Cell Division and the Cell Cycle

What is 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, repair, and reproduction in all living organisms.

• Multicellular organisms develop from a single cell (zygote) through repeated cell divisions.

• Cell division enables tissue growth and maintenance.

Key Terms: Chromatin, Chromatids, Centromere, Genome

• Chromatin: The complex of DNA and proteins that forms chromosomes within the nucleus.

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

• Centromere: The region where sister chromatids are joined; essential for chromosome movement during cell division.

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

The Cell Cycle

The cell cycle is the ordered sequence of events that leads to cell division and duplication.

• Consists of Interphase (G1, S, G2) and M phase (mitosis and cytokinesis).

• G1 phase: Cell growth.

• S phase: DNA synthesis (replication).

• G2 phase: Preparation for mitosis.

• M phase: Division of nucleus (mitosis) and cytoplasm (cytokinesis).

Mitosis: Somatic Cell Division

Mitosis is the process by which somatic (non-reproductive) cells divide, producing two genetically identical daughter cells.

• Occurs in all body cells except gametes.

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

Phases of the Cell Cycle and Mitosis

• Prophase: Chromatin condenses into visible chromosomes; mitotic spindle forms.

• Metaphase: Chromosomes align at the cell's equator.

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

• Telophase: Nuclear envelope reforms; chromosomes decondense.

• Cytokinesis: Division of cytoplasm; in animals, a cleavage furrow forms; in plants, a cell plate forms.

Mitotic Spindle and Chromosome Movement

• Mitotic spindle: Structure made of microtubules that segregates chromosomes during mitosis.

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

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

Binary Fission

Binary fission is the method by which prokaryotes (bacteria and archaea) reproduce, involving the division of a single cell into two identical cells.

Cell Cycle Checkpoints

• G1 checkpoint: Determines if cell will proceed to division.

• S checkpoint: Ensures DNA is replicated correctly.

• G2 checkpoint: Checks for DNA damage before mitosis.

• M checkpoint: Ensures chromosomes are properly attached to spindle before separation.

• G0 phase: Non-dividing state.

Growth Factors and Regulation

• Growth factors: Proteins released by cells 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: Offspring are genetically identical to parent (clones).

• Sexual reproduction: Offspring inherit a combination of genes from two parents, increasing genetic diversity.

Genes, Gametes, and Chromosomes

• Genes: Units of heredity, made of DNA.

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

• Humans have 46 chromosomes (23 pairs).

Karyotype and Chromosome Types

• Karyotype: Ordered display of an individual's chromosomes.

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

• Sex chromosomes: X and Y chromosomes determine sex.

• Autosomes: Non-sex chromosomes.

Diploid and Haploid Cells

• Diploid (2n): Cells with two sets of chromosomes.

• Haploid (n): Cells with one set of chromosomes (gametes).

Sexual Life Cycles in Animals, Plants, Fungi

• Animals: Diploid dominant; gametes produced by meiosis.

• Plants: Alternation of generations; both haploid and diploid multicellular stages.

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

Meiosis: Chromosome Reduction

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

• Sister chromatid cohesion: Holds chromatids together until separation.

• Synapsis: Homologous chromosomes pair up during prophase I.

• Crossing over: Exchange of genetic material between homologous chromosomes, increasing genetic diversity.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Genetic Variation Mechanisms

• Independent assortment: Random distribution of homologous chromosomes.

• 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 inheritance in pea plants.

• Disproved the 'blending hypothesis' with experiments showing discrete inheritance.

True Breeding, Hybridization, Generations

• True breeding: Organisms that produce offspring identical to themselves.

• Hybridization: Crossing two different true-breeding organisms.

• 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 when at least one allele is present.

• Recessive trait: Expressed only when two recessive alleles are present.

• Mendel's F2 ratio: 3:1 (dominant:recessive).

Alleles, Law of Segregation, Law of Independent Assortment

• Alleles: Different versions of a gene, located on homologous chromosomes.

• Law of Segregation: Each organism carries two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits are inherited independently.

Phenotype, Genotype, Punnett Squares, Testcross

• Phenotype: Observable traits.

• Genotype: Genetic makeup.

• Punnett Square: Diagram used to predict genetic outcomes.

• Testcross: Crossing an individual with a dominant phenotype with a homozygous recessive to determine genotype.

Monohybrid, Dihybrid, Trihybrid Crosses

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

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

• Trihybrid cross: Three traits.

Probability Rules

• Multiplication Rule: Probability of two 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.

Example: Probability of getting two heads in two coin tosses:

Types of Dominance

• Complete dominance: One allele completely masks the other.

• Incomplete dominance: Heterozygote shows intermediate phenotype.

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

Multiple Alleles and Blood Type

• Blood type is determined by three alleles: IA, IB, and i.

• Possible genotypes: IAIA, IAi, IBIB, IBi, IAIB, ii.

Pleiotropy, Epistasis, Polygenic Inheritance

• Pleiotropy: One gene affects multiple traits.

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

• Polygenic inheritance: Multiple genes contribute to a single trait (e.g., skin color).

Nature vs. Nurture

• Environment can influence phenotype (e.g., hydrangea flower color depends on soil pH).

Pedigree Analysis and Genetic Disorders

• Pedigree: Diagram showing inheritance of traits in a family.

• Recessive disorders: Require two recessive alleles (e.g., cystic fibrosis).

• Sickle cell disease: Caused by a single allele; heterozygotes may show mild symptoms.

• Dominant disorders: Require only one dominant allele (e.g., Huntington's disease).

• Multifactorial disorders: Caused by genetic and environmental factors (e.g., heart disease).

Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

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

Morgan's Fruit Fly Experiments

• Thomas Hunt Morgan used Drosophila melanogaster to study inheritance.

• Discovered sex-linked traits (e.g., white eyes).

Sex-Linked Genes, X-Linked Genes, X Inactivation

• Sex-linked genes: Located on sex chromosomes.

• X-linked genes: Found on the X chromosome.

• X inactivation: In females, one X chromosome is inactivated, forming a Barr body.

Linked Genes and Genetic Recombination

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

• Genetic recombination: Production of offspring with combinations of traits different from either parent.

• Crossing over: Contributes to recombination.

Genetic Maps and Linkage Maps

• Genetic map: Shows the relative positions of genes on a chromosome.

• Linkage map: Based on recombination frequencies.

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

Alterations of Chromosome Number and Structure

• Nondisjunction: Failure of chromosomes to separate properly 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: Repetition of a chromosome segment.

• Inversion: Reversal of a segment within a chromosome.

• Translocation: Movement of a segment to another chromosome.

Aneuploidy of Sex Chromosomes and Disorders

• Examples: Turner syndrome (XO), Klinefelter syndrome (XXY).

• Structural alterations can cause genetic disorders (e.g., Cri du chat syndrome).

Summary Table: Types of Chromosomal Alterations

Additional info: This guide expands on brief points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.