Cell cycle: Mitosis

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MITOSIS

Why mitosis?

  • For cell repair ( by replacing new cells with old and  worn out cells)
  • For growth of multi-cellular organism
  • At the end of mitosis we end up with two daughter cells with same number of chromosome as that of parent

Summary-

Before replication, one chromosome is composed of one DNA molecule,not single chromatid .As one child of a pair of twins is not referred to as a single twin likewise there is no auch thing as single chromatid.

At the beginning of interphase ,chromosome are made up of one DNA molecule.During S-phase of interphase , there is doubling of DNA amount i.e,formation of new DNA strand from parent DNA strand (Compact DNA forms chromosome). So this new DNA strand when condensed during Prophase (during mitotic phase)give rise to sister chromatids.

Now ,the chromosomes are made up of 2 chromatids (sister chromatids) joined by centromere . This centromere spilts in anaphase to form future chromosomes of daughter cell.

  • M Phase /Mitosis phase(phase of actual cell division)- Last for only an hour in a average cell cycle duration of 24 hrs in humans. Cell is preparing for division (cell growth + DNA replication)

                    Cytokinesis follows M- phase

  • Interphase /resting Phase (phase between 2 M phase)-last for the rest of 23 hours (i.e) >95% duration during an average cell cycle duration of 24 hrs in humans . Proper cell division (equational type of cell division) occurs during this phase.

Interphase is further divided into :

  • G0 Phase( quiescent stage)              –

Cells in this stage remain metabolically active but no longer proliferate as in  cells which do not exhibit mitosis(heart cells) or occasionally  divide ,as and when needed to replace worn out/dead cells

  • G1 phase (Pre-Synthetic growth)

 Metabolically active and growing stage.The cell makes a variety of proteins that are needed for DNA replication.

  • S Phase (DNA sysnthesis)

DNA amount doubles (N → 2N) in nucleus and centriole divide in cytoplasm .Chromosome no. remains same.

  • G2 Phase(Pre-mitotic growth)

Synthesis of proteins needed for cell division (M Phase).During this time, the cell builds up its supply of a protein complex called the maturation promotion factor, or MPF. If the cell has damaged DNA, MPF halts the cell’s transition from G2 to mitosis — the G2/M checkpoint. 

Before replication, one chromosome is composed of one DNA molecule,not single chromatid .As one child of a pair of twins is not referred to as a single twin likewise there is no thing such as single chromatid.

G1 Phase –

  • no. of chromosomes- 46
  • amount of DNA – n

S Phase –

  • no. of chromosomes -still 46
  • amount of DNA – 2n (doubles)
  • Replication of DNA make a new strand which gets compacted with histones to form 2 DNA molecule / sister chromatids .
  • Consider chromosome as vessel and DNA amount as tea .Vessel remains same only amount of tea doubles!

G2 Phase-

  • Same as that of S Phase

M Phase is further divided into :

  • Prophase 

(1)Chromatin condensation occurs i.e untangling of chromosomal material to form compact chromosomes

(2) centriole begin to move towards opposite poles in cell

(3) mitotic spindle ,microtubule ,proteinaceous structure in cell cytoplasm begin to assemble (4) Cell organelles disappear

  • Metaphase

(1) chromosome condensation completes

(2) chromosomes spread in cytoplasm as the nuclear membrane disintegration occurs ; morphology best viewed at this stage under microscope

(3) spindle fibers,formed by centioles attach to disk like structure (kinetochores) on centriole (holds sister chromatids together) of chromosome and align them at the center/equator of the cell k/a Metaphasic Plate

  • Telophase

(1)At metaphasic Plate, cetrioles split ; sister chromatids {future chromosome}split and migrate towards opp. pole .This is known as karyokinesis

{Additional point – During the movement of chomatids splited centiole faces pole and trailing behind chromatid face equator}

  • Anaphase

(1) Chromosomes reach respective pole and condense/tangle to form chromatin material

(2) Nuclear   membrane formed around clustered chromosome

(3) Cell organelles like ER, golgi apparatus reform and distribute among daughter cell during cytokinesis which follows.

Cytokinesis

  1. May or may not occur. Faliure of cytokinesis leads to multinucleate condition ,forming syncytium.
  2. furrow method in animal cell [periphery to centre]
  3. cell plate method in plant cell [center to periphery Cell wall formation]
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