Download Sexual Reproduction

SEXUAL REPRODUCTION
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When somatic cells undergo
mitosis as part of the cell cycle
they are reproducing
asexually the daughter cells
are exact copies of the parent
cell
however, human children are
not exact copies of their
parents and this is because
humans undergo sexual
reproduction
Sexual reproduction involves
two parents and leads to the
production of genetically
distinct offspring
HAPLOID & DIPLOID CELLS IN SEXUAL
REPRODUCTION
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Sexual reproduction involves
the fusion of a male
reproductive cell with a female
reproductive cell
The reproductive cells are
called gametes and the cell
that results from this fusion is
called a zygote, the process
through which gametes are
combined to form a zygote is
called fertilization
In humans the male gamete is
the sperm cell and the female
gamete is the egg cell
HAPLOID & DIPLOID
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Gametes, which contain single, unpaired
chromosomes are said to be haploid (from a
Greek word meaning single), the haploid
number of chromosomes in an organism is
represented by n
Cells that contain pairs of chromosomes, which
includes all somatic cells, are said to be
diploid (from a Greek word meaning double),
the diploid number of chromosomes in an
organism is represented by 2n
Each human gamete is haploid, with n = 23 after fertilization, the zygote
cell is diploid with a total of 2n chromosomes, that is n chromosomes
from the female parent plus n chromosomes from the male parent the
diploid number in humans, is therefore 46 chromosomes notice that n
also describes the number of pairs of chromosomes in an organism, when
two human gametes combine, 23 pairs of homologous chromosomes
are formed
MEIOSIS: PRODUCING HAPLOID GAMETES
Meiosis is the process through which gametes with a haploid
number of chromosomes are produced
Meiosis has 2 key outcomes:
1) Genetic Reduction:
 daughter cells are produced with half the number of
chromosomes per cell
2) Genetic Recombination:
 the products of meiosis have different combinations of alleles,
genetic recombination gives rise to offspring that are
genetically different from one another and their parents
 this greatly increases the genetic variation in a population
INTERPHASE
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Cells that divide by meiosis (that
is sperm and egg cells) go through
the growth and synthesis phases
of interphase before dividing
This includes the replication of
chromosomes and therefore at the
beginning of meiosis a cell
contains duplicated chromosomes,
each of which are made up of a
pair of identical sister chromatids
held together at the centromere
PHASES OF MEIOSIS
**Like Mitosis, Meiosis involves specific phases: prophase,
metaphase, anaphase and telophase...however, these four
phases are repeated twice, once in Meiosis I and the second
time in Meiosis II
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Each pair of chromosomes lines
up side by side, this lining up of
homologous chromosomes is
called synapsis
It is during at this point that
genetic material can be
exchanged between the two
homologous chromosomes
Throughout this phase the
centrosomes move to the poles of
the cell and the spindle
apparatus forms
METAPHASE I
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The pairs of
chromosomes line up
along the equator line
or metaphase plate
and the spindle fibers
have attached to the
centromere of each
homologous
chromosome
ANAPHASE 1
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The homologous chromosomes
separate and move to opposite
poles of the cell
Since the entire chromosome
moves, the sister chromatids
are in this case held together,
so the centromeres do not split
as they do in mitosis
As a result one chromosome
moves to each pole of the cell
and the chromosome number
is reduced from 4n to 2n
(diploid)
TELOPHASE I
The homologous chromosomes
now begin to uncoil and the
spindle fibers disappear
 Cytokinesis takes place, and a
nuclear membrane forms
around each group of
homologous chromosomes and
two cells form
 each of these new cells is now
diploid
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MEIOSIS II
The phases of Meiosis
II are similar to the
phases of Mitosis, the
key difference is that
the starting cells are
diploid and are not
replicated again
before dividing
 The chromosomes
condense, the
centrosomes move to
the poles of the cell
and the spindle
apparatus forms
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METAPHASE II
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The spindle fibers guide the
chromosomes to the center
line of the cell, they then
attach to the centromere of
each chromosome
The centromeres split apart and
the sister chromatids separate
from each other
 The spindle fibers shorten and the
chromosomes are pulled towards
the poles of the cell
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The chromosomes have reached
the opposite poles of the cell, they
then start to unwind into
chromatin
The spindle fibers break down and
a nuclear membrane forms around
the new set of chromosomes
Cytokinesis follows, splitting the
cell into two haploid gametes
GAMETE FORMATION IN ANIMALS
The products of Meiosis are gametes: in humans these gametes are sperm and
eggs, the process of sperm formation is called spermatogenesis and the
process of egg production is called oogenesis both processes involve meiosis,
but they happen in slightly different ways
In most male animals, meiosis takes place in the testes
• the process of spermatogenesis begins with a diploid
cell called a spermatogonium
• beginning at puberty, spermatogonia reproduce by
mitosis and the resulting cells undergo meiosis to
form 4 haploid cells
• The cells then develop into mature sperm after
Meiosis II
• The nucleus and certain molecules required by the
cell are organized into a "head" region, the
midsection holds many mitochondria, which are an
energy source for the cell and finally a long tail-like
flagellum develops for locomotion
OOGENESIS
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In most female animals, meiosis takes place
in the ovaries
Oogenesis starts with a diploid cell called an
oogonium
Before birth, the oogonia reproduce by
mitosis, and they begin meiosis, but they
stop at Prophase I
Meiosis will continue for one cell each month
Beginning at puberty Oogenesis involves an
unequal division of cytoplasm, the cell that
