Cell Cycle

Cell Division

  • Cell Theory:

    • All cells derive from one another.

    • Significance: Cell division is CRUCIAL for reproduction and continuity of life.

  • Three Main Purposes of Cell Division:

    1. Reproduction:

    • Key process for unicellular organisms known as binary fission.

    1. Growth and Development.

    2. Tissue Renewal.

  • General Outcome:

    • Typically, cell division creates two identical “daughter” cells. This process is coordinated through the cell cycle.

DNA: Chromatin vs. Chromosome

  • Genome:

    • Refers to the full complement of a cell’s DNA (all its genes).

  • Chromatin:

    • Long, thin strands of unwound DNA, packaged with proteins.

    • Typically invisible under a standard microscope.

  • Chromosomes:

    • Individual strands of chromatin in a condensed form.

    • Only visible when a cell is actively dividing.

DNA: Prokaryotes vs. Eukaryotes

  • Difference #1:

    • Prokaryotes have one circular chromosome.

    • Eukaryotes have many linear chromosomes.

  • Difference #2:

    • Prokaryotes have their genome in the cytosol.

    • Eukaryotes have their genome within the nucleus.

  • Difference #3:

    • Eukaryotes can be multicellular with different cell types.

    • Somatic (Body) Cells: Diploid cells containing two copies of the genome.

    • Gametic (Gamete) Cells: Haploid cells with only one copy of the genome.

  • Examples of Prokaryotic and Eukaryotic Cells:

    • Prokaryotic Cell: Bacteria

    • Eukaryotic Cell: Egg, Sperm, Epithelial (Somatic) Cells

Questions for Consideration

  • What kind of cell is this?

    • Prokaryotic or Eukaryotic?

  • What form of DNA is present?

    • Chromatin or Chromosomes?

Chromosomes

  • Function:

    • Chromosomes represent the condensed form of DNA. Most of the time, DNA is in the readable form of chromatin.

  • Presence:

    • Chromosomes are only present when a cell prepares to divide,

    • This form helps to protect DNA during cell division.

  • S Phase (DNA Synthesis):

    • The entire genome is duplicated.

    • After replication, each chromosome has two identical sister chromatids.

  • M Phase:

    • Follows S Phase and refers to Mitosis, the process of nucleus division.

    • Sister chromatids separate and are passed to daughter cells.

  • Cytokinesis:

    • Division of the cytoplasm occurs after mitosis.

Stages of the Cell Cycle

  • G0 Stage:

    • Most cells exist in the G0 stage until a signal to divide is received.

  • Cell Cycle Sequence:

    • G1 Phase: First “gap” phase where the cell grows and produces organelles for two cells.

    • S Phase: Involves replication/synthesis of DNA and centrosomes.

    • G2 Phase: Second gap phase involving further growth.

    • M Phase: Actual phase of cell division (mitosis).

  • Interphase:

    • Refers to all phases except mitosis: Interphase = G0 + G1 + S + G2.

  • Duration of Phases:

    • Longest Phase: Depends on genome complexity; G1 is typically the longest, but complex genomes may extend S phase.

    • Shortest Phase: M phase, where actual cell division occurs quickly (<1 hour).

Mitosis (M Phase)

  • Definition: Nuclear division of somatic cells.

  • Distinction: Gametes undergo meiosis, not mitosis.

  • Outcome: Each copy of the genome is segregated equally into two genetically identical daughter cells.

Five Phases of Mitosis

  1. Prophase:

    • Chromosomes condense and become visible.

    • Sister chromatids are visually identifiable.

    • Nucleoli disappear, but the nuclear membrane remains.

    • Mitotic spindle begins to form, with “asters” appearing (composed of microtubules extending from centrosomes).

    • Centrosome pair separates.

  2. Prometaphase:

    • The nuclear envelope dissolves.

    • Chromosomes fully condensed, forming kinetochores at centromeres.

    • Microtubules of the spindle invade the nuclear area, grabbing chromosomes at kinetochores.

  3. Metaphase:

    • The longest phase of mitosis (~20 minutes).

    • Centrosomes align opposite each other.

    • Chromosomes line up at the metaphase plate; centromeres align here.

    • Kinetochores of sister chromatids attached to opposite spindle poles.

    • Spindle apparatus fully formed; microtubules fully extended and connected to kinetochores.

  4. Anaphase:

    • The shortest phase of mitosis.

    • Sister chromatids fully separate.

    • Spindles pull sister chromatids towards centrosomes.

