Cell Cycle and Cell Division

Growth and reproduction are characteristics of cells. All cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time. Cycles of growth and division allow a single cell to form a structure consisting of millions of cells.

Cell Cycle

The sequence of events by which a cell duplicates its genome, synthesises the other
constituents of the cell and eventually divides into two daughter cells is termed cell cycle.

• Cell growth (in terms of cytoplasmic increase) is a continuous process.
• DNA synthesis occurs only during one specific stage in the cell cycle.
• Replicated chromosomes (DNA) are distributed to daughter nuclei by a complex series of
  events during cell division.
• Upper events are themselves under genetic control.

Phases of Cell Cycle

A human eukaryotic cell divide once in approximately every 24 hours.

Duration of cell cycle can vary from organism to organism and also from cell type to cell type.
• In Yeast cell cycle in only about 90 minutes.
• Cell cycle is divided into two basic phases:

1. Interphase 2. M Phase (Mitosis phase)

1. Interphase (resting phase) :-

The time during which the cell is preparing for division by undergoing both cell growth and
DNA replication in an orderly manner.

• The interphase lasts more than 95% of the duration of cell cycle.

Divided into three further phases:
(i) G¹ phase (Gap 1)
(ii) S phase (Synthesis)
(iii) G² phase (Gap 2)
Gap 1 (G¹ phase) - corresponds to the interval between mitosis and initiation of DNA replication.

Cell is metabolically active and continuously grows but does not replicate its DNA.

Gap 1 (G¹ phase)

corresponds to the interval between
mitosis and initiation of DNA replication. Cell is metabolically active and continuously grows but does
not replicate its DNA.

Synthesis (S phase)

During which DNA synthesis or replication takes place and the amount of DNA per cell doubles.
Ex. - If the initial amount of DNA is denoted as 2C then it increases to 4C.

• There is no increase in the chromosome number; if the cell had diploid or 2n number of
  chromosomes at G¹ after S phase the chromosome still 2n.
• DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.

Gap 2 (G² phase)

Proteins are synthesised in preparation for mitosis while cell growth continues.
• Sequence of Cell Cycle = G¹ — S — G² — M phase

2. M phase (Mitosis phase)

When the actual cell division or mitosis occurs.

• It is significant to note that in the 24 hour average duration of cell cycle of a human cell, cell
  division proper lasts for only about an hour.
• M Phase starts with the nuclear division.
• to the separation of daughter chromosomes (karyokinesis)
• ends with division of cytoplasm (cytokinesis).
• Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many
  other cells divide only occasionally, as needed to replace cells that have been lost because of
  injury or cell death.
• Cells that do not divide further exit Gap 1 phase to enter the inactive stage called Quiescent phase (G⁰ phase).
• In animals - mitotic cell division is only seen in the diploid somatic cells.
• few exceptions to this where haploid cells divide by mitosis. Ex. male honey bees.
• In plants - mitotic divisions in both haploid and diploid cells.

M Phase

Most dramatic period of the cell cycle, involving a major reorganisation of virtually all
components of the cell.
• Number of chromosomes in the parent and progeny cells is the same, it is also called as
equational division.
• Divided into four stages of nuclear division (karyokinesis) (NEET 2017 in detail)

1. Prophase 2. Metaphase 3. Anaphase 4. Telophase

1. Prophase

first stage of karyokinesis of mitosis.

• Marked by the initiation of condensation of chromosomal material.
• chromosomal material becomes untangled during the process of chromatin condensation.
• Centrosome, which had undergone duplication during S phase of interphase, now begins to
  move towards opposite poles of the cell.
• Completion of prophase can thus be marked by the following characteristic events:
  (i) Chromosomal material condenses to form compact mitotic chromosomes.
• Chromosomes are seen to be composed of two chromatids attached together at the
  centromere.
  (ii) Centrosome which had undergone duplication during interphase, begins to move towards
  opposite poles of the cell.
• Each centrosome radiates out microtubules called asters.
• Two asters together with spindle fibres forms mitotic apparatus.
  (iii) Cells at the end of prophase, when viewed under the microscope, do not show golgi
  complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.

2.Metaphase

• Complete disintegration of the nuclear envelope marks the start of the second phase of mitosis.
• Chromosomes are spread through the cytoplasm of the cell.
• Condensation of chromosomes is completed and they can be observed clearly under the microscope.
• Stage at which morphology of chromosomes is most easily studied.
• Metaphase chromosome is made up of two sister chromatids, which are held together by the centromere.
• Small disc-shaped structures at the surface of the centromeres are called kinetochores serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell. (NEET 2016)
• Metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole.
• Plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
  • The key features of metaphase are:
  (i) Spindle fibres attach to kinetochores of chromosomes.
  (ii) Chromosomes are moved to spindle equator and get aligned along
  metaphase plate through spindle fibres to both poles. (NEET 2011)

3.Anaphase

Each chromosome arranged at the metaphase plate is split simultaneously and the two
daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei,
begin their migration towards the two opposite poles. Each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of
the chromosome trailing behind.

