Cell Signaling, Cell Division, Mendelian Genetics, and Chromosomal Inheritance: Study Guide

Cell Signaling

Synaptic Signaling and Signal Molecules

Cell signaling is a fundamental process by which cells communicate with each other to coordinate biological functions. In synaptic signaling, neurons transmit signals across synapses using specific molecules.

• Neurotransmitters: Molecules that carry signals between neurons in synaptic signaling. Examples include acetylcholine, dopamine, and serotonin.

• Ligands: General term for molecules that carry information to a new cell by binding to receptors.

Signal Transduction

Signal transduction refers to the process by which a cell converts an extracellular signal into a functional response. Transduction does not always involve amplification.

• Transduction (without amplification): The signal is relayed through a series of molecular changes, but the number of molecules involved does not increase.

• Phosphorylation Cascades: Most commonly involve protein kinases (add phosphate groups) and protein phosphatases (remove phosphate groups).

• Hydrophobic/Nonpolar Signaling Molecules: Activate intracellular receptors (e.g., steroid hormone receptors) because they can cross the cell membrane.

G Protein-Coupled Receptors (GPCRs)

GPCRs are a large family of membrane receptors that activate G proteins upon ligand binding.

• Activation: GPCR binds ligand, changes shape, and activates a G protein by exchanging GDP for GTP.

• Deactivation: G protein hydrolyzes GTP to GDP, returning to its inactive state.

Direct DNA Interaction and Cellular Response

• Transcription Factors: Molecules that interact directly with DNA after being activated at the end of signal transduction, regulating gene expression.

• Cellular Specificity: Different cells respond differently to the same signaling molecule due to variations in receptor types, signal transduction pathways, and gene expression profiles.

Enzymes in Phosphorylation Cascades

• Protein Kinases: Enzymes that add phosphate groups to proteins, often activating them.

• Protein Phosphatases: Enzymes that remove phosphate groups, often deactivating proteins.

Mitosis and Cell Division

Chromosome Behavior During Mitosis

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells. Chromosomes undergo specific changes during each phase.

• Prophase: Chromosomes condense and become visible; spindle forms.

• Metaphase: Chromosomes align at the metaphase plate (characteristic event).

• Anaphase: Sister chromatids separate and move toward opposite poles; they become individual chromosomes.

• Telophase: Chromosomes decondense; nuclear envelope reforms.

Interphase Events

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

• S Phase: DNA replication occurs.

• G2 Phase: Further growth and preparation for mitosis.

Mitotic Spindle and Microtubules

• Roles of Microtubules:

  • Form the spindle apparatus

  • Attach to chromosomes via kinetochores

  • Pull chromosomes apart during anaphase

• Structures Used: Kinetochores and spindle fibers move chromosomes during mitosis/meiosis.

Cell Cycle Regulation

• Cyclins and CDKs: Cyclins bind to cyclin-dependent kinases (CDKs) to regulate cell cycle progression.

• Emergent Property: All life exhibits reproduction, a property linked to cell division.

• DNA Replication: Occurs during S phase of interphase.

• Uncontrolled Division: When a cell gains the ability to divide uncontrollably, it is called cancer or transformation.

Outcomes of Mitosis

• Number and Ploidy: Mitosis produces two diploid (2n) cells.

Meiosis and Sexual Life Cycles

Comparison of Mitosis and Meiosis

Meiosis is a specialized cell division that produces gametes with half the chromosome number of the original cell, increasing genetic diversity.

• Similarities: Both involve chromosome duplication and separation.

• Differences: Meiosis has two divisions (I and II), produces four haploid cells, and includes crossing over.

Genetic Diversity Mechanisms

• Independent Assortment

• Crossing Over: Occurs during prophase I of meiosis.

• Random Fertilization

Chromosome Separation in Meiosis

• Anaphase I: Homologous chromosomes are pulled apart.

• Anaphase II: Sister chromatids are pulled apart.

Gamete Production and Ploidy

• Gametes: Fusion of haploid gametes (n) produces diploid (2n) cells.

• Meiosis Outcome: Four haploid (n) cells are produced.

• Human Gametes: Each has 23 chromosomes.

• Nondisjunction: Results in gametes with abnormal ploidy (e.g., n+1 or n-1).

• Somatic vs. Gamete Cells: Somatic cells are diploid (46 chromosomes); gametes are haploid (23 chromosomes).

Mendelian Inheritance

Blood Type Determination

• Blood Types: Determined by alleles IA, IB, and i. Parental genotypes can predict offspring blood types.

X-linked Recessive Disorders

• Male Susceptibility: Males (XY) are more affected because they have only one X chromosome.

Mendel's Pea Plant Experiments

• F1 Generation: All had purple flowers due to dominance of the purple allele.

• Monohybrid Cross: Crossing heterozygote and dominant homozygote yields all dominant phenotype.

• Epistatic Genes: Genes that mask the effect of other genes.

• Genotype vs. Phenotype: Genotype is genetic makeup; phenotype is observable traits.

• Test Cross: Used to determine if an organism is homozygous dominant or heterozygous.

• Mosaic Dominance: Codominance occurs when both alleles are expressed (e.g., AB blood type).

• Dihybrid Cross: Supported the hypothesis of independent assortment.

• Probability of Recessive Disorder: If both parents are carriers (heterozygous), the probability is 25% ($P = 0.25$).

X-linked Trait Crosses

• Cat Fur Color: X-linked recessive traits can result in different phenotypes depending on parental genotypes.

• Genotype Determination: Offspring genotypes can be used to infer parental genotypes, especially for X-linked traits.

Chromosomal Inheritance

Aneuploidy and Chromosome Number

• Aneuploidy: Event where a fertilized egg has too few or too many chromosomes.

• Genome: The complete set of genes or genetic material in an organism.

X-linked vs. Y-linked Genes

• X-linked Genes: Present in both sexes; can be inherited from either parent.

• Y-linked Genes: Only present in males; inherited from father.

Sex vs. Gender

• Sex: Biological classification based on chromosomes (XX or XY).

• Gender: Social and cultural identity.

Morgan's Experiments

• Main Traits: Eye color and wing shape in fruit flies.

Gene Linkage

• Physical Basis: Genes located close together on the same chromosome tend to be inherited together.

Process	Number of Cells Produced	Ploidy	Chromosome Number (Human)
Mitosis	2	Diploid (2n)	46
Meiosis	4	Haploid (n)	23

Phase	Chromosome Behavior
Prophase	Chromosomes condense
Metaphase	Chromosomes align at metaphase plate
Anaphase	Sister chromatids separate
Telophase	Chromosomes decondense

Enzyme	Function
Protein Kinase	Adds phosphate group to proteins
Protein Phosphatase	Removes phosphate group from proteins