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GAMETOGENESISGAMETOGENESIS
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1. To describe events occurring in the normal:
1.1 Meiosis
1.2 Gametogenesis1.2.1. Spermatogenesis
1.2.2. Oogenesis
Aims and Objective
2. To describe the abnormal gametes and the
clinical correlations.
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Lecture Outline
Nomenclature
Meiosis
Gametogenesis Spermatogenesis; spermiogenesis
Oogenesis
Spermatogenesis: different stages
Abnormal gametes and clinical correlates
Review questions
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1 pairofdouble-structured chromosomes
contain 4 chromatids
1 single-structured chromosomecontains 1 chromatid
1 double-structured chromosome
contains 2 chromatids
Short arm
Long arm
Centromere
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Thus gametes (spermatozoa, ova) only have half
the normal complement of chromosomes
Cells having two sets of chromosomes - 46 in number,
(i.e. 23 pairs) are referred to as diploid or2n cells
Gametes have one set of chromosomes - 23 in number,
are referred to as haploid or1n cells
Haploid cells can only be produced by meiosis
Haploid cells cannot be produced by mitosis
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Normal Female Karyotype (46, XX)
1 2 3 4 5
6 7 8 9 10 11 12
13 1514 16 17 18
19 20 21 22X X
A B
C (minus X)
D E
GF
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Normal Male Karyotype (46, XY)
1 2 3 4 5
6 7 8 9 10 11 12
13 1514 16 17 18
19 20 21 22X Y
A B
D E
GF
C
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Cell TypeNo. of chromosomes,
Amount of DNA
Primordial germ cells Spermatogonia
Oogonia
Zygote
Blastomeres
All normal somatic cells
46, 2n
Primary spermatocytes
Primary oocytes46, 4n
Secondary spermatocytes
Secondary oocytes23, 2n
Spermatid, spermatozoa
Mature oocytes (Ova)
23, n
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Homologous chromosomes (Homologs)
Pairs of chromosomes (one from each parent)
In the form of double chromatid chromosomes.
Crossing over
Random exchange of genetic materialsbetween homologous chromosomes.
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Homologous chromosomes (Homologs)
Pairs of chromosomes (one from each parent)
In the form of double chromatid chromosomes.
Crossing over
Random exchange of genetic materialsbetween homologous chromosomes.
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1st Meiotic Division (Reductional Division)
2nd Meiotic Division (Equatorial Division)
A special type of cell division consisting of two meiotic
cell divisions.
Takes place in germ cells only
MEIOSIS
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1st Meiotic Division(Reductional Division)
MEIOSIS
2nd Meiotic Division(Equatorial Division)
Prophase II
Metaphase II Anaphase II
Telophase II
Prophase I
Metaphase I Anaphase I
Telophase I
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1st Meiotic Division(Reductional Division)
MEIOSIS
Metaphase I Anaphase I
Telophase I
2nd Meiotic Division(Equatorial Division)
Prophase II
Metaphase II Anaphase II
Telophase II
Prophase I
Leptotene Zygotene
Pachytene
Diplotene
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Zygotene
Pairing-synapsing of chromosomes
Prophase I
DNA duplication
Leptotene
Condensation of chromosomes
DictyoteneSeparation of chromosome pairs
revealing chiasmata
S Phase
Pachytene
Crossing over of chromatids
MEIOSIS - I
Crossing over:
Random exchange of genetic materials
between homologous chromosomes.
Homologous chromosomes (Homologs)
Pairs of chromosomes (one from each parent)
Each is a double chromatid chromosome
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Zygotene
Pairing-synapsing of chromosomes
DNA duplication
Metaphase I Anaphase I Telophase I
Leptotene
Condensation of chromosomes
DictyoteneSeparation of chromosome pairs
revealing chiasmata
S Phase
Metaphase I
Homologous
chromosome
pairs at equatorial
plane
Anaphase I
Homologous chromosomes
migrate to opposite poles
Telophase I
Migrating chromosomesreach the poles
Pachytene
Crossing over of chromatids
Prophase I
MEIOSIS - I
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DNA duplication
Metaphase I Anaphase I Telophase IS Phase Prophase I
At the end of 1st meiotic (reductional) division,each daughter cell (2o spermatocyte or 2o oocyte) has
a haploid chromosome number butstill as double chromatid chromosomes
2nd meiotic (equatorial) division follows 1st meiotic divisionwithout DNA replication
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DNA duplication
Metaphase I Anaphase I Telophase IS Phase Prophase I
Prophase II
Condensation again of chromosomes of daughter cells
Metaphase IIMigration of chromosomes toward the equator
Anaphase II
Chromosomes divide longitudinally
Separation (disjunction) of chromosomes
Telophase II
Each half of chromosome or chromatid is drawn
or migrates toward and reach the opposite poles
Cytokinesis
Constriction of cell across the nuclear membrane and
division into two cells
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DNA duplication
Metaphase I Anaphase I Telophase IS Phase Prophase I
Prophase II
Condensation again of chromosomes of daughter cells
Metaphase IIMigration of chromosomes toward the equator
Telophase II
Each half of chromosome or chromatid is drawn
or migrates toward the opposite poles
Cytokinesis
Constriction of cell across the nuclear membrane and
division into two cells
Anaphase II
Separation (disjunction) of chromosomes
Chromosomes divide longitudinally
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It is known as reductional division because the
chromosome number is reduced from diploid tohaploid.
