The Cell Cycle and Mitosis

The Cell Cycle and Mitosis

The Cell Theory

  • All living things are composed of one or more cells.

  • Cells serve as the basic unit of life.

  • All cells originate from pre-existing cells, which occurs through cell division.

Binary Fission

  • Prokaryotic reproduction through binary fission, exemplified by bacteria.

  • Process Details: Similar to mitosis where chromosomes replicate, and two daughter chromosomes migrate apart as the cell elongates, but it occurs without a nucleus.

Mitosis: Division of the Nucleus

  • Major processes involved:   - Breakdown of the nuclear membrane.   - Separation of sister chromatids.   - Loosely coiled replicated chromosomes.   - Appearance of spindle fibers.   - Chromosomes condense for visibility.   - Spindle fibers attach to chromosomes.

  • Stages of Mitosis:   - Prophase: Chromosomes condense, spindle fibers form, and the nuclear envelope breaks down.   - Prometaphase: Spindle fibers attach to chromosomes; nuclear membrane continues to dissolve.   - Metaphase: Chromosomes align at the cell's equatorial plane.   - Anaphase: Sister chromatids are pulled apart towards opposite poles.   - Telophase: Chromosomes decondense, and nuclear envelope reforms.   - Cytokinesis: Cytoplasm divides, forming two identical daughter cells.

Overview of the Cell Cycle

  • Purpose: To replicate cells, yielding two genetically identical daughter cells from a parent cell.

  • Phases: Comprises interphase (preparatory phases) and mitosis followed by cytokinesis (actual cell division).

Cell Division vs. Mitosis

  • Cell Division: Refers generally to the splitting of one cell into two (occurs in prokaryotes).

  • Mitosis: Specific to the division of the nucleus in eukaryotic cells, resulting in each daughter cell having its own nucleus.

Vocabulary

  • Chromosome: Long DNA molecule, typically containing thousands of genes.

  • Chromatin: The form of genetic material during interphase; "thread-like", uncoiled, long, and thin.

  • Chromosome (replicated): Coiled, short & thick, X-shaped; visible under microscope during mitosis.

Purpose of Cell Division

  • Enables a parent cell to divide and create two genetically identical diploid daughter cells.

  • Diploid Cells: Contain two copies of chromosomes (one from mother, one from father)—23 pairs in humans.

DNA, Diploid & Haploid

  • Sexually reproducing organisms receive genetic material from both parents (diploid).

  • To reproduce, organisms must form gametes (haploid) with half the chromosome number.

  • Fusion of two haploid gametes forms a diploid zygote.

Diploid vs. Haploid

  • Diploid (2n): Nuclei with paired homologous chromosomes; all somatic (body) cells are diploid.

  • Haploid (n): Nuclei with only one set of chromosomes; sex cells (gametes) are haploid, formed from diploid cells via meiosis.

Differences between Diploid and Haploid

  • Diploid (2n): Two copies of each chromosome.

  • Haploid (n): One copy of each chromosome.

Function of Mitosis

  • Mitosis ensures the maintenance of chromosome number from one cell to the next (2n = 46 in humans).

  • Occurs in somatic cells, while meiosis leads to gamete formation (n = 23 in humans).

Karyotype

  • Chromosomes arranged in pairs; one from each parent.

  • Chromosome pairs 1-22 are autosomes; 23rd pair are sex chromosomes (XX for female, XY for male).

Interphase

  • Divided into phases:   - G1 Phase: Cell grows and prepares for mitosis by building proteins.   - S-phase: Duplication of genetic material.   - G2 Phase: Second growth phase that produces organelles and structures (e.g., centrioles).

  • DNA Replication: Occurs only in the S phase. DNA is chromatin, uncoiled during interphase, and is double-stranded.

Changes in Genetic Material

  • Interphase: DNA is not duplicated or condensed (chromatin).

  • Mitosis: DNA is duplicated and condensed into chromosomes (X-shaped).

Prophase

  • Chromatin condenses into visible chromosomes.

  • The nuclear membrane and nucleolus break down; centrioles move apart and form spindle fibers.

Metaphase

  • Nuclear membrane has disappeared; chromosomes align at the equatorial plate, attached by spindle fibers to centrioles.

Anaphase

  • Centromeres split, and spindle fibers pull sister chromatids to opposite poles; these are now referred to as daughter chromosomes.

Telophase

  • Daughter chromosomes uncoil and lengthen; nuclear membrane and nucleolus reform.

  • Chromosomes decondense, and cytoplasm begins to divide (cytokinesis).

Cytokinesis

  • Division of cytoplasm and organelles; in animals, forms a cleavage furrow; in plants, creates a cell plate.

Summary of Mitosis Stages (IPMAT)

  • I: Interphase

  • P: Prophase

  • M: Metaphase

  • A: Anaphase

  • T: Telophase

  • C: Cytokinesis

Differences Between Animal and Plant Cells During Mitosis

  • Animal Cells: Cell membrane pinches to create cleavage furrow.

  • Plant Cells: Vesicles deliver materials to create a cell plate between two nuclei.

Regulation of Cell Division

  • Different cells divide at different rates; example: skin cells divide more frequently than nerve or muscle cells.

  • Cell division is controlled by specific chemical signals and a distinct cell cycle control system akin to a clock.

Cell Cycle Control System

  • Internal Controls: Chemical signaling within the cytoplasm.

  • External Controls: Availability of nutrients and growth factors.

  • Checkpoints: G1, G2, and M checkpoints, where the cycle pauses until a go-ahead signal is received.

Importance of Checkpoints

  • The G1 checkpoint is crucial; a "green light" signal allows completion of S, G2, and M phases, leading to division.

  • If the signal isn't received, the cell enters G0 phase, a non-dividing state, which can lead to apoptosis (programmed cell death).

Consequences of Unchecked Cell Cycle

  • Healthy cells undergo periodic division and repair DNA damage; they can apoptose if necessary.

  • Cancer cells divide uncontrollably, accruing DNA damage and errors during mitosis; this leads to tumor formation.