Ch(10) cell divison

Cell Division

Functions of Cell Division

  • Cell division serves two key functions:

    • In multicellular organisms, it is essential for:

    • Growth

    • Maintenance of cells

    • Repair of tissues

    • In single-celled organisms, it is used for reproductive purposes.

Overview of the Genome

  • The cell's DNA constitutes its genome.

    • In Prokaryotes:

    • The genome is made up of a single double-stranded, circular DNA molecule.

    • This molecule resides within the nucleoid region of the cell.

    • Additional smaller loops of DNA, known as plasmids, may also be present:

      • Plasmids are not essential for normal growth but can carry important genetic information.

      • They allow for gene transfer between prokaryotic cells via the exchange of plasmids.

The Eukaryotic Genome

  • Eukaryotic organisms possess a more complex genomic structure:

    • It consists of several double-stranded DNA molecules organized into chromosomes.

Eukaryotic Chromosomes

  • The number of chromosomes in the cell nucleus can vary significantly across species.

  • Within a single species, the chromosome number remains consistent, but it may differ depending on the developmental stage or specific cell type.

Types of Cells

  • Somatic Cells:

    • Typically contain 2 matched sets of chromosomes, making them diploid (2n).

  • Gametes (reproductive cells e.g., eggs and sperm):

    • These cells contain only half the number of chromosomes, classifying them as haploid (1n).

Karyotype and Chromosomal Organization

  • Arranging chromosomes by size leads to the creation of a karyotype.

  • Homologous chromosomes:

    • Chromosomes that come in pairs during the reproduction of diploid cells.

  • Heterologous pairs:

    • These are non-matching chromosome pairs, exemplified by the human X and Y chromosomes.

Organization of Eukaryotic Chromosomes

  • Eukaryotic DNA must be compacted to fit within the nucleus,

    • This is achieved through several structural modifications:

    • The DNA double helix wraps around a core of 8 histone proteins, resembling a string of beads.

    • This complex (the bead) is termed a nucleosome, while the connecting DNA (the string) is referred to as linker DNA.

    • These nucleosomes coil to form chromatin fibers, which further condense to become distinct chromosomes.

The Cell Cycle

  • Defined as an ordered series of events in a cell's life.

    • Two major phases exist:

    • Interphase: Time allotted for normal growth and preparation for cell division.

    • Mitotic phase: Involves the separation of replicated DNA and cytoplasmic material leading to daughter cells.

Stages of Interphase

  • Interphase is divided into three distinct stages:

    • G1 Phase (first gap):

    • No visible changes occur, but the cell is biochemically active.

    • S Phase:

    • Occurs synthesis of DNA, resulting in identical copies of DNA molecules called sister chromatids which are joined at the centromere.

    • Centrosomes are produced to help in chromosome movement; in animal cells, they are associated with centrioles that assist in organizing cell division.

    • G2 Phase (second gap):

    • Involves energy replenishment, reproduction of organelles, and breakdown of the cytoskeleton.

The Mitotic Phase

  • Comprises two essential steps:

    • Karyokinesis: Also termed mitosis or nuclear division, which is the first part of the mitotic phase.

    • Cytokinesis: The physical separation of cellular components into two daughter cells.

Process of Karyokinesis (Mitosis)

  • Phases of Karyokinesis:

    • Prophase:

    • Chromosomes condense and become visible.

    • Spindle fibers emerge from the centrosomes.

    • The nuclear envelope dissolves and the nucleolus disappears.

    • Sister chromatids coil tightly with the aid of condensin proteins.

    • Prometaphase:

    • Formation of kinetochores at the centromeres, which connect sister chromatids to spindle fibers.

    • Metaphase:

    • Chromosomes align along the metaphase plate with sister chromatids attached by cohesion proteins.

    • Anaphase:

    • Cohesin proteins disintegrate, allowing chromatids to separate and move toward opposite poles of the cell aided by microtubules.

    • The cell elongates during this process.

