AP Biology Unit 6: Gene Expression

Overview

• Complex concepts in gene expression: production of RNA, modification into mRNA, and translation into protein.
• Key topics: DNA/RNA structure and function, replication, transcription, translation, genetic code, gene expression regulation in prokaryotes and eukaryotes, mutation, horizontal gene transfer, biotechnology.

6.1 DNA and RNA Structure

• DNA: Double-stranded helix, composed of nucleotide monomers (deoxyribose sugar, phosphate group, nitrogenous base).
  • Base pairing: Adenine (A) - Thymine (T), Guanine (G) - Cytosine (C).
  • Anti-parallel strands: 5' end to 3' end orientation.
• RNA: Single-stranded, uses uracil (U) instead of thymine.
• Prokaryotic vs. Eukaryotic DNA
  • Prokaryotic: Circular, not wrapped around proteins.
  • Eukaryotic: Linear, wrapped around histones.

6.2 DNA Replication

• Semiconservative Replication: Each new DNA molecule consists of one old strand and one new strand.
• Key Enzymes:
  • Helicase: Unwinds DNA.
  • DNA Polymerase: Adds nucleotides to the 3’ end.
  • Primase: Synthesizes RNA primers.
  • Ligase: Joins Okazaki fragments on the lagging strand.
• Leading vs. Lagging Strand
  • Leading: Continuous synthesis.
  • Lagging: Discontinuous synthesis via Okazaki fragments.

6.3 Transcription

• Central Dogma: DNA → RNA → Protein.
• RNA Types:
  • mRNA: Carries genetic code.
  • rRNA: Catalytic part of ribosomes.
  • tRNA: Transfers amino acids during protein synthesis.
• Process: RNA polymerase transcribes DNA template strand into RNA.

6.4 Translation and Genetic Code

• Genetic Code: Triplet codons specifying amino acids.
• Process:
  • Initiation: Ribosome assembles on mRNA.
  • Elongation: Amino acids added one by one.
  • Termination: Stop codon ends translation.
• Key Players: mRNA, ribosomes, tRNA, amino acids.

6.5 Regulation of Gene Expression: Prokaryotes

• Operons: Cluster of genes transcribed together.
  • Trp Operon: Repressible system, turns off in presence of tryptophan.
  • Lac Operon: Inducible system, turns on in presence of lactose.
• Negative Feedback: Output of system inhibits the process.

6.6 Regulation of Gene Expression: Eukaryotes

• Complex Regulation: Involves enhancers, silencers, and epigenetic changes (methylation/acetalation).
• Epigenetics: Reversible changes in gene expression without altering DNA sequence.
• Introns and Exons: Alternative splicing increases protein diversity.

6.7 Mutation and Horizontal Gene Transfer

• Mutations: Changes in DNA sequence.
  • Point mutations: Silent, missense, nonsense.
  • Frameshift: Insertion/deletion causing shift in reading frame.
• Horizontal Gene Transfer: Gene transfer between organisms.
  • Transformation, conjugation, transduction.

6.8 Biotechnology

• Recombinant DNA: DNA from different sources combined.
• Techniques
  • Gel Electrophoresis: Separates DNA fragments by size.
  • PCR (Polymerase Chain Reaction): Amplifies DNA sequences.
  • DNA Sequencing: Determines order of nucleotides.
• Applications: Genetic engineering, forensic analysis, evolutionary studies.