Cell Cycle and Cell Division

Cell Cycle

Introduction

• All living organisms start life from a single cell.
• Growth and reproduction in multicellular organisms occur through repeated cell divisions.
• Each parental cell divides to form two daughter cells.
  • These daughter cells can further grow and divide, producing millions of cells from a single original cell.
• Therefore, cell division is essential for growth, development, repair, and reproduction.

• During cell division, three processes must occur in a coordinated manner:
  • Cell growth
  • DNA replication
  • Cell division
• The orderly sequence of events through which a cell grows, duplicates its DNA, and divides into two daughter cells is called the cell cycle.

Phases of Cell Cycle

• The cell cycle has two main phases:

1. Interphase

• Phase between two successive M phases.
• Longest and most active phase of the cell cycle, occupying more than 95% of the total duration.
• Cell prepares for division by growing and replicating its DNA in an orderly manner.
• Interphase is divided into three phases:
• a. G₁ Phase (Gap 1 phase)
  • This phase occurs between mitosis and the initiation of DNA replication.
  • The cell is metabolically active and grows continuously.
  • RNA and proteins are synthesized, but DNA replication does not occur.
• b. S Phase (Synthesis phase)
  • During this phase, DNA synthesis or replication takes place.
  • The DNA content of the cell doubles from 2C to 4C, but the number of chromosomes remains unchanged (2n).
  • In animal cells, centriole duplication also occurs during this phase.
• c. G₂ Phase (Gap 2 phase)
  • This phase involves final preparation for mitosis.
  • Proteins required for cell division are synthesized, and cell growth continues.

• Some cells, such as heart cells, exit the cell cycle from the G₁ phase and enter a resting stage/non-dividing state called the quiescent stage (G₀).
• These cells remain metabolically active but do not divide unless required, such as in response to injury.

2. M Phase (Mitosis phase)

• The M phase is the phase of actual cell division.
• In a typical human cell cycle of 24 hours, the M phase lasts about one hour.
• This phase includes:
  • Karyokinesis – division of the nucleus
  • Cytokinesis – division of the cytoplasm
• During the M phase, replicated chromosomes are equally distributed to the two daughter cells.
• These events are under strict genetic control to ensure proper distribution of DNA to daughter cells.

Duration of Cell Cycle

• The duration of the cell cycle varies among organisms and cell types:
  • Human cells divide approximately once every 24 hours.
  • Yeast cells can complete the cell cycle in about 90 minutes.

Cell Division in Animals and Plants

• In animals, mitotic cell division occurs only in diploid somatic cells (except haploid male honey bees).
• In plants, mitotic division occurs in both haploid and diploid cells.

M Phase

Overview

• M Phase is the most dramatic phase of the cell cycle.
• It involves a major reorganisation of almost all cellular components.
• It is called an equational division because the number of chromosomes remains the same in the parent cell and the daughter cells.
• M phase completes cell division and includes nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis).
• Although mitosis is a continuous process, for convenience it is divided into four stages of karyokinesis:
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

1. Prophase

• First stage of mitosis.
• Follows S and G2 phases of interphase.
• DNA molecules formed during the S phase are initially intertwined.
• Key events:
  • Chromatin material condenses to form compact, visible chromosomes.
  • Each chromosome consists of two sister chromatids joined at a centromere.
  • The centrosome (duplicated during S phase) moves towards opposite poles.
  • Each centrosome gives rise to asters; together with spindle fibres they form the mitotic apparatus.
  • Golgi apparatus, endoplasmic reticulum, nucleolus, and nuclear envelope gradually disappear.
  • Completion of prophase is marked by the absence of the nuclear membrane and nucleolus.

2. Metaphase

• Metaphase begins with the complete disintegration of the nuclear envelope.
• Chromosomes are fully condensed and clearly visible, making this the best stage for studying chromosome morphology.
• Key events:
  • Each chromosome has two sister chromatids joined at the centromere.
  • Disc-shaped structures called kinetochores develop on centromeres.
  • Spindle fibres attach to kinetochores, thus attaching chromosomes to spindle fibers..
  • All chromosomes align at the equatorial plane of the cell, forming the metaphase plate.
  • Each chromatid is connected to spindle fibres from opposite poles.

• Key features of metaphase:
  • Spindle fibres attach to kinetochores.
  • Chromosomes are arranged at the spindle equator.

3. Anaphase

• Anaphase is the shortest stage of mitosis and begins simultaneously for all chromosomes.
• Key events:
  • Chromosomes (Centromeres) split simultaneously at the metaphase plate..
  • Sister chromatids separate and are now called daughter chromosomes.
  • Daughter chromosomes move towards opposite poles.
  • Centromeres lead the movement, with chromosome arms trailing behind.

