chapter 14

• One gene, one protein


  Each gene codes for at least one polypeptide
• Polypeptide


  A single chain of amino acids bound together by peptide bonds
• Protein


  Can be a single polypeptide or a complex of several polypeptides
• Eukaryotic genes


  • Use alternative splicing to make different polypeptides from the same gene
  • Violates the original one gene, one protein rule
• Gene expression


  1. Transcription

  2. Translation
• Gene


  A segment of DNA that controls production of a protein and/or an RNA
• Gene expression


  The processes by which a gene's information is transferred to the RNA and the making of the protein
• Central Dogma of Gene Expression


  1. DNA

  2. RNA

  3. Polypeptide
• Transcription


  DNA is transcribed into RNA
• Transcription


  • Takes place in the nucleus of eukaryotic cells
  • Takes place in the nucleoid space in prokaryotic cells
• Translation


  RNA is translated by a ribosome into a polypeptide
• Translation


  • Takes place in a ribosome in both prokaryotic and eukaryotic cells
• RNA molecules required for gene expression


  • Messenger RNA (mRNA)
  • Ribosomal RNA (rRNA)
  • Transfer RNA (tRNA)
• Messenger RNA (mRNA)


  RNA molecule produced during transcription that carries the gene code for a specific polypeptide to the ribosome
• Ribosomal RNA (rRNA)


  • Part of a ribosome, involved in catalyzing peptide bonds during translation
  • Made in the nucleolus in eukaryotic cells
  • Made in the nucleoid space in prokaryotic cells
• Transfer RNA (tRNA)


  Carries (transfers) amino acids from cytoplasm to the ribosome
• Some viruses have RNA as the genetic material instead of DNA
• Central dogma of RNA viruses

  1. RNA

  2. RNA

  3. Polypeptide
• Central dogma of retroviruses


  1. DNA

  2. RNA

  3. Polypeptide
• Transcription


  Synthesis of pre-messenger RNA (pre-mRNA) and processing into mature mRNA
• Transcription


  • Involves DNA (template strand), promoter, coding, and termination sequence, and the enzyme RNA polymerase
• RNA polymerase


  • Enzyme that polymerizes RNA nucleotides into long polymers
  • Can only add nucleotides in the 5' to 3' direction, like DNA polymerase
  • Does not require a primer to polymerize mRNA
• Transcription


  1. Initiation

  2. Elongation

  3. Termination
• Transcription initiation


  Begins when RNA primase binds to the promoter
• Promoter


  • DNA control sequences that tell RNA polymerase where to start transcription and which strand of DNA to transcribe (the template DNA strand)
  • Includes the initiation site
• Prokaryotic promoters


  • Often start transcription for several genes
  • Example: Lac Operon promoter controls three genes
• Sigma proteins


  Proteins that help RNA polymerase bind to the template DNA strand in prokaryotic cells
• Eukaryotic promoters


  • Start transcription for single gene
• Transcription factors


  Proteins that help bind the RNA polymerase to the template DNA strand in eukaryotic cells
• Transcription elongation


  1. DNA unwinds approximately 10 base pairs

  2. RNA polymerase adds nucleotides in a 5' – 3' direction

  3. RNA transcription does not have a proofreading step

  4. Approximately 1 error per 100,000 base pairs
• Transcription termination


  1. Requires a termination sequence that defines the termination site

  2. RNA polymerase detaches from the template DNA strand

  3. The newly synthesized pre-mRNA transcript is released
• Genetic code


  A chart of all possible codons and the amino acids each codon codes for
• Codon


  A triplet of mRNA nucleotides
• Start codon (AUG)


  The initiation codon for translation
• Stop codons


  Three codons that signal translation termination
• The genetic code chart is based on messenger RNA (mRNA) only
• Genetic code


  • Nearly universal for all living organisms' nuclear DNA
  • Differs slightly in mitochondria and chloroplast DNA
  • Has 64 possible codons that code for 20 different amino acids
  • Redundant - more than one codon codes for most amino acids
• The genetic code is universal - all organisms have the same genetic code
• The universality of the genetic code allows researchers to insert genes from one organism to another, distantly related organism
• GFP - a fluorescent protein found in jellyfish, inserted in other living organisms due to the universality of the genetic code