i.e. diploid germ cells give rise to haploid gametes
(sperms and oocytes)
1st Meiotic Division (Reductional Division)
MEIOSIS
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2nd Meiotic Division (Equatorial Division)
Follows the 1st meiotic division withoutinterphase
or DNA replication.
Each chromosome divides longitudinally.
Each half of chromosome or chromatid is drawn
or migrates toward the opposite poles.
MEIOSIS
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Each daughter cell so formed .
now has a single chromatid chromosome. retains a haploid number (23) of chromosomes.
contains a representative of each of the
chromosome pair.
2nd Meiotic Division (Equatorial Division)
MEIOSIS
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Replication (of DNA) always precedes 1st meiotic division
No replication precedes the 2nd meiotic division
MEIOSIS
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Significance of meiosis
Provides forconstancy of chromosomal number
from generation to generation.
Allows random assortment of maternal and paternal
chromosomes between gametes.
MEIOSIS
Relocates segments of maternal and paternal
chromosomes by crossing over of chromosome
segments resulting in recombination of genetic
material.
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2nd maturation division1n/23 single
Replication4n/46 double
1st maturation division
2n/23 double
Before meiosis
1n/46 single
Replication4n/46 double
(4n/23 pairs double)
Before meiosis
2n/46 single
(2n/23 pairs single)
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication
4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication
4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Before meiosis
2n/46 single
(2n/23 pairs single)
Replication
4n/46 double
(4n/23 pairs double)
1st maturation division
2n/23 double
2nd maturation division1n/23 single
Events Occurring During Gametogenesis
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Primordial germ cells
Yolk sac
The first human germ cells (primordial germ cells) appear
in the wall of the yolk sac (3rd
week)
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The germ cells, through amoeboid movement, move
towards the gonads where they arrive at about 5th
week
Primordial cells later differentiate into mature gametes
i.e. spermatogonia (male) oroogonia (female)
Primordial germ cells
Yolk sac
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It is the process of formation and development of
gametes
Prepares the gametes for fertilization.
Reduces the numberof chromosomes.
Alters shape of the cells.
GAMETOGENESIS
Consists of:
Spermatogenesis (in male)
Oogenesis (in female)
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Begins at puberty (13- 16 years).
Continues into old age.
Entire process of spermatogenesis including thespermiogenesis, takes about 2 months.
Process which transforms spermatogonia into
spermatozoa (sperms).
SPERMATOGENESIS
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Dormant primordial germ cell differentiates
into spermatogonium (at thetime of puberty)Spermatogonium(46/2n single)
Primary Spermatocyte
(46/4n double)
2o
Spermatocytes
(23/2n double)
Spermatogonium differentiatesinto a primary spermatocyte.
1o
spermatocyte enters 1stmeiotic division,resulting in formation of 2daughtercells (2
ospermatocytes).
2o
spermatocytes enter2ndmeiotic division,
resulting in formation of2 daughtercells (spermatids).
Spermatids undergo spermiogenesis andfinally become the spermatozoa.
Events occurring during Spermatogenesis
Spermatids
(23/1n single)
1stmeiotic division
2
nd
meioticdivision
Spermatozoa(23/1n single)
Spermiogenesis
Primordial germ cell(46/2n single)
Replication
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Condensation ofnucleus(formingmostof the head)
SPERMIOGENESIS
Formation ofacrosome or acrosomal cap
(which covers the nucleus)
Shedding of most of the cytoplasm
Formation of a tail
(which includes the middle piece or
mitochondrialsheath)
Spermatid
cytoplasm shedded
Spermatozoa
tail
head
nucleus
acrosome
The last phase of spermatogenesis.
Rounded spermatids are transformed intoelongated spermatozoa (sperms).
Changes taking place:
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Parts of a Spermatozoa (Sperm)
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Begins at birth.
Completed after puberty
Process which transforms oogonia into mature ova.
Oogenesis
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1st polarbody
2nd polar bodies
Primordial germ cell differentiates intooogonium.
Primary oocyte enters 1stmeiotic divisionresulting in formation ofone 2
ooocyte and one 1stpolarbody
2o
oocyte enters 2ndmeiotic division
If fertilization does not occur:2
ooocyte degenerates
If fertilization occurs:2ndmeiotic division is completed, resulting inthe formation ofone mature oocyte + one 2ndpolarbody
Oogonium laterdifferentiates into primaryoocyte.
