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: Identical copies of a chromosome, joined at the centromere.

The Cell Cycle

The cell cycle is the ordered sequence of events that leads to cell division and duplication. It consists of interphase (G1, S, G2) and the mitotic phase (M).

• Interphase: Cell grows (G1), DNA replicates (S), prepares for division (G2).

• Mitosis: Division of the nucleus, followed by cytokinesis (division of cytoplasm).

• Somatic Cells: Body cells that undergo mitosis.

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

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: Resting phase; cells exit the cycle and do not divide.

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 pull them apart during anaphase.

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

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

• Prometaphase: Nuclear envelope breaks down, spindle attaches to chromosomes.

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Nuclear envelope reforms, chromosomes decondense.

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

Binary Fission

Binary fission is the process by which prokaryotes (bacteria and archaea) reproduce, involving DNA replication and division.

Cell Cycle Checkpoints

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

• G2 Checkpoint: Ensures DNA is replicated correctly.

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

Regulation of Cell Division

• Growth Factors: Proteins released by cells to stimulate 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 produces genetically identical offspring; sexual reproduction combines genetic material from two parents.

• Genes: Units of heredity, made of DNA.

• Gametes: Reproductive cells (sperm and egg).

• Humans: 46 chromosomes (23 pairs).

Karyotype and Chromosome Types

• Karyotype: Ordered display of chromosomes.

• Homologous Chromosomes: Chromosome pairs with same genes.

• Sex Chromosomes: X and Y; determine sex.

• Autosomes: Non-sex chromosomes.

Diploid and Haploid Cells

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

• Haploid (n): One set of chromosomes.

Variety of Sexual Life Cycles

• Animals: Diploid dominant; gametes produced by meiosis.

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

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

Meiosis

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

• 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 is the father of genetics, known for his experiments with pea plants.

• Blending Hypothesis: Traits mix; disproven by Mendel.

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

• Hybridization: Crossing different varieties.

• P Generation: Parental generation.

• F1 Generation: First filial generation.

• F2 Generation: Second filial generation.

Dominant and Recessive Traits

• Dominant Trait: Expressed in the phenotype.

• Recessive Trait: Masked unless homozygous.

• F2 Ratio: 3:1 (dominant:recessive).

Alleles and Laws of Inheritance

• Alleles: Different forms 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.

Genetic Crosses

• Monohybrid: One trait.

• Dihybrid: Two traits.

• Trihybrid: Three traits.

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

• 3:1 Ratio: Monohybrid cross result.

Probability Rules

• Multiplication Rule: Probability of independent events:

• Addition Rule: Probability of mutually exclusive events:

Types of Dominance

• Complete Dominance: One allele masks the other.

• Incomplete Dominance: Heterozygote is intermediate.

• Codominance: Both alleles are expressed.

Multiple Alleles and Blood Type

• Blood Type: Determined by multiple alleles (A, B, O).

Pleiotropy, Epistasis, and 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 in a family.

Genetic Disorders

• Recessive Disorders: Require two copies of mutant allele.

• Sickle Cell Disease: Caused by a single allele; heterozygotes have mild symptoms.

• Dominant Disorders: Require only one mutant allele.

• Multifactorial Disorders: Caused by genetic and environmental factors.

Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

This theory states that genes are located on chromosomes, and chromosomes are the basis for inheritance.

• Law of Segregation: Repeated from Mendel.

• Law of Independent Assortment: Repeated from Mendel.

Morgan's Fruit Fly Experiments

• Trait Studied: Eye color; white-eyed mutation.

• Sex-Linked Genes: Genes located on sex chromosomes.

• X-Linked Genes: Genes on the 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 new combinations of traits.

• Crossing Over: Contributes to recombination.

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

• 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 a chromosome segment.

• Duplication: Repeat of a segment.

• Inversion: Reversal of a segment.

• Translocation: Segment moves to another chromosome.

Aneuploidy of Sex Chromosomes and Disorders

• Aneuploidy of Sex Chromosomes: Abnormal number of X or Y chromosomes.

• Disorders: Caused by structural changes (e.g., Turner syndrome, Klinefelter syndrome).

Additional info: Some explanations and examples were expanded for clarity and completeness, as the original guide was a list of concepts.