Cell Division and the Cell Cycle

Introduction to Cell Division

Cell division is the process by which a single cell divides to produce two daughter cells. This process is fundamental for growth, development, and repair in multicellular organisms, as well as reproduction in unicellular organisms.

• Cell Division: The process of creating new cells from a parent cell.

• Multicellular Organisms: Develop from a single cell (zygote) through repeated cell divisions.

Key Terms in Cell Division

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

• Centromere: The region where sister chromatids are joined.

• Chromatin: DNA and protein complex that forms chromosomes.

• Genome: The complete set of genetic material in an organism.

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.

Mitosis and Somatic Cells

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.

Mitotic Spindle and Chromosome Movement

The mitotic spindle is a structure made of microtubules that separates chromosomes during mitosis.

• Components: Microtubules, centrosomes, kinetochores.

• Function: Ensures accurate chromosome segregation.

• Experiment: Kinetochore microtubules shorten during anaphase, pulling chromatids apart.

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

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

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Cytokinesis

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

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

Binary Fission

Binary fission is the process by which prokaryotes (bacteria and archaea) divide.

• Process: DNA replicates, cell splits into two.

Cell Cycle Checkpoints

• G0: Non-dividing state.

• G1: Checks for cell size, nutrients.

• S: Checks for DNA replication.

• G2: Checks for DNA damage.

• M: Checks for chromosome attachment to spindle.

Growth Factors and Regulation

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

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

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

Meiosis and Sexual Life Cycles

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Offspring are genetically identical to parent.

• Sexual Reproduction: Offspring inherit genetic material from two parents.

Genes and Gametes

• Genes: Units of heredity, made of DNA.

• Gametes: Reproductive cells (sperm and egg).

Chromosome Number and Species Differences

• Humans: 46 chromosomes (23 pairs).

• Species Differences: Chromosome number varies among species.

Karyotype and Chromosome Types

• Karyotype: Ordered display of chromosomes.

• Homologous Chromosomes: Chromosome pairs with same genes.

• Sex Chromosomes: X and Y; determine sex.

• Autosomes: Non-sex chromosomes.

Diploid and Haploid Cells

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

• Haploid (n): One set of chromosomes.

Sexual Life Cycles

• Animal Life Cycle: Diploid dominant; gametes produced by meiosis.

• Plant Life Cycle: Alternation of generations; both haploid and diploid stages.

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

• Alternation of Generations: Life cycle alternates between haploid and diploid forms.

Meiosis and Chromosome Reduction

Meiosis reduces chromosome number by half, producing four haploid cells from one diploid cell.

• Sister Chromatid Cohesion: Sister chromatids held together during meiosis.

• Synapsis: Homologous chromosomes pair up; occurs during prophase I.

• Crossing Over: Exchange of genetic material between homologs.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Genetic Variation Mechanisms

• 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: Father of genetics; studied 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, and F2 Generations

• P Generation: Parental generation.

• F1 Generation: First filial generation (offspring of P).

• F2 Generation: Second filial generation (offspring of F1).

Dominant and Recessive Traits

• Dominant Trait: Expressed in the phenotype if present.

• Recessive Trait: Masked unless both alleles are recessive.

• F2 Ratio: Mendel observed a 3:1 ratio in F2 generation.

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: Observable traits.

• Genotype: Genetic makeup.

Punnett Squares and Testcrosses

• Punnett Square: Diagram to predict genetic outcomes.

• Testcross: Cross with homozygous recessive to determine genotype.

Homozygous and Heterozygous

• Homozygous: Two identical alleles.

• Heterozygous: Two different alleles.

Monohybrid, Dihybrid, and Trihybrid Crosses

• Monohybrid Cross: One trait; results in 3:1 ratio.

• Dihybrid Cross: Two traits; results in 9:3:3:1 ratio.

Probability Rules

• Multiplication Rule: Probability of two independent events:

• Addition Rule: Probability of either event:

Types of Dominance

• Complete Dominance: One allele completely masks the other.

• Incomplete Dominance: Heterozygote shows intermediate phenotype.

• 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 single 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

• Chromosome Theory: Genes are located on chromosomes.

• Law of Segregation: Repeated; alleles separate during gamete formation.

• Law of Independent Assortment: Repeated; genes assort independently.

Morgan's Fruit Fly Experiments

• Thomas Hunt Morgan: Used fruit flies to study inheritance.

• Common Trait: White eyes; named as a mutant trait.

Sex-Linked and X-Linked Genes

• Sex-Linked Genes: Located on sex chromosomes.

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

X Inactivation and Barr Bodies

• X Inactivation: One X chromosome in females is inactivated.

• Barr Body: Inactivated X chromosome.

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 on chromosomes.

• Linkage Map: Based on recombination frequencies.

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

Alterations of Chromosome Number

• Nondisjunction: Failure of chromosomes to separate properly.

• 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 chromosome segment.

• Inversion: Reversal of a segment.

• Translocation: Segment moves to another chromosome.

Aneuploidy of Sex Chromosomes and Disorders

• Aneuploidy of Sex Chromosomes: Abnormal number of X or Y chromosomes.

• Disorders: Caused by structural changes (e.g., Turner syndrome, Klinefelter syndrome).

Summary Table: Types of Genetic Crosses and Outcomes

Summary Table: Chromosome Alterations and Effects

Key Equations

• Probability of Independent Events:

• Probability of Either Event:

Additional info:

• For coat color in Labradors, epistasis involves two genes: one for pigment color (B = black, b = brown) and one for pigment deposition (E = allows color, e = no color, resulting in yellow).

• Trisomy 21 (Down syndrome) is caused by an extra chromosome 21.

• Pedigree analysis is used to track inheritance of traits and disorders in families.