Meiosis. Gametogenesis
Maria Kazakova, PhD
Department of Medical Biology
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Asexual reproduction:
simple and direct
offspring genetically
identical to the parent
organism
Sexual reproduction:
involves the mixing of DNA
from two individuals
offspring genetically distinct
from one another and from
their parents
Organisms that reproduced sexually
are diploid
Diploid:
- each cell contains two sets of
chromosomes – on inherited from
each parent
- the somatic cells – leave no
progeny of their own; help the cells
of the germ line to survive and
propagate
Haploid:
- the specialized cells that perform
the central process in sexual
reproduction
- the germ cells or gametes - leave
progeny of their own
Sexual reproduction generates genetic diversity
Sexual reproduction:
Produces novel chromosome combinations
- Each gamete will receive a mixture of maternal and paternal
homologs
Generates genetic diversity through genetic recombination
- Crossing-over
Gives organisms a competitive advantage in a changing
environment
- Can help a species survive in an unpredictably variable
environment
- Can speed up the elimination of deleterious alleles and to
prevent them from accumulating in the population.
Meiosis
Theodor Boveri – 1883 – the fertilized egg of a parasitic
roundworm contains four chromosomes, whereas the worm’s
gametes - only two.
Meiosis - cell division by which diploid gamete precursors
produce haploid gametes
reduction division
involves one round of DNA replication followed by two
rounds of cell division
- Meiotic division I
- Meiotic division II
Molecular Biology of the Cell (© Garland Science 2008)
Meiotic division I consists of:
1. Prophase I
- Leptotene - the chromosomes become visible under the light
microscope
- Zygotene - homologous chromosomes
are paired into
synapsis; synaptonemal complex – involved in the process of
genetic recombination
- Pachytene - bivalents are formed; crossing-over –
recombination between homologous chromosomes
- Diplotene - the bivalents begin to separate and appear
cross-shaped structure – chiasmata
- Diakinesis - maximal condensation
2. Metaphase I
3. Anaphase I
3. Telophase I
Homologs – carry the same set of genes.
Pairing – each duplicated paternal chromosome first locates
and then attaches itself to the corresponding duplicated
maternal homolog.
Bivalent – contains four sister chromatids.
Crossing-over - the recombination of genetic material
between the non-sister chromatids within each duplicated
chromosome.
Synaptonemal complex – hold the bivalent together and
align the homologs so that the strand exchange can readily
occur between the non-sister chromatids.
Chiasma (Greek letter χ) – structure that corresponds to
a crossover between non-sister chromatids.
Homolog alignment and crossing-over
Before the synaptonemal complex forms,
recombination complexes assemble on dsDNA
breaks on sister chromatids and help catalyze
crossing-over between nonsister chromatid loops
Molecular Biology of the Cell (© Garland Science 2008)
Synapsis – a process in which the paired homologous
chromosomes become intimately associated with one
another.
- begins in zygotene and is complete throughout
pachytene
- the synaptonemal complex mediates this process.
Molecular Biology of the Cell (© Garland Science 2008)
An electron micrograph of
synaptonemal complex at
pachytene in a lily flower
Immunofluorescence micrographs of prophase I cells of the fungus
Sordaria.
(C) – partially synapsed bivalents and fully synapsed bivalents (D)
Molecular Biology of the Cell (© Garland Science 2008)
Synapsis and centromere pairing in mouse spermatocytes.
Synaptonemal complex is highly conserved among different species.
Molecular Biology of the Cell (© Garland Science 2008)
Meiosis requires the pairing of duplicated chromosomes
Interphase
Replication
Pachytene
Crossing-over
A bivalent with three chiasmata resulting
from three crossover events.
Molecular Biology of the Cell (© Garland Science 2008)
2. Metaphase I -
The nuclear membrane disappears; Bivalents align on a
equator; A spindle connects the centromeres to centrioles.
3. Anaphase I -
The homologous chromosomes comprising each bivalent
separate from each other.
4. Telophase I - The two haploid sets of chromosomes reach opposite poles.
Summary of Meiosis I: Crossing-over occurs between
homologous chromosomes. Homologous chromosomes separate
from each other and 2 haploid cells are formed.
Meiosis ІІ
• DNA synthesis does not occur
• Is similar to mitosis
• The difference is that there are only 23 chromosomes, no 46
• Consists of:
1. Prophase II - Chromatin once again condenses into discrete
chromosomes. The spindle apparatus forms.
2. Metaphase II - Chromosomes are lined up along the
equatorial plate, similar to metaphase in mitosis.
