AP Biology Unit 6: Gene Expression

Overview

• Complex topic covering gene expression.
• Preparation for AP Bio exam and unit 6 test.
• Topics: DNA/RNA structure, replication, transcription, translation, gene regulation, mutation, horizontal gene transfer, biotechnology.

DNA and RNA Structure and Function

DNA Structure

• Double-stranded helical molecule.
• Composed of nucleotide monomers: five-carbon sugar (deoxyribose), phosphate group, four nitrogenous bases (A, T, C, G).
• Base pairing: A-T, G-C. Anti-parallel strands (5' to 3', 3' to 5').

Function

• Information storage: sequence determines code for RNA and proteins.
• Replicability: specific base pairing allows each strand to serve as a template.
• Stability: double helix protects sequence, allows for mutability (mutations enable evolution).

RNA Function

• Hereditary molecule in some viruses (e.g., HIV, SARS-CoV-2).
• Involved in protein synthesis (mRNA, tRNA, rRNA).
• Regulatory functions in eukaryotes (e.g., splicing).

Genetic Information Storage

• Prokaryotes: looped circular chromosomes, DNA is naked.
• Eukaryotes: multiple linear chromosomes, DNA wrapped around histones.

Plasmids

• Extra-chromosomal DNA loops, common in bacteria, involved in horizontal gene transfer and genetic engineering.

DNA Replication

Semiconservative Replication

• Each new DNA double helix has one conserved and one new strand.

Key Enzymes

• Helicase: Unzips DNA strands.
• DNA Polymerase: Synthesizes new DNA, needs RNA primer to start.
• Primase: Lays down RNA primers.
• Ligase: Seals gaps between DNA fragments.

Leading vs Lagging Strand

• Leading Strand: Continuous replication.
• Lagging Strand: Discontinuous, forms Okazaki fragments.

Transcription

Overview

• Central Dogma: DNA → RNA → Protein.
• Transcription: synthesis of RNA from DNA.

RNA Types

• mRNA: Carries genetic code to ribosomes.
• rRNA: Catalytic component of ribosomes.
• tRNA: Transfers amino acids.

Process

• RNA polymerase binds to promoter, transcribes DNA to RNA.
• Ends at terminator region.

Translation and Genetic Code

Genetic Code

• Triplets of nucleotides (codons) code for amino acids.
• Universal, specific, redundant.

Protein Synthesis

• Ribosomes read mRNA, tRNAs bring amino acids.
• Peptide bonds formed, polypeptide chain grows.

Translation Stages

• Initiation: mRNA binds to ribosome, start codon recognized.
• Elongation: tRNAs bring amino acids, peptide bonds form.
• Termination: Stop codon reached, polypeptide released.

Gene Expression Regulation

Operons (Prokaryotic)

• Cluster of genes with shared control mechanisms.
• trp Operon: Repressible, turned off by tryptophan.
• lac Operon: Inducible, turned on by lactose.

Eukaryotic Regulation

• Complex due to multicellularity.
• Gene expression influenced by acetylation (activates) and methylation (silences).
• Enhancers, silencers, and transcription factors involved.

Alternative Splicing

• Allows one gene to code for multiple proteins.

Mutations

Types

• Point Mutations: Change in a single nucleotide.
• Silent: No amino acid change.
• Missense: Different amino acid.
• Nonsense: Stop codon introduced.
• Frameshift: Insertions/deletions alter reading frame.

Impact

• Mutations can be positive, negative, or neutral.
• Sickle cell disease: example of missense mutation.

Horizontal Gene Transfer

Mechanisms

• Conjugation: DNA transfer between bacteria through pili.
• Transformation: Uptake of DNA from environment.
• Transduction: Virus-mediated DNA transfer.

Biotechnology

Recombinant DNA

• Combining DNA from different sources.
• Created using restriction enzymes and ligase.

Techniques

• Gel Electrophoresis: Separation of DNA by size.
• PCR: Amplifies DNA samples.
• Sequencing: Determines DNA nucleotide order.

This lecture provides an extensive overview of gene expression, focusing on essential concepts for understanding genetic processes and the tools used in modern biotechnology.