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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
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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Figure 13.1
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
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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
A clone is a group of genetically identical
individuals from the same parent
In sexual reproduction, two parents give rise
to offspring that have unique combinations of
genes inherited from the two parents
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Figure 13.2
(a) Hydra (b) Redwoods
Bud
Parent
0.5 mm
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
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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
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: are the cells that are responsible for
reproduction
All the rest of the body’s cells are somatic cells
Gametes = reproductive cells
Sperm and eggs are reproductive cells –
gametes
The cells that divide to produce gametes
undergo meiosis
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
How do gametes overcome this problem?
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
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Figure 13.3
Pair of homologous duplicated chromosomes
Centromere
Sister chromatids
Metaphase chromosome
5 m
APPLICATION
TECHNIQUE
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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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Sex 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
For humans, the diploid number is 46 (2n = 46)
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Sets of Chromosomes in Human Cells 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
Meiosis is when a diploid cell divides to produce haploid reproductive cells
Meiosis
First the chromosomes (DNA) are
duplicated during Interphase
Then there are two cell divisions
Remember that mitosis had chromosome
(DNA) duplication followed by one cell
division
DNA Replication
In a cell in which DNA synthesis has
occurred, each chromosome is replicated
Each replicated chromosome consists of
two identical sister chromatids
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Remember that there are pairs
chromosomes, each chromosome has two
chromatids just after DNA replication
Figure 13.4
Sister chromatids of one duplicated chromosome
Key
Maternal set of chromosomes (n 3)
Paternal set of chromosomes (n 3)
Key
2n 6
Centromere
Two nonsister chromatids in a homologous pair
Pair of homologous chromosomes (one from each set)
Meiosis
The DNA has already replicated during interphase – the chromosomes have become duplicated
In Meiosis the chromosome homologous pairs separate and the cell divides = 1st cell division
Then the chromatids separate and cell divide = 2cd cell division
The figures are going to show only one pair of chromosomes – but there are 23 pairs at the start
Figure 13.7-1
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Interphase
Figure 13.7-2
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Homologous chromosomes separate
Haploid cells with duplicated chromosomes
Meiosis I
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Interphase Figure 13.7-3
Pair of homologous chromosomes in diploid parent cell
Duplicated pair of homologous chromosomes
Chromosomes duplicate
Sister chromatids
Diploid cell with duplicated chromosomes
Homologous chromosomes separate
Haploid cells with duplicated chromosomes
Sister chromatids separate
Haploid cells with unduplicated chromosomes
Interphase
Meiosis I
Meiosis II
2
1
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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
Meiosis Overview
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)
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BioFlix: Meiosis
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
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
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Prophase I
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Metaphase I
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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)
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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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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Telophase I
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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
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End of Telophase I - Cytokinesis
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
Interphase between Meiosis I and II is brief.
The S phase does not take place
Preparation for Meiosis II: centrosome
replicates
Meiosis II
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 Anaphase II Telophase II and
Cytokinesis
Sister chromatids separate
Haploid daughter cells forming
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Prophase II
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Metaphase II
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Anaphase II
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Telophase II
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Figure 13.8
MEIOSIS I: Separates homologous chromosomes
Prophase I Metaphase I Anaphase I Telophase I and
Cytokinesis
Centrosome (with centriole pair)
Sister chromatids
Chiasmata
Spindle
Homologous chromosomes
Fragments of nuclear envelope
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example.
Centromere (with kinetochore)
Metaphase plate
Microtubule attached to kinetochore
Chromosomes line up by homologous pairs.
Sister chromatids remain attached
Homologous chromosomes separate
Each pair of homologous chromosomes separates.
Cleavage furrow
Two haploid cells form; each chromosome still consists of two sister chromatids.
MEIOSIS I: Separates sister chromatids
Prophase II Metaphase II Anaphase II Telophase II and
Cytokinesis
Sister chromatids separate
Haploid daughter cells forming
During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.
Figure 13.8a
Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis
Centrosome (with centriole pair)
Sister chromatids
Chiasmata
Spindle
Homologous chromosomes
Fragments of nuclear envelope
Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n 6 in this example.
Centromere (with kinetochore)
Metaphase plate
Microtubule attached to kinetochore
Chromosomes line up by homologous pairs.
Sister chromatids remain attached
Homologous chromosomes separate
Each pair of homologous chromosomes separates.
Cleavage furrow
Two haploid cells form; each chromosome still consists of two sister chromatids.
At the end of Meiosis I how many cells are there?
1. One
2. Two
3. Three
4. Four
One
Tw
o
Thre
e
Four
25% 25%25%25%
At the end of Meiosis I are these cells haploid or diploid?
1. Haploid
2. Diploid
Hap
loid
Dip
loid
50%50%
At the end of Meiosis I, how many chromosomes
are there in each cell?
1. 23 chromosomes
2. 46 chromosomes
23
chro
mos
omes
46
chro
mos
omes
50%50%
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At the end of Meiosis I, are the chomosomes
in the duplicated state?
1. Yes
2. No
Yes N
o
50%50%
At the end of Meiosis II how many cells are
there?
1. One
2. Two
3. Three
4. Four
One
Tw
o
Thre
e
Four
25% 25%25%25%
At the end of Meiosis II are these cells
haploid or diploid?
1. Haploid
2. Diploid
Hap
loid
Dip
loid
50%50%
At the end of Meiosis II, how many
chromosomes are there in each cell?
1. 23 chromosomes
2. 46 chromosomes
23
chro
mos
omes
46
chro
mos
omes
50%50%
At the end of Meiosis II, are the
chromosomes in the duplicated state?
1. Yes
2. No
Yes N
o
50%50%
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
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
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Genetic diversity through meiosis
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
Independent Assortment
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Figure 13.10-1
Possibility 1 Possibility 2
Two equally probable arrangements of chromosomes at
metaphase I
Figure 13.10-2
Possibility 1 Possibility 2
Two equally probable arrangements of chromosomes at
metaphase I
Metaphase II
Figure 13.10-3
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
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
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Figure 13.11-1 Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Figure 13.11-2 Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Figure 13.11-3 Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Figure 13.11-4 Prophase I of meiosis
Nonsister chromatids held together during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Figure 13.11a
Chiasma
Centromere
TEM
Random Fertilization
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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© 2011 Pearson Education, Inc.
Animation: Genetic Variation Right-
click slide / select “Play”
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
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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
Gametophytes produce gametes (eggs
and sperm)
Sporophyte:
Produce spores
Are dependent on the gametophytes
(they grow out of the gametophytes)
Fig. 11.2c
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 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
Mitosis
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
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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?