General Biology Exam Review: Molecular Basis of Inheritance, Gene Expression, Regulation, and Viruses

Chapter 16: The Molecular Basis of Inheritance

16.1 DNA is the Genetic Material

DNA is the hereditary molecule responsible for storing and transmitting genetic information in all living organisms.

• Basic Structure of DNA: DNA is a double helix composed of two antiparallel strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine).

• Chargaff's Rules: The amount of adenine equals thymine, and the amount of cytosine equals guanine in DNA ($[A] = [T]$, $[C] = [G]$).

• Biochemical Makeup: DNA's structure allows for the storage and transmission of genetic information through complementary base pairing.

• Example: The double helix model proposed by Watson and Crick explains how DNA replicates and encodes genetic information.

16.2 Many Proteins Work Together in DNA Replication and Repair

DNA replication is a complex process involving multiple enzymes and proteins to ensure accurate copying of genetic material.

• Key Proteins: DNA polymerase synthesizes new DNA strands; helicase unwinds the DNA; primase synthesizes RNA primers; ligase joins Okazaki fragments.

• Replication Fork: The site where DNA unwinds and replication occurs, forming leading and lagging strands.

• Base Pairing: Ensures fidelity in DNA replication by matching complementary bases.

• Example: DNA polymerase proofreads and corrects errors during replication.

16.3 Chromosome Structure

Chromosomes are DNA molecules packaged with proteins, allowing efficient storage and regulation of genetic material.

• Nucleosome: The basic unit of chromatin, consisting of DNA wrapped around histone proteins.

• Chromatin Forms: Euchromatin (loosely packed, active in transcription) and heterochromatin (densely packed, inactive).

• Example: Human cells contain 46 chromosomes, each with distinct regions of euchromatin and heterochromatin.

Chapter 17: Gene Expression—From Gene to Protein

17.1 Genes Specify Proteins via Transcription and Translation

Gene expression involves converting genetic information in DNA into functional proteins through transcription and translation.

• Transcription: The process by which RNA polymerase synthesizes messenger RNA (mRNA) from a DNA template.

• Translation: The process by which ribosomes synthesize proteins using the mRNA sequence.

• Genetic Code: The set of rules by which nucleotide sequences are translated into amino acids. Each codon (three nucleotides) specifies an amino acid.

• Codon Table: Used to interpret mRNA sequences into amino acids.

• Example: The codon AUG codes for methionine and serves as the start codon for translation.

17.2 Transcription in DNA-Directed Synthesis of RNA

Transcription involves several steps and regulatory elements to ensure accurate RNA synthesis.

• Stages: Initiation, elongation, and termination.

• Promoters: DNA sequences (e.g., TATA box) where RNA polymerase binds to initiate transcription.

• Transcription Factors: Proteins that regulate the binding and activity of RNA polymerase.

• Example: In eukaryotes, transcription factors are required for RNA polymerase II to bind to the promoter.

17.3 Eukaryotic mRNA Modification After Transcription

After transcription, eukaryotic mRNA undergoes several modifications before translation.

• RNA Splicing: Removal of introns and joining of exons to produce mature mRNA.

• 5' Cap and Poly-A Tail: Added to mRNA for stability and export from the nucleus.

• Example: Alternative splicing allows a single gene to code for multiple proteins.

17.4 Translation: Synthesis of Polypeptides

Translation is the process of assembling amino acids into polypeptides based on the mRNA sequence.

• Ribosome Structure: Composed of rRNA and proteins, with large and small subunits.

• tRNA: Transfers specific amino acids to the ribosome, matching codons with anticodons.

• Polypeptide Targeting: Signal sequences direct proteins to specific cellular locations.

• Example: Secretory proteins are directed to the endoplasmic reticulum by signal peptides.

17.5 Mutations Affect Protein Structure and Function

Mutations are changes in DNA sequence that can alter protein structure and function.

• Types of Mutations: Point mutations (substitution, insertion, deletion), frameshift mutations.

• Effects: Mutations can be silent, missense, or nonsense, affecting protein function in various ways.

• Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.

Chapter 18: Regulation of Gene Expression

18.1 Bacterial Gene Regulation

Bacteria regulate gene expression primarily at the transcriptional level using operons.

• Operon Model: Consists of structural genes, promoter, operator, and regulatory genes.

• Inducible vs. Repressible Operons: Inducible operons (e.g., lac operon) are activated in response to specific substrates; repressible operons (e.g., trp operon) are inhibited when the end product is abundant.

• Example: The lac operon is induced in the presence of lactose.

18.2 Eukaryotic Gene Regulation

Gene expression in eukaryotes is regulated at multiple levels, including chromatin structure, transcription, and post-transcriptional modifications.

• Chromatin Modification: Histone acetylation and DNA methylation affect gene accessibility.

• Transcription Factors: Regulate the initiation and rate of transcription.

• Cell Differentiation: Differential gene expression leads to specialized cell types.

• Example: Muscle cells and nerve cells express different sets of genes despite having the same DNA.

18.3 Noncoding RNAs and Gene Regulation

Noncoding RNAs, such as microRNAs (miRNAs), play important roles in regulating gene expression post-transcriptionally.

• miRNAs: Bind to complementary mRNA sequences to inhibit translation or promote degradation.

• Example: miRNAs are involved in development and disease processes.

Chapter 19: Viruses

19.1 Virus Structure

Viruses are infectious agents composed of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid.

• Components: Nucleic acid genome, capsid, sometimes an envelope derived from host cell membranes.

• Variation: Viruses vary in shape, size, and genome type (single-stranded or double-stranded, DNA or RNA).

• Comparison: Viruses differ from bacteria in structure and replication; viruses are nonliving and require host cells to reproduce.

• Example: Influenza virus is an enveloped RNA virus; bacteriophage T4 is a DNA virus that infects bacteria.

19.2 Viral Replication Cycles

Viruses replicate only within host cells, using either lytic or lysogenic cycles.

• Lytic Cycle: Virus injects its genome, replicates, and lyses the host cell to release new virions.

• Lysogenic Cycle: Viral genome integrates into host DNA and replicates with the host cell without immediate destruction.

• Example: HIV is a retrovirus that integrates its RNA genome into host DNA via reverse transcription.

19.3 Viruses and Prions as Pathogens

Viruses and prions can cause diseases in animals and plants, often with significant health impacts.

• Prions: Infectious proteins that cause neurodegenerative diseases (e.g., mad cow disease).

• Emerging Viral Diseases: New viral pathogens can spread rapidly in human populations due to mutation and transmission.

• Example: COVID-19 is caused by the novel coronavirus SARS-CoV-2.

Feature	Virus	Bacteria	Prion
Genetic Material	DNA or RNA	DNA	None (protein only)
Cellular Structure	Non-cellular	Cellular (prokaryote)	Non-cellular
Replication	Requires host cell	Binary fission	Converts normal proteins
Diseases	Flu, HIV, COVID-19	Strep throat, tuberculosis	Mad cow disease

Additional info: Some explanations and examples have been expanded for clarity and completeness.