Untitled

• Features are shared with parents through heredity, where traits pass from parents to offspring.
• Variation demonstrates differences among individuals, making each sibling unique despite sharing parents.
• Genetics: The study of heredity and variation, aiming to understand how traits pass to the next generation and how variation arises.
• Reproduction is essential for all living things, providing energy through metabolic processes for growth, development, and reproduction.
• Cellular reproduction, as cell division, provides the basis for organismal-level reproduction.
• Chromosomes contain the genetic material that instructs cell activities and functions.
• Deoxyribonucleic acid (DNA), the genetic material, passes from one generation to ensure life's continuity.
• In eukaryotic cells, DNA binds with proteins and organizes into chromosomes.

Chromosome Numbers

• The number of chromosomes in a cell is species-specific.
• Humans have 46 chromosomes; rice has 24.

Cell Cycle Stages

• Chromosomes change form as a cell transitions through its cell cycle.
• Interphase: Chromosomes are long and extended, referred to as chromatin.
• Cell Division: Chromosomes condense and thicken.
• Cell Cycle: An ordered sequence of events resulting in cell growth and division into daughter cells.
• Non-dividing cells are not considered in the cell cycle.
• Eukaryotic cells can only divide after genome doubling and halving.
• S phase (synthesis phase): DNA doubles.
• Mitosis (M phase): Genome halves.
• G1 stage: The cell grows and prepares chromosomes for replication.
• S stage: DNA replication occurs.
• G2 stage: Cell prepares for mitosis.
• M stage: Nuclear (chromosome separation) and cytoplasmic (cytokinesis) division occur.

Regulation of the Cell Cycle

• Cell division control is complex; errors can lead to cancer.
• Cancer: Cell cycle regulation fails, leading to abnormal cell growth.
• Cdk (cyclin-dependent kinase) with cyclins: Control switches for the cell cycle, moving cells from G1 to S or G2 to M.
• MPF (Maturation Promoting Factor): Includes CdK and cyclins, triggering cell cycle progression.
• p53: Blocks the cell cycle if DNA is damaged or induces apoptosis (cell death) if damage is severe.
• High p53 levels in damaged cells allow DNA repair.
• p53 mutation: The most frequent mutation leading to cancer.
• Li Fraumeni syndrome: Genetic defect in p53, causing high cancer frequency.
• p27: Binds to cyclin and cdk, blocking entry into S phase.
• Reduced p27 levels: Predict poor outcomes for breast cancer patients.

Cell Division: Mitosis and Meiosis

• Mitosis: Nuclear division plus cytokinesis, producing identical daughter cells.
• Interphase: Includes G1, S, and G2 phases of the cell cycle, but is technically not part of mitosis.
• Mitosis: Occurs among somatic or body cells.
• Animal cells: Mitosis takes about an hour.

Mitosis Stages

• Prophase: Nuclear membrane breaks down, and the nucleolus disintegrates.
• Centrosome duplicates: Forms two daughter centrosomes migrating to opposite cell ends.
• Microtubules: Organized by centrosomes and form spindle fibers, constituting the mitotic spindle.
• Chromosomes condense: Compact structures observed.
• Replicated chromosomes: Consist of two identical chromatids (or sister chromatids) held by the centromere.
• Centromere divides chromosomes: Forms p arms (shorter) and q arms (longer).
• Giemsa staining: Alternating dark (heterochromatin) and light (euchromatin) regions appear.
• Heterochromatin: More coiled and dense than euchromatin.
• Prometaphase: Chromosomes migrate to the cell's midline, led by their centromeres.
• Metaphase plate: The region of the mitotic spindle formed by centrosomes.
• Kinetochore: Spindle fibers bind to the centromere.
• Metaphase: Chromosomes align along the metaphase plate of the spindle apparatus.
• Anaphase: Centromeres divide, and sister chromatids separate ('disjoin').
• Spindle fibers pull chromatids: Move to opposite cell ends, attached to kinetochore regions.
• Daughter chromosomes: Separated sister chromatids.
• Telophase: Final mitosis stage; reverses prophase processes.
• Nuclear membrane reforms: Chromosomes at cell poles uncoil, and spindle fibers disappear.
• Cytokinesis: Final cell division to form two new cells.
• Cell plate forms in plants; animals have cytoplasm constriction.
• The cell then enters interphase - the interval between mitotic divisions.

Meiosis

• Meiosis: Eukaryotic cell division that produces haploid sex cells or gametes from diploid cells.
• Single DNA replication: Followed by two nuclear and cellular divisions (Meiosis I and Meiosis II).
• Meiosis results in gametogenesis: Production of sperm(spermatogenesis) and eggs (oogenesis).
• Meiosis I: Reduction division, halves the number of chromosomes, making daughter cells haploid.
• Genetic recombination: Primarily occurs during Meiosis I through crossing over.

Meiosis I Stages

• Prophase I: Has sub-stages.
• Leptotene: Chromosomes consist of two long threads of sister chromatids due to replication during the S phase of the cell cycle.
• Zygotene: Chromosomes pair off with homologous chromosomes with exact pairing.
• Pachytene: Chromosomes contract and coil.
• Crossing over: Segments are exchanged between sister chromatids of homologous chromosomes via chiasma formation.
• Diplotene: Chromosomes begin to uncoil.
• Diakinesis: Paired chromosomes disperse in the nucleus.
• Metaphase I: Tetrads line up at the equator; the spindle is fully formed.
• Anaphase I: Spindle fibers attach to centromeres.
• Homologous chromosomes: Separate and move towards the poles.
• Sister chromatids remain together: The centromere does not divide.
• Telophase I: Chromosomes with two chromatids decondense and the nuclear envelope reforms.
• The nucleus is now haploid.

Meiosis II

• Meiosis II: Reduces DNA to normal by splitting chromosomes for one chromatid per daughter cell.
• Prophase II: Chromosomes with two chromatids condense, with nuclear envelope and nucleus disappearing.
• Metaphase II: Chromosomes with two chromatids line up at the equator; the spindle is fully formed.
• Anaphase II: Chromosomes split, a chromosome with only one chromatid heads toward each pole.
• Telophase II: Chromosomes with one chromatid decondense and are surrounded by new nuclear envelopes.
• Four daughter cells now all haploid: Ready to develop into sperm or eggs.