Cell Division, Inheritance, and the Molecular Basis of Genetics

Cell Division and the Cell Cycle

Roles of Cell Division in Organisms

• Growth: Cell division increases the number of cells, allowing organisms to grow from a single cell to multicellular complexity.

• Repair and Maintenance: Damaged or old cells are replaced through cell division, maintaining tissue health.

• Reproduction: Cell division enables both asexual (offspring genetically identical to parent) and sexual reproduction (offspring with genetic variation).

• Development: Cell division is essential for the formation of tissues and organs during development.

Key Terms and Definitions

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

• Chromosome: A structure composed of DNA and proteins that carries genetic information.

• Chromatin: The complex of DNA and proteins (mainly histones) that forms chromosomes; less condensed than chromosomes.

• Gamete: A reproductive cell (sperm or egg) containing half the number of chromosomes (haploid).

• Somatic Cell: Any body cell other than gametes; typically diploid (2n).

• Sister Chromatids: Two identical copies of a chromosome, joined at the centromere.

• Centromere: The region where sister chromatids are attached.

Structural Organization of Genomes

• Prokaryotic Genome: Single, circular DNA molecule located in the nucleoid region; may contain plasmids (small, circular DNA).

• Eukaryotic Genome: Multiple, linear DNA molecules (chromosomes) within a membrane-bound nucleus; DNA is wrapped around histone proteins, forming chromatin.

Phases of the Cell Cycle

• Interphase: Period of cell growth and DNA replication.

  • G1 phase: Cell grows, synthesizes proteins and organelles, and performs normal functions.

  • S phase: DNA is replicated; each chromosome now consists of two sister chromatids.

  • G2 phase: Further growth, preparation for division, and error checking.

• M phase (Mitotic phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Phases of Mitosis

• Prophase: Chromatin condenses into visible chromosomes; mitotic spindle forms; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the metaphase plate; spindle fibers attach to centromeres.

• Anaphase: Sister chromatids are separated and pulled to opposite poles; each chromatid is now an individual chromosome.

• Telophase: Chromosomes arrive at poles, decondense into chromatin; nuclear envelope reforms; spindle apparatus disassembles.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

The Mitotic Spindle Apparatus

• Centrosomes: Microtubule-organizing centers at opposite poles during mitosis.

• Mitotic Spindle: Structure of microtubules guiding chromosome movement.

• Kinetochore Microtubules: Attach from centrosomes to kinetochores on centromeres.

• Asters: Star-shaped microtubule arrays radiating from centrosomes, anchoring the spindle.

Cytokinesis: Animals vs. Plants

• Animal Cells: Cytokinesis occurs by cleavage; a contractile ring of actin filaments forms a cleavage furrow.

• Plant Cells: Vesicles from the Golgi apparatus coalesce at the cell center to form a cell plate, which develops into a new cell wall.

Binary Fission in Bacteria

• DNA Replication: Begins at the origin of replication; circular chromosome is copied.

• Chromosome Segregation: DNA molecules move to opposite ends of the cell.

• Cell Elongation: Cell grows, separating chromosomes further.

• Septum Formation: New cell wall forms, dividing the cell into two.

Cell Cycle Checkpoints and Control

• G1 Checkpoint: Checks for cell size, nutrients, and growth signals.

• G2 Checkpoint: Ensures DNA is fully replicated and undamaged.

• Metaphase Checkpoint: Confirms all chromosomes are properly attached to the spindle.

• Regulation: Controlled by cyclins and cyclin-dependent kinases (Cdks).

• Loss of Control: Failure of checkpoints can lead to uncontrolled cell division, cancer, and mutations.

Types of Tumors

• Benign: Non-cancerous, localized, does not invade nearby tissues.

• Malignant: Cancerous, invades surrounding tissues, can damage organs.

• Metastatic: Cancer cells travel through the bloodstream to form new tumors elsewhere.

Meiosis and Sexual Life Cycles

Key Terms and Definitions

• Ploidy: Number of chromosome sets in a cell (haploid = 1 set, diploid = 2 sets).

• Heredity: Transmission of traits from parents to offspring via genes.

• Gene: DNA segment coding for a specific trait or protein.

• Homologous Chromosomes: Chromosome pairs with the same genes at the same loci, possibly different alleles.

• Karyotype: Organized profile of all chromosomes in a cell.

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

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

• Zygote: Fertilized egg formed by fusion of sperm and egg.

• Gamete: Haploid reproductive cell (sperm or egg).

• Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis, increasing genetic diversity.

• Chiasmata: Physical sites where crossing over occurs.

• Tetrad: Group of four chromatids formed during pairing of homologous chromosomes in meiosis.

• Synapsis: Pairing of homologous chromosomes during meiosis.

Comparisons and Distinctions

• Somatic Cell vs. Gamete: Somatic cells are diploid; gametes are haploid.

• Autosome vs. Sex Chromosome: Autosomes are non-sex chromosomes; sex chromosomes determine gender.

• Haploid vs. Diploid: Haploid cells have one set of chromosomes; diploid have two.

• Asexual vs. Sexual Reproduction: Asexual produces identical offspring; sexual increases genetic diversity.

• Sister Chromatids vs. Homologous Chromosomes: Sister chromatids are identical copies; homologous chromosomes are similar but not identical (one from each parent).

Meiosis: Definition, Location, and Products

• Definition: Specialized cell division producing gametes (sperm, eggs) with half the chromosome number.

• Location: Animals: testes (sperm), ovaries (eggs); Plants: anthers (pollen), ovules (egg cells).

Phases of Meiosis

• Meiosis I:

  • Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis) to form tetrads, crossing over occurs at chiasmata, nuclear envelope breaks down.

  • Metaphase I: Tetrads align at the metaphase plate.

  • Anaphase I: Homologous chromosomes separate to opposite poles; sister chromatids remain together.

  • Telophase I & Cytokinesis: Chromosomes reach poles; cell divides into two haploid cells.

• Meiosis II:

  • Prophase II: Chromosomes condense again; new spindle forms in each haploid cell.

  • Metaphase II: Chromosomes align at the metaphase plate.

  • Anaphase II: Sister chromatids separate and move to opposite poles.

  • Telophase II & Cytokinesis: Chromatids reach poles; cells divide, resulting in four haploid gametes.

Comparison: Mitosis vs. Meiosis

• Mitosis: Produces two genetically identical diploid cells; one division; no crossing over.

• Meiosis: Produces four genetically unique haploid cells; two divisions; includes crossing over and independent assortment.

Sources of Genetic Variation

1. Crossing Over: Exchange of DNA between homologous chromosomes during prophase I.

2. Independent Assortment: Random alignment of homologous pairs during metaphase I.

3. Random Fertilization: Any sperm can fertilize any egg, creating unique combinations.

Trisomy 21 (Down Syndrome)

• Definition: Genetic disorder caused by an extra copy of chromosome 21.

• Cause: Usually due to non-disjunction during meiosis.

Mendelian Genetics and Chromosomal Inheritance

Key Terms and Definitions

• Homologous Chromosomes: Chromosome pairs with the same genes.

• Locus: Specific location of a gene on a chromosome.

• Allele: Different versions of a gene at a locus.

• Trait: Specific variant of a character (e.g., blue eyes).

Genetic Crosses and Generations

• True Breeding: Organisms homozygous for a trait, always producing the same phenotype.

• Hybridization: Crossing two true-breeding organisms with different traits.

• P Generation: Parental generation in a genetic cross.

• F1 Generation: First filial generation, offspring of P generation.

• F2 Generation: Offspring of F1 generation.

• Monohybrid Cross: Cross involving one gene; both parents heterozygous.

• Dihybrid Cross: Cross involving two genes; both parents heterozygous for both.

Dominance and Genotype/Phenotype

• Dominant Allele: Expressed in phenotype if present.

• Recessive Allele: Expressed only if two copies are present.

• Heterozygous: Two different alleles for a gene.

• Homozygous: Two identical alleles for a gene.

• Genotype: Genetic makeup (allele combination).

• Phenotype: Observable traits.

Mendel's Laws

• Law of Segregation: Each parent passes only one allele for each gene to offspring; alleles separate during meiosis.

• Law of Independent Assortment: Alleles of different genes are distributed independently to gametes.

Testcross

• Definition: Cross between an individual with unknown genotype and a homozygous recessive individual to determine genotype.

Punnett Squares and Pedigrees

• Punnett Squares: Used to predict genotypic and phenotypic ratios in crosses.

• Pedigree Charts: Diagrams showing inheritance patterns in families; used to determine dominance/recessiveness and genotypes.

Genetic Disorders and Carriers

• Recessive Disorders: Require two copies of the mutant allele; carriers have one copy and are phenotypically normal.

• Dominant Disorders: Expressed with one mutant allele; lethal dominant genes are rare because affected individuals often do not reproduce.

