.If a meiocyte has 24 chromosomes in G1 phase,
what will be the number of chromatids in each of
the cells formed after meiosis II?
(1) 12
(2) 24
(3) 48
(4) 96
Answer ▽ ✅Verified
Answer (1)
G1-24 chromatids
S- 48 chromatids
G2- 48 chromatids
Meiotic I products- 24 chromatids
Meiotic II products – 12 chromatids
chromatid
👉Every chromosome (visible only in dividing cells)
essentially has a primary constriction or the centromere
on the sides of which disc shaped structures called
kinetochores are present . Centromere holds
two chromatids of a chromosome. Based on the position
of the centromere, the chromosomes can be classified into
four types . The metacentric chromosome
has middle centromere forming two equal arms of the
chromosome. The sub-metacentric chromosome has
centromere slightly away from the middle of the
chromosome resulting into one shorter arm and one
longer arm. In case of acrocentric chromosome the
centromere is situated close to its end forming one
extremely short and one very long arm, whereas the
telocentric chromosome has a terminal centromere.
👉Karyokinesis involves
following four stages:
Prophase
Metaphase
Anaphase
Telophase
👉Prophase
#Chromosomal material condenses to form compact mitotic
chromosomes. Chromosomes are seen to be composed of two
chromatids attached together at the centromere.
# Centrosome which had undergone duplication during interphase,
begins to move towards opposite poles of the cell. Each centrosome
radiates out microtubules called asters. The two asters together
with spindle fibres forms mitotic apparatus.
#Cells at the end of prophase, when viewed under the
microscope, do not show golgi complexes, endoplasmic
reticulum, nucleolus and the nuclear envelope.
👉Metaphase
#Spindle fibres attach to kinetochores of
chromosomes.
#Chromosomes are moved to spindle equator and get
aligned along metaphase plate through spindle fibres
to both poles.
👉Anaphase
Centromeres split and chromatids separate.
Chromatids move to opposite poles.
👉Telophase
#Chromosomes cluster at opposite spindle poles and their
identity is lost as discrete elements.
#Nuclear envelope develops around the chromosome
clusters at each pole forming two daughter nuclei.
#Nucleolus, golgi complex and ER reform.
👉MEIOSIS
#Meiosis involves two sequential cycles of nuclear and cell division called
meiosis I and meiosis II but only a single cycle of DNA replication.
#Meiosis I is initiated after the parental chromosomes have replicated
to produce identical sister chromatids at the S phase.
#Meiosis involves pairing of homologous chromosomes and
recombination between non-sister chromatids of homologous
chromosomes.
# Four haploid cells are formed at the end of meiosis II.
Meiotic events can be grouped under the following phases:
Meiosis I Meiosis II
Prophase I Prophase II
Metaphase I Metaphase II
Anaphase I Anaphase II
Telophase I Telophase II
👉Meiosis II
👉Prophase I:
Prophase of the first meiotic division is typically longer and
more complex when compared to prophase of mitosis.
It has been further
subdivided into the following five phases based on chromosomal
behaviour, i.e., Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
During leptotene stage the chromosomes become gradually visible
under the light microscope. The compaction of chromosomes continues
throughout leptotene.
This is followed by the second stage of prophase
I called zygotene. During this stage chromosomes start pairing together
and this process of association is called synapsis. Such paired
chromosomes are called homologous chromosomes. Electron
micrographs of this stage indicate that chromosome synapsis is
accompanied by the formation of complex structure called
synaptonemal complex. The complex formed by a pair of synapsed
homologous chromosomes is called a bivalent or a tetrad.
However,
these are more clearly visible at the next stage. The first two stages of
prophase I are relatively short-lived compared to the next stage that is
pachytene. During this stage, the four chromatids of each bivalent
chromosomes becomes distinct and clearly appears as tetrads. This stage
is characterised by the appearance of recombination nodules, the sites
at which crossing over occurs between non-sister chromatids of the
homologous chromosomes. Crossing over is the exchange of genetic
material between two homologous chromosomes. Crossing over is also
an enzyme-mediated process and the enzyme involved is called
recombinase. Crossing over leads to recombination of genetic material
on the two chromosomes. Recombination between homologous
chromosomes is completed by the end of pachytene, leaving the
chromosomes linked at the sites of crossing over.
The beginning of diplotene is recognised by the dissolution of the
synaptonemal complex and the tendency of the recombined
homologous chromosomes of the bivalents to separate from each other
except at the sites of crossovers. These X-shaped structures, are called
chiasmata. In oocytes of some vertebrates, diplotene can last for
months or years.
The final stage of meiotic prophase I is diakinesis. This is marked by
terminalisation of chiasmata. During this phase the chromosomes are
fully condensed and the meiotic spindle is assembled to prepare the
homologous chromosomes for separation. By the end of diakinesis, the
nucleolus disappears and the nuclear envelope also breaks down.
Diakinesis represents transition to metaphase.
👉Metaphase I:
The bivalent chromosomes align on the equatorial plate
. The microtubules from the opposite poles of the spindle
attach to the kinetochore of homologous chromosomes.
👉Anaphase I:
The homologous chromosomes separate, while sister
chromatids remain associated at their centromeres
👉Telophase I:
The nuclear membrane and nucleolus reappear, cytokinesis
follows and this is called as dyad of cells . Although in many
cases the chromosomes do undergo some dispersion, they do not reach
the extremely extended state of the interphase nucleus. The stage between
the two meiotic divisions is called interkinesis and is generally short lived.
There is no replication of DNA during interkinesis. Interkinesis is followed
by prophase II, a much simpler prophase than prophase I.
👉Meiosis II
👉Prophase II:
Meiosis II is initiated immediately after cytokinesis, usually
before the chromosomes have fully elongated. In contrast to meiosis I,
meiosis II resembles a normal mitosis. The nuclear membrane disappears
by the end of prophase II . The chromosomes again become
compact.
👉Metaphase II:
At this stage the chromosomes align at the equator and
the microtubules from opposite poles of the spindle get attached to the
kinetochores of sister chromatids.
👉Anaphase II:
It begins with the simultaneous splitting of the centromere
of each chromosome (which was holding the sister chromatids together),
allowing them to move toward opposite poles of the cell by
shortening of microtubules attached to kinetochores.
👉Telophase II:
Meiosis ends with telophase II, in which the two
groups of chromosomes once again get enclosed by a nuclear
envelope; cytokinesis follows resulting in the formation of tetrad
of cells i.e., four haploid daughter cells
👉Name the stage of cell cycle at which one of the following events occur:(i) Chromosomes are moved to spindle equator.(ii) Centromere splits and chromatids separate.(iii) Pairing between homologous chromosomes takes place.(iv) Crossing over between homologous chromosomes takes place.
👉Chromosomal Disorders
The chromosomal disorders on the other hand are caused due to absence
or excess or abnormal arrangement of one or more chromosomes.
Failure of segregation of chromatids during cell division cycle results
in the gain or loss of a chromosome(s), called aneuploidy. For example,
Down’s syndrome results in the gain of extra copy of chromosome 21.
Similarly, Turner’s syndrome results due to loss of an X chromosome in
human females. Failure of cytokinesis after telophase stage of cell division
results in an increase in a whole set of chromosomes in an organism and,
this phenomenon is known as polyploidy. This condition is often seen in
plants.
___@organised notes_____
