Molecular Genetic Mechanisms

Chapter 5: Molecular Genetic Mechanisms

Central Dogma of Gene Expression

  • Information flow: DNA -> RNA -> Proteins

  • Key molecules: tRNA (3%), mRNA (2%), rRNA (~95%)

Eukaryotic Gene Expression

  • Transcription occurs in the nucleus, producing pre-mRNA.

  • RNA Processing modifies the pre-mRNA (5' capping, splicing introns, and adding poly(A) tail).

  • Translation occurs in the cytosol, converting mRNA into proteins using ribosomes and tRNA.

Nucleotide Structure

  • Consists of a nitrogenous base, ribose or deoxyribose, and 1-3 phosphates.

  • Key Differences: RNA has 2'-OH group (ribose), DNA has 2'-H (deoxyribose).

Base Pairing Rules

  • A pairs with T (or U in RNA) with 2 hydrogen bonds; G pairs with C with 3 hydrogen bonds.

  • Strands are antiparallel and have sugar-phosphate backbones on the outside.

DNA Conformation

  • B-form DNA: Right-handed helix with major/minor grooves; stabilizes via base stacking.

  • A-form DNA: Occurs under low humidity; RNA-RNA hybrids tend to adopt A-form.

  • Z-form DNA: Left-handed helix, usually features alternating G and C bases; uncommon.

RNA Properties

  • Generally single-stranded, can form secondary/tertiary structures.

  • Has catalytic activity (ribozymes) and is sensitive to degradation due to 2'-OH.

DNA Replication

  • Replication is semi-conservative: each new DNA consists of one original and one new strand.

  • Leading vs. Lagging Strand: Continuous vs. discontinuous synthesis, respectively.

  • Primers are required for DNA polymerase to start synthesis (RNA primers).

DNA Repair Mechanisms

  • Various pathways: base excision, mismatch repair, nucleotide excision.

  • Non-homologous end joining and homologous recombination are also critical for repairing double-strand breaks.

Translation Overview

  • tRNA: Transfers amino acids to ribosomes during protein synthesis; recognizes mRNA coding via anticodons.

  • Ribosomes: Composed of rRNA and proteins; site for translation, includes A, P, and E sites for tRNA binding.

Genetic Code

  • Codons (triplet RNA sequences) specify amino acids; some redundancy allows multiple codons for same AA.

  • AUG serves as the initiation codon, while UAA, UAG, and UGA are stop codons.

Viral Genetics

  • Viruses can have narrow host ranges and undergo lytic or lysogenic cycles. Retroviruses integrate their genome into host DNA, leading to chronic infections (e.g., HIV).