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: Begin as a single cell (zygote) and develop into complex beings 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: Two identical copies of a chromosome connected by a centromere.

The Cell Cycle

The cell cycle is the 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).

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 that microtubules shorten at the kinetochore during anaphase.

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Cytokinesis

• Animal Cells: Cleavage furrow forms.

• Plant Cells: Cell plate forms.

Binary Fission

Binary fission is the process by which prokaryotes (bacteria and archaea) reproduce. The cell grows, replicates its DNA, and divides into two identical cells.

Cell Cycle Checkpoints

• G1 Checkpoint: Determines if cell will divide.

• G2 Checkpoint: Ensures DNA is replicated correctly.

• M Checkpoint: Ensures chromosomes are properly attached to spindle.

Growth Factors and Regulation

• Growth Factors: Proteins 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.

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

Genes, Gametes, and Chromosomes

• Genes: Units of heredity, made of DNA.

• Gametes: Reproductive cells (sperm and egg).

• Humans: 46 chromosomes (23 pairs).

• Karyotype: Ordered display of chromosomes.

• Homologous Chromosomes: Chromosome pairs with same genes.

• Sex Chromosomes: X and Y; Autosomes: non-sex chromosomes.

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

• Haploid (n): One set of chromosomes.

Variety of 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 consecutive divisions: Meiosis I and Meiosis II.

• Sister Chromatid Cohesion: Holds chromatids together.

• Synapsis: Homologous chromosomes pair up (Prophase I).

• Crossing Over: Exchange of genetic material between homologs.

Stages of Meiosis

• Meiosis I: Homologs separate.

• Meiosis II: Sister chromatids separate.

Genetic Variation

• 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, the father of genetics, used pea plants to study inheritance. He disproved the 'blending hypothesis' and established the particulate nature of genes.

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

• Hybridization: Crossing two different true-breeding plants.

• P Generation: Parental generation.

• F1 Generation: First filial generation.

• F2 Generation: Second filial generation.

Dominant and Recessive Traits

• Dominant Trait: Expressed in the F1 generation.

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

• 3:1 Ratio: Typical ratio in F2 generation for monohybrid cross.

Alleles and Laws of Inheritance

• Alleles: Different versions 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 crosses.

• Testcross: Cross with homozygous recessive to determine genotype.

Homozygous vs. Heterozygous

• Homozygous: Two identical alleles.

• Heterozygous: Two different alleles.

Monohybrid, Dihybrid, Trihybrid Crosses

• Monohybrid: One trait.

• Dihybrid: Two traits.

• Trihybrid: Three traits.

• 9:3:3:1 Ratio: Dihybrid cross result.

Probability Rules

• Multiplication Rule: Probability of two independent events:

• Addition Rule: Probability of either event:

Types of Dominance

• Complete Dominance: One allele masks the other.

• Incomplete Dominance: Heterozygote shows intermediate phenotype.

• Codominance: Both alleles are fully expressed.

Multiple Alleles and Blood Type

• Blood Type: Determined by three alleles: , , .

Pleiotropy, Epistasis, 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

• Pedigree: Diagram showing inheritance patterns in families.

Genetic Disorders

• Recessive Disorders: Require two copies of mutant allele.

• Sickle Cell Disease: Caused by a single allele; heterozygotes show partial phenotype.

• Dominant Disorders: Caused by 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: Chromosomes separate during meiosis.

• Law of Independent Assortment: Chromosomes assort independently.

Morgan's Fruit Fly Experiments

• Thomas Hunt Morgan: Used Drosophila melanogaster to show genes are on chromosomes.

• Common Trait: White eyes; named as a sex-linked trait.

Sex-Linked and X-Linked Genes

• Sex-Linked Genes: Located on sex chromosomes.

• X-Linked Genes: Located on X chromosome.

• X Inactivation: One X chromosome in females is inactivated (Barr body).

Linked Genes and Genetic Recombination

• Linked Genes: Genes located close together on a chromosome; inherited together.

• Genetic Recombination: Production of offspring with combinations of traits different from parents.

• Crossing Over: Exchange of genetic material during meiosis.

Genetic and Linkage Maps

• Genetic Map: Shows gene locations based on recombination frequency.

• Linkage Map: Based on recombination data.

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

Alterations of Chromosome Number and Structure

• Nondisjunction: Failure of chromosomes to separate 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 chromosome segment.

• Duplication: Repeat of segment.

• Inversion: Reversal of segment.

• Translocation: Segment moves to another chromosome.

Aneuploidy of Sex Chromosomes and Disorders

• Aneuploidy: Can affect sex chromosomes (e.g., Turner syndrome, Klinefelter syndrome).

• Structural Alterations: Can cause genetic disorders.

Summary Table: Types of Chromosomal Alterations

Summary Table: Cell Cycle Checkpoints

Summary Table: Types of Genetic Crosses

Additional info: Academic context and examples were added to clarify concepts and provide self-contained explanations for exam preparation.