Chapter 13 GENE FUNCTION

A. Comparison of DNA & RNA

B. Transcription Process by which a DNA sequence ( gene ) is converted to an RNA sequence. F Occurs in the nucleus of eukaryotic cells & cytoplasm of prokaryotic cells. F Is regulated by operons (bacterial cells) or transcription factors (multicellular organisms). F 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. 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 123

Three types of RNA are transcribed. ] mRNA (messenger RNA) - encodes genetic information from DNA & carries it into the cytoplasm. Each three consecutive mRNA bases forms a genetic code word (codon) that codes for a particular amino acid. codon 5’3’

] rRNA (ribosomal RNA) - associates with proteins to form ribosomes. Subunits are separate in the cytoplasm, but join during protein synthesis (translation). large subunit small subunit

] 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). F Occurs in the cytoplasm of eukaryotic & prokaryotic cells. F Requires: mRNA, tRNAs, amino acids & ribosomes. F 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. 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 AlaGlnSTOPMet

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. F May not affect phenotype (silent mutation). F Can affect somatic cells (somatic mutation) or sex cells (germinal mutation). F Can form spontaneously or be induced by a mutagen.

1. Point mutation - replacement of one DNA nucleotide with another. F missense mutation - point mutation that changes a codon so that a different amino acid is specified. Ex. sickle cell anemia

F 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