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; often involves membrane-bound molecules.

• Paracrine signaling: Cells release signaling molecules that affect nearby cells.

• Synaptic signaling: Specialized form of paracrine signaling in neurons; neurotransmitters cross synapses.

• Endocrine/hormonal signaling: Hormones are released into the bloodstream and act on distant target cells.

Signal Receptors

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

• Receptor tyrosine kinases (RTKs): Bind ligands, dimerize, and autophosphorylate, triggering downstream signaling.

• G protein coupled receptors (GPCRs): Ligand binding activates G proteins, which relay signals to effectors.

• Ligand-activated channels: Ligand binding opens ion channels, allowing ions to flow across membranes.

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

Only cells with the appropriate receptor for a signal are considered target cells and can respond.

Second Messengers

• cAMP: Produced from ATP by the enzyme adenylyl cyclase.

• Ca2+ ions: Released from the endoplasmic reticulum (ER) into the cytosol, often triggered by IP3 (inositol trisphosphate).

• Other second messengers: IP3 and DAG (diacylglycerol) are produced from phospholipids by phospholipase C.

Apoptosis in C. elegans

Apoptosis is programmed cell death, crucial for development and homeostasis. In C. elegans, specific genes regulate apoptosis, ensuring proper cell removal.

The Cell Cycle

Chromosome Numbers

• 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

The cell cycle consists of ordered phases:

• G1 phase: Cell growth.

• S phase: DNA replication occurs.

• G2 phase: Preparation for division.

• 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: Form the contractile ring during cytokinesis 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 and Cell Cycle 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, formed after DNA replication.

Karyotype

A karyotype is an ordered display of an individual's chromosomes, used to detect chromosomal abnormalities.

Mitosis vs. Meiosis

• Goal of mitosis: Produce two identical diploid cells for growth and repair.

• Goal of meiosis I: Separate homologous chromosomes, reducing chromosome number by half.

• Goal of meiosis II: Separate sister chromatids, similar to mitosis.

• Unique to meiosis: Crossing over, independent assortment, reduction division.

• Shared events: Chromosome alignment, separation, cytokinesis.

• Mitosis is similar to meiosis II: Both involve separation of sister chromatids.

Independent Assortment

Occurs during metaphase I of meiosis; homologous pairs align randomly, creating genetic variation.

Mendelian Genetics

Dominant and Recessive Traits

Dominant traits mask recessive traits in heterozygotes.

Key Vocabulary

• Homozygote: Individual with two identical alleles.

• Heterozygote: Individual with two different alleles.

• 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 ratio: Ratio of observed traits.

• Genotypic ratio: Ratio of genetic combinations.

Mendel’s Experiments

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

• Dihybrid cross: Yellow, round seeds x green, wrinkled seeds; F2 shows 9:3:3:1 ratio.

Deduction from Phenotypic Ratios

• 1:1 ratio → parents are Aa x aa

• 3:1 ratio → parents are Aa x Aa

Punnett Squares

Punnett squares are used to predict genotypic and phenotypic ratios.

Independent Assortment

Traits are inherited independently if genes are on different chromosomes.

Probability Laws

• 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: (A from parent 1) × $\frac{1}{2}$ (a from parent 2) = .

Additional info: Probability rules and Punnett square methods are fundamental for predicting inheritance patterns and understanding genetic variation.