CELL DIVISION: MITOSIS AND MEIOSIS
What is Cell Division?
Cell division is the biological process by which a parent cell divides into two or more daughter cells. It is essential for growth, repair, reproduction, and genetic continuity in all living organisms.
There are two primary types of cell division in eukaryotes:
- Mitosis: for growth and tissue repair
- Meiosis: for sexual reproduction and genetic diversity
INTERPHASE: The Preparation Phase
Before any form of division begins, a cell goes through a long and crucial preparatory phase called interphase. Although not officially part of mitosis or meiosis, it is vital for preparing the cell.
Stages of Interphase:
G1 Phase (Gap 1)
- Cell grows physically larger
- Increases number of organelles
- Synthesizes proteins needed for DNA replication
- Normal metabolic activities take place
S Phase (Synthesis)
- DNA replication occurs
- Each chromosome becomes two identical sister chromatids
- Centrosome is also duplicated (important for spindle formation)
G2 Phase (Gap 2)
- Further growth
- Final preparation for mitosis or meiosis
- Checks for DNA damage and ensures all proteins for mitosis are available
MITOSIS: Division of Somatic Cells
Mitosis is the process by which a eukaryotic cell separates its duplicated genome into two identical sets. It is used for growth, repair, and asexual reproduction.
Importance of Mitosis:
- Maintains chromosome number (diploid to diploid)
- Produces genetically identical daughter cells
- Enables tissue regeneration and organismal growth
Stages of Mitosis:
Prophase
- Chromatin condenses into visible chromosomes
- Each chromosome has two sister chromatids
- Nuclear envelope begins to break down
- Spindle fibers start forming from centrosomes
- Centrosomes migrate to opposite poles
Metaphase
- Chromosomes align at the metaphase plate (cell equator)
- Spindle fibers attach to kinetochores (protein structures on centromeres)
Anaphase
- Sister chromatids are pulled apart to opposite poles
- Now each chromatid is an individual chromosome
- Cell elongates
Telophase
- Chromosomes decondense back into chromatin
- Nuclear envelopes re-form around each set of chromosomes
- Spindle fibers disappear
Cytokinesis (separate but overlaps telophase)
- Division of the cytoplasm
- Animal cells: Cleavage furrow forms and pinches the cell in two
- Plant cells: A cell plate forms, eventually becoming the new cell wall
Result of Mitosis:
Two identical diploid (2n) daughter cells
MEIOSIS: Division for Sexual Reproduction
Meiosis occurs in germ cells (sperm and egg) and reduces the chromosome number by half, creating haploid (n) cells for sexual reproduction.
Importance of Meiosis:
- Introduces genetic variation via independent assortment and crossing over
- Reduces chromosome number from diploid to haploid
- Essential for maintaining species' chromosome number over generations
Meiosis occurs in two sequential divisions: Meiosis I and Meiosis II.
MEIOSIS I: Reductional Division
1. Prophase I (Longest and most complex phase)
- Chromosomes condense
- Homologous chromosomes pair up (synapsis)
- Crossing over occurs between non-sister chromatids at the chiasmata
- Nuclear envelope dissolves
- Spindle forms
Key feature: Genetic recombination!
2. Metaphase I
- Homologous pairs (tetrads) line up at the metaphase plate
- Orientation is random (independent assortment)
3. Anaphase I
- Homologous chromosomes (not chromatids) are pulled to opposite poles
- Sister chromatids remain attached
4. Telophase I and Cytokinesis
- Nuclear envelopes may briefly reform
- Cytoplasm divides
Two haploid cells are formed (each with half the chromosome number)
MEIOSIS II: Equational Division
This phase resembles mitosis.
1. Prophase II
- Chromosomes re-condense
- New spindles form
- Nuclear envelope (if reformed) dissolves
2. Metaphase II
- Chromosomes align at the metaphase plate
3. Anaphase II
- Sister chromatids are separated and pulled to opposite poles
4. Telophase II and Cytokinesis
- Chromosomes decondense
- Nuclear membranes reform
- Cytoplasm divides
Result of Meiosis:
Four genetically unique haploid (n) cells