Events occurring during Oogenesis
Oogonium
(46/2n single)
Primordial germ cell
(46/2n single)
1stmeiotic division
2ndmeioticdivision
Primary Oocyte
(46/4n double)
2o
Oocyte
(23/2n double)
Mature
oocyte
(23/1n single)
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After replication4n/46 double
1st
meiotic division2n/23 double
2nd meiotic division1n/23 single
Before meiosis2n/46 single
Spermatogonium
Primordial
germ cell
Primordialgerm cell
Oogonium
Primary
spermatocyte
Secondary
spermatocyte
Secondary oocyte
1st Polar body
Mature oocyte
2nd
Polar bodies
Spermatids
Events occurring during Gametogenesis
Primary
oocyte
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Before DNA replication (S Phase)
Chromosomes exist as single chromatid chromosomes.
After DNA replication
Chromosomes exist as double chromatid chromosomes.
FURTHER READING!
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Ovulation
Primary oocyte resumes and completes 1st meiotic division
just days before ovulation.
Primary oocytes do not finish 1st meiotic division before
puberty.
No primary oocytes are formed after puberty in females,
but in contrast..
Primary spermatocytes are continuously formed after
puberty in males.
FURTHER READING!
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When secondary oocyte is shed from the ovary ........
mitotic spindle is formed.
2nd meiotic division in 2o oocyte begins at ovulation.
Ovulation
2nd meiotic division becomes arrested at metaphase.
Maturation of oocyte is completed when 2nd polar body
is extruded.
2nd
meiotic division is completed only when oocyte isfertilized (when sperm penetrates the seondary oocyte)
FURTHER READING!
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2nd meiotic division is completed when sperm penetrates
the seondary oocyte.
Maturation of oocyte is completed when 2nd polar body
is extruded.
2nd meiotic division is completed only when oocyte isfertilized.
Ovulation
FURTHER READING!
1st meiotic division is completed just days before ovulation.
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Diplotene Stage
It is a resting stage between Prophase I and Metaphase I
Some primary oocytes may be at rest (dormant) in thediplotene stage for 40 years or more.
This arrested state is produced by a peptide, oocyte
maturation inhibitor (OMI) secreted by follicular cells.
FURTHER READING!
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Primary oocytes in the suspended Prophase I (diplotene)
are vulnerable to environmental agents such as radiation.
Long duration of the 1st meiotic division may account for
high frequency of meiotic errors such as nondisjunction
that occur with increasing maternal age.
Diplotene Stage
Nondisjunction = failure of paired chromatids to dissociate
CLINICAL CORRELATES
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Likelihood of chromosomal abnormalities in the embryo
increases significantly after the mother is 35.
Ideal maternal age for reproduction is 18 -35 years.
CLINICAL CORRELATES
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Abnormal Sperms
Up to 10% of sperms ejaculated may be grossly abnormal
(double heads).
Abnormal sperms do not fertilize due to:
Lack of normal motility.
Inability to pass through mucus in the cervical canal.
CLINICAL CORRELATES
Such abnormally shaped sperms affect fertility if their
number exceeds 20%.
X-rays, severe allergic reactions and antispermatogenic
agents may increase the % of abnormally shaped sperms
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Abnormal Gametogenesis
Due to disturbances of meiosis during gametogenesis,
chromosomally abnormal gametes may be formed.
If involved in fertilisation, gametes with numericalchromosome abnormalites cause abnormal development
e.g. Downsyndrome.
Nondisjunction
Failure of paired chromatids to dissociate.
CLINICAL CORRELATES
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Normal
Spermatogenesis
Abnormal
Spermatogenesis
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Abnormal
Oogenesis
Normal
Oogenesis
Secondaryoocyte23, Xmature follicle
First polarbody23, X
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Further Reading
Chromosomal abnormalities
Numerical abnormalities
(Trisomies, monosomy, mosaicsm)
Structural abnormalities(Translocations, microdeletions)
CLINICAL CORRELATES
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State what is:
a single-structured chromosome
a double-structured chromosome
Review questions
State which type of cell division can give rise
to haploid cells.
State what is:
a haploid cell a diploid cell
Explain the terms:
homologous chromosomes
crossing over
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N
ame the stage/phase of meiosis at which thefollowing events occur:
condensation of chromosomes
pairing of chromosomes
crossing over of chromatids separation of chromosome pairs
Review questions
Name the stage/phase of meiosis at which the
chromosomes; lie at the equatorial plane
migrate to opposite poles
reach the poles
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State the difference between Anaphase I and
Anaphase II.
Review questions
State the number and type of daughter cellsproduced by one mother cell, at the end of:
spermatogenesis
oogenesis (if fertilisation occurs)
State the significances of meiosis.
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Review questions
How do abnormal sperms affect fertility.
Explain nondisjunction.
What is diplotene stage and state its clinical
significance.
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End ofGAMETOGENESIS
End ofGAMETOGENESIS