Different shapes of chromosomes (V, L, J, or I shaped) may be seen depending on the position of the centromere (metacentric, submetacentric, acrocentric, or telocentric).

4. Telophase

• Final stage of nuclear division.
• Key events:
  • Chromosomes reach opposite poles and begin to decondense.
  • Distinct chromosomes lose their distinct shapes/identity, and form chromatin masses.
  • Nuclear envelope re-forms around each chromatin mass.
  • Nucleolus, Golgi apparatus, and endoplasmic reticulum reappear.
  • Final result: Two daughter nuclei are formed.

Cytokinesis

• Cytokinesis is the division of the cytoplasm and completes cell division.
• It usually begins during late anaphase and is completed during telophase.
• Key events:
  • In animal cells:
    • A cleavage furrow appears in the plasma membrane.
    • The furrow deepens centripetally and divides the cell into two daughter cells.
  • In plant cells:
    • Cytokinesis occurs by cell plate formation.
    • A cell plate forms at the centre and grows outward to meet lateral walls.
    • The cell plate develops into the middle lamella between the two daughter cells.
  • During cytokinesis, organelles such as mitochondria and plastids are distributed between the daughter cells.
  • Special Case: Sometimes, cells divide their nuclei (karyokinesis) but not their cytoplasm, resulting in multinucleate conditions called syncytium (e.g., liquid endosperm of coconut).

Additional Notes

• Amitosis:
  • A simple form of direct cell division without spindle formation.
  • Occurs in organisms like Paramecium, Chara, and in ageing or diseased cells.

• Mitotic poisons:
  • Chemicals that inhibit normal mitosis.
  • Colchicine inhibits spindle formation and arrests cells in metaphase.
  • Cyanide and azide inhibit prophase.
  • Mustard gas causes chromosome fragmentation and agglutination.

Significance of Mitosis

What is Mitosis?

• Mitosis, also called equational division, usually occurs in diploid cells where the chromosome number of parent and daughter cells remains the same.
• However, in some lower plants and social insects, haploid cells can also divide by mitosis.

Why is Mitosis Important?

• Growth:
  • Mitosis produces somatic cells and is essential for growth and development of multicellular organisms.
• Nucleo-Cytoplasmic Ratio:
  • Cell growth disturbs the balance between nuclear content and cytoplasm.
  • Mitosis restores the proper nucleo-cytoplasmic (surface-volume) ratio.
• Maintenance of Chromosome Number:
  • Mitosis ensures equal distribution of chromosomes so that all cells of an organism have the same number and type of chromosomes.
• Cell Repair, Healing, and Regeneration:
  • Mitosis replaces worn-out or damaged cells such as epidermal cells, gut lining cells, and blood cells.
  • It also helps in healing injuries and regeneration of lost parts.
• Reproduction:
  • In unicellular organisms, mitosis is the mode of reproduction.
• Plant Growth:
  • In plants, mitosis in meristematic tissues (like the apical meristem, shoot apical meristem, vascular cambium, and cork cambium) allows continuous growth throughout their life.

Meiosis

What is Meiosis?

• Meiosis is a special type of cell division in which the chromosome number is reduced to half, resulting in the formation of haploid daughter cells.
• It is essential for sexual reproduction as fertilisation restores the diploid chromosome number.

Key features of Meiosis:

• Meiosis involves two sequential divisions: Meiosis I and Meiosis II, but only one cycle of DNA replication.
  • Meiosis I is the reductional division.
• Homologous chromosomes pair and undergo recombination (exchange of genetic material).
• Results in four haploid cells at the end of Meiosis II.

When Does Meiosis Happen?

• During gametogenesis (formation of gametes) in plants and animals, leading to the formation of haploid gametes.

Meiosis I (Reduction Division)

• Meiosis I starts after interphase and reduces the chromosome number to half.
• It consists of four stages: Prophase I, Metaphase I, Anaphase I, and Telophase I.

1. Prophase I:

Prophase I is the longest and most complex stage of meiosis. It is divided into five substages:

1. Leptotene:
   • Chromosomes begin to condense and become visible.
   • In this stage, chromosomes may appear arranged in a bouquet-like manner.
2. Zygotene:
   • Homologous chromosomes pair with each other in a process called synapsis.
   • A synaptonemal complex forms.
   • Paired homologous chromosomes are called bivalents or tetrads.
3. Pachytene:
   • Each bivalent shows four chromatids.
   • Crossing over occurs between non-sister chromatids at recombination nodules with the help of the enzyme recombinase.
   • Genetic recombination is completed by the end of this stage.
4. Diplotene:
   • Synaptonemal complex dissolves.
   • Homologous chromosomes begin to separate but remain connected at crossover points called chiasmata.
   • In some vertebrate oocytes, this stage may last for years.
   • Lampbrush chromosomes represent diplotene chromosomes.
5. Diakinesis:
   • Chiasmata undergo terminalisation.
   • Chromosomes are fully condensed.
   • Meiotic spindle forms.
   • Nuclear envelope and nucleolus disappear, marking transition to metaphase I.

