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 of producing new cells from a parent cell.

• Multicellular Development: Begins with a single cell (zygote) that divides repeatedly to form a multicellular organism.

Key Terms in Cell Division

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

• Centromere: The region where sister chromatids are joined.

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

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

The Cell Cycle

The cell cycle is the sequence 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).

• Phases: G1 (growth), S (DNA synthesis), G2 (preparation for division), M (mitosis).

• Duration: Each phase varies in length depending on cell type.

Mitosis and Somatic Cells

Mitosis is the division of the nucleus in somatic (body) cells, resulting in two genetically identical daughter cells.

• Somatic Cells: All cells except gametes; found throughout the body.

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

Phases of the Cell Cycle

• G1: Cell grows and prepares for DNA replication.

• S: DNA is replicated.

• G2: Cell prepares for mitosis.

• M: Mitosis and cytokinesis occur.

Mitotic Spindle and Chromosome Movement

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

• Components: Microtubules, centrosomes, kinetochores.

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

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

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

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

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

• Telophase: Nuclear envelope reforms, chromosomes decondense.

Cytokinesis: Division of the cytoplasm. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.

Binary Fission

Binary fission is the method by which prokaryotes (bacteria and archaea) divide.

• Process: DNA is replicated, cell elongates, and divides into two.

Cell Cycle Checkpoints

• G0: Non-dividing state.

• G1, S, G2, M: Checkpoints ensure proper progression and DNA integrity.

Growth Factors and Regulation

• 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 offspring genetically identical to the parent. Sexual reproduction involves fusion of gametes, resulting in genetic diversity.

• Genes: Units of heredity.

• Gametes: Reproductive cells (sperm and egg).

Chromosome Number and Karyotypes

• Humans: 46 chromosomes (23 pairs).

• Karyotype: Ordered display of chromosomes.

Homologous Chromosomes, Sex Chromosomes, Autosomes

• Homologous Chromosomes: Chromosome pairs with the 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.

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 and Chromosome Reduction

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

• Sister Chromatid Cohesion: Holds chromatids together.

• Synapsis: Homologs pair up during prophase I.

• Crossing Over: Exchange of genetic material between homologs.

Stages of Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Genetic Variation Mechanisms

• 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. He used pea plants to study inheritance patterns.

• Blending Hypothesis: Traits mix; disproven by Mendel.

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

• Hybridization: Crossing different varieties.

P, F1, and F2 Generations

• P Generation: Parental generation.

• F1 Generation: First filial; offspring of P.

• F2 Generation: Second filial; offspring of F1.

Dominant and Recessive Traits

• Dominant: Expressed trait.

• Recessive: Masked trait.

• F2 Ratio: 3:1 dominant to 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: Physical appearance.

• Genotype: Genetic makeup.

Punnett Squares and Testcrosses

• Punnett Square: Diagram to predict genetic outcomes.

• Testcross: Cross with homozygous recessive to determine genotype.

Homozygous and Heterozygous

• Homozygous: Two identical alleles.

• Heterozygous: Two different alleles.

Monohybrid, Dihybrid, Trihybrid Crosses

• Monohybrid: One trait; 3:1 ratio.

• Dihybrid: Two traits; 9:3:3:1 ratio.

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 is intermediate.

• Codominance: Both alleles are expressed.

Multiple Alleles and Blood Type

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

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 in a family.

Genetic Disorders

• Recessive Disorders: Require two recessive alleles.

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

• Dominant Disorders: Require only one dominant 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.

Morgan's Fruit Fly Experiments

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

• Conclusion: Genes are on chromosomes.

Sex-Linked and X-Linked Genes

• Sex-Linked Genes: Located on sex chromosomes.

• X-Linked Genes: Located on the X chromosome.

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

Linked Genes and Genetic Recombination

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

• Genetic Recombination: New combinations of genes due to crossing over.

• Frequency Calculation: Based on data from crosses.

Genetic and Linkage Maps

• Genetic Map: Shows gene locations.

• Linkage Map: Based on recombination frequencies.

• Map Units: 1 unit = 1% recombination.

Alterations of Chromosome Number

• Nondisjunction: Failure of chromosomes to separate; can occur in meiosis I or II.

• Aneuploidy: Abnormal chromosome number.

• 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

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

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

Additional info: This study guide covers foundational concepts in cell division, meiosis, Mendelian genetics, and chromosomal inheritance, suitable for General Biology students preparing for exams.