receives most of the cytoplasm after the first
division continues through meiosis I and II
to form a viable egg, the egg needs a large
quantity of nutrients that will support the
zygote after fertilization
The other, smaller cell is called a polar
body and it will degenerate
The final stages of meiosis II are not
completed unless fertilization by a sperm
cell occurs
When meiosis II is completed, the mature
egg and another polar body is produced
MULTIPLE BIRTHS
Fraternal
twins are formed
from two eggs being
fertilized by two
sperm cells
Identical twins are
born when a single zygote
divides into two separate
bodies in the first few days
of development
INDEPENDENT ASSORTMENT
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During metaphase I,
chromosomes are arranged in
homologous pairs along the
metaphase plate (equatorial)
In each pair, the chromosome of
maternal origin is oriented
toward one pole of the cell and
the chromosome of paternal
origin is oriented toward the
other pole of the cell
The orientation of each pair of
chromosomes is independent of
the orientation of the other pairs
Depending on how the
chromosomes line up, a number
of different combinations are
possible
• The number of genetically distinct gametes that can be produced from a
diploid cell is 2n
• Therefore each human can produce 223 or
8 388 608 genetically distinct gametes
CROSSING OVER
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While homologous
chromosomes are lined up
during Prophase I, non sister
chromatids may exchange
pieces of chromosome, this is
known as crossing over
Crossing over can occur at
several points along the sister
chromatids, a section of
chromosome that is crossed
over may contain hundreds or
even thousands of genes
As a result individual
chromosomes contain some
genes of maternal origin and
some of paternal origin
This once again dramatically
increases the genetic diversity
of the gametes produced
ERRORS DURING MEIOSIS
While independent assortment and
crossing over provide opportunities for
genetic diversity they also provide the
potential for chromosomal
abnormalities
 Many of the errors during meiosis
produce gametes that will not survive;
however, some do and can potentially
pass those errors on to the zygote if
fertilized
 Since every cell in the offspring is
produced from that one fertilized zygote
cell therefore all of the cells in the
embryo will contain the chromosomal
abnormality
 There are two types of chromosomal
errors that can occur during meiosis:
1) changes in chromosome structure
2) changes to chromosome number
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ERRORS CAUSED BY CHANGES IN
CHROMOSOME STRUCTURE
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To facilitate crossing
over the chemical bonds
that hold the DNA
together in the
chromosome must be
broken and reformed,
sometimes this does not
happen correctly
Also, non homologous
pairs may cross over,
producing chromosomes
that contain genes not
normally on that
chromosome
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Errors include:
1) Deletion
2) Duplication
3) Inversion
4) Translocation
DELETION
A
piece of
chromosome is
deleted
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Example of Genetic Disorder:
Cri du Chat (French for " cry
of a cat") syndrome is caused
by a deletion in chromosome 5
Many children with this
syndrome cry with a high
pitched, catlike sound, other
symptoms include: low birth
weight, widely spaced eyes,
recessed chin and
developmental and cognitive
delays
There is no cure for this
disorder
DUPLICATION
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A section of a
chromosome appears
two or more times in a
row
Example of Genetic Disorder:
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Charcot MarieTooth Disease
is caused by duplication of a gene
on chromosome 7
The most common symptoms are
muscle weakness and loss of
some sensation in the lower legs,
feet, and hands
A high foot arch with constantly
flexed toes is often present
There is no cure for this disorder
INVERSION
A Section of the
chromosome is inverted
Example of Genetic Disorder:
 FG Syndrome, a form of
FG syndrome is caused by
the inversion of the X
Chromosome
 This syndrome occurs
almost exclusively in males
 Symptoms include
intellectual disabilities of
varying degrees, delayed
motor development, low
muscle tone and broad toes
and thumbs
 There is no cure for this
disorder
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TRANSLOCATION
A segment of one chromosome
becomes attached to a
different chromosome
Example of Genetic Disorder:
 Chronic Myelogenous
Leukemia, most cases of
CML are are caused by a
translocation between
chromosome 9 and 22, which
results
 in an abnormal gene this is a
cancer of the white blood cells
 Treatment involves using a
drug that stops the increased
production of white blood cells
that the abnormal gene
causes
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Some disorders are
associated with more
than one type of error
For example: autism can
be caused by various
duplications, inversions
and translocations, it is a
complex developmental
disorder that is found in 1
in 165 children
ERRORS CAUSED BY CHANGES IN CHROMOSOME
NUMBER
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Sometimes homologous
chromosomes or sister
chromatids do not separate as
This is known as
nondisjunction
This phenomenon occurs in
anaphase I or II of meiosis
In anaphase I, nondisjunction
occurs when homologous
chromosome pairs do not
separate to opposite poles,
instead one entire pair is
pulled to the same pole
In anaphase II, nondisjunction
occurs when sister chromatids
do not separate to opposite
poles, instead they are pulled
toward the same pole
Nondisjunction always results in
gametes with too few or too many
chromosomes as seen above
GENETIC DISORDERS ASSOCIATED WITH
CHROMOSOME NUMBER
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Many of the genetic disorders that
have been identified to date are due
to an individual having an incorrect
number of chromosome
Many individuals born with this
syndrome have an extra
chromosome or an extra piece of
chromosome 21
The incidence of nondisjunction
leading to Down syndrome
increases with maternal age,
women ages 20 to 24 the chances
are 1 in 1490, for women at age 40
the chances are 1 in 106 and for
women at age 49 it increases to 1 in
11
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One example is Down
Syndrome
PHYSICAL FEATURES OF BABIES WITH
DOWN SYNDROME
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