The Expression of Genetic Information: an Overview

• All cells contain enormous amounts of DNA. Even the smallest known cell, a bacterium named Mycoplasma genitalium, contains nearly 500,000 base pairs (500 kilo base pairs, or 500 kbp).
• What do cells do with all this information? In bacteria (the simplest cells), DNA serves two major functions:
  1. encoding structural information that can be converted into RNA and (usually) thence into protein sequence.
  2. encoding regulatory signals that allow certain proteins to decide where to begin or terminate reading DNA
• In eukaryotic cells, where DNA also has to undergo condensation into chromosomes and mitotic or meiotic events, DNA also contains specialized sequences used for centromeres, telomeres, and other functions.
• The "central dogma" of molecular biology: information flows from nucleic acids to proteins, not the reverse:

  DNA RNA polypeptides (proteins)
  View graphic summary of information flow

• Transcription = the process of making RNA from DNA templates
• Translation = the process of making polypeptides
• View example of how information is coded from DNA to peptide

Transcription: the Synthesis of RNA

Structure of RNA

• Components: Ribose, Phosphate, A, C, G & U.
• View RNA components (protected).
• U has same base pairing properties as T (forms U=A base pairs)
• RNA is not double stranded (except in some viruses)
• RNA can have extensive "hairpin" loops
• RNA can have modified bases (after transcription) -- find unusual bases such as inosine, pseudouridine. These still have base pairing properties, and contribute to stability of molecule,
• Messenger RNA has half-life of only 3 minutes in bacteria, so cell must constantly make new messages to make new proteins-- allows rapid adaptation to new environments.
• Eucaryotic RNA is controlled very differently. RNA synthesized in nucleus, modified (to be discussed), and exported to cytoplasm before it is translated.

Types of RNA

• messenger RNA -- carries codons to RNA
• ribosomal RNA -- part of ribosome structure, catalyzes peptide bond formation
• transfer RNA -- set of small RNAs, transport amino acids to ribosome for incorporation into growing polypeptides

Transcription Process

• View animation of transcription (protected).
• RNA polymerase enzyme opens up DNA helix for short stretch (~ 15 base pairs)
• selects one of two strands as template strand
• RNA synthesized in 5' to 3' direction
• RNA synthesis begins at promoters: sites on DNA that are recognized as "start" signals for RNA synthesis.
• Terminators: regions where RNA synthesis stops, RNA is released from DNA.
• Practice making RNA (Campbell website activity)