Cell cyle and dvision 2

Question 1

define meiosis

Answer 1

This specialised kind of cell
division that reduces the chromosome number by half results in the
production of haploid daughter cells. This kind of division is called
meiosis.

Question 2

list the 4 characs of meiosis

Answer 2

• 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.

Question 3

what happpen in Prophase 1

Answer 3

It is typically for longer and more complex than mitotic prophase.
The first two stages of
prophase I are relatively short-lived
Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis

Question 4

what happens in leptotene

Answer 4

During leptotene stage the chromosomes become gradually visible
under the light microscope. The compaction of chromosomes continues
throughout leptotene.
The chromosomes become distinct visible threads.
Centrioles move to opposite poles

• condensationof chromosomes begin
• the er, nuclear envelope, nucleolus and golgi bodies begin to disintegrate
• ## the centrioles begin to move towards opposite poles

Question 5

what happens in zygotene

Answer 5

In this stage, the pairing of homologous chromosomes occur.

Homologous chromosomes refer to chromosomes that have the same genes in the fixed position or loci, coding for the same trait

The process of pairing or association of homologus choromomes is called synapsis. Synapsis is due to the formation of a network of proteinaceous threads that join the two chromomesomes called Synaptonemal Complex.

The complex formed by a pair of synapsed homologous chromsomes is called a bivalent or a tetrad.

Question 6

what happens in pachytene

Answer 6

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 .
Crossing over is the exchange of genetic
material between 2 non-sister chromatids of the
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.

Question 7

what happens in diplotene stage

Answer 7

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.
Certain cellls ,like oocytes of certain vertebreates, stay arrested in the diplotene stage for months/years.

Question 8

what happens in diakinesis

Answer 8

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.

Question 9

what happens in metaphase and anaphase and telophase 1

Answer 9

Metaphase I: The bivalent chromosomes align on the equatorial plate
(Figure 10.3). 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 l: The nucleolus and nuclear membrane reaform and the cytokinesis results in the formation of a pair of haploid cellls which are called dyad of cells

Question 10

what is interkinesis

Answer 10

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.

Question 11

explain meiosis II

Answer 11

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 (Figure 10.4). 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 (Figure 10.4) 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 (Figure 10.4) 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

Question 12

what is the significance of meiosis

Answer 12

The production of offspring by sexual reproduction includes the fusion
of two gametes, each with a complete haploid set of chromosomes. Gametes
are formed from specialised diploid cells.
Meiosis ensures the production of haploid phase in the life cycle
of sexually reproducing organisms whereas fertilisation restores the diploid
phase. We come across meiosis during gametogenesis in plants and
animals.

Meiosis is the mechanism by which conservation of specific
chromosome number of each species is achieved across
generations in sexually reproducing organisms, even though the
process, per se, paradoxically, results in reduction of chromosome
number by half.

It also increases the genetic variability in the
population of organisms from one generation to the next. Variations
are very important for the process of evolution.

Question 13

why is meisosis reductional division

Answer 13

It is called the reduction division since it reduces the chromosome
number by half while making the gametes.