Cell Division and the Cell Cycle

Overview of 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, and repair in multicellular organisms, and for reproduction in unicellular organisms.

• Multicellular Organisms: Develop from a single cell (zygote) 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 at a region called the 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 cells (e.g., neurons) exit the cycle into G0 phase.

Mitosis

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

• Somatic Cells: All body cells except gametes.

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

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Mitotic Spindle

• Function: Separates chromosomes during mitosis.

• Components: Microtubules, centrosomes, kinetochores.

• Kinetochore Microtubules: Shorten during anaphase, pulling chromatids apart.

Cytokinesis

• Animal Cells: Cleavage furrow forms, pinching the cell in two.

• Plant Cells: Cell plate forms, dividing the cell.

Binary Fission

• Definition: A form of asexual reproduction in prokaryotes.

• Process: DNA replicates, cell splits into two.

Cell Cycle Checkpoints

• G1, S, G2, M: Control points where the cell assesses readiness to proceed.

• G0: Non-dividing state.

Growth Factors and Regulation

• Growth Factors: Proteins that 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 combines genetic material from two parents.

• Genes: Units of heredity.

• 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; 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.

Alternation of Generations

• Definition: Life cycle alternates between haploid and diploid stages.

Meiosis

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

• Sister Chromatid Cohesion: Holds chromatids together.

• Synapsis: Homologous chromosomes pair up (prophase I).

• Crossing Over: Exchange of genetic material between homologs.

Stages of Meiosis

• Meiosis I: Homologs separate.

• Meiosis II: Sister chromatids separate.

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, known for his experiments with pea plants.

• Blending Hypothesis: Outdated idea that traits mix in offspring.

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

• Hybridization: Crossing two different true-breeding plants.

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

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 outcomes.

• Testcross: Cross with homozygous recessive to determine genotype.

Genetic Crosses

• Monohybrid: One trait.

• Dihybrid: Two traits.

• Trihybrid: Three traits.

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

Probability in Genetics

• Multiplication Rule: Probability of independent events: multiply probabilities.

• Addition Rule: Probability of mutually exclusive events: add probabilities.

Types of Dominance

• Complete Dominance: One allele masks the other.

• Incomplete Dominance: Heterozygote is intermediate.

• Codominance: Both alleles are fully expressed.

Multiple Alleles and Blood Type

• Blood Type: Determined by multiple alleles (A, B, O).

Pleiotropy, Epistasis, and Polygenic Inheritance

• 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.

Pedigree Analysis

• Pedigree: Diagram showing inheritance patterns in families.

Genetic Disorders

• Recessive Disorders: Require two recessive alleles.

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

• Dominant Disorders: Require only one dominant allele.

• Multifactorial Disorders: Caused by genetic and environmental factors.

Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

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

• Law of Segregation: Repeated from Chapter 11.

• Law of Independent Assortment: Repeated from Chapter 11.

Morgan's Fruit Fly Experiments

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

Sex-Linked and X-Linked Genes

• Sex-Linked Genes: Located on sex chromosomes.

• X-Linked Genes: Located on 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 new combinations of traits.

• Crossing Over: Contributes to recombination.

Genetic and Linkage Maps

• Genetic Map: Shows gene locations based on recombination frequency.

• Linkage Map: Based on recombination data.

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

Alterations of Chromosome Number

• Nondisjunction: Failure of chromosomes to separate during meiosis.

• Aneuploidy: Abnormal number of chromosomes.

• 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.

Summary Table: Types of Chromosomal Alterations

Summary Table: Types of Genetic Crosses

Key Equations

• Multiplication Rule:

• Addition Rule:

Additional info: Academic context and examples were added to clarify concepts and provide self-contained explanations for exam preparation.