Anaphase stage is characterised by :-
(i) Centromeres split and chromatids separate. (NEET 2017)
(ii) Chromatids move to opposite poles

4.Telophase

Beginning of the final stage of karyokinesis, i.e., telophase.

• Chromosomes that have reached their respective poles decondense and
  lose their individuality.
• Individual chromosomes can no longer be seen and each set of
  chromatin material tends to collect at each of the two poles.
  • Shows the following key events:
  (i) Chromosomes cluster at opposite spindle poles and their identity is
  lost as discrete elements.
  (ii) Nuclear envelope develops around the chromosome clusters at each
  pole forming two daughter nuclei.
  (iii) Nucleolus, golgi complex and ER reform.

Cytokinesis

Cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis
at the end of which cell division gets completed.
• In an animal cell :- Achieved by the appearance of a furrow in the plasma membrane.

• furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.
  • Plant cells :- enclosed by a relatively inextensible cell wall, therefore they undergo cytokinesis
  by a different mechanism.
• Wall formation starts in the centre of the cell and grows outward to meet the existing lateral
  walls.
• Formation of the new cell wall begins with the formation of a simple precursor, called the cell plate
  that represents the middle lamella between the walls of two adjacent cells.

Cytoplasmic division, organelles like mitochondria and plastids get distributed between the
two daughter cells.

Some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate
condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).

Significance of Mitosis

Equational division is usually restricted to the diploid cells only.
• Some lower plants and in some social insects haploid cells also divide by mitosis.
• Mitosis usually results in the production of diploid daughter cells with identical genetic
complement.
• Gowth of multicellular organisms is due to mitosis.

• Cell growth results in disturbing the ratio between the nucleus and the cytoplasm So, cell to
  divide to restore the nucleo-cytoplasmic ratio.
• Cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being
  constantly replaced by Mitosis.
• Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a
  continuous growth of plants throughout their life.

Meiosis

Gametes are formed from specialised diploid cells and meiosis division that reduces the
chromosome number by half results in the production of haploid daughter cells.

• Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing
  organisms whereas fertilisation restores the diploid phase. (NEET 2013)

Key Points

(I) Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and
meiosis II but only a single cycle of DNA replication.
(ii) Meiosis I is initiated after the parental chromosomes have replicated to produce identical
sister chromatids at the S phase.
(iii) Meiosis involves pairing of homologous chromosomes and recombination between nonsister
chromatids of homologous chromosomes.
(iv) Four haploid cells are formed at the end of meiosis II.

Meiotic events can be grouped under the following phases:
• Meiosis I = Prophase I , Metaphase I , Anaphase I , Telophase I
• Meiosis II = Prophase II ,Metaphase II , Anaphase II , Telophase II

Meiosis I

1. Prophase I = Division is typically longer and more complex when compared to prophase of
   mitosis.
   • Subdivided into five phases based on chromosomal behaviour
   (i) Leptotene (ii) Zygotene (iii) Pachytene (iv) Diplotene (v) Diakinesis
   (II) Leptotene - chromosomes become gradually visible under the light microscope.

• Compaction of chromosomes continues throughout leptotene.
  (ii) Zygotene - chromosomes start pairing together and this process of association is called
  synapsis. (NEET 2009,2016)
• 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.
• Complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a
  tetrad. (NEET 2013)
  ☆ Leptotene and Zygotene stages of prophase I are relatively short-lived.
  (iii) Pachytene - four chromatids of each bivalent chromosomes becomes distinct and clearly
  appears as tetrads. (NEET 2016)
• Appearance of recombination nodules, the sites at which crossing over occurs between nonsister
  chromatids of the homologous chromosomes. (NEET 2015)
• 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. (NEET 2012,2014)
• 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

(iv) Diplotene - 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. (NEET 2018)

• X-shaped structures, are called chiasmata.
• In oocytes of some vertebrates, diplotene can last for months or years.
• (v) Diakinesis - marked by terminalisation of chiasmata. (NEET 2015,2016)
• 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.

2. Metaphase I - bivalent chromosomes align on the equatorial plate. Microtubules from the opposite poles of the spindle attach to the kinetochore of homologous chromosomes.

• 3.Anaphase I - Homologous chromosomes
• separate, while sister chromatids remain
• associated at their centromeres.

4. Telophase I - Nuclear membrane and nucleolus reappear, cytokinesis follows and
   this is called as dyad of cells.

In many cases the chromosomes do undergo some dispersion, they do not reach
the extremely extended state of the interphase nucleus.
• Stage between the two meiotic divisions is called interkinesis and is generally short lived.

No replication of DNA during interkinesis.

Meiosis 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.

Prophase II - Nuclear membrane disappears by the end of prophase II. Chromosomes again become compact.

Metaphase II - Chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.

Anaphase II

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:- Resulting in the formation of tetrad of cells i.e., four haploid daughter cells

Significance of Meiosis

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