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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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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.



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


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


Prophase II

Condensation again of chromosomes of daughter cells

Metaphase IIMigration of chromosomes toward the equator

Telophase II


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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Follows the 1st meiotic division withoutinterphase

or DNA replication.

Each chromosome divides longitudinally.


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.


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


(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 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single





2n/23 double



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Before meiosis

2n/46 single


Replication

4n/46 double



2n/23 double



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Before meiosis

2n/46 single


Replication

4n/46 double



2n/23 double



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Before meiosis

2n/46 single


Replication

4n/46 double



2n/23 double



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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

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AL-03 Gamete (10-11)