Question? • How does DNA control a cell? • By controlling Protein Synthesis. • Proteins are the link between genotype and phenotype.
Central Dogma DNA Transcription RNA Translation Polypeptide
Explanation • DNA - the Genetic code or genotype. • RNA - the message or instructions. • Polypeptide - the product for the phenotype.
DNA vs RNA DNARNA Sugar – deoxyribose ribose Bases – ATGC AUGC Backbones – 2 1 Size – very large small Use – genetic code varied
Genetic Code • Sequence of DNA bases that describe which Amino Acid to place in what order in a polypeptide. • The genetic code gives the primary protein structure.
Genetic Code • Is based on triplets of bases. • Has redundancy; some AA's have more than 1 code. • Proof - make artificial RNA and see what AAs are used in protein synthesis (early 1960’s).
Codon • A 3-nucleotide “word” in the Genetic Code. • 64 possible codons known.
Codon Dictionary • Start- AUG (Met) • Stop- UAA UAG UGA • 60 codons for the other 19 AAs.
Code Redundancy • Wobble effect: Third base in a codonshows "wobble”. • First two bases are the most important in reading the code and giving the correct AA. The third base often doesn’t matter.
Code Evolution • The genetic code is nearly universal. • Ex: CCG = proline (all life) • Reason - The code must have evolved very early. Life on earth must share a common ancestor.
Reading Frame and Frame Shift • The “reading” of the code is every three bases (Reading Frame) • Ex: the red cat ate the rat • Frame shift – improper groupings of the bases • Ex: thredcatatet her at • The “words” only make sense if “read” in this grouping of three.
Transcription • Process of making RNA from a DNA template.
Transcription Steps 1. RNA Polymerase Binding 2. Initiation 3. Elongation 4. Termination
RNA Polymerase • Enzyme for building RNA from RNA nucleotides.
Binding • Requires that the enzyme find the “proper” place on the DNA to attach and start transcription.
Binding • Is a complicated process • Uses Promoter Regions on the DNA (upstream from the information for the protein) • Requires proteins called Transcription Factors.
TATA Box • Short segment of T,A,T,A • Located 25 nucleotides upstream from the initiation site. • Recognition site for transcription factors to bind to the DNA.
Transcription Factors • Proteins that bind to DNA before RNA Polymerase. • Recognizes TATA box, attaches, and “flags” the spot for RNA Polymerase.
Initiