Comprehensive Study Notes for Cell Biology Exam Topics

Overall Test Format

Your exam will consist of multiple formats, including multiple choice, short answer, and discussion questions. Success requires not only memorization but also the ability to explain mechanisms and draw diagrams.

• Multiple Choice: 50%

• Short Answer: 30%

• Discussion: 20%

Discussion questions may ask you to:

• Illustrate gene regulation levels

• Draw cell cycle checkpoints

• Describe G-protein signaling pathways

• Explain COVID-19 viral entry into cells

• Present standard scientific conclusions

Key advice: Understand mechanisms, not just facts.

Major Topics You Must Know

mRNA Translation (Chapter 19)

mRNA translation is the process by which ribosomes synthesize proteins using messenger RNA as a template. This is a central mechanism in gene expression.

• Ribosome: Cellular machinery for protein synthesis, composed of rRNA and proteins.

• tRNA: Transfer RNA, brings amino acids to the ribosome.

• mRNA: Messenger RNA, carries genetic code from DNA.

• Codons: Triplets of nucleotides on mRNA specifying amino acids (e.g., AUG for methionine/start).

• Ribosome sites: A (aminoacyl), P (peptidyl), E (exit) sites.

• Release factors: Proteins that recognize stop codons and terminate translation.

• Mutations: Changes in DNA sequence; can be missense, nonsense, frameshift, or silent.

• Post-translational modifications: Chemical changes to proteins after synthesis (e.g., phosphorylation).

Example: The start codon AUG initiates translation, while stop codons (UAG, UAA, UGA) signal termination.

Gene Regulation (Chapters 19–20)

Gene regulation controls when and how genes are expressed, allowing cells to respond to environmental and developmental signals.

• Activation: Turning a gene ON.

• Repression: Turning a gene OFF.

• Cis elements: DNA sequences (e.g., enhancers, promoters) that regulate gene expression on the same DNA molecule.

• Trans elements: Proteins (e.g., transcription factors, activators) that bind to cis elements to regulate transcription.

• Operon models: lac operon (inducible, bacteria), trp operon (repressible, bacteria).

Levels of gene control:

1. Genome level: Chromatin structure, methylation, X-inactivation.

2. Transcriptional level: Transcription factors, enhancers, silencers.

3. Post-transcriptional level: RNA splicing, editing, transport.

4. Translational level: Regulation of mRNA translation.

5. Post-translational level: Protein modification, degradation.

Example: The lac operon is activated in the presence of lactose and repressed when glucose is available.

Methods in Cell Biology (Chapter 21)

Cell biology uses various methods to study cellular structure, function, and genetics. Understanding the purpose and application of each method is essential.

• Genomics: Study of entire genomes.

• Transcriptomics: Analysis of RNA expression.

• Proteomics: Study of protein levels and modifications.

• Microscopy: Visualization of cells and organelles.

• Western blot: Protein detection technique.

Manipulation techniques:

• PCR: Polymerase chain reaction, amplifies DNA.

• Cloning: Inserting DNA into vectors for replication.

• Gel electrophoresis: Separates DNA by size.

• Restriction enzymes: Cut DNA at specific sequences.

• CRISPR/Cas9: Genome editing tool.

Example: PCR is used to amplify DNA, while CRISPR/Cas9 can edit specific genes.

Nerve Cells & Action Potentials (Chapter 22)

Nerve cells (neurons) transmit electrical impulses called action potentials, which are essential for communication in the nervous system.

• Neuron: Basic unit of the nervous system.

• Dendrite: Receives signals.

• Axon: Transmits signals.

• Resting potential: -70 mV, maintained by ion gradients.

• Action potential: Rapid change in membrane potential.

• Depolarization: Na+ influx.

• Repolarization: K+ efflux.

• Voltage-gated channels: Open/close in response to voltage changes.

• Propagation: Movement of action potential along axon.

Sequence of events:

1. Resting potential

2. Ligand-gated channel opens

3. Threshold reached

4. Depolarization

5. Repolarization

6. Action potential jumps (nodes of Ranvier)

Example: Saltatory conduction allows action potentials to jump between nodes, speeding transmission.

Signal Transduction (Chapter 23)

Signal transduction is the process by which cells respond to external signals via receptors and intracellular pathways. The G protein pathway is a key example.

• Ligand: Signal molecule that binds receptor.

• G protein: Intracellular protein activated by receptor.

• Second messengers: Molecules like cAMP, IP3, Ca2+ that relay signals.

• Signaling amplification: One signal can activate many downstream molecules.

G protein pathway sequence:

1. Ligand binds receptor

2. G protein activates (GTP loaded)

3. Activates phospholipase C (PLC)

4. PLC cleaves PIP2 → DAG + IP3

5. IP3 releases Ca2+ from ER

6. Ca2+ triggers cellular effects

Example: Muscle contraction is regulated by Ca2+ signaling.

Cell Cycle & Checkpoints (Chapter 25)

The cell cycle is a series of phases that cells go through to divide and replicate. Checkpoints ensure proper progression and prevent errors.

• Restriction point (G1–S): Cyclin D1 + CDK4; cell commits to DNA replication.

• G2–M checkpoint: Checks for DNA damage before mitosis; ensures chromosomes are properly aligned.

• Vocabulary: Cycle, CDKs, Cyclins, Chromosomes, Mitosis, Spindle, Checkpoints.

Example: DNA damage at G2 checkpoint halts cell division until repair occurs.

COVID-19 Molecular Pathology

COVID-19 is caused by the SARS-CoV-2 virus, which enters human cells and hijacks cellular machinery for replication.

• RNA genome: Genetic material of the virus.

• Lipid shell: Viral envelope.

• Spike protein: Binds to ACE2 receptor on host cells.

• Fusion: Viral RNA enters host cell.

• Hijacking ribosomes: Viral RNA uses host ribosomes to make viral proteins.

Example: SARS-CoV-2 spike protein binds ACE2, allowing entry and replication in human cells.

Ethics in Science

Ethics in science involves understanding the moral implications of research and its impact on society.

• Topics: HIV/AIDS, CRISPR gene editing, research misconduct, medical ethics.

Example: CRISPR raises ethical questions about gene editing in embryos.