grade)

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The Importance of Meiosis
 Meiosis consists of two sets of divisions
 Produces four haploid daughter cells that
are not identical
10.1 Meiosis
 Results in genetic variation
Chapter
10

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Meiosis Provides Variation
 Depending on how the
chromosomes line up at the
equator, four gametes with
four different combinations
of chromosomes can result.
 Genetic variation also is
produced during crossing
over and during
fertilization, when gametes
randomly combine.
10.1 Meiosis
Parent Cell

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Meiosis in Human beings
Meiosis in males (Called Spermatogenesis)
Making sperm cells by means of meiosis
Diploid cell with
2n chromosomes
From Dad
From mom
23
23
Meiosis – I
23 23
Haploid cell
1n chromosomes
Haploid cell
1n chromosomes
Meiosis – II
23 23 23 23
4 Haploid
daughter
cells
The 4 haploid daughter cells then develop a flagellum called
tail and are able to move freely. These are called sperms.
Spermatogenesis is a lifelong process.

32
Meiosis in Human beings
Meiosis in females (Called Oogenesis)
Making egg cells or ova by means of meiosis.
Diploid cell with
2n chromosomes
From Dad
From mom
23
23
Meiosis – I
23 23
Haploid cell
1n chromosomes
Haploid cell
1n chromosomes
Meiosis – II
23 23 23 23
4 Haploid
daughter
cells
3 of the 4 haploid daughter cells disintegrate, only one
develops in to an egg cell. The disintegrated cells are called
Bar bodies.
Oogenesis takes place once a month.

33
Human chromosomes
• Humans have 23 chromosomes.
• 22 of these chromosomes determine the various
traits. These are called autosomes.
• Any disorders caused by problems in these
chromosomes are called autosomal disorders.
• One of the chromosomes either x or y, are called
“sex determining chromosomes.”
• Human males have one x and one y
chromosomes.
• Human females have 2 x chromosomes.
• Any disorders caused by problems in x or y
chromosomes are called “sex linked disorders”.

34
These paired human chromosomes separate during
Meiosis. Each gamete will have only one set (23) of
chromosomes.

35
Differences between Mitosis and
Meiosis
• MITOSIS
1. Consist of 4 phases
2. Two identical daughter
cells are produced.
3. Daughter cells are
diploid.
4. Takes place in somatic
(body) cells.
5. No new genetic
combination is
produced.
• MEIOSIS
1. Consist of 8 phases
2. 4 un-identical daughter
cells are produced.
3. Daughter cells are
haploid.
4. Takes place in sex or
reproductive cells.
5. New genetic
combinations are
produced.
LT: 4 I can differentiate between Mitosis and Meiosis.

36
Mistakes in
Meiosis
1. Non Disjunction
(a) Trisomy
(b) Monosomy
2. Polyploidy
3. Chromosomal mutations
(1) Non-Disjunction
• One of the homologous pairs of chromosomes fails
to separate during Anaphase – 1
• As a result, two of the daughter cell has one extra
chromosome (called trisomy).
• And the other two have one less chromosome
(called monosomy)
LT5: I can determine types of errors in Meiosis
and how they lead to variations.

37
Monosomy
Trisomy
Mistakes in Meiosis
“Non-Disjunction”

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Examples of Trisomy in Humans
• Trisomy 21 (Down syndrome) Most common
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome)
• Trisomy 9
• Trisomy 8 (Warkany syndrome 2)
• Trisomy 22

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There are three # 21
Chromosome.
A normal human has two #21
Chromosomes
Down syndrome – Trisomy 21

40
Monosomy in Humans VIDEO
• Turner syndrome - Women with Turner
syndrome typically have one X chromosome instead
of the usual two sex chromosomes. Turner syndrome
is the only full monosomy that is seen in humans—all
other cases of full monosomy are lethal and the
individual will not survive development.
• Cri du chat syndrome -- (French for "cry of the cat"
after the distinctive noise by affected persons'
malformed larynx) a partial monosomy caused by a
deletion of the end of the short p (from the word petit,
French for small) arm of chromosome 5
• 1p36 Deletion Syndrome -- a partial monosomy
caused by a deletion at the end of the short p arm
of chromosome 1

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(2) Polyploidy
Polyploidy is the occurrence of one or more extra
sets of all chromosomes in an organism.
 A triploid
organism, for
instance, would be
designated 3n,
which means that
it has three complete
sets of
chromosomes.
Organisms with polyplidy are bigger in size.
Natural polyploidy is observed in some plants.

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(3) Chromosomal Mutations
1.Deletion - A large section of a chromosome can be
deleted resulting in the loss of a number of genes.
2. Insertion or Duplication - In this mutation, some genes are
duplicated and displayed twice on the same chromosome.

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Inversion - A region of DNA on the chromosome can
flip its orientation with respect to the rest of the
chromosome.
Translocation - Translocations are the transfer of a piece of one
chromosome to a non-homologous chromosome. They are often
reciprocal, with the two chromosomes swapping segments with
each other.
Chromosomal Mutations
LT5: I can determine types of errors in Meiosis and how they lead to
variations.

44
Translocation in Humans
Philadelphia chromosome
•In most cases of chronic myelogenous leukemia
(CML), blood cancer, the leukemia cells share a
chromosomal abnormality known as Philadelphia
chromosome.
•This abnormality is the result of a reciprocal
translocation between chromosomes 9 and 22.
•An abnormal hybrid gene is created leading to the
production of a novel protein that is not normally
found in the cell.
•This protein prevents normal growth and
development, leading to leukemia.
LT5: I can determine types of errors in Meiosis and how
they lead to variations.

Now complete the following
family chart using slide #40-44
45

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 Needs only Mitosis, no Meiosis cell division.
 does not require another partner
 is quicker than sexual reproduction
 the resulting organism is genetically identical to
the parent organism.
 Organisms that reproduce asexually can produce
many identical offspring in a short period of time.
 Asexual reproduction is an advantage in a stable
environment where the parental genotype is well-
suited.
 Many colonizers of new environments reproduce
asexually.
Asexual reproduction
LT 6: I can compare and contrast sexual and asexual
reproduction.

47
 Involves much more time than asexual reproduction.
 Gametes must be formed through the process of meiosis
 Mating must occur between two organisms of different
sexes.
 There is also time involved in the growth and development of
the offspring.
 The advantage of sexual reproduction is the genetic
variability that results from meiosis.
 Genetic recombination allows offspring greater diversity
 And increases the likelihood that some offspring will have
more advantageous traits than the parents.
 Sexual reproduction is an advantage in a rapidly changing
environment
 Because the diversity of the population increases the
possibility that some organisms will both survive and
reproduce.
Sexual reproduction

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Sexual Reproduction
1.Chromosomes from
both parents.
2.Involves Meiosis
3.Offspring are
genetically un-
identical
4.New varieties result.
5.Varieties will be
stronger.
Asexual Reproduction
1. Chromosomes from
single parent.
2. Involves mitosis only
3. Offspring are
genetically identical
4. No new varieties,
Clones result.
5. Varieties may not be
stronger.
Sexual Reproduction v. Asexual Reproduction
LT 6: I can compare and contrast sexual and asexual
reproduction.

Meiosis and Sexual Reproduction

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