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GENE FUNCTION Chapter 13 A. Comparison of DNA & RNA B. Transcription Process by which a DNA sequence (gene) is converted to an RNA sequence. Occurs in the nucleus of eukaryotic cells & cytoplasm of prokaryotic cells. Is regulated by operons (bacterial cells) or transcription factors (multicellular organisms). Involves 3 processes: initiation, elongation & termination 1. Initiation RNA polymerase attaches to a promoter on DNA strand. Helicase unzips a short section of DNA. Free RNA nucleotides move in & H-bond to complementary bases on DNA template strand. 2. Elongation RNA polymerase links RNA nucleotides together in a 5’ to 3’ direction. Growing RNA strand peels away from DNA template. 3. Termination RNA polymerase detaches when it reaches a terminator. Completed RNA molecule is released from DNA template. Usually, several copies of RNA are made at a time. 1 2 3 Determine the base sequence of RNA transcribed from the following DNA template strand. DNA template C A G T A A G C C RNA strand G T C A U U C G G Three types of RNA are transcribed. mRNA (messenger RNA) - encodes genetic information from DNA & carries it into the cytoplasm. 5’ 3’ codon Each three consecutive mRNA bases forms a genetic code word (codon) that codes for a particular amino acid. rRNA (ribosomal RNA) - associates with proteins to form ribosomes. large subunit small subunit Subunits are separate in the cytoplasm, but join during protein synthesis (translation). tRNA (transfer RNA) - transports specific amino acids to ribosome during protein synthesis (translation). Anticodon - specific sequence of 3 nucleotides; complementary to an mRNA codon. Amino acid accepting end Anticodon sequence determines the specific amino acid that binds to tRNA. Eukaryotic mRNA must be processed before it exits nucleus & enters cytoplasm. nucleotide cap is added “poly A tail” is added introns are removed C. Translation Process by which an mRNA sequence is translated into an amino acid sequence (polypeptide/protein). Occurs in the cytoplasm of eukaryotic & prokaryotic cells. Requires: mRNA, tRNAs, amino acids & ribosomes. Involves 3 processes: initiation, elongation & termination 1. Initiation Small ribosomal subunit binds to “start codon” [AUG] on mRNA molecule. AUG codon attracts initiator tRNA. 2. Elongation Large ribosomal subunit binds to small subunit. A second tRNA anticodon binds to the next mRNA codon. A peptide bond forms between the two amino acids. Initiator tRNA is released. Ribosome moves down mRNA by 1 codon. A third tRNA anticodon binds to the next mRNA codon. A peptide bond forms between 2nd & 3rd amino acids. tRNAs continue to add amino acids; polypeptide lengthens. 3. Termination Occurs when ribosome reaches an mRNA stop codon (UGA, UAG or UAA). Stop codons do NOT specify an amino acid. Last tRNA is released, ribosomal subunits separate & new polypeptide/protein is released. Usually, several copies of the polypeptide/protein are made at a time. 6 5 4 3 2 1 Some polypeptides must be altered before they can function. Determine the amino acid sequence a ribosome would translate from the following mRNA strand. mRNA C A U G G C U C A A U G A Met Ala Gln STOP Review: Genetic information flows in cell from DNA RNA protein. Each gene on DNA codes for production of a specific polypeptide/amino acid. D. Mutation A physical change in the nucleotide sequence of DNA. May not affect phenotype (silent mutation). Can affect somatic cells (somatic mutation) or sex cells (germinal mutation). Can form spontaneously or be induced by a mutagen. 1. Point mutation - replacement of one DNA nucleotide with another. missense mutation - point mutation that changes a codon so that a different amino acid is specified. Ex. sickle cell anemia nonsense mutation - point mutation that changes an amino acid-specifying codon into a stop codon. 2. Frameshift mutation - the insertion or deletion of DNA nucleotides; results in disruption of the reading frame. Ex. cystic fibrosis 3. Expanding repeat - the # of copies of a 3 nucleotide sequence increases over several generations. Ex. myotonic dystrophy