    • Chromatids migrate toward opposite spindle poles and are now independent chromosomes.

    • The cell elongates.

  5. Telophase and Cytokinesis:

    • Telophase:

      • Two separate “daughter” nuclei form (with nuclear envelopes).

      • Chromosomes start to de-condense.

    • Cytokinesis:

      • Begins during telophase.

      • Cleavage furrow forms and pinches the cell into two parts.

    • Note: Though these phases occur simultaneously, they represent distinct processes during mitosis.

Importance of DNA Copying

  • Prior to entering M phase, the parent cell (2n) has twice the normal genetic information within the nucleus (4n).

  • After mitosis, each daughter cell produced will have the same genome as the parent cell (2n).

The Mitotic Spindle

  • Definition: The mitotic spindle comprises microtubules that grow during mitosis from the centrosomes.

  • Centrosome Classifications:

    • Classified as a Microtubule Organizing Center (MTOC).

    • Non-dividing cells typically have only one centrosome; during S phase, the centrosome duplicates.

  • Spindle Fibers:

    • Microtubules extend to attach to chromosomes at kinetochores.

    • Kinetochores are attachment points located at the centromere where sister chromatids join.

Highlight: Animal vs. Plant Cells

  • Animal Cells:

    • A cleavage furrow results from a ring of microtubules that form between separating cells, pinching to form two daughter cells.

  • Plant Cells:

    • A cell plate forms as vesicles from the Golgi apparatus deposit cellulose fibrils, extending to the cell wall which divides the two daughter cells.

Various Stages of the Cell Cycle in Tissues

  • Cells within a tissue can exist in various stages of the cell cycle.

Prokaryotic Cell Division: Binary Fission

  • Bacteria can undergo a process called binary fission to divide; this results in two genetically identical offspring.

  • Mechanism:

    • Bacteria possess one circular chromosome attached to the cell membrane.

    • After DNA replication, the two chromosome copies connect to the plasma membrane at opposite cell sides.

    • As the cell grows, the plasma membrane and cell membrane form a furrow and pinch into two cells.

Cell Cycle Regulation

  • During periods of growth and development, there is extensive cell division.

  • In Mature Animals:

    • Some cells grow but rarely divide (e.g., nerve, muscle cells).

    • Some are constantly dividing (e.g., skin, gut, blood cells).

    • Some divide only at specific times (e.g., wound repair, tissue remodeling).

  • Importance of Regulation:

    • Errors in the mechanisms controlling the cell cycle can have catastrophic consequences, such as abnormal cell division and uncontrolled growth, which can lead to cancer.

Checkpoints in Cell Cycle Regulation

  • A checkpoint is a point in the cell cycle where cells are checked for DNA damage or other abnormalities.

    • If the cell passes the checkpoint, it proceeds through the cell cycle.

    • If it does not pass, it may undergo apoptosis (programmed cell death).

  • Critical Checkpoint:

    • The most vital checkpoint is the G1 Restriction Point, occurring between G0 and G1.

    • If the cell passes this checkpoint, it is committed to progressing through the cell cycle (given the proper external signals, such as growth factors).

    • Prior to this checkpoint, the cell can revert to G0.

Cell Cycle Control: CDKs and Cyclins

  • The progression of a cell into subsequent phases of the cell cycle is regulated by two families of proteins:

    • Cyclins: Proteins expressed only during specific times/phases of the cell cycle.

    • Cyclin-dependent kinases (CDKs): Enzymes that are consistently present but require cyclins to be activated.

Cell Cycle Regulation: TSGs and Oncogenes

  • Regulator Genes: Many genes play roles in controlling the cell cycle.

    • Oncogenes: Genes that promote cell growth and division.

    • Tumor Suppressor Genes (TSGs): Genes that restrain cell growth and division.

  • Balance: the activity between these genes assists in regulating the cell cycle, and abnormal gene function is implicated in various types of cancer.

Cell Cycle Control and Cancer

  • Cancer Mechanism: Cancer is a result of abnormal and uncontrolled cellular growth and division, typically caused by 2 or more mutations in regulating genes within the cell cycle.

  • P53 Protein:

    • Produced by the P53 gene (a TSG).

    • Plays a role in slowing down cell growth, facilitating DNA repair, and inducing apoptosis to prevent passing mutations to daughter cells.

    • Abnormal p53 is found in many different cancer types.

  • Significance: Understanding the cell cycle is crucial for comprehending the development and progression of cancer.