Cell Communication (Chapter 11)

Stages of Intercellular Cell Signaling

Cell signaling is a fundamental process that allows cells to communicate and coordinate their activities. The process typically involves three main stages:

• Reception: The cell detects a signaling molecule (ligand) from the outside environment via a receptor protein.

• Transduction: The signal is relayed and amplified inside the cell, often through a series of molecular changes known as a signal transduction pathway.

• Response: The cell responds to the signal, which may involve changes in gene expression, enzyme activity, or cell behavior.

Types of Cell Surface and Intracellular Receptors

• G-Protein Coupled Receptor (GPCR): A membrane receptor that activates a G-protein upon ligand binding, initiating a signaling cascade. GPCRs have seven transmembrane domains.

• Receptor Tyrosine Kinase (RTK): Membrane receptors with intrinsic kinase activity. Ligand binding causes dimerization and autophosphorylation, triggering downstream signaling.

• Ligand Gated Ion Channel: Membrane channels that open in response to ligand binding, allowing ions to flow across the membrane and alter cell activity.

• Steroid Hormone Receptor: Intracellular receptors that bind steroid hormones, then act as transcription factors to regulate gene expression.

Role of G-Proteins in Signaling

G-proteins act as molecular switches inside cells. When activated by GPCRs, they exchange GDP for GTP and trigger downstream effectors such as enzymes or ion channels. G-proteins are inactivated when GTP is hydrolyzed to GDP.

Protein Kinases and Phosphorylation Cascades

Protein kinases transfer phosphate groups from ATP to target proteins, often activating or deactivating them. Phosphorylation cascades amplify signals and allow for complex regulation.

Second Messengers

• cAMP (Cyclic Adenosine Monophosphate): Generated from ATP by adenylyl cyclase; activates protein kinase A (PKA).

• Ca2+ (Calcium ions): Released from intracellular stores; triggers various cellular responses.

• IP3 (Inositol Triphosphate): Produced by phospholipase C; stimulates Ca2+ release from the endoplasmic reticulum.

Key Enzymes in Signaling

• Adenylyl Cyclase: Converts ATP to cAMP.

• Receptor Tyrosine Kinase: Catalyzes phosphorylation of tyrosine residues.

• G-protein (GTPase): Hydrolyzes GTP to GDP, inactivating the G-protein.

• Phosphodiesterase: Breaks down cAMP to AMP, terminating the signal.

• Phospholipase C: Cleaves phospholipids to generate IP3 and DAG (diacylglycerol).

Herceptin and Trikafta: Mechanisms of Action

• Herceptin: A monoclonal antibody that targets the HER2 receptor (a type of RTK) in breast cancer cells, blocking signaling and inhibiting cell proliferation.

• Trikafta: A combination therapy for cystic fibrosis that improves the function of the CFTR protein, enhancing chloride ion transport.

The Cell Cycle and Mitosis (Chapter 12)

Stages of the Eukaryotic Cell Cycle

The cell cycle consists of a series of phases that prepare the cell for division:

• G1 phase: Cell growth and preparation for DNA replication.

• S phase: DNA synthesis; chromosomes are duplicated.

• G2 phase: Further growth and preparation for mitosis.

• M phase (Mitosis): Division of the nucleus and cytoplasm.

Structure of a Duplicated Chromosome

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

• Centromere: The region where sister chromatids are attached.

• Kinetochore: Protein complex at the centromere that attaches to spindle microtubules.

Mitotic Spindle Organization

• Kinetochore Microtubules: Attach to kinetochores and pull chromatids apart.

• Non-Kinetochore Microtubules: Overlap and push against each other to elongate the cell.

Significance and Stages of Mitosis

Mitosis ensures equal distribution of genetic material to daughter cells. The stages are:

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclear envelope reforms, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm.

Cell Cycle Control: Cdk and Cyclin

• Cyclin-dependent kinases (Cdks): Enzymes that drive cell cycle transitions when bound to cyclins.

