Transcription and Translation From Gene to Protein

Central Dogma How might DNA serve as the genetic code for the synthesis of our proteins? This chain occurs in all living things Our question for the day…

RNA Distinctions from DNA Three types Ribose sugar Uracil instead of thymine Usually single-stranded Three types mRNA (messenger) – formed complimentary to a strand of DNA (stay tuned!) rRNA (ribosomal) – ribosome formation tRNA (transfer) – transport amino acids during translation Think of our worksheet, DNA is double stranded!

Transcription We’re still in the nucleus! Synthesizes a strand of mRNA from the DNA template mRNA is the basis for making proteins Helicase unzips the DNA template RNA polymerase binds and brings the nucleotides to form the mRNA strand Reads in 3’ to 5’, new strand is created 5’ to 3’ Non-template strand Finished mRNA enters cytoplasm via nuclear pores That’ll change come Monday

RNA Processing Final mRNA code is significantly shorter than the DNA template – why? Introns and Exons – explain the “missing pieces” Introns – sequences “spliced” out of the original mRNA (before entering cytoplasm) Exons – sequences that code for proteins Addition of cap to 5’ end (for recognition) Addition of poly-A tail to 3’ end Found huge gaps in the sequences that are not in the final mRNA Poly-A – many adenines! Unclear purpose. Cap – recognition by the ribosomes

Translation mRNA transcript is split into groups of three base pairs called codons Each codon corresponds to an amino acid More than one codon can indicate the same amino acid STOP codons – UAA, UGA, UAG START codons - AUG Scientists knew these things

Translation Processed mRNA moves to the Ribosome tRNA molecules act as the interpreters of the mRNA codon sequence What is a codon? (Three letters) On each tRNA there is an anticodon Codon is read 5’ to 3’ tRNA has respective amino acid attached to it Stop codons Amino acids are linked by peptide bonds to form proteins

So what is going to bind at the promoter?

Gene Regulation and Mutations Chapter 12, Sections 4-5 Depending on time?

Prokaryotic Gene Regulation Ability of an organism to control which genes are transcribed in response to changes in the environment Remember introns? Operon – section of DNA that contains the genes for the proteins needed for a specific metabolic pathway.

Lac Operon Lac – lactose used by E. coli as an energy source! Repressor Lactose is an energy source! Trp is another repressor operon

Eukaryotic Gene Regulation Transcription factors – ensure the right time and right amount for expression. Homeobox (Hox) genes – controls the differentiation of cells (in development). Code for certain transcription factors RNAi (interference) – double-stranded RNA is cut/modified, eventually binds mRNA preventing translation

Mutations Gene mutations Point(s) mutations Frameshift mutations Missense substitution – wrong amino acid Nonsense substitution – stops sequences early Can also be silent Frameshift mutations Insertion Deletion

Mutations (continued) Chromosomal mutations Deletion – of more than just one base Duplication – of a segment Inversion – part of sequence is flipped Translocation – segments are exchanged between DNA strands (like crossing over…but not on homologous chromosomes)