Cell Cycle and Division

Overview

This lecture covers the cell cycle, including mitosis and meiosis, detailing their phases, significance, and differences in plant and animal cells.

All organisms start life as a single cell that divides to form more cells.
The cell cycle is the series of events leading to cell duplication and division.
DNA synthesis occurs during a specific stage in the cycle, not continuously.
The typical cell cycle includes two phases: interphase and M phase (mitosis).
The duration of the cell cycle varies, e.g., about 24 hours in humans and 90 minutes in yeast.

Interphase is the non-dividing phase where cell growth and DNA replication occur.
It consists of three stages: G1 (cell grows, normal functions, no DNA replication), S (DNA replicates, DNA amount doubles), and G2 (growth continues, proteins for division synthesized).
Some cells exit G1 into G0, a quiescent stage, and only divide when needed.

Mitosis (karyokinesis) is the division of the nucleus, typically followed by cytokinesis (division of cytoplasm).
Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase.
In animals, cytokinesis occurs by furrow formation; in plants, by forming a cell plate.
Mitosis produces two identical daughter cells (equational division) with the same chromosome number as the parent.

Ensures growth, tissue repair, and replacement of dead cells.
Occurs in somatic (body) cells in animals; both haploid and diploid cells in plants.
Maintains chromosome number and restores the nuclear-cytoplasmic ratio.

Meiosis occurs in diploid germ cells to form haploid gametes (sperm/egg).
Involves two sequential divisions: meiosis I (homologous chromosomes separate) and meiosis II (sister chromatids separate).
Meiosis I includes prophase I (with crossing over), metaphase I, anaphase I, telophase I.
Meiosis II resembles mitosis: prophase II, metaphase II, anaphase II, telophase II.
Results in four genetically diverse haploid cells due to crossing over in prophase I.

Reduces chromosome number by half, maintaining species chromosome number after fertilization.
Introduces genetic variation, important for evolution.

Cell Cycle — Series of events in which a cell grows, duplicates DNA, and divides.
Interphase — Growth phase between cell divisions; includes G1, S, and G2.
Mitosis — Division of the nucleus into two identical daughter nuclei.
Cytokinesis — Division of the cytoplasm to form two daughter cells.
G0 Phase — Resting phase where cells exit the cell cycle.
Meiosis — Reduction division creating four haploid cells from one diploid germ cell.
Crossing Over — Exchange of genetic material between homologous chromosomes during prophase I of meiosis.
Karyokinesis — Division of the cell’s nucleus.
Synaptonemal Complex — Protein structure facilitating synapsis and crossing over.
Chiasmata — Sites where crossing over occurs between chromosomes.

Review details of mitosis and meiosis phases for diagrams and key differences.
Prepare for questions on the significance of cell division in various cell types.
Study the definitions and steps of interphase, mitosis, and meiosis thoroughly.