Cell Division and the Cell Cycle

What is Cell Division?

Cell division is the process by which a parent cell divides into two or more daughter cells. It is fundamental for growth, development, repair, and reproduction in all living organisms.

• Multicellular Organisms: Begin as a single cell (zygote) and develop into complex beings through repeated cell divisions.

• Genome: The complete set of genetic material in a cell.

• Chromatin: DNA and protein complex that condenses to form chromosomes during cell division.

• Sister Chromatids: Identical copies of a chromosome connected by a centromere.

The Cell Cycle

The cell cycle is a series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).

• Phases: G1 (growth), S (DNA synthesis), G2 (preparation for division), M (mitosis).

• Duration: Varies by cell type; some phases are longer than others.

• Checkpoints: G1, G2, and M checkpoints ensure proper progression; G0 is a resting phase.

Mitosis

Mitosis is the division of the nucleus in somatic (body) cells, resulting in two genetically identical daughter cells.

• Somatic Cells: All cells except gametes; found throughout the body.

• DNA Content: After mitosis, each daughter cell has the same DNA content as the parent cell.

• Stages: Prophase, Metaphase, Anaphase, Telophase (with descriptions and illustrations).

• Mitotic Spindle: Structure made of microtubules that separates chromosomes; includes kinetochore microtubules (shorten during anaphase).

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate cells.

• Animal Cells: Occurs via cleavage furrow.

• Plant Cells: Occurs via cell plate formation.

Binary Fission

Binary fission is a form of asexual reproduction in prokaryotes, where the cell divides into two identical cells.

Cell Cycle Regulation

• Growth Factors: Proteins released by cells to stimulate cell division.

• Density-Dependent Inhibition: Cells stop dividing when crowded.

• Anchorage Dependence: Cells must be attached to a substrate to divide.

Meiosis and Sexual Life Cycles

Asexual vs. Sexual Reproduction

Asexual reproduction produces genetically identical offspring; sexual reproduction involves fusion of gametes and genetic variation.

• Genes: Units of heredity; segments of DNA.

• Gametes: Reproductive cells (sperm and egg).

Chromosome Number and Types

• Humans: 46 chromosomes (23 pairs).

• Karyotype: Ordered display of chromosomes.

• Homologous Chromosomes: Chromosome pairs with the same genes.

• Sex Chromosomes: X and Y; determine sex.

• Autosomes: Non-sex chromosomes.

• Diploid (2n): Two sets of chromosomes.

• Haploid (n): One set of chromosomes.

Sexual Life Cycles

• Animals: Diploid dominant; gametes produced by meiosis.

• Plants: Alternation of generations; both haploid and diploid stages.

• Fungi: Haploid dominant; zygote is only diploid stage.

Meiosis

Meiosis reduces chromosome number by half, producing four haploid cells.

• Sister Chromatid Cohesion: Holds chromatids together.

• Synapsis: Homologs pair up; occurs in prophase I.

• Crossing Over: Exchange of genetic material between homologs.

• Stages: Meiosis I (separates homologs), Meiosis II (separates sister chromatids).

Genetic Variation

• Independent Assortment: Random distribution of homologs.

• Crossing Over: Produces recombinant chromosomes.

• Random Fertilization: Any sperm can fertilize any egg.

Mendelian Genetics

Gregor Mendel and His Experiments

Gregor Mendel is the father of genetics; he used pea plants to study inheritance.

• Blending Hypothesis: Traits mix; disproven by Mendel.

• True Breeding: Plants that produce offspring identical to themselves.

• Hybridization: Crossing different varieties.

• P, F1, F2 Generations: Parental, first filial, second filial.

Dominant and Recessive Traits

• Dominant: Expressed trait.

• Recessive: Masked trait.

• F2 Ratio: 3:1 dominant:recessive.

Alleles and Laws of Inheritance

• Alleles: Different forms of a gene; located on homologous chromosomes.

• Law of Segregation: Alleles separate during gamete formation.

• Law of Independent Assortment: Genes on different chromosomes assort independently.

Phenotype and Genotype

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

Punnett Squares and Testcrosses

• Punnett Square: Diagram to predict genetic crosses.

• Testcross: Cross with homozygous recessive to determine genotype.

Types of Crosses

• Monohybrid: One trait.

• Dihybrid: Two traits.

• Trihybrid: Three traits.

• 9:3:3:1 Ratio: Dihybrid cross result.

Probability Rules

• Multiplication Rule: Probability of two independent events:

• Addition Rule: Probability of either event:

Patterns of Inheritance

• Complete Dominance: One allele masks the other.

• Incomplete Dominance: Heterozygote is intermediate.

• Codominance: Both alleles are expressed.

• Multiple Alleles: More than two alleles (e.g., blood type).

• Pleiotropy: One gene affects multiple traits.

• Epistasis: One gene affects expression of another (e.g., Labrador coat color).

• Polygenic Inheritance: Multiple genes affect a trait (e.g., skin color).

Nature vs. Nurture

• Environmental Effects: Environment can influence phenotype (e.g., hydrangea flower color).

Pedigree Analysis

• Pedigree: Diagram showing inheritance in a family.

Genetic Disorders

• Recessive Disorders: Require two recessive alleles (e.g., cystic fibrosis).

• Sickle Cell Disease: Caused by a single allele; heterozygotes have mild symptoms.

• Dominant Disorders: Require one dominant allele (e.g., Huntington's disease).

• Multifactorial Disorders: Caused by genetic and environmental factors.

Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

Genes are located on chromosomes, and chromosomes undergo segregation and independent assortment.

Morgan's Fruit Fly Experiments

• Trait Studied: Eye color; discovered sex-linked inheritance.

Sex-Linked Genes

• X-Linked Genes: Genes on the X chromosome.

• X Inactivation: One X chromosome in females is inactivated (Barr body).

Linked Genes and Genetic Recombination

• Linked Genes: Genes located close together on a chromosome; inherited together.

• Genetic Recombination: Production of offspring with combinations different from parents.

• Crossing Over: Contributes to recombination.

Genetic Maps

• Genetic Map: Shows gene locations.

• Linkage Map: Based on recombination frequencies.

• Map Units: 1 unit = 1% recombination frequency.

Alterations of Chromosome Number

• Nondisjunction: Failure of chromosomes to separate; occurs in meiosis I or II.

• Aneuploidy: Abnormal chromosome number.

• Monosomic: Missing one chromosome.

• Trisomic: Extra chromosome (e.g., Trisomy 21 = Down syndrome).

Alterations of Chromosome Structure

• Deletion: Loss of a chromosome segment.

• Duplication: Repeat of a segment.

• Inversion: Reversal of a segment.

• Translocation: Segment moves to another chromosome.

Aneuploidy of Sex Chromosomes and Disorders

• Examples: Turner syndrome (XO), Klinefelter syndrome (XXY).

• Disorders: Caused by structural changes in chromosomes.

Additional info: This study guide covers foundational concepts in cell division, meiosis, Mendelian genetics, and chromosomal inheritance, suitable for exam preparation in a General Biology course.