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 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): Membrane receptors that, upon ligand binding, dimerize and autophosphorylate, triggering downstream signaling.

• G Protein Coupled Receptors (GPCRs): Ligand binding activates a G protein, which then modulates other proteins or enzymes.

• Ligand-activated channels: Ion channels 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

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 inositol triphosphate (IP3).

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

Apoptosis in C. elegans

Apoptosis is programmed cell death, essential for development and homeostasis. In C. elegans, specific genes regulate the process, 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

• G2 phase: Preparation for mitosis

• M phase: Mitosis and cytokinesis

DNA Replication: Occurs in S phase. After replication, 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.

Signs of Cytokinesis

• Animal cells: Formation of a cleavage furrow.

• Plant cells: Formation of a 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

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

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

Mitosis vs. Meiosis

Unique Events in Meiosis

• Crossing over: Exchange of genetic material between homologous chromosomes.

• Independent assortment: Random distribution of homologous chromosomes during metaphase I.

Independent Assortment

Occurs during metaphase I of meiosis. It increases genetic variation by randomly distributing maternal and paternal chromosomes.

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 certain traits.

• 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 observable traits in offspring.

• Genotypic ratio: Ratio of genetic combinations in offspring.

Mendel's Experiments

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

• Dihybrid cross: True-breeding 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 in crosses.

Independent Assortment

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 getting Aa offspring from Aa x Aa cross: (A from parent 1) × (a from parent 2) = .