Cell Communication

Types of Cell Signaling

Cells communicate through various signaling mechanisms, each suited to different distances and contexts. Understanding these types is crucial for grasping how multicellular organisms coordinate activities.

• Cell-cell contact-dependent signaling: Direct interaction between membrane-bound molecules on adjacent cells.

• Paracrine signaling: Local signaling where cells release signals affecting nearby cells.

• Synaptic signaling: Specialized paracrine signaling in neurons, involving neurotransmitter release at synapses.

• Endocrine/hormonal signaling: Long-distance signaling via hormones released into the bloodstream, affecting distant target cells.

Signal Receptors

Signal receptors are proteins that bind signaling molecules and initiate cellular responses. There are three main types:

• Receptor tyrosine kinases (RTKs):

  • Bind ligands, dimerize, and autophosphorylate.

  • Trigger downstream signaling cascades via phosphorylation.

• G protein-coupled receptors (GPCRs):

  • Bind ligands, activate G proteins by exchanging GDP for GTP.

  • G proteins activate enzymes or ion channels, leading to second messenger production.

• Ligand-activated channels:

  • Ligand binding opens ion channels, allowing ions to flow across the membrane.

Kinase vs. Phosphatase

• 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 target cells and can respond.

Second Messengers

• 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, often triggered by inositol triphosphate (IP3), another second messenger.

Apoptosis in C. elegans

• Programmed cell death is regulated by signaling pathways; in C. elegans, specific genes and proteins control apoptosis.

The Cell Cycle

Chromosome Numbers

Organisms have characteristic chromosome numbers, which are important for understanding cell division.

• Diploid number (2n): Total chromosomes in somatic cells (e.g., 2n = 46 in humans).

• Haploid number (n): Chromosomes in gametes (e.g., n = 23 in humans).

• Sets of chromosomes: Diploid cells have two sets; haploid cells have 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.

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.

First Sign 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 damage.

• G2 checkpoint: Checks for DNA replication completion and damage.

• M checkpoint: Ensures chromosomes are properly attached to the spindle.

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: Chromosome pairs, 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 image of all chromosomes in a cell, arranged by size and shape.

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.

• Heterozygote: Individual with two different alleles.

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

• Allele: Variant form of a gene.

• Monohybrid/dihybrid: Cross involving one/two traits.

• 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.

• Genotypic ratio: Ratio of genetic combinations.

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 from Phenotypic 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.

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 Calculation

• For a monohybrid cross Aa x Aa, probability of aa offspring:

• For a dihybrid cross AaBb x AaBb, probability of aabb offspring:

Additional info: Second messengers like IP3 are produced from phospholipids in the membrane and trigger Ca2+ release. Mendel's laws of segregation and independent assortment are fundamental to classical genetics.