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 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).

• Interphase: Cell grows (G1), replicates DNA (S), and prepares for division (G2).

• Mitosis: Division of the nucleus; produces two genetically identical somatic cells.

• Cytokinesis: Division of the cytoplasm; differs in animal (cleavage furrow) and plant cells (cell plate formation).

• Binary Fission: A form of asexual reproduction in prokaryotes.

Phases of the Cell Cycle

• G1 Phase: Cell growth.

• S Phase: DNA synthesis.

• G2 Phase: Preparation for mitosis.

• M Phase: Mitosis and cytokinesis.

• G0 Phase: Non-dividing state.

Mitotic Spindle and Chromosome Movement

The mitotic spindle is a structure made of microtubules that segregates chromosomes during mitosis. Kinetochore microtubules attach to chromosomes and shorten during anaphase, pulling chromatids apart.

• Components: Microtubules, centrosomes, kinetochores.

• Experiment: Demonstrated microtubule shortening at kinetochores during anaphase.

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

• Prometaphase: Nuclear envelope breaks down, spindle attaches to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Cell Cycle Regulation

• Checkpoints: G1, G2, M; ensure proper division.

• Growth Factors: External signals 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, increasing genetic diversity.

• Genes: Units of heredity.

• Gametes: Reproductive cells (sperm, egg).

Chromosome Number and Types

• Humans: 46 chromosomes (23 pairs).

• 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.

• Plants: Alternation of generations (haploid and diploid stages).

• Fungi: Haploid dominant.

Meiosis

Meiosis reduces chromosome number by half, producing four haploid gametes. It involves two divisions: Meiosis I and Meiosis II.

• Sister Chromatid Cohesion: Holds chromatids together.

• Synapsis: Homologs pair up during Prophase I.

• Crossing Over: Exchange of genetic material between homologs.

• Independent Assortment: Random distribution of homologs.

• Random Fertilization: Any sperm can fertilize any egg.

Stages of Meiosis

• Meiosis I: Homologs separate.

• Meiosis II: Sister chromatids separate.

Genetic Variation

• Recombinant Chromosomes: Result from crossing over.

Mendelian Genetics

Gregor Mendel and His Experiments

Gregor Mendel is the father of genetics. He used pea plants to study inheritance, disproving the blending hypothesis.

• 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.

• 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.

Types of Crosses

• Monohybrid: One trait.

• Dihybrid: Two traits; 9:3:3:1 ratio.

• Trihybrid: Three traits.

Probability Rules

• Multiplication Rule: Probability of independent events:

• Addition Rule: Probability of mutually exclusive events:

Types of Dominance

• Complete Dominance: One allele masks the other.

• Incomplete Dominance: Heterozygote is intermediate.

• Codominance: Both alleles are expressed.

Multiple Alleles and Blood Type

• Blood Type: Determined by three 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 and Genetic Disorders

• Pedigree: Diagram of family inheritance.

• Recessive Disorders: Require two alleles.

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

• Dominant Disorders: Require one 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 Mendel.

• Law of Independent Assortment: Repeated from Mendel.

Morgan's Fruit Fly Experiments

• Common Trait: Red eyes; mutation led to white eyes.

• Sex-Linked Genes: Located on sex chromosomes.

• X-Linked Genes: Located on 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: Located close together; inherited together.

• Genetic Recombination: New combinations from crossing over.

• Frequency Calculation: Based on offspring ratios.

Genetic and Linkage Maps

• Genetic Map: Shows gene locations.

• Linkage Map: Based on recombination frequencies.

• Map Units: 1 unit = 1% recombination.

Alterations of Chromosome Number

• Nondisjunction: Failure of chromosomes to separate.

• 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 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.

Table: Types of Chromosomal Alterations

Additional info: Table examples inferred for completeness.