    • Telophase:

    • Chromosomes arrive at opposite poles and start to decondense.

    • Nuclear envelopes regenerate around each set of chromosomes.

    • The mitotic spindle disassembles, and nucleoli reappear.

Cytokinesis

  • The end stage of the mitotic phase entails the division of the cytoplasm which varies between cell types:

    • Animal Cells:

    • Form a cleavage furrow facilitated by a contractile ring.

    • Plant Cells:

    • Form a cell plate from Golgi vesicles.

Regulation of Cell Cycle

  • Ensures that new cells replicate the original cell correctly and checks for potential functional errors, such as mutations or incorrect chromosome numbers.

  • This regulation occurs at three checkpoints in the cell cycle:

    1. Near the end of G1 Phase.

    2. At the transition from G2 to Mitosis.

    3. In Metaphase of Mitosis.

G1 Checkpoint

  • Determines if environment conditions are suitable for division:

    • Evaluates external influences, cell reserves, and cell size.

    • Checks for DNA damage;

    • If requirements are unmet, the cell can:

    • Halt the cycle and attempt repairs.

    • Enter G0 phase and await favorable conditions.

G2 Checkpoint

  • Prevents entry into mitosis if conditions are unfavorable:

    • Verifies cell size and protein reserves.

    • Ensures all chromosomes are correctly replicated and DNA is undamaged:

    • If issues arise, the cycle halts for replication or repair attempts.

M Checkpoint (Spindle Checkpoint)

  • Occurs near the end of metaphase:

    • Confirms correct attachment of sister chromatids to spindle microtubules.

    • The cycle cannot progress until kinetochores are firmly anchored to two spindle fibers,

    • Non-correction may lead to non-disjunction of chromatids.

Molecular Regulators of the Cell Cycle

  • Two main groups of molecules operate within cells to regulate the cell cycle:

    • Positive Regulators: Promote progression to subsequent steps of the cycle.

    • Negative Regulators: Inhibit advancement through the cycle.

Cyclins and Cyclin-Dependent Kinases (Cdks)

  • The levels of cyclins and Cdks fluctuate predictably throughout the cell cycle.

  • Internal or external signals may stimulate increases in cyclin levels.

Mechanism of Positive Regulators

  • The functionality of positive regulators:

    • Cdk binds with Cyclin to form an active complex.

    • The complex adds phosphate groups to target proteins to advance the cell cycle.

Negative Regulatory Molecules

  • Some well-studied negative regulators include:

    • Retinoblastoma protein (Rb),

    • p53, and

    • p21.

    • Primarily active at G1 checkpoint to prevent undesirable cycle progression.

Cancer and the Cell Cycle

  • Cancer entails disparate diseases linked to uncontrolled cell proliferation:

    • Gene mutations lead to malfunctioning proteins that oversee cell reproduction.

    • Resulting tumors emerge when the proliferation of mutated cells outpaces normal cell growth.

Proto-Oncogenes

  • Normal genes that encode proteins for positive cell cycle regulators:

    • Certain mutations can transmute proto-oncogenes into oncogenes, leading to excessive promotion of cell division.

Tumor Suppressor Genes

  • Segments of DNA that code for proteins inhibiting uncontrolled cell division:

    • An active form can curb division, whereas mutations in these genes may hinder regulation, exemplified by cervical cancer risks.

Prokaryotic Cell Division

  • Binary fission is the sole method of cell propagation in prokaryotes (e.g., bacteria).

Steps of Binary Fission in Prokaryotes

  1. Replication of the circular prokaryotic chromosome initiates at the origin and moves in both directions.

  2. As the cell elongates, FtsZ proteins migrate to the midpoint.

  3. Duplicated chromosomes separate, progressing toward opposite ends of the cell,

  4. FtsZ ring forms at the periphery of the midpoint, driving septum formation.

  5. The cell pinches into two daughter cells, dispersing FtsZ throughout the new cells.