Cell Division and the Cell Cycle

Mitosis and the Cell Cycle

Mitosis is the process by which eukaryotic cells divide to produce two genetically identical daughter cells. The cell cycle is the series of events that cells go through as they grow and divide.

• Cell Cycle Phases: G1 (cell growth), S (DNA synthesis), G2 (preparation for mitosis), and M (mitosis and cytokinesis).

• Interphase: Consists of G1, S, and G2 phases; the cell grows and replicates its DNA.

• Mitosis Stages: Prophase, Metaphase, Anaphase, Telophase.

• Cytokinesis: Division of the cytoplasm, forming two separate cells.

• Cleavage Furrow: The indentation that begins the process of cytokinesis in animal cells.

• Cell Plate: Structure that forms in plant cells during cytokinesis to divide the cell.

• Centrosomes and Centrioles: Organize the mitotic spindle; centrioles are found in animal cells.

• Mitotic Spindle: Structure made of microtubules that separates chromosomes during mitosis.

• Kinetochores: Protein complexes on chromosomes where spindle fibers attach.

• Metaphase Plate: Imaginary plane where chromosomes align during metaphase.

Example: In human somatic cells (diploid), mitosis results in two identical diploid cells.

Binary Fission

Binary fission is the process by which prokaryotic cells (such as bacteria) divide.

• DNA is replicated, and the cell splits into two identical cells.

Cell Cycle Checkpoints and Regulation

Checkpoints ensure the cell cycle progresses only when conditions are favorable.

• Four Main Checkpoints: G1, S, G2, and M checkpoints.

• Apoptosis: Programmed cell death; removes damaged or unnecessary cells.

• P53: Tumor suppressor protein that can trigger apoptosis if DNA is damaged.

Cancer and Cell Cycle Control

• Cancer: Uncontrolled cell division due to mutations in genes regulating the cell cycle.

• Proto-oncogenes: Normal genes that promote cell division; mutations can convert them to oncogenes.

• Tumor Suppressor Genes: Genes that inhibit cell division; mutations can lead to cancer.

• Benign Tumor: Non-invasive, non-cancerous growth.

• Malignant Tumor: Invasive, cancerous growth that can spread (metastasize).

• Metastasis: Spread of cancer cells to other parts of the body.

• Multiple Hit Hypothesis: Multiple mutations are usually required for cancer to develop.

Chromosomes and Genetic Material

Chromosome Structure

• Chromosomes: Structures made of DNA and proteins (histones) that carry genetic information.

• Chromatin: DNA-protein complex; less condensed form of genetic material.

• Histone Proteins: Proteins around which DNA winds to form nucleosomes.

• Sister Chromatids: Identical copies of a chromosome, connected at the centromere.

• Centromere: Region where sister chromatids are joined and spindle fibers attach.

Meiosis and Sexual Reproduction

Overview of Meiosis

Meiosis is the process by which diploid cells produce haploid gametes (egg and sperm), introducing genetic diversity.

• Gametes: Reproductive cells (egg/sperm) with half the chromosome number (haploid).

• Fertilization: Fusion of gametes to form a diploid zygote.

• Haploid (n): One set of chromosomes; Diploid (2n): Two sets of chromosomes.

• Homologous Chromosomes: Chromosome pairs, one from each parent, with the same genes.

• Tetrad: Structure of four chromatids formed during synapsis in prophase I.

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I.

• Synapsis: Pairing of homologous chromosomes during meiosis I.

• Independent Assortment: Random distribution of homologous chromosomes during meiosis I.

• Autosomes: Non-sex chromosomes; Sex Chromosomes: X and Y chromosomes.

• Gene: Unit of heredity; Allele: Different forms of a gene.

• Karyotype: Display of an individual's complete set of chromosomes.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate (includes prophase I, metaphase I, anaphase I, telophase I).

• Meiosis II: Sister chromatids separate (similar to mitosis).

• Interkinesis: Short resting phase between meiosis I and II (no DNA replication).

Gametogenesis and Life Cycles

• Gametogenesis: Formation of gametes (spermatogenesis in males, oogenesis in females).

• Life Cycles: Diploid dominant (animals), haploid dominant (fungi), alternation of generations (plants).

Differences Between Mitosis and Meiosis

• Mitosis produces two identical diploid cells; meiosis produces four genetically unique haploid cells.

• Meiosis involves two divisions and crossing over; mitosis does not.

Chromosomal Disorders

• Nondisjunction: Failure of chromosomes to separate properly during meiosis.

• Aneuploidy: Abnormal number of chromosomes (e.g., trisomy, monosomy).

• Trisomy: Three copies of a chromosome (e.g., Down syndrome).

• Monosomy: One copy of a chromosome (e.g., Turner syndrome).

Mendelian Genetics and Patterns of Inheritance

Gregor Mendel and His Experiments

Gregor Mendel is known as the father of genetics. He studied inheritance patterns using pea plants.

• Mendel's Two Main Ideas: Law of Segregation and Law of Independent Assortment.

• Heredity: Transmission of traits from parents to offspring.

• Trait: Specific characteristic (e.g., flower color).

• Genetics: Study of heredity and variation.

• Pea Plant Traits: Mendel studied seven traits (e.g., seed shape, flower color).

• Pure Breeding (True Breeding): Organisms that always pass down certain traits.

Genetic Crosses and Patterns

• Monohybrid Cross: Cross involving one trait.

• Dihybrid Cross: Cross involving two traits.

• Dominant: Trait expressed when at least one allele is present.

• Recessive: Trait expressed only when two recessive alleles are present.

• Genotype: Genetic makeup (e.g., AA, Aa, aa).

• Phenotype: Observable trait (e.g., purple flowers).

• Homozygous: Two identical alleles (AA or aa).

• Heterozygous: Two different alleles (Aa).

• Punnett Square: Diagram to predict genetic crosses.

Extensions of Mendelian Genetics

• Complete Dominance: Dominant allele masks recessive allele.

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., red x white = pink flowers).

• Co-dominance: Both alleles are expressed (e.g., AB blood type).

• Sex-Linked Traits: Genes located on sex chromosomes (often X-linked).

• Test Cross: Cross with homozygous recessive to determine unknown genotype.

• Multiple Alleles: More than two alleles for a gene (e.g., ABO blood group).

Gene Linkage and Mapping

• Gene Linkage: Genes located close together on the same chromosome tend to be inherited together.

• Recombinant Genotypes: New combinations of alleles due to crossing over.

• Linkage Mapping: Determining the relative positions of genes on a chromosome.

• Map Distance: Measured in centimorgans (cM); 1% recombination frequency = 1 cM.

Complex Patterns of Inheritance

• Pleiotropy: One gene affects multiple traits (e.g., phenylketonuria).

• Quantitative Traits: Traits influenced by multiple genes (polygenic), showing continuous variation (e.g., height).

• Expressivity: Degree to which a trait is expressed.

• Gene-Gene Interaction (Epistasis): One gene affects the expression of another gene.

Pedigrees and Human Inheritance

• Pedigree: Diagram showing inheritance patterns in families.

• Autosomal Recessive Traits: Trait appears only when both alleles are recessive (e.g., cystic fibrosis).

• Autosomal Dominant Traits: Trait appears when at least one dominant allele is present (e.g., Huntington's disease).

• Diseases: Various genetic diseases are associated with each inheritance pattern.

Example: A Punnett square can be used to predict the probability of offspring inheriting a genetic disorder.

Additional info: For more details on specific diseases, refer to clinical genetics resources. The above content covers foundational concepts for exam preparation in cell division and genetics.