The Cell Cycle and Cell Division Notes

Cell Division: Process where a single cell divides to form two cells.
Asymmetrical Division: One mother cell produces a daughter cell while the mother cell remains unchanged. Example: yeast.
Symmetrical Division: Mother cell divides to produce two identical daughter cells. Example: bacteria and protists.
Multicellular organisms can exhibit both types of division.
Asexual Reproduction:
Offspring are clones, genetically identical to the parent (variations arise from mutations).
Prokaryotes typically reproduce via binary fission.
Eukaryotes reproduce via mitotic division in single-celled organisms and can also reproduce asexually in many multicellular forms.
Sexual Reproduction:
Occurs in eukaryotes involving the production of gametes (sperm and egg) that fuse, resulting in genetically diverse offspring.
Gametes are formed through meiosis, a division process that halves the chromosome number and increases genetic variation.

Key Events in Cell Division:
Initiation: Signals trigger cell division.
DNA Replication: Duplicates genetic material for two new cells.
DNA Segregation: Distribution of DNA into the daughter cells.
Cytokinesis: Division of cytoplasm, resulting in two separate cells.
Binary Fission in Prokaryotes:
Cell division initiated by environmental signals (e.g., nutrients).
Starts from a single origin (ori) on the circular chromosome.
After replication, DNA separates by moving to opposite ends of the cell.
Eukaryotic Cell Cycle:
Cell cycle stages:
- G1 phase: Normal cell functions; cells may enter a resting state (G0).
- S (Synthesis) phase: DNA replication occurs.
- G2 phase: Preparation for mitosis.
- M phase: Mitosis and cytokinesis take place.
Cytokinesis differs in plant (cell plate formation) vs. animal cells (contractile ring formation).
Chromosomes:
Compact structures formed by DNA wrapped around histones.
During M phase, chromosomes are condensed and easily visualized.

Meiosis: Process that produces haploid gametes, essential for sexual reproduction.
Involves two nuclear divisions with only one DNA replication.
Results in four genetically unique haploid nuclei.
Meiosis I: Halves the chromosome number; homologous chromosomes pair and separate but sister chromatids remain attached.
Meiosis II: Similar to mitosis; sister chromatids separate creating haploid cells that are not genetically identical due to crossing over during prophase I.
Crossing Over: Genetic exchange between homologous chromosomes during prophase I, increasing genetic diversity among offspring.

Nondisjunction: Failure of homologous chromosomes to separate properly during cell division, leading to aneuploidy (abnormal chromosome number). Examples include Down syndrome (trisomy 21).
Aneuploidy Effects: Can lead to serious developmental issues; most human embryos with aneuploidy do not survive.
Polyploidy: Organisms possessing more than two complete sets of chromosomes, commonly seen in plants, and can arise from errors in cell cycle processes.

Regulation of the Cell Cycle:
Eukaryotic cell cycles are tightly regulated. The transition from G1 to S phase (restriction point) controls whether a cell will divide.
Internal signals (Cyclin-dependent kinases or CDKs) trigger various checkpoints within the cycle:
- G1 checkpoint: Checks for DNA damage.
- S checkpoint: Ensures complete DNA replication, free of damage.
- G2 checkpoint: Checks for DNA damage prior to mitosis.
- M checkpoint: Ensures all chromosomes are attached to the spindle apparatus.
Cell Death:
Necrosis: Uncontrolled cell death due to damage or lack of nutrients, often results in inflammation.
Apoptosis: Programmed cell death, occurs in cases like aging, infection, or DNA damage.
- Apoptotic cells undergo specific morphological changes and are often removed by phagocytosis.
Cancer Cells: Often bypass normal regulatory mechanisms, leading to uncontrolled division. Common mutations in various cancer types affect checkpoints and apoptosis processes.