meiosis

MEIOSIS
HISTORY
 DIFFERENT STAGES IN MEIOSIS
oMEIOSIS I
oMEIOSIS II
 TYPES OF MEIOSIS
SIGNIFICANCE OF MEIOSIS
REFERENCE

Meiosis is a type of cell division occur only in eukaryote
They are also called as Reduction division
 Diploid(2n)  Haploid(n)
 Two stages in meiosis: meiosis I and meiosis II
 The two different sex cells are: sperm and egg
 Sex cells produce from germ cells

 Gametes are haploid
 The fusion of gamete lead to production of zygote
 Zygote attaining maturity capable of dividing in to
daughter cells
 In plants, meiosis is observed after spore production whereas in
animals meiosis take place during gamete formation
 Meiosis is similar to mitosis but there is a significant difference
 Meiosis consist of two type division:
•Heterotypic division
•Homotypic division

 Meiosis was discovered and described for the first time
in sea urchin eggs , German biologist Oscar Hertwig,1876
 At the level of chromosomes, by the Belgian
zoologist Edouard Van in Ascaris roundworm eggs,1883
 The significance of meiosis for reproduction and
inheritance, by German biologist August Weismann, 1890

 Noted that two cell divisions were necessary to
transform one diploid cell into four haploid cells if the
number of chromosomes had to be maintained
 In 1911 the American geneticist Thomas Hunt
Morgan detected crossovers in meiosis in the fruit
fly Drosophilla Melanogaster
 This helped to establish that genetic traits are
transmitted on chromosomes.

 The term meiosis (originally spelled "maiosis")
was introduced to biology by J.B. Farmer and J.E.S
Moore in 1905
 It is derived from a Greek word , meaning
'lessening'

 After chromosomes duplicate, two divisions follow
 Meiosis I (reduction division)
 Meiosis II (equational division)

 The parent cell or the dividing cell undergoes a
preparatory phase, known as interphase
 The parent cell synthesizes more DNA and proteins,
increasing the overall size and mass of the cell
 The dividing cell duplicates or doubles its
chromosomes

 In animal cells, Outside the nucleus there are two
centrosomes each containing a pair of centrioles
 The two centrosomes are produced by the duplication
of single centrosomes during premeiotic interphase.
 The centrosomes serve as a microtubule organizing
centers (MTOCs)
 Microtubules readily extend from centrosomes forming
an aster
 Plant cells do not have centrosomes

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 kinetochores)
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.

(a)Prophase I:
 The chromosomes have
already duplicated and they
coil and become shorter and
thicker.
 The duplicated homologous
chromosomes pair, and
crossing-over occurs

 Crossing–over is the process that can give rise to
genetic recombination
 The sites of crossing over are seen as
crisscrossed non-sister chromatids and are called as
chiasmata
 Prophase I is divided into five phases:
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis

1) LEPTOTENE[THIN -THREADSTAGE]
 Chromosomes starts to condense
 Homologous dyads find each other and align
themselves from end to end with the aid of an axial
element.

2) ZYGOTENE[YOKE-THREAD STAGE]
 Homologues chromosomes become closely
associated to form pairs of chromosome
 It is known as synapsis
 Paired homologous chromosome join with thick
protein containing synaptonemal Complex (SC)

3) PACHYTENE[THICK-THREAD STAGE]
 Crossing over take place between pairs of
homologous chromosome to form chiasmata
 There represent point where DNA recombination
is occurring
 The steps in recombining DNA continue to the end
of pachytene

4) DIPLOTENE[DOUBLE-THREAD STAGE]
 Homologous chromosome start to separate but
remain attached to chiasmata
 DNA recombination is complete
 The SC begins to breakdown
 At first chiasmata are located at the sites of the
recombination nodules, But later they migrate towards
the end of the chromatids

5) DIAKINESIS
 Homologous chromosome continue to separate, and
chiasmata move to the end of chromosomes
 In some organisms, the chromosomes decondense
and begin to be transcribed for a time

Fig. Showing sub division of prophase I

(b)Metaphase I
 The pairs of homologous chromosomes,
as tightly coiled and condensed
 It arranged on a plane equidistant from the
poles called the metaphase plate.
 Spindle fibers from one pole of the cell
attach to one chromosome of each pair
 Spindle fibers from the opposite pole
attach to homologous chromosome.

(C)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 towards the pole

(d) 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
 Nuclear envelope reforms around each
chromosome set, the spindle disappears, and
cytokinesis follows.
 Cytokinesis usually occurs
simultaneously, forming two haploid
daughter cells

 In animal cells, a cleavage furrow forms; in plant
cells, a cell plate forms
 No chromosome replication occurs between the
end of meiosis I and the beginning of meiosis II
because the chromosomes are already replicated

 Division in meiosis II also occurs as four phases
Prophase II
Metaphase II
Anaphase II
Telophase II and cytokinesis
 Meiosis II is very similar to mitosis

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.

(a)Prophase II
 The nuclear envelope breaks down and
the spindle apparatus forms.
 Here the centrioles duplicate
 occurs by the separation of two members
of the pair, then the formation of a daughter
centrioles perpendicular to each original
centriole
 The two pairs of centrioles separate in to
two centrosomes

(b) Metaphase II
 In metaphase II, the sister chromatids are
arranged at the metaphase plate
 Because of crossing over in meiosis I, the
two sister chromatids of each chromosome
are no longer genetically identical
 The kinetochores of sister chromatids
attach to microtubules extending from
opposite poles

(c)Anaphase I:
 The centromeres separate
 The two chromatids of each chromosome
move to opposite poles on the spindle
 The separated chromatids are now called
as chromosomes

(d)Telophase II
 A nuclear envelope forms around each set of
chromosomes
 Cytokinesis take place, producing four daughter
cells, each with a haploid set of chromosomes because
of crossing-over
 Some chromosomes have recombined segments of
original parental chromosomes.

 Gametic/Terminal meiosis
 Zygotic/Initial meiosis
 Sporic or Intermediate meiosis

GAMETIC/TERMINAL MEIOSIS
In male vertebrates,

Meiosis facilitates stable sexual reproduction.
If there is no meiosis, the chromosome number is
doubled. This would result in the formation of
abnormal forms.
Constant number of chromosome in a given
species is maintained by meiosis
During crossing over, the hereditary factors from
female and male parent get mixed. This can lead to
the genetical variation among the species

 Cooper, G. M., & Hausman, R. E. 2009. The cell: a
molecular approach. Washington, D.C., ASM Press.
 Lodish,Berk,Kaiser,Kreiger,Scott,Bretscher,Ploegh,Matsu
daura,6thedition,Molecular Cell Biology.
 Albert,Johnson,Lewis,Raff,Roberts,Walton,5thedition,The
Molecular Biology Of The Cell
 Gerald Karp, Cell Biology, 6thedition, John Wiley &
Sons, Inc.

meiosis

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