Gene  Protein Chapter 17

Protein Synthesis / Gene Expression Gene expression: The translation of information encoded in a gene into protein or RNA. Expressed genes include genes that are transcribed into messenger RNA (mRNA) and then translated into protein, as well as genes that are transcribed into types of RNA such as transfer RNA (tRNA) and ribosomal RNA (rRNA) that are not translated into protein.

Gerrod - 1909 genes dictate phenotype through enzymes that catalyze specific chemical reactions in the cell symptoms of an inherited disease reflect a person’s inability to synthesize a particular enzyme

one gene - one enzyme hypothesis

Development of the Theory one gene - one protein not all proteins are enzymes one gene - one polypeptide many proteins are composed of several polypeptides, each of which has its own gene

RNA contains ribose as its sugar substitutes the base uracil for thymine consists of a single strand

transcription and translation Transcription  DNA strand provides a template for the synthesis of a complementary RNA strand Translation  the information contained in the order of nucleotides in mRNA is used to determine the amino acid sequence of a polypeptide

Transcription template strand of DNA provides a template for the sequence of nucleotides in RNA The complementary RNA molecule is synthesized according to base-pairing rules except that uracil is the complementary base to adenine

RNA polymerase separates the DNA strands at the appropriate point bonds the RNA nucleotides as they base-pair along the DNA template

http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html

At the 5’ end of the pre-mRNA molecule, a modified form of guanine is added, the 5’ cap At the 3’ end, an enzyme adds 50 to 250 adenine nucleotides, the poly(A) tail

RNA splicing Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides Spliceosome  removes introns and joins exons to create an mRNA molecule with a continuous coding sequence

What’s the point of splicing? at least some introns contain sequences that control gene activity in some way splicing itself may regulate the passage of mRNA from the nucleus to the cytoplasm one clear benefit of split genes is to enable a one gene to encode for more than one polypeptide.

Translation blocks of three nucleotides, codons, are decoded into a sequence of amino acids codons are read in the 5’->3’ direction along the mRNA ribosome adds each amino acid carried by tRNA to the growing end of the polypeptide chain

tRNA carries a specific amino acid at one end and has a specific nucleotide triplet, an anticodon, at the other The anticodon base-pairs with a complementary codon on mRNA

http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html

tRNA molecule

binding site for mRNA The P site holds the tRNA carrying the polypeptide chain The A site carries the tRNA with the next amino acid Discharged tRNAs leave the ribosome at the E site

Initiation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits

Codons

Elongation each amino acid is added to the proceeding one codon recognition peptide bond formation translocation

Elongation

Termination Stop codon Releasing factor

polyribosomes

Fig. 17-23 Wild-type DNA template strand 3 5 5 3 mRNA 5 3 Protein Stop Amino end Carboxyl end A instead of G Extra A 3 5 3 5 5 3 5 3 U instead of C Extra U 5 3 5 3 Stop Stop Silent (no effect on amino acid sequence) Frameshift causing immediate nonsense (1 base-pair insertion) T instead of C missing 3 5 3 5 5 3 5 3 A instead of G missing 5 3 5 3 Stop Missense Frameshift causing extensive missense (1 base-pair deletion) A instead of T missing 3 5 3 5 5 3 5 3 U instead of A missing 5 3 5 3 Stop Stop Nonsense No frameshift, but one amino acid missing (3 base-pair deletion) (a) Base-pair substitution (b) Base-pair insertion or deletion

Producing a protein