Cell Division and The Cell Cycle

Cell Division Overview

• Cell division is the process where a cell divides into two or more daughter cells.
• Cell division is essential for growth and reproduction in living organisms.
• The cell cycle encompasses all processes from the birth of a cell to its division into new cells.
• After formation, daughter cells grow, duplicate organelles and DNA, and then divide.

Cell Cycle Definition

• It's a repetitive process allowing for the creation of millions of cells from a single cell.
• The cell cycle includes cell growth, organelle duplication, DNA replication, and cell division.

The Need for Cell Division

• Cytoplasmic growth is a continuous process in active cells.
• As cells grow, the nucleus-to-cytoplasm ratio decreases.
• Cell division restores the appropriate nucleus-to-cytoplasm ratio.
• Dividing the cytoplasm and nuclear content ensures the nucleus can manage the cell effectively.

Events and Coordination

• Organelle duplication, DNA replication, and actual cell division must occur in a sequence and harmony.
• Checkpoints, regulated by cyclin proteins and cyclin-dependent kinases (CDKs), ensure proper event completion.
• Checkpoints prevent mistakes during DNA replication.
• These checkpoints ensure the daughter cells receive an intact genome.

Time Variation in cell cycle

• Different cells divide at diverse rates.
• E. coli has a 20-minute cell cycle.
• Yeast cells divide in 90 minutes.
• Human cheek cells in culture take 24 hours for a complete cell cycle.
• Heart and neuron cells in adult humans lack the capability to divide.
• Stomach lining cells regenerate every 3-4 days due to acid exposure.

Cell Cycle Phases

• The cell cycle has two main phases: interphase and M phase (mitosis).
• M phase splits into karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).

Interphase

• It is a preparation phase where the cell grows, duplicates organelles, and replicates DNA.
• It is divided into G1 (gap 1), S (synthesis), and G2 (gap 2) phases.
• More than 95% of the cell cycle is spent in interphase.
• It lasts about 23 hours in a 24-hour human cell cycle.
• Cell size increases during interphase

M Phase

• It is the actual division phase where the nucleus and cytoplasm divide.
• It is a smaller time frame of the cell cycle.
• M phase lasts about 1 hour in a 24-hour human cell cycle.
• M phase is divided into prophase, metaphase, anaphase, and telophase (PMAT).

Interphase Subphases: G1 Phase

• It precedes the S phase (DNA synthesis).
• The cell accumulates energy (ATP) required for DNA replication.
• RNA, amino acids, and proteins are produced.
• Most cell organelles duplicate.

Interphase Subphases: S Phase

• DNA replication occurs and the DNA content doubles.
• Centriole duplication also happens.
• Histone proteins are synthesized to pack the newly replicated DNA.
• DNA content doubles (from 2c to 4c), but the number of chromosomes remains unchanged.

Interphase Subphases: G2 Phase

• Remaining organelles like mitochondria and chloroplasts duplicate.
• Synthesis of RNA, amino acids, and proteins, including tubulin for spindle fibers.
• Tubulin protein forms mitotic spindle

Significance of Mitosis

• Growth of the body from a single-celled zygote occurs via mitosis.
• Repair of injuries and replacement of lost cells.
• Red blood cells are replaced after 120 days.
• Plants are mostly diploid, but mosses and algae are haploid, hence mitosis differs.
• Mitosis in diploid cells results in two identical diploid daughter cells and vice versa for haploid cells.
• Some social insects undergo mitosis in haploid cells.

Mitotic Apparatus

• Centrioles move to opposite poles of the cell, forming asters in animal cells.
• Plant cells lack centrioles and asters.
• The mitotic apparatus includes asters (if present) and spindle fibers.

The Mechanism of Mitosis: Prophase

• DNA condenses into visible chromosomes.
• Centrioles move toward opposite poles.
• The nuclear envelope, nucleolus, endoplasmic reticulum, and Golgi apparatus disintegrate.

The Mechanism of Mitosis: Metaphase

• Chromosomal condensation is complete.
• Chromosomes align at the metaphase plate (equatorial plate).
• Spindle fibers attach to kinetochores of the centromeres.
• Chromosome morphology is most visible at metaphase.

The Mechanism of Mitosis: Anaphase

• Centromeres split, separating sister chromatids, which move to opposite poles.
• Separated chromatids are now called chromosomes.
• The centromere faces the poles, with the arms trailing behind.

The Mechanism of Mitosis: Telophase

• Chromosomes decondense and lose their identity.
• The nuclear envelope, ER, Golgi complex, and nucleolus reappear.
• Two nuclei form at opposite poles.

Cytokinesis

• Division of the cytoplasm after nuclear division.
• In animal cells, a cleavage furrow forms from the outside, pinching the cell into two.
• In plant cells, a cell plate forms from the center, creating a new cell wall.
• The cell plate consists of ER and Golgi complex fragments.
• The cell plate becomes the middle lamella between two plant cells.
• Cytokinesis is a centripetal division via the fusion of vesicles

Abnormalities in Cytokenisis

• If cytokinesis does not occur after karyokinesis, it leads to a multinucleated condition (syncytium).
• Liquid endosperm in coconut is an example of syncytium in plants.

Meiosis: Reduction Division

• Meiosis is the reductional division that halves the chromosome number in daughter cells.
• One round of DNA replication (S phase) is followed by two cell divisions (meiosis I and meiosis II).
• Meiosis produces four genetically different daughter cells.
• It maintains chromosome number consistency across generations in sexually reproducing organisms.

The Importance of Reduction Division

• Maintains constant chromosome numbers from one generation to the next.
• The haploid gametes fuse to restore the diploid number in the offspring.
• Creates genetic variation through recombination and crossing over during prophase I.

Recombination and Crossing Over

• Occurs between non-sister chromatids of homologous chromosomes in prophase I (pachytene stage).
• Recombinase enzyme facilitates the exchange of genetic material.

Meiosis in Comparison to Mitosis

• In mitosis, diploid cell --> two diploid daughter cells.
• In meiosis, diploid cell --> four haploid daughter cells.

Meiosis I and Meiosis II

• Meiosis I is the reductional division, halving the chromosome number.
• Meiosis II is an equational division, similar to mitosis.

Meiosis I

• Contains prophase I, metaphase I, anaphase I, and telophase I.
• The same subphases as mitosis are still present

Interkinesis

• A short rest phase between meiosis I and meiosis II.
• The chromosomes partially decondense but do not replicate DNA.

Meiosis II

• Equational division just like mitosis.
• A continuation of the steps of meiosis

Phases and Functions

• The interphase before meiosis is similar to that before mitosis.
• A cell doubles its DNA content, duplicates organelles, and accumulates necessary proteins.

Detailed Prophase One

• One of the longest phases that is further divided: leptotene, zygotene, pachytene, diplotene, and diakinesis.

Prophase One Subphases: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis

• Leptotene: Chromosomes begin to condense and become visible under a light microscope.
• Zygotene: Homologous chromosomes pair up (synapsis) via the synaptonemal complex (bivalent or tetrad formation).
• Pachytene: Crossing over occurs, and the tetrad is visible.
• Diplotene: Synaptonemal complex dissolves, and chiasmata (X-shaped structures) become visible.
• Diakinesis: Further chromosome condensation, nuclear envelope breakdown, and spindle fiber formation.

Prophase Mnemonic

• The phrase "La Zara Pani De De" is a mnemonic device used to remember the stages of phrophase one

Chromosal Arrest

• Some oocytes can remain arrested in the diplotene stage for months or years.