Unit 6: Gene Expression and Regulation

Introduction

• Focus on DNA and RNA, which are nucleic acids involved in gene expression.
• Nucleic Acids: Composed of nucleotides, each with a phosphate group, pentose sugar, and nitrogenous base.
• Sugar-Phosphate Backbone: Backbone of nucleotides connected by sugar and phosphate groups.

DNA and RNA Basics

• DNA (Deoxyribonucleic Acid): Double helix, contains A (adenine), T (thymine), C (cytosine), G (guanine).
• RNA (Ribonucleic Acid): Single-stranded, contains A, U (uracil), C, G.

DNA Structure

• Double Helix: Two strands twisted with nitrogenous bases paired (A-T, C-G) via hydrogen bonds.
• Antiparallel Strands: One strand runs 5' to 3', the other runs 3' to 5'.
• Base Pairing: Purines (A, G) pair with pyrimidines (T in DNA, U in RNA, C).

DNA Replication

• Semiconservative Replication: Each new DNA has one old and one new strand.
• Leading and Lagging Strands: Continuous synthesis on the leading strand; Okazaki fragments on the lagging strand.
• Enzymes Involved:
  • Helicase: Unwinds DNA.
  • DNA Polymerase: Adds nucleotides to the 3' end.
  • Ligase: Joins Okazaki fragments.
  • Topoisomerase: Relieves strain ahead of replication fork.
  • Single-Strand Binding Proteins: Maintain strand separation.

Transcription and Translation

• Central Dogma: DNA → mRNA → Protein.
• Transcription: Formation of mRNA from DNA template.
• RNA Processing: Addition of 5' cap, poly-A tail, and splicing to remove introns.
• Translation: Ribosomes translate mRNA codons into amino acids to form proteins.
  • Ribosome Sites: A site (aminoacyl), P site (peptidyl), E site (exit).
  • Codons: Triplets of bases coding for amino acids; AUG is start codon.

Regulation of Gene Expression

• DNA Methylation: Decreases gene expression by tightening DNA winding.
• Histone Acetylation: Increases gene expression by loosening DNA winding.
• Operon System (e.g., Lac Operon):
  • Promoter: Initiation site for transcription.
  • Operator: Site where repressors bind to block transcription.
  • Inducers and Repressors: Control gene expression.

Mutations

• Point Mutations: Change in a single nucleotide.
• Frameshift Mutations: Insertions or deletions altering reading frame.
• Aneuploidy: Abnormal number of chromosomes.

Genetic Diversity in Bacteria

• Conjugation: Transfer of DNA through direct contact.
• Transformation: Uptake of DNA from the environment.
• Transduction: Transfer of DNA by bacteriophages.

Biotechnology

• Gene Cloning: Insertion of genes into plasmids for replication in bacteria.
• PCR (Polymerase Chain Reaction): Amplifies DNA using cycles of denaturation and synthesis.
• Gel Electrophoresis: Separates DNA fragments by size using an electrical field.
• DNA Sequencing: Determines the exact sequence of nucleotides.
  • Shotgun Sequencing and Next-Gen Sequencing: Modern methods for sequencing large genomes.