Cell Cycle and Cell Division

Cell Cycle

Introduction

• All organisms start life from a single cell.
• A single cell divides to form large organisms.
• Cells grow and reproduce by dividing into two daughter cells.
• This cycle of growth and division leads to millions of cells from one cell.
• Cell division is crucial for all living organisms.
• During cell division:
  • DNA replication occurs
  • Cell growth occurs
• These processes must be coordinated to ensure proper division.
• The sequence of events in cell growth, DNA duplication, and division is called the cell cycle.

Phases of Cell Cycle

• The cell cycle has two main phases:
  1. Interphase
     • The cell grows and prepares for division.
     • DNA synthesis occurs only during this phase.
  2. M Phase (Mitosis phase)
     • The replicated DNA is distributed to daughter nuclei.
     • Controlled by genetic factors.
• Example: Human cells divide once every 24 hours.
• Yeast cells can complete the cycle in about 90 minutes.

M Phase (Mitosis Phase)

• M Phase is when actual cell division (mitosis) happens.
• In a human cell cycle of 24 hours, cell division lasts about 1 hour.
• It starts with nuclear division (karyokinesis) and ends with cytoplasm division (cytokinesis).

Interphase

• Interphase is the phase between two M phases.
• It lasts more than 95% of the cell cycle.
• The cell prepares for division through growth and DNA replication.
• Interphase has three phases:
  1. G1 Phase (Gap 1)
  2. S Phase (Synthesis)
  3. G2 Phase (Gap 2)

G1 Phase (Gap 1)

• Occurs between mitosis and DNA replication.
• The cell is metabolically active and grows continuously.
• No DNA replication happens here.

S Phase (Synthesis)

• DNA synthesis or replication occurs.
• DNA amount doubles from 2C to 4C, but chromosome number remains the same (2n).

G2 Phase (Gap 2)

• Proteins are synthesized in preparation for mitosis.
• Cell growth continues.

Special Cases

• Some cells, like heart cells, do not divide and enter a resting stage called G0 Phase (quiescent stage).
• These cells are metabolically active but do not divide unless necessary.

Cell Division in Animals and Plants

• In animals, mitotic cell division occurs in diploid somatic cells.
• In plants, mitosis can happen in both haploid and diploid cells.

M Phase

Overview

• M Phase is the most dramatic period of the cell cycle.
• It involves major reorganization of the cell.
• Also called equational division because the number of chromosomes remains the same in parent and progeny cells.
• Divided into four stages:
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

Prophase

• First stage of mitosis.
• Follows S and G2 phases of interphase.
• Key events:
  • Chromosomal material condenses into compact chromosomes.
  • Chromosomes consist of two chromatids joined at the centromere.
  • Mitotic spindle starts forming.
  • Centrioles move towards opposite poles.
  • Golgi complexes, endoplasmic reticulum, nucleolus, and nuclear envelope disappear.

Metaphase

• Second stage of mitosis.
• Key events:
  • Nuclear envelope disintegrates.
  • Chromosomes fully condense and are visible under a microscope.
  • Each chromosome is made up of two sister chromatids joined by a centromere.
  • Kinetochores form on centromeres, attaching chromosomes to spindle fibers.
  • Chromosomes align at the cell’s equator, forming the metaphase plate.
  • Spindle fibers from opposite poles attach to kinetochores of sister chromatids.

Anaphase

• Third stage of mitosis.
• Key events:
  • Chromosomes split at the metaphase plate.
  • Daughter chromatids, now chromosomes, move to opposite poles.
  • Centromeres lead the way, with chromosome arms trailing behind.

Telophase

• Final stage of mitosis.
• Key events:
  • Chromosomes reach the poles and decondense.
  • Chromosomes lose their distinct shapes, forming a chromatin mass.
  • Nuclear envelope reassembles around chromosome clusters.
  • Nucleolus, golgi complex, and endoplasmic reticulum reform.

Cytokinesis

• Division of the cell’s cytoplasm.
• Key events:
  • In animal cells:
    • A furrow appears in the plasma membrane.
    • Furrow deepens and splits the cell into two daughter cells.
  • In plant cells:
    • Cell wall formation starts in the center and grows outward.
    • A cell plate forms, becoming the middle lamella between new cell walls.
  • Organelles like mitochondria and plastids are distributed between the daughter cells.
  • Sometimes, cells divide their nuclei (karyokinesis) but not their cytoplasm, forming multinucleate cells (e.g., liquid endosperm in coconut).

Significance of Mitosis

What is Mitosis?

• Mitosis, also called equational division, usually occurs in diploid cells.
• Some lower plants and social insects can also have haploid cells divide by mitosis.