Microtubules from opposite poles attach to each sister
chromatid of a chromosome.
3. Anaphase II - Sister chromatids separate and move toward
opposite poles as individual chromosomes.
4. Telophase II - Chromosomes decondense and nuclear
envelopes reform.
Each cell with a haploid set of
chromosomes and each chromosome has only one
chromatid.
Summary of Meiosis II: Sister chromatids separate from each
other (similar to mitosis) and 4 haploid gamete cells are
formed.
Molecular Biology of the Cell (© Garland Science 2008)
The genetic consequences of meiosis are:
1. Reduction of the chromosome
number from 2n to n
2. Independent assortment of homologous
chromosome
3. Crossing-over
Meiosis is not flawless
Meiotic errors
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Nondisjunction – mistakes in separation of
chromosome or chromatids is referred to
as nondisjunction
Haploid cells lack a particular chromosome
Haploid cells have more than one copy of
particular chromosome
– Results in aneuploidy
– Usually embryo lethal
– Trisomy 21, exception leading to
Downs syndrome
– Sex chromosomes
• Turner syndrome: monosomy, XO
• Klinefelter syndrome: XXY
Translocation and deletion: transfer of a
piece of one chromosome to another or
loss of fragment of a chromosome
Molecular Biology of the Cell (© Garland Science 2008)
Behavior of the sex chromosomes in Meiosis
46 human chromosomes:
- 22 pairs of autosomes
- Two sex chromosomes
The behavior of sex chromosomes and autosomes
exhibits important differences during meiosis.
X and Y chromosome have a short region of
homologous sequence
only this region does pair and undergo genetic
recombination – Pseudoautosomal region
about 2.6 million base pairs in humans
• Gametogenesis - the process of formation of
gametes from the germ cells in the testes and
ovaries.
Many principles of gametogenesis are the same in both
males and females. Gametogenesis is divided into
several phases:
1.
2.
3.
4.
Multiplication
Growth
Maturation
Differentiation – only presented in spermatogenesis
Spermatogenesis
• Spermatocytogenesis - comprises
the cells from the spermatogonium
up to and including the secondary
spermatocyte.
• Spermiogenesis - comprises the
differentiation/maturation of the
sperm cell, starting with the
spermatid phase and is termed
• Takes place within the testes in long
tubes called seminiferous tubules.
testes
• Starts during puberty
• The ends is quite individual (old age)
• Multiplication, growth, maturation,
differentiation
• No interruptions
• Hormonal control - Luteinizing
hormone, testosterone, follicle–
stimulating hormone
Molecular Biology of the Cell (© Garland Science 2008)
Sertoli cells - initiate and regulate sperm
production. Probably the most important
are molecules called inhibin and activin,
proteins that control the actions of FSH
Leydig cells – secrete testosterone
Structure of sperm
Head – nucleus, acrosomal
vesicle
Tail
Oogenesis
• Takes place in the ovary
• Starts early in the embryogenesis
• The ends of the process – during
menopause
• Hormonal control:
- Follicle-stimulating hormone
- Luteinizing hormone
- Estrogen
• Interrupted process
Interrupted processes:
1. Meiosis I is interrupted at the diplotene stage of prophase I –
at the moment of birth a female baby – till puberty.
2. The secondary oocytes is arrested in metaphase II.
Meiosis II is accomplished after fertilization!!!
Human follicle development stages
Primary (primordial); Developing; Antral; Dominant antral follicle
Basic terms:
Corpus luteum the temporary structures that secretes
hormones that prepares the uterus for an embryo.
Ovarian cycle – the cycle in which a primary oocyte matures.
Ovulation – release of a secondary oocyte from an ovary.
Electron micrograph of
developing primary oocytes in
the rabbit ovary
Spermatogenesis vs Oogenesis
• Spermatogenesis is fast
process
• Oogenesis is slow process
primordial germ cell
primordial germ cell
spermatogonia
oogonia
primary spermatocytes
primary oocytes
secondary spermatocytes
secondary oocyte + the first polar
body
4 spermatids - spermatozoa
After fertilization
1 mature egg (ovum) + the second
polar body
Apoptosis in germ cells
• To maintain germline tissue homeostasis
• Out of ~7,000,000 germ cells occurring in the ovaries
of 5 months old human embryos - ~300,000 remain
at an age of 7 years, and less are left in the years
before ovarian senescence
• Multiple pathways are required:
1. Physiological germ cell apoptosis
2. DNA damage-induced apoptosis
Apoptosis during embryogenesis
Apoptosis in adult
spermatogenesis