Cell and Molecular Biology Final Review Notes

Cell Cycle

  • All organisms are made of cells, which divide to produce more cells.
    • Essence of life: male sperm and female eggs.
    • Cells function as units of heredity.

How do Cells Divide?

  • One mother cell divides into two daughter cells (classic cell theory).
    • Both daughter cells must inherit necessary properties to survive.
    • Vital components of the cell include: proteins, carbohydrates, lipids, Golgi apparatus, endoplasmic reticulum, mitochondria, etc.
    • Significant energy expenditure occurs during chromosome division.

Chromosomes

  • Karyotype: method to stain and organize chromosomes.
    • Chromosomes are formed during cell division, becoming visible only then.
    • Number of chromosomes is species-specific (e.g., bacteria = 1; humans = 46).
    • Eukaryotic chromosomes consist of homologous pairs (humans have 22 homologs and 2 sex chromosomes = 23 pairs).
    • Cell division results in chromosome duplication, leading to chromatids.
    • Centromere: proteins holding chromatids together, ensuring equal distribution to daughter cells.
    • Mitotic chromosomes represent a specific configuration observed during karyotyping.
    • Chromosome structure varies: circular (bacteria) vs. linear (eukaryotes).

Cell Division Process

  • Importance of equal chromosome distribution; each daughter cell must receive one of each chromosome.
    • Rare exceptions occur, such as Down syndrome, resulting from chromosomal anomalies during meiosis.
    • All chromosomes are required for organism survival, necessitating precise mechanisms for distribution.

Organizing Cell Division: The Cell Cycle

  • Ensures all cells have the correct number of chromosomes through a series of steps:
    1. DNA Replication (S-phase)
    2. M-Phase (Mitosis): cells copy themselves with one chromosome copy.
    3. Meiosis: produces non-identical daughter cells with half the DNA content.
    4. Cytokinesis: division of cytoplasm.
    5. Interphase includes G1, S-phase, and G2 stages.
    • G1 Phase: rest period before S-phase.
    • G2 Phase: preparation for mitosis after S-phase.

Cell Cycle Checkpoints

  • The cell cycle includes checkpoints to ensure readiness:
    • S-phase Checkpoint: verifies DNA replication; improper replication halts G2 initiation.
    • Proteins involved: cyclins and cyclin-dependent kinases (cdks) regulate cycle progression via concentration changes.

Cell Cycle and Cancer

  • Normal cellular division is regulated; cells do not divide continuously without necessity.
    • Unregulated division (cancer) can result from checkpoint failures, causing cells to divide at inappropriate times.
    • Understanding cell cycle regulation may help in cancer treatment.

Mitosis and Meiosis

Mitosis Process:

  • Involves stages:
    1. G2: centrosome replication.
    2. Prophase: centrosomes move, spindle forms, and chromosomes condense.
    3. Prometaphase: nuclear envelope breaks down; polar and kinetochore microtubules form.
    4. Metaphase: chromosomes align at the metaphase plate.
    5. Anaphase: centromeres separate, pulling chromatids apart.
    6. Telophase: spindle disassembles, nuclear envelope reforms, and chromosomes decondense.

Cytokinesis

  • In animals: actin and myosin pinch the cell.
  • In plants: vesicles fuse to form cell plate and new cell wall.

Alternatives to Mitosis

  • Suitable for organisms requiring clones; mitosis is asexual.
  • Diversity in offspring comes from meiosis, the process of genetic material mixing through gamete formation.

Meiosis Overview

  • Begins with a diploid cell, proceeds through DNA replication, then undergoes two rounds: Meiosis I and II.

Meiosis I Phases:

  1. Prophase I: homologous chromosomes pair and form chiasmata.
  2. Prometaphase I: spindle forms, and nuclear envelope breaks down.
  3. Metaphase I: chromosomes align at the metaphase plate, with microtubules attaching.
  4. Anaphase I: homologous chromosomes separate.
  5. Telophase I: cell divides into two haploid cells, each with separate chromatids.

Meiosis II Phases:

  • Similar to mitosis; chromosomes align and then separate into four haploid cells.