2. Metaphase I:

• Bivalent chromosomes align at the equatorial plate.
• Spindle fibres from opposite poles attach to homologous chromosomes.

3. Anaphase I:

• Homologous chromosomes separate and move to opposite poles.
• Sister chromatids remain attached at their centromeres.

4. Telophase I:

• Nuclear membrane and nucleolus reappear.
• Cytokinesis occurs, forming two haploid cells called a dyad.
• Chromosomes may partially decondense but do not reach interphase state.

Interkinesis:

• Interkinesis is a short resting phase between Meiosis I and Meiosis II.
• No DNA replication occurs during interkinesis.
• It is followed by Prophase II, which is much simpler than Prophase I.

Meiosis II

• Meiosis II begins immediately after cytokinesis of Meiosis I and resembles a normal mitotic division.
• Unlike Meiosis I, it does not involve reduction in chromosome number.

1. Prophase II:

• Meiosis II starts immediately after cytokinesis, usually before the chromosomes have fully elongated.
• Nuclear membrane disappears.
• Chromosomes again become compact.
• In contrast to Meiosis I, no synapsis or crossing over occurs in this stage.

2. Metaphase II:

• Chromosomes align at the equatorial plate.
• Spindle microtubules from opposite poles attach to the kinetochores of sister chromatids.

3. Anaphase II:

• Centromeres split simultaneously.
• Sister chromatids separate and move towards opposite poles due to shortening of spindle fibres.
• Each chromatid now behaves as an independent chromosome.

4. Telophase II:

• Chromosomes reach the poles and get enclosed by a newly formed nuclear envelope.
• Cytokinesis follows, resulting in the formation of four haploid daughter cells, collectively called a tetrad.

Significance of Meiosis

• Meiosis maintains the specific chromosome number of a species across generations by reducing the chromosome number to half in gametes and restoring it during fertilisation.
• It produces haploid cells required for sexual reproduction.
• Meiosis increases genetic variability in populations due to crossing over and independent assortment of chromosomes.
  • This variability is essential for adaptation and evolution.

Key Points

• Meiosis I = reductional division.
• Meiosis II = equational division.
• Final outcome of meiosis is 4 haploid, genetically different daughter cells.

Chapter Summary

• According to cell theory, new cells arise from pre-existing cells.
  • This process is known as cell division.
• In sexually reproducing organisms, life begins from a single-celled zygote.
• Cell division continues throughout the life cycle of an organism and is essential for growth, repair, and reproduction.

• The cell cycle is the sequence of events from one cell division to the next.
• It consists of two main phases: Interphase and Mitosis (M phase).

• Interphase is the preparatory phase for cell division. It includes:
  • G1 Phase: The cell grows and carries out normal metabolic activities.
    • Cell organelles duplicate during this phase.
  • S Phase: DNA replication occurs, resulting in duplication of chromosomes.
  • G2 Phase: Further cytoplasmic growth occurs, and the cell prepares for mitosis.

• Mitosis (M Phase) is the phase of actual cell division. It is divided into four stages:
• Prophase:
  • Chromosomes condense and become visible.
  • Centrioles move to opposite poles.
  • The nuclear envelope and nucleolus disappear, and spindle fibres begin to form.
• Metaphase:
  • Chromosomes align at the equatorial plate of the cell.
• Anaphase:
  • Centromeres divide, and sister chromatids separate and move towards opposite poles.
• Telophase:
  • Chromatids reach the poles, chromosomes elongate, and the nuclear membrane and nucleolus reappear.

• Cytokinesis follows mitosis and involves division of the cytoplasm, resulting in two daughter cells.
• Mitosis conserves the chromosome number, producing daughter cells genetically identical to the parent cell.

• Meiosis is a type of cell division that occurs in diploid cells to form gametes and is therefore called reduction division.
• It reduces the chromosome number to half in gametes, which is restored during fertilisation in sexual reproduction.

• Meiosis I is the reductional division.
  • Homologous chromosomes pair to form bivalents and undergo crossing over.
• Prophase I is divided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
• During metaphase I, bivalents align at the equatorial plate.
• In anaphase I, homologous chromosomes separate and move to opposite poles.
• Telophase I results in the formation of two haploid cells.

• Meiosis II is similar to mitosis and is an equational division.
• During anaphase II, sister chromatids separate.
• At the end of meiosis II, four haploid daughter cells are formed.