Patterns of Inheritance

• Complete Dominance: Heterozygote shows dominant phenotype.

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

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

Complex Inheritance

• Pleiotropy: One gene affects multiple traits (e.g., sickle cell anemia).

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

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

Chromosomal Basis of Inheritance

Chromosomal Theory of Inheritance

• Concept: Genes are located on chromosomes, which are inherited from parents.

• Discovery: Walter Sutton and Theodor Boveri.

Chromosome Behavior and Mendel's Laws

• Law of Segregation: Homologous chromosomes separate during meiosis I, so each gamete receives one allele.

• Law of Independent Assortment: Homologous pairs align randomly during metaphase I, explaining independent inheritance of genes on different chromosomes.

Sex-Linked Inheritance

• Sex-Linked Genes: Located on sex chromosomes (X or Y); X-linked traits are more common in males (only one X chromosome).

• X-Inactivation: In female mammals, one X chromosome per cell is randomly inactivated to balance gene dosage.

Linked Genes

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

Genetic Recombination and Linkage Maps

• Recombinant Phenotypes: Produced by crossing over and independent assortment during meiosis.

• Linkage Maps: Constructed by crossing individuals, calculating recombination frequencies, and determining gene order and distances.

  • Recombination frequency formula: $\text{Recombination\ Frequency} = \frac{\text{Number\ of\ Recombinant\ Offspring}}{\text{Total\ Offspring}} \times 100$

Chromosomal Mutations

• Non-disjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

• Aneuploidy: Abnormal number of chromosomes in a cell.

• Trisomy: Cell has one extra chromosome (e.g., Down syndrome).

• Monosomy: Cell is missing a chromosome.

• Polyploidy: More than two complete sets of chromosomes.

• Deletions: Loss of a chromosome segment.

• Duplications: Extra copies of a chromosome segment.

• Inversions: Reversal of a chromosome segment.

• Translocations: Segment moves to a non-homologous chromosome.

Molecular Basis of Inheritance

Key Experiments and Discoveries

• Hershey and Chase: Used bacteriophages to show DNA, not protein, is genetic material.

• Watson and Crick: Developed the double helix model of DNA; explained base pairing (A=T, C=G).

• Franklin and Wilkins: Used X-ray diffraction to study DNA structure.

Hershey-Chase Experiment

• Labeled DNA with radioactive phosphorus and protein with radioactive sulfur in bacteriophages.

• After infection, only radioactive phosphorus entered bacteria, proving DNA is the genetic material.

Structure of DNA

• Double Helix: Two antiparallel strands twisted into a helix.

• Sugar-Phosphate Backbone: Joined by covalent phosphodiester bonds.

• Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G); A pairs with T, C pairs with G via hydrogen bonds.

• Directionality: Strands run 5' to 3' and 3' to 5'.

• Complementarity: Base pairing ensures accurate replication.

DNA Replication

• Replication Bubble: DNA opens at origins to form bubbles.

• Replication Fork: Y-shaped region where DNA is unwound and copied.

• Helicase: Unwinds and separates DNA strands.

• Topoisomerase: Relieves tension ahead of the fork.

• Primase: Synthesizes short RNA primers for DNA polymerase to start synthesis.

• DNA Polymerase: Adds nucleotides to the primer in the 5' to 3' direction.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork in short Okazaki fragments; requires multiple primers.

• Okazaki Fragments: Short DNA segments on the lagging strand.

• DNA Ligase: Joins Okazaki fragments into a continuous strand.

Enzymes of DNA Replication and Their Functions

Enzyme	Function
Helicase	Unwinds DNA double helix
Topoisomerase	Relieves tension ahead of replication fork
Primase		Synthesizes RNA primers
DNA Polymerase		Adds new nucleotides to growing DNA strand
DNA Ligase	Joins Okazaki fragments

Base Pairing Rules

• Adenine (A) pairs with Thymine (T)

• Guanine (G) pairs with Cytosine (C)

Differences Between Leading and Lagging Strand Synthesis

• Leading Strand: Synthesized continuously in the same direction as the replication fork (5' to 3').

• Lagging Strand: Synthesized discontinuously in short fragments (Okazaki fragments) opposite to fork movement; requires multiple primers.

Summary Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Genetic Identity	Identical	Unique
Chromosome Number	Diploid (2n)	Haploid (n)
Role	Growth, repair	Gamete production

Additional info: This guide expands on the original notes with academic context, definitions, and structured explanations to provide a comprehensive overview suitable for exam preparation in introductory biology.