Cell Division and the Cell Cycle

Introduction to Cell Division

Cell division is a fundamental biological process by which a single cell divides to produce two or more daughter cells. This process is essential for growth, development, and tissue repair in multicellular organisms.

• Cell Division: The process by which a parent cell divides into two or more daughter cells.

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

Key Terms in Cell Division

• Sister Chromatids: Identical copies of a chromosome connected by a centromere, formed during DNA replication.

• Centromere: The region where sister chromatids are most closely attached.

• Chromatin: The complex of DNA and proteins that makes up chromosomes.

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

The Cell Cycle

The cell cycle is the ordered sequence of events that a cell undergoes from its formation to its own division.

• G1 phase: Cell grows and carries out normal functions.

• S phase: DNA is replicated.

• G2 phase: Preparation for mitosis; cell grows more and produces proteins.

• M phase (Mitosis): Division of the nucleus and cytoplasm.

• G0 phase: A resting state where the cell is not actively dividing.

Duration: The length of each phase varies by cell type; S phase and M phase are typically shorter than G1 and G2.

Mitosis

Mitosis is the process by which somatic (body) cells divide, resulting in two genetically identical daughter cells.

• Somatic Cells: All body cells except gametes (sperm and egg).

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

Phases of Mitosis

• Prophase: Chromatin condenses into visible chromosomes; mitotic spindle begins to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Nuclear envelopes reform; chromosomes decondense.

Cytokinesis follows mitosis, dividing the cytoplasm:

• Animal Cells: Cleavage furrow forms and pinches the cell in two.

• Plant Cells: Cell plate forms, eventually becoming the cell wall.

Mitotic Spindle and Chromosome Movement

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

• Kinetochore Microtubules: Attach to chromosomes and shorten during anaphase, pulling chromatids apart.

• Experiment: Demonstrated that microtubules shorten at the kinetochore end during anaphase.

Binary Fission

• Binary Fission: A form of asexual reproduction in prokaryotes where the cell divides into two genetically identical cells.

Cell Cycle Regulation

• Checkpoints: Control points where the cell cycle can be stopped or allowed to proceed (G1, G2, M).

• Growth Factors: Proteins released by certain cells that stimulate other cells to divide.

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

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

Meiosis and Sexual Life Cycles

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Offspring are genetically identical to the parent (e.g., binary fission, mitosis).

• Sexual Reproduction: Offspring inherit a unique combination of genes from two parents.

Genes, Gametes, and Chromosomes

• Genes: Units of heredity made of DNA.

• Gametes: Reproductive cells (sperm and egg) that carry half the number of chromosomes.

• Humans: 46 chromosomes (23 pairs); different species have different chromosome numbers.

Karyotype and Chromosome Types

• Karyotype: An ordered display of an individual's chromosomes.

• Homologous Chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Sex Chromosomes: Determine sex (X and Y in humans).

• Autosomes: All other chromosomes.

Diploid and Haploid Cells

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

• Haploid (n): One set of chromosomes (gametes).

Sexual Life Cycles

• Animal Life Cycle: Gametes (n) fuse to form a zygote (2n), which grows by mitosis.

• Plant Life Cycle (Alternation of Generations): Alternates between multicellular haploid (gametophyte) and diploid (sporophyte) stages.

• Fungi and Some Protists: Mostly haploid; zygote is the only diploid stage.

Meiosis

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

• Sister Chromatid Cohesion: Holds sister chromatids together until anaphase II.

• Synapsis: Homologous chromosomes pair up during prophase I.

• Crossing Over: Exchange of genetic material between homologous chromosomes, creating recombinant chromosomes.

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Genetic Variation in Meiosis

• Independent Assortment: Random orientation of homologous pairs during metaphase I.

• Crossing Over: Produces recombinant chromosomes.

• Random Fertilization: Any sperm can fertilize any egg, increasing genetic diversity.

Mendelian Genetics

Gregor Mendel and His Experiments

• Gregor Mendel: Father of genetics; studied inheritance in pea plants.

• Blending Hypothesis: Outdated idea that genetic material mixes like paint.

• True Breeding: Plants that produce offspring of the same variety when self-pollinated.

• Hybridization: Mating of two different true-breeding varieties.

Generations and Inheritance Patterns

• P Generation: Parental generation.

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

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

• Dominant Trait: Expressed in the F1 generation.

• Recessive Trait: Masked in the F1, reappears in F2.

• 3:1 Ratio: Typical ratio of dominant to recessive traits in F2 generation.

Genes, Alleles, and Chromosomes

• Alleles: Alternative versions of a gene.

• Law of Segregation: Alleles separate during gamete formation.

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

Genotype and Phenotype

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

• Phenotype: Observable traits.

• Punnett Square: Diagram to predict genetic crosses.

• Testcross: Cross with a homozygous recessive to determine genotype.

• Homozygous: Two identical alleles.

• Heterozygous: Two different alleles.

Monohybrid and Dihybrid Crosses

• Monohybrid Cross: Involves one trait; results in a 3:1 ratio.

• Dihybrid Cross: Involves two traits; results in a 9:3:3:1 ratio.

Probability in Genetics

• Multiplication Rule: Probability of two independent events both occurring is the product of their probabilities.

• Addition Rule: Probability of either of two mutually exclusive events is the sum of their probabilities.

Extensions of Mendelian Genetics

• Complete Dominance: One allele completely masks the other.

• Incomplete Dominance: Heterozygotes have an intermediate phenotype.

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

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

• Pleiotropy: One gene affects multiple traits.

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

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

• Nature vs. Nurture: Environment can influence phenotype.

Pedigree Analysis and Human Genetics

• Pedigree: Family tree showing inheritance of traits.

• Recessive Disorders: Require two copies of the mutant allele (e.g., cystic fibrosis).

• Sickle Cell Disease: Caused by a single amino acid change; heterozygotes have some resistance to malaria.

• Dominant Disorders: Caused by a single copy of the mutant allele (e.g., Huntington's disease).

• Multifactorial Disorders: Influenced by multiple genes and environment (e.g., heart disease).

Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

• Genes are located on chromosomes, which segregate and independently assort during meiosis.

Sex-Linked and Linked Genes

• Sex-Linked Genes: Genes located on sex chromosomes (X or Y).

• X-Linked Genes: Genes on the X chromosome; often show different inheritance patterns in males and females.

• X Inactivation: In female mammals, one X chromosome is inactivated (Barr body).

• Linked Genes: Genes located close together on the same chromosome; tend to be inherited together.

Genetic Recombination and Mapping

• Genetic Recombination: Production of offspring with combinations of traits differing from either parent.

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

• Genetic Map: Ordered list of genetic loci along a chromosome.

• Linkage Map: Based on recombination frequencies; measured in map units (centimorgans).

Alterations of Chromosome Number and Structure

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

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

• Trisomy 21: Down syndrome; caused by an extra chromosome 21.

Alterations of Chromosome Structure

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a segment.

• Inversion: Reversal of a segment within a chromosome.

• Translocation: Segment moves from one chromosome to another.

• Aneuploidy of Sex Chromosomes: Can result in disorders such as Turner syndrome (XO) or Klinefelter syndrome (XXY).

• Disorders from Structural Alterations: Can cause developmental and health problems depending on the genes affected.

Example Table: Types of Chromosomal Alterations

Key Equations

• Probability of Independent Events:

• Probability of Mutually Exclusive Events:

• Recombination Frequency:

Additional info: Some details, such as specific experiment descriptions and illustrations, were inferred or expanded for completeness and clarity.