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Question:-
Explain about mitosis and meiosis cell division in detail.
(More than 250 words)

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Answer 1

Answer:

Mitosis

Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.

The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells.

In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation.

Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase.

38 mitotis phases.jpg

The phases of mitosis

Prophase

Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere.

Prometaphase

The chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense.

s, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids.

Meiosis I

Meiosis I separates the pairs of homologous chromosomes.

39-meiosis_I_males.gif

In Meiosis I a special cell division reduces the cell from diploid to haploid.

Prophase I

The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:

Leptotene: chromosomes start to condense.

Zygotene: homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes (bivalents) consisting of four chromatids (tetrads).

Pachytene: crossing over between pairs of homologous chromosomes to form chiasmata (sing. chiasma).

Diplotene: homologous chromosomes start to separate but remain attached by chiasmata.

Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.

Prometaphase I

Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.

Metaphase I

Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)

Anaphase I

The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell

Telophase I

The chromosomes become diffuse and the nuclear membrane reforms.

Cytokinesis

The final cellular division to form two new cells, followed by Meiosis II. Meiosis I is a reduction division: the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome.

Meiosis II

Meiosis II separates each chromosome into two chromatids.

40-meiosis_II_males.gif

The events of Meiosis II are analogous to those of a mitotic division, although the number of chromosomes invol

Answer 2

$\text{\textbf{\bf{\large Mitosis} (\textit{mitos} : thread, referring to chromatin thread)}}$

    $\textsf{Mitosis is the cell division in which one parent cell cell divides into two}\\\textsf{identical daughter cells.}\\\textsf{The most important aspect of mitosis is that \textit{the same normal chromosome}}\\\textsf{\textit{number is maintained at each cell division.}}$

    $\textsf{\textbf{Phases of mitosis:} Mitosis is completed in two steps:}\\\textsf{\hspace{8em}\:\: Karyokinesis and Cytokinesis.}$

    $\textbf{\textsf{Karyokinesis {\small (Division of nucleus)}}}\\\textsf{\:\: Karyokinesis of mitosis occurs in four main phases although each of these}\\\textsf{\:\: phases merges into the next phase thereby making it a continuous process.}\\\textsf{\:\:\:\: The four phases of mitosis are:}\\\textsf{\hspace{2em}(I) \textbf{Prophase}, (II) \textbf{Metaphase}, (III) \textbf{Anaphase} and, (IV) \textbf{Telophase}.}$

    $\textsf{\textbf{Significance of Mitosis}}$

       $\textsf{\hspace{1em} \textemdash \: \textit{Growth} or increase in the body size due to formation of new}\\\textsf{\hspace{3em} cells in the tissues.}\\\textsf{\hspace{1em} \textemdash \: \textit{Repair} of damaged and wounded tissues by renewal of the lost cells}\\\textsf{\hspace{1em} \textemdash \: \textit{Replacement} of the old and dead cells such as the replacement of the }\\\textsf{\hspace{3em} blood cells and the epidermal cells of the skin.}$

       $\textsf{\hspace{1em} \textemdash \: \textit{Asexual reproduction} in which the unicellular organisms, such as}\\ \textsf{\hspace{3em} amoeba or the yeast cell, divide into two.}\\\textsf{\hspace{1em} \textemdash \: Maintains \textit{same chromosome number} in daughter cells.}$

$\text{\textbf{\bf{\large Meiosis} (reduction division producing gametes)}}$

     $\textsf{Meiosis (meion = to lessen, referring to the reduction of the chromosome}\\\textsf{number) is the kind of cell division that produces the sex cells or the gametes.}\\\textsf{It takes place in the reproductive organs (\textit{testis} and \textit{ovary}) in humans to}\\\textsf{produce sperms and ova. In the flowering plants, it takes place in the}\\\textsf{anthers and the ovule to produce \textit{pollen grains} and \textit{female gametophyte}.}$

     $\textsf{The most significant aspect of meiosis (meiosis diminution) is that the}\\\textsf{\textit{number of chromosomes} in the sex cells is halved.}$

     $\textsf{For example, out of the 23 pairs of chromosomes in humans, only}\\ \textsf{chromosomes i.e. one member of each pair \textit{(haploid)} are passed on to}\\\textsf{the sex cells. This is essential because when the male and female}\\ \textsf{gametes fuse during fertilisation, the normal double \textit{(diploid)} number}\\ \textsf{on chromosomes in pairs) is reacquired. The diploid number, as a rule,}\\ \textsf{is expressed as \textbf{"2n"} and the haploid number as \textbf{"n"}.}$

     $\textsf{Meiosis is completed in two divisions meiosis I and meiosis II, the first is}\\\textsf{the \textit{reduction division} and the second is \textit{mitotic division}. The G2 phase}\\\textsf{of the cell cycle is absent in the case of meiosis. Prophase of meiosis I}\\\textsf{corresponds most closely to the G, phase of the mitotic cell cycle.}$

     $\textsf{\textbf{Significance of Meiosis}}$

        $\textsf{\hspace{1em} \textemdash \: \textit{Chromosome number is halved} in gametes (sex cells),}\\\textsf{\hspace{3em} so that on fertilization, the normal number (2n) is restored.}$

        $\textsf{\hspace{1em} \textemdash \: It provides for mixing up of genes which occurs in two ways :}\\\textsf{\hspace{3em} (i) The maternal and paternal chromosomes get mixed up}\\\textsf{\hspace{5em} during the first (reduction) division as they separate from}\\\textsf{\hspace{5em} the homologous pairs.}$

               $\textsf{\hspace{3em} (ii) While the maternal and paternal chromosomes are separating,}\\\textsf{\hspace{5em} the chromatid material very often gets exchanged between the}\\\textsf{\hspace{5em} two members of a homologous pair. This is known as crossing}\\\textsf{\hspace{5em} over which results in genetic recombination. Chiasma is the}\\\textsf{\hspace{5em} X-shaped structure formed due to crossing over between the }\\\textsf{\hspace{5em} non-sister chromatids of the paired homologous chromosomes.}$

                  $\textsf{\large \textbf{Differences between Mitosis and Meiosis}}$

  $\begin{footnotesize} \begin{tabular} {l | l | l}&\textsf{\bf Mitosis} & \textsf{\bf Meiosis}\\Where it occurs & \textsf{in the somatic (body) cells.} & \textsf{in reproductive cells.}\\What for & \textsf{to provide for growth and development} & \textsf{only for gamete formation.}\\When it occurs & \textsf{continuously throughout life.} & \textsf{only in reproductively active age.}\\Daughter cells & \textsf{two daughter cells.} & \textsf{four daughter cells.}\\\end{tabular}\end{footnotesize}$