Why is Mitosis Important?

• Growth: Mitosis helps multicellular organisms grow.
• Nucleo-Cytoplasmic Ratio: Cell growth disturbs the balance between the nucleus and the cytoplasm. Mitosis helps restore this balance.
• Cell Repair: Mitosis replaces cells in the epidermis (skin), lining of the gut, and blood cells.
• Plant Growth: In plants, mitosis in meristematic tissues (like the apical and lateral cambium) allows continuous growth throughout their life.

Meiosis

What is Meiosis?

• Meiosis is a special cell division that reduces the chromosome number by half, producing haploid daughter cells.
• It is essential for sexual reproduction, ensuring that offspring have the correct chromosome number.

Key features of Meiosis:

• Involves two cycles of division: Meiosis I and Meiosis II, but only one cycle of DNA replication.
• Starts after chromosomes replicate to form identical sister chromatids.
• Involves pairing of homologous chromosomes and 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

Prophase I:

• Leptotene: Chromosomes become visible and start compacting.
• Zygotene: Chromosomes pair up (synapsis), forming homologous pairs (bivalents or tetrads).
• Pachytene: Tetrads clearly visible, crossing over (exchange of genetic material) happens at recombination nodules with the help of the enzyme recombinase.
• Diplotene: Synaptonemal complex dissolves, homologous chromosomes start to separate but remain connected at crossover points (chiasmata).
• Diakinesis: Chromosomes fully condense, meiotic spindle forms, nucleolus and nuclear envelope disappear, transitioning to metaphase.

Metaphase I:

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

Anaphase I:

• Homologous chromosomes separate.
• Sister chromatids remain connected at centromeres.

Telophase I:

• Nuclear membrane and nucleolus reappear.
• Cytokinesis occurs, forming two cells (diad of cells).
• Chromosomes may slightly disperse but not fully.
• Followed by a short stage called interkinesis before the next division.

Interkinesis:

• A short resting phase between the two meiotic divisions.
• Followed by Prophase II, which is simpler than Prophase I.

Meiosis II

Prophase II:

• Starts right after cytokinesis.
• Chromosomes do not fully elongate before starting.
• Nuclear membrane disappears.
• Chromosomes become compact again.

Metaphase II:

• Chromosomes align at the equator.
• Microtubules attach to kinetochores of sister chromatids.

Anaphase II:

• Centromeres split, separating sister chromatids.
• Sister chromatids move to opposite poles.

Telophase II:

• Chromosomes get enclosed by a nuclear envelope.
• Cytokinesis follows, forming four haploid daughter cells.

Significance of Meiosis

• Maintains the specific chromosome number in species across generations.
• Reduces chromosome number by half in sexually reproducing organisms.
• Increases genetic variability, which is important for evolution.

Chapter Summary:

• According to cell theory, cells arise from preexisting cells.
• This process is called cell division.
• Any sexually reproducing organism starts from a single-celled zygote.
• Cell division continues throughout the life cycle.
• The cell cycle includes stages from one division to the next.
• Cell cycle has two phases: Interphase and Mitosis (M phase).
• Interphase: Preparation for cell division.
  • G1 Phase: Cell grows and carries out normal metabolism; organelles duplicate.
  • S Phase: DNA replication and chromosome duplication occur.
  • G2 Phase: Cytoplasmic growth happens.
• Mitosis (M Phase): Actual cell division.
  • Divided into four stages: Prophase, Metaphase, Anaphase, Telophase.
  • Prophase: Chromosomes condense, centrioles move to opposite poles, nuclear envelope and nucleolus disappear, spindle fibers appear.
  • Metaphase: Chromosomes align at the equatorial plate.
  • Anaphase: Centromeres divide, chromatids move to opposite poles.
  • Telophase: Chromatids reach poles, chromosomes elongate, nuclear membrane and nucleolus reappear.
• Cytokinesis: Cytoplasmic division follows nuclear division.
• Mitosis conserves the chromosome number in daughter cells.
• Meiosis: Occurs in diploid cells forming gametes, called reduction division.
  • Reduces chromosome number by half in gametes.
  • Chromosome number is restored in offspring during sexual reproduction.
  • Meiosis I: Homologous chromosomes pair, form bivalents, and undergo crossing over.
    • Prophase I has five stages: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
    • Metaphase I: Bivalents align on the equatorial plate.
    • Anaphase I: Homologous chromosomes move to opposite poles.
    • Telophase I: Nuclear membrane and nucleolus reappear.
  • Meiosis II: Similar to mitosis.
    • Anaphase II: Sister chromatids separate.
    • Four haploid cells are formed at the end.