Chapter 17 From Gene to Protein

Connection Between Genes and Proteins Transcription – synthesis of RNA using DNA Template Messenger RNA is transcribed from DNA with the help of RNA polymerase; separates DNA and inserts the complementary base pairs to the 3’ end. The genetic code to produce proteins contain adenine, uracil, guanine, and cytosine. Transcription begins at initiation and ends at the termination region Transcription factors- DNA binding proteins that bind to specific DNA sequences at the promoter to let RNA polymerase bind to the promoter so transcription can begin. TATA box – nucleotide sequence in eukaryotes at the promoter that RNA polymerase II recognizes and binds to the DNA so transcription can begin.

Connection Between Genes and Proteins (Continued) Elongation of the RNA strand RNA polymerase II moves along the DNA exposing 10 nucleotides at a time inserting the appropriate bases to the 3’ end Eukaryotic mRNA grows at 30-60 nucleotide bases per second; as the strand elongates, it peels away fro the DNA template Many RNA polymerase II enzymes may be working on the same DNA template Mass production of proteins in a series. Termination of Transcription RNA polymerase II recognizes a termination sequence, usually AAUAAA in the mRNA strand Prokaryote RNA is ready for translation, eukaryote RNA must be processed further.

Modification of mRNA in Eukaryotes Alteration of pre-mRNA ends Pre-mRNA – primary transcript that will be processed to functional mRNA 5’ cap – modified guanine nucleotide (guanine triphosphate that is added to the 5’ end Protects growing mRNa from hydrolytic enzymes Help ribosome subunits to recognize the 5’ end and the leader sequence (unstranslkated strand from the 5’ end to the start codon Poly (A) Tail – sequence of 200-300 adenine nucleotides to the 3’ end Inhibits degradation in cytoplasm Facilitate attachment to the small ribosomal subunit Facilitate export of mRNA from nucleus to cytoplasm Protects stop codon

Split Genes and RNA Splicing Introns – noncoding sequences of mRNA Exons – coding strands of mRNA RNA Splicing – removal of introns and splicing of exons to form a mature mRNa strand that will move into the cytoplasm Introns contain short boundary sequences that signal RNA splicing sites Small nuclear ribonucleoproteins (snRNP’s)- proteins found in the nucleus that participate in Rna splicing; called “snurps” Spliceosome – molecular complex that catalyzes splicing reactions that involve snurps Cuts the 5’ and 3’ ends to splice exons together

Split Genes and RNA Splicing (Continued) Ribozymes – RNA molecules that can break and from covalent bonds to produce mature rRNA and tRNA Evolutionary importance of Introns Cutting of introns and the splicing of exons may regulate export into the cytoplasm Introns may allow a single gene to direct the synthesis of different proteins All introns may be removed from a transcript in one case; but in another, one or more introns may be left in to be translated; each resulting protein may be different With introns in, coding sequences have a higher recombination frequency than if introns were always left out

The Synthesis of Protein Translation – RNA directed synthesis of a polypeptide Transfer RNA (tRNA) – carries the anticodon to the mRNA and the amino acid Obtains amino acid from cytoplasm’s pool Recognizes the correct codon of mRNA 3’ bind the amino acid Single-stranded with 80 base pairs that is double stranded at regions by hydrogen bonding between complimentary nucleotide sequences Wobble Position – ability of a tRNA molecule to base pair with two or three different mRNA codons due to the 5’ end of the anitocodon

The Synthesis of Protein (Continued) Ribosomes – coordinate the pairing of tRNA anticodons to mRNA codons Two subunits, large and small, that are separated when not involved in synthesis During synthesis, the small subunit is on the bottom Ribosomes are constructed by the nucleolus Ribosomes contain three bonding sites P site – holds tRNA carrying the growing polypeptide chain A site – holds next tRNA with the next amino acid E site – discharges tRNA’s

The Synthesis of Protein (Continued) Initiation Small ribosome subunit binds to mRNA and initiator tRNA carrying methionine and the anticodon UAC (start codon) Large subunit binds to the small u nit producing a translation complex. 5’ cap helps bind the leader to the small subunit in eukaryotes Initiator tRNA binds to the P site and a site is ready for the next tRNA Elongation Codon Recognition – mRNA codon in the A site forms hydrogen bonds with the entering tRNA Peptide bond formation – peptide bond is formed between the polypeptide in the P site and the new amino acid in the A site Translocation – A site tRNA is moved to the P site, P site tRNA is moved to E site where it is detached Termination Termination Codon – triplet of UAA, UAG, and UGA that signal the end of translation These codons have nothing to do with termination sequences on DNA during transcription of mRNA. Hydrolysis of the peptide chain from the P site tRNA releases the protein from the ribosome

The Synthesis of Protein (Continued) Mutations Point – one or two base pairs Substitutions – a base pair is replaced by another base pair according to base pairing rules Silent Substitution – base pair substitution may transform one codon into another that codes for the same amino acid Conservative Substitution – base pair substitution that may transform one codon into anther that codes for a different amino acid, but it has no effect May produce an improved protein that will enhance mutant organisms and descendants Frameshift Mutation – insertion of deletion of a base pair that causes the reading frame to shift Causes premature termination May cause nonfunctional protein unless the mutation is near the end of the gene

The Synthesis of Protein (Continued) Mutagens Cause errors in DNA replication, repair, or recombination of chromosomes that result in substitution and frameshift mutations Radiation Base Analogues – chemicals that mimic DNA bases, but base pair incorrectly