Exam 3

• Gene expression


  The process by which DNA directs protein synthesis, includes two stages: transcription and translation
• Transcription and translation


  • They are the two stages of gene expression
• Beadle and Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media
• Identifying nutritional mutants in Neurospora

  1. Cells placed on complete medium

  2. Cells subjected to X-rays

  3. Surviving cells form colonies

  4. Cells placed in minimal medium

  5. Nutritional mutants identified

  6. Nutritional mutants placed in vials with a variety of media

  7. Observe for growth
• Class I mutants


  Mutation in gene A, can grow only on citrulline or arginine
• Class II mutants


  Mutation in gene B, can grow with or without any supplements
• Class III mutants


  Mutation in gene C, can grow on ornithine, citrulline, or arginine
• RNA is the bridge between genes and the proteins for which they code
• Transcription


  The synthesis of RNA using information in DNA
• Translation


  The synthesis of a polypeptide, using information in the mRNA
• Ribosomes


  The sites of translation
• In prokaryotes, translation of mRNA can begin before transcription has finished
• In a eukaryotic cell, the nuclear envelope separates transcription from translation
• Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA
• Primary transcript


  The initial RNA transcript from any gene prior to processing
• Central dogma


  Cells are governed by a cellular chain of command: DNA → RNA → protein
• The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words
• The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA
• These words are then translated into a chain of amino acids, forming a polypeptide
• Codon


  A triplet of nucleotides that specifies an amino acid
• Of the 64 triplets, 61 code for amino acids; 3 triplets are "stop" signals to end translation
• The genetic code is redundant (more than one codon may specify a particular amino acid) but not ambiguous (no codon specifies more than one amino acid)
• Codons must be read in the correct reading frame (correct groupings) for the specified polypeptide to be produced
• Transcription is the first stage of gene expression
• RNA polymerase


  The enzyme that catalyzes RNA synthesis
• The RNA is complementary to the DNA template strand
• RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
• Transcription


  1. Initiation

  2. Elongation

  3. Termination
• In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification
• In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence
• RNA processing


  Enzymes in the eukaryotic nucleus modify pre-mRNA before the genetic messages are dispatched to the cytoplasm
• 5' cap


  A modified guanine nucleotide added to the 5' end of pre-mRNA
• Poly-A tail


  A region of 50-250 adenine nucleotides added to the 3' end of pre-mRNA
• Introns


  Long noncoding stretches of nucleotides that lie between coding regions in eukaryotic genes and their RNA transcripts
• Exons


  The regions that are eventually expressed, usually translated into amino acid sequences
• RNA splicing


  The process that removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
• Ribozymes


  Catalytic RNA molecules that function as enzymes and can splice RNA
• Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing, called alternative RNA splicing
• Consequently, the number of different proteins an organism can produce is much greater than its number of genes
• Domains


  Modular regions that make up the structure of a protein