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: Sister Chromatids, Centromere, Chromatin, Genome

• 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.

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

• 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).

• Each phase has distinct functions and durations.

Mitosis – Somatic Cells

• Mitosis: Division of the nucleus in somatic (body) cells, resulting in two genetically identical daughter cells.

• Somatic cells are found throughout the body, except in reproductive tissues.

• 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 (replication)

• G2 phase: Preparation for mitosis

• M phase: Mitosis and cytokinesis

Mitotic Spindle and Its Function

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

• Components include centrosomes, spindle microtubules, and kinetochores.

• During anaphase, kinetochore microtubules shorten, pulling chromatids apart.

Stages of Mitosis

• Prophase: Chromatin condenses, spindle forms.

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

• Metaphase: Chromosomes align at the metaphase plate.

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

• Telophase: Nuclear envelopes reform, chromosomes decondense.

Cytokinesis

• Animal Cells: Occurs via cleavage furrow.

• Plant Cells: Occurs via cell plate formation.

Binary Fission

• Prokaryotes divide by binary fission, a simpler process than mitosis.

Cell Cycle Checkpoints

• Checkpoints: G0, G1, S, G2, M

• Ensure proper cell cycle progression; G0 is a resting phase.

Growth Factors, Density-Dependent Inhibition, Anchorage Dependence

• Growth Factors: Proteins that stimulate cell division.

• Density-Dependent Inhibition: Cells stop dividing when crowded.

• Anchorage Dependence: Cells require attachment to a surface to divide.

Meiosis and Sexual Life Cycles

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Offspring are genetically identical to the parent.

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

Genes and Gametes

• Genes: Units of heredity, made of DNA.

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

Chromosome Number and Species Differences

• Humans have 46 chromosomes (23 pairs).

• Chromosome number varies by species.

Karyotype

• A karyotype is an ordered display of chromosomes, arranged by size and shape.

Homologous Chromosomes, Sex Chromosomes, Autosomes

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

• Sex Chromosomes: Determine sex (X and Y in humans).

• Autosomes: Non-sex chromosomes.

Diploid and Haploid Cells

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

• Haploid (n): One set of chromosomes.

Sexual Life Cycles: Animals, Plants, Fungi

• Animal Life Cycle: Diploid dominant; gametes produced by meiosis.

• Plant Life Cycle: Alternation of generations; both haploid and diploid multicellular stages.

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

Alternation of Generations

• Alternation between haploid and diploid stages in plants and some algae.

Meiosis: Chromosome Reduction

• Meiosis reduces chromosome number by half, producing haploid gametes.

Sister Chromatid Cohesion, Synapsis, Crossing Over

• Sister Chromatid Cohesion: Sister chromatids held together by proteins.

• Synapsis: Homologous chromosomes pair up during prophase I of meiosis.

• 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: Independent Assortment, Crossing Over, Random Fertilization

• Independent Assortment: Chromosomes are randomly distributed to gametes.

• Crossing Over: Produces recombinant chromosomes.

• Random Fertilization: Any sperm can fertilize any egg.

Mendelian Genetics

Gregor Mendel and His Experiments

• Mendel used pea plants to study inheritance.

• Disproved the 'blending hypothesis' and established the particulate nature of genes.

True Breeding, Hybridization, Generations

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

• Hybridization: Crossing two different true-breeding varieties.

• 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.

Inheritance Patterns and Ratios

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

• 3:1 and 9:3:3:1 ratios arise from monohybrid and dihybrid crosses, respectively.

Alleles, Law of Segregation, Law of Independent Assortment

• 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, Genotype, Punnett Square, Testcross

• Phenotype: Observable traits.

• Genotype: Genetic makeup.

• Punnett Square: Diagram to predict genetic crosses.

• Testcross: Crossing with a homozygous recessive to determine genotype.

Homozygous, Heterozygous, Monohybrid, Dihybrid, Trihybrid

• Homozygous: Two identical alleles.

• Heterozygous: Two different alleles.

• Monohybrid: One gene cross.

• Dihybrid: Two gene cross.

• Trihybrid: Three gene cross.

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 is determined by multiple 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 single trait (e.g., skin color).

Nature vs. Nurture

• Environment can influence phenotype (e.g., hydrangea flower color).

Pedigree Analysis

• Pedigrees track inheritance in families.

Recessive and Dominant Disorders

• Recessive Disorders: Require two copies of the mutant allele (e.g., cystic fibrosis).

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

• Dominant Disorders: Require only one copy (e.g., Huntington's disease).

• Multifactorial Disorders: Caused by genetic and environmental factors.

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 show that genes are on chromosomes.

• 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: Located on the X chromosome.

• X Inactivation: In females, one X chromosome is inactivated (Barr body).

Linked Genes and Genetic Recombination

• Linked Genes: Genes located close together on the same chromosome, inherited together.

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

• Frequency of recombination can be used to map genes.

Genetic and Linkage Maps

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

• Linkage Map: A type of genetic map based on crossing over.

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

Alterations of Chromosome Number and Structure

• Nondisjunction: Failure of chromosomes to separate during meiosis I or II.

• Aneuploidy: Abnormal number of chromosomes (e.g., monosomic, trisomic).

• Trisomy 21: Down syndrome, caused by an extra chromosome 21.

• Alterations of Structure: Deletion, duplication, inversion, translocation.

Aneuploidy of Sex Chromosomes and Disorders

• Abnormal sex chromosome number can cause disorders (e.g., Turner syndrome, Klinefelter syndrome).

• Structural changes can cause genetic disorders.

Summary Table: Types of Chromosomal Alterations

Summary Table: Cell Cycle Checkpoints

Key Equations

• Multiplication Rule:

• Addition Rule:

Additional info: Academic context and examples were added to clarify concepts and provide a self-contained study guide.