Chapter 17

• Genetic Information in DNA

  DNA consists of specific nucleotide sequences. Genes contain the information for synthesizing proteins, which link genotype to phenotype.
• Gene Expression


  1. Transcription (DNA to RNA)

  2. Translation (RNA to protein)
• Central Dogma


  The flow of genetic information goes from DNA to RNA to protein.
• Codons


  mRNA base triplets, read in the 5' to 3' direction, specify amino acids. There are 64 codons, with 61 coding for amino acids and 3 acting as "stop" signals for translation termination.
• Transcription Process


  RNA polymerase binds to the promoter and initiates transcription. It unwinds the DNA helix and synthesizes an RNA transcript in the 5' to 3' direction.
• Stages of Transcription


  1. Initiation: Transcription factors and RNA polymerase form the transcription initiation complex at the promoter (e.g., the TATA box in eukaryotes)

  2. Elongation: RNA polymerase untwists the DNA and adds nucleotides to the growing RNA strand

  3. Termination: In bacteria, transcription stops at the terminator; in eukaryotes, it continues beyond pre-mRNA and eventually stops
• mRNA Modifications


  The 5' cap and poly-A tail protect mRNA and aid its export from the nucleus.
• RNA Splicing


  Removes introns (noncoding sequences) and joins exons (coding sequences). Spliceosomes (snRNPs) carry out this process. Alternative splicing allows one gene to code for multiple proteins.
• Ribozymes


  Catalytic RNA molecules that can function as enzymes, involved in RNA splicing.
• tRNA


  Carries specific amino acids and contains anticodons to pair with mRNA codons. The anticodon base-pairs with mRNA to ensure correct amino acid placement.
• Ribosomes


  Composed of two subunits (large and small), each with proteins and rRNA. They have three binding sites for tRNA (P site, A site, E site).
• Stages of Translation

  1. Initiation: The small ribosomal subunit, mRNA, and initiator tRNA assemble, followed by the large subunit

  2. Elongation: Amino acids are added one by one through codon recognition, peptide bond formation, and translocation

  3. Termination: The A site accepts a release factor, causing a water molecule to be added instead of an amino acid, releasing the polypeptide
• Polyribosomes


  Multiple ribosomes can translate a single mRNA simultaneously.
• Protein Folding and Post-Translational Modifications


  Polypeptides fold into their 3D shape. Post-translational modifications include cleavage, assembly, and targeting to specific cellular locations.
• Targeting Polypeptides


  Free ribosomes synthesize proteins for the cytosol, while bound ribosomes synthesize proteins for secretion or the endomembrane system. Signal peptides and signal-recognition particles (SRP) help guide ribosomes to the correct location.
• Mutations


  Changes in DNA sequences, which can affect protein structure and function.
• Types of Mutations


  • Substitutions: Replace one nucleotide pair with another. This can be silent (no change in the protein), missense (change to a different amino acid), or nonsense (creates a stop codon, leading to nonfunctional proteins)
  • Insertions/Deletions: Add or remove nucleotide pairs, potentially causing frameshift mutations, which can have severe effects on proteins
• Mutagens


  Physical or chemical agents that cause mutations. Spontaneous mutations can occur during DNA replication, recombination, or repair.
• Bacteria can simultaneously transcribe and translate the same gene, whereas eukaryotic transcription and translation are separated by the nuclear envelope. Archaea are likely to couple these processes similarly to bacteria.
• Gene Definition


  A gene is a unit of inheritance, a specific nucleotide sequence in a chromosome, or a DNA sequence coding for a specific polypeptide or RNA molecule.