• Cyclins: Regulatory proteins whose levels fluctuate during the cell cycle.

Transformation to Cancer Cells

• Transformation: Process by which normal cells become cancerous.

• Metastasis: Spread of cancer cells to distant sites.

• Angiogenesis: Formation of new blood vessels to supply tumors.

Oncogenes vs. Tumor Suppressors

Unusual Features of Cancer Cells

• Loss of cell cycle control

• Ability to invade other tissues

• Immortality (avoidance of apoptosis)

• Induction of angiogenesis

Categories of Chemotherapy Drugs

• Antimetabolites: Interfere with DNA synthesis.

• Mitotic inhibitors: Block mitosis.

• DNA-damaging agents: Cause DNA breaks.

Meiosis (Chapter 13)

Haploid vs. Diploid Cells and Homologous Chromosomes

• Haploid (n): One set of chromosomes (gametes).

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

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

Key Events in Meiosis: Reduction Division

• Pairing of Homologous Chromosomes: Occurs during Prophase I.

• Recombination (Crossing Over): Exchange of genetic material between homologues.

• Replication Only Before Prophase I: Chromosomes replicate once before meiosis begins.

• Sister Chromatid Separation Only in Meiosis II: Sister chromatids are separated during the second division.

Crossing Over (Synapsis) and Consequences

Crossing over occurs during synapsis in Prophase I, increasing genetic diversity by exchanging DNA between homologous chromosomes.

Significance and Diagram of Meiosis

• Reduces chromosome number by half

• Produces genetically unique gametes

Diagram: (Not shown; students should refer to textbook figures for meiosis stages.)

Contrast: Mitosis vs. Meiosis

Non-Disjunction and Aneuploidy

Non-disjunction is the failure of chromosomes to separate properly during meiosis, leading to aneuploidies such as Down syndrome (trisomy 21).

Genetics: Mendelian and Chromosomal (Chapters 14/15)

Key Terms in Genetics

• Phenotype: Observable traits of an organism.

• Genotype: Genetic makeup of an organism.

• Locus: Location of a gene on a chromosome.

• Allele: Alternative form of a gene.

• Dominant Allele: Expressed when present.

• Recessive Allele: Expressed only when homozygous.

• Homozygous: Two identical alleles at a locus.

• Heterozygous: Two different alleles at a locus.

Mendel’s Laws

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Solving Genetics Problems

• Monohybrid Cross: Involves one gene; typical ratio is 3:1 for dominant:recessive phenotype.

• Dihybrid Cross: Involves two genes; typical ratio is 9:3:3:1.

• Trihybrid Cross: Involves three genes; more complex ratios.

• Test Cross: Cross with a homozygous recessive to determine genotype.

Probability in Genetic Crosses

Use multiplication rule to predict outcomes:

Example: Probability of two independent events both occurring is the product of their individual probabilities.

Autosomal Dominant vs. Autosomal Recessive Disorders

Complex Patterns of Inheritance

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers).

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

• Multiple Alleles: More than two alleles exist for a gene (e.g., blood type).

• Epistasis: One gene affects the expression of another.

Pleiotropy

A single gene affects multiple traits (e.g., sickle cell gene affects blood, organs, and resistance to malaria).

X-linked Inheritance in Mammals

• X-linked genes: Genes located on the X chromosome; males are hemizygous (one X), females can be homozygous or heterozygous.

• Hemizygous: Having only one allele for a gene (e.g., males for X-linked genes).

• Mosaic: Females with two X chromosomes may express different alleles in different cells due to X-inactivation.

• Barr Body: Inactivated X chromosome in female cells.

• Dosage Compensation: Mechanism to equalize gene expression from X chromosomes between males and females.

Additional info: Academic context and explanations have been expanded for clarity and completeness. Diagrams referenced should be reviewed in the textbook for visual understanding.