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.

• 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.

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

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

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

• Centromere: The region where sister chromatids are joined together.

The Cell Cycle

The cell cycle is an ordered sequence of events that extends from the formation of a cell to its own division into two daughter cells.

• Phases of the Cell Cycle:

  • 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 non-dividing state that some cells enter from G1.

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

Mitosis and Cytokinesis

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

• Somatic Cells: Body cells that undergo mitosis.

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

• Stages of Mitosis:

  1. Prophase: Chromatin condenses into chromosomes; spindle forms.

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

  3. Metaphase: Chromosomes align at the metaphase plate.

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

  5. Telophase: Nuclear envelopes reform; chromosomes decondense.

• Cytokinesis:

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

  • Plant Cells: Cell plate forms, leading to separation.

Mitotic Spindle and Chromosome Movement

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

• Components: Centrosomes, spindle microtubules, kinetochores.

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

• Experiment: Marking microtubules showed that shortening occurs at the kinetochore end.

Binary Fission

Binary fission is the process by which prokaryotes (bacteria and archaea) reproduce by dividing in half.

• Process: DNA replicates, cell elongates, and divides into two identical cells.

Cell Cycle Regulation

Cell cycle progression is controlled by checkpoints and regulatory proteins.

• Checkpoints: G1, G2, and M checkpoints ensure proper division.

• Growth Factors: Proteins released by cells that stimulate division in other cells.

• 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

Organisms reproduce either asexually (one parent, identical offspring) or sexually (two parents, genetically diverse offspring).

• Asexual Reproduction: Offspring are clones of the parent.

• Sexual Reproduction: Involves fusion of gametes (sperm and egg).

Genes, Chromosomes, and Karyotypes

• Genes: Units of heredity made of DNA.

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

• Human Chromosome Number: 46 chromosomes (23 pairs).

• Karyotype: Ordered display of chromosomes from a cell.

• Homologous Chromosomes: Chromosome pairs with the same genes.

• Sex Chromosomes: X and Y chromosomes determine sex.

• Autosomes: Non-sex 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).

• Plant Life Cycle: Alternation of generations between multicellular haploid and diploid stages.

• Fungi Life Cycle: Mostly haploid, with a brief diploid stage.

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

Meiosis

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

• Sister Chromatid Cohesion: Sister chromatids are held together after DNA replication.

• Synapsis: Homologous chromosomes pair up during prophase I.

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

• Stages of Meiosis:

  1. Meiosis I: Homologous chromosomes separate.

  2. Meiosis II: Sister chromatids separate.

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

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

Mendelian Genetics

Gregor Mendel and the Laws of Inheritance

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

• Blending Hypothesis: Outdated idea that offspring are a blend of parents' traits.

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

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

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

• Dominant and Recessive Traits: Dominant traits mask recessive ones.

• 3:1 Ratio: Observed in F2 generation for a single trait cross.

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

Mendel's Laws

• Law of Segregation: Alleles separate during gamete formation.

• Law of Independent Assortment: Genes for different traits 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: One trait; 3:1 ratio in F2.

• Dihybrid Cross: Two traits; 9:3:3:1 ratio in F2.

• Trihybrid Cross: Three traits.

Probability in Genetics

• Multiplication Rule: Probability of independent events occurring together 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 another.

• Incomplete Dominance: Heterozygote has intermediate phenotype.

• Codominance: Both alleles are 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 (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 genes and environment (e.g., heart disease).

Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

• Chromosome Theory: Genes are located on chromosomes, which segregate and assort independently.

Sex-Linked and Linked Genes

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

• X-Linked Genes: Genes on the X chromosome; males are more likely to express recessive X-linked traits.

• X Inactivation: In females, 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 gene loci along a chromosome.

• Linkage Map: Based on recombination frequencies.

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

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; three copies of chromosome 21.

• Alterations of Structure:

  • Deletion: Loss of a chromosome segment.

  • Duplication: Repetition of a segment.

  • Inversion: Reversal of a segment.

  • Translocation: Segment moves to a nonhomologous chromosome.

• 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.

Summary Table: Types of Chromosomal Alterations

Key Equations

• Probability of Independent Events:

• Probability of Mutually Exclusive Events:

• Recombination Frequency:

Example: In a dihybrid cross, the expected phenotypic ratio is 9:3:3:1 if the genes assort independently.

Additional info: These notes synthesize the key concepts from chapters 9-12, providing definitions, explanations, and examples relevant to cell division, genetics, and chromosomal inheritance.