Cell Communication

Types of Cell Signaling

Cells communicate through various signaling mechanisms, each suited to different biological contexts.

• Cell-cell contact-dependent signaling: Requires direct contact between cells. Example: Immune cell interactions.

• Paracrine signaling: Cells release signals that affect nearby cells. Example: Growth factors in tissue repair.

• Synaptic signaling: Specialized for neurons; neurotransmitters cross synapses to target cells.

• Endocrine/hormonal signaling: Hormones travel through the bloodstream to distant target cells. Example: Insulin regulation of blood sugar.

Signal Receptors

Cells detect signals via specific receptors, each with unique mechanisms.

• Receptor Tyrosine Kinases (RTKs): Membrane receptors that phosphorylate tyrosine residues upon ligand binding, triggering a cascade of cellular responses.

• G Protein Coupled Receptors (GPCRs): Activate G proteins upon ligand binding, which then modulate intracellular signaling pathways.

• Ligand-activated channels: Ion channels that open in response to ligand binding, allowing ions to flow across the membrane.

Order of Steps: Each receptor type follows a sequence: ligand binding → receptor activation → signal transduction → cellular response.

Kinases vs. Phosphatases

• Kinase: Enzyme that adds phosphate groups to proteins (phosphorylation).

• Phosphatase: Enzyme that removes phosphate groups from proteins (dephosphorylation).

Target Cells and Signal Specificity

• Only cells with the appropriate receptor for a signal are target cells.

• Signal specificity is determined by receptor presence and downstream signaling machinery.

Second Messengers: cAMP and Ca2+

• cAMP (cyclic AMP): Produced from ATP by the enzyme adenylyl cyclase, often activated by GPCRs.

• Ca2+ ions: Released from the endoplasmic reticulum (ER) into the cytosol via ligand-gated channels.

• Other second messengers involved: IP3 (inositol trisphosphate) and DAG (diacylglycerol), both produced from phospholipids by phospholipase C.

Apoptosis in C. elegans

• Apoptosis: Programmed cell death, essential for development and tissue homeostasis.

• In C. elegans, apoptosis is regulated by specific genes and signaling pathways.

The Cell Cycle

Diploid and Haploid Numbers

Chromosome numbers are fundamental to understanding cell division.

• Diploid (2n): Two sets of chromosomes; in humans, 2n = 46.

• Haploid (n): One set of chromosomes; in humans, n = 23.

• Somatic cells: Diploid; contain both sets.

• Gametes: Haploid; contain one set.

Phases of the Cell Cycle

• G1 phase: Cell growth.

• S phase: DNA replication occurs.

• G2 phase: Preparation for mitosis.

• M phase: Mitosis and cytokinesis.

After S phase, each chromosome consists of two sister chromatids.

Cytoskeletal Components in Cell Division

• Microtubules: Form the mitotic spindle, separating chromosomes.

• Microfilaments/Actin filaments: Involved in cytokinesis, forming the contractile ring in animal cells.

First Signs of Cytokinesis

• Animal cells: Formation of the cleavage furrow.

• Plant cells: Formation of the cell plate.

Cell Cycle Checkpoints

• G1 checkpoint: Checks for cell size, nutrients, and DNA integrity.

• G2 checkpoint: Ensures DNA replication is complete and undamaged.

• M checkpoint: Verifies chromosome attachment to spindle before anaphase.

Cancer Cells and Checkpoints

• Cancer cells often bypass checkpoints, leading to uncontrolled division.

Meiosis and Sexual Life Cycles

Homologous Chromosomes vs. Sister Chromatids

• Homologous chromosomes: Pair of chromosomes, one from each parent, with similar genes but possibly different alleles.

• Sister chromatids: Identical copies of a chromosome, joined after DNA replication.

Karyotype

• Karyotype: Visual representation of all chromosomes in a cell, used to detect chromosomal abnormalities.

Mitosis vs. Meiosis

Independent Assortment

• Occurs during Metaphase I of meiosis.

• Homologous chromosomes align randomly, creating genetic variation.

Mendelian Genetics

Dominant and Recessive Traits

• Dominant trait: Expressed when at least one dominant allele is present.

• Recessive trait: Expressed only when both alleles are recessive.

Key Vocabulary

• Homozygote: Individual with two identical alleles for a gene.

• Heterozygote: Individual with two different alleles for a gene.

• True breeding: Organism that always passes down the same trait.

• Allele: Variant form of a gene.

• Monohybrid/dihybrid: Crosses involving one or two traits, respectively.

• Self-cross: Crossing an organism with itself.

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

• P/F1/F2 generations: Parental, first filial, and second filial generations.

• Phenotype: Observable traits.

• Genotype: Genetic makeup.

• Phenotypic/genotypic ratio: Ratios of observed traits/genotypes in offspring.

Mendel’s Experiments

• Monohybrid cross: True-breeding purple x white flowers; F1 all purple, F2 ratio 3:1 purple:white.

• Dihybrid cross: True-breeding yellow, round seeds x green, wrinkled seeds; F2 ratio 9:3:3:1.

Deduction of Parental Genotypes from Ratios

• 1:1 ratio → parents are Aa x aa.

• 3:1 ratio → parents are Aa x Aa.

Punnett Squares

• Used to predict genotypic and phenotypic ratios.

• Each box represents a possible genotype of offspring.

Independent Assortment of Traits

• Traits are inherited independently if genes are on different chromosomes.

Probability Laws in Genetics

• Multiplication rule: Probability of two independent events both occurring is the product of their probabilities.

• Addition rule: Probability of either of two mutually exclusive events occurring is the sum of their probabilities.

Example: Probability of Aa offspring from Aa x Aa cross:

Example: Probability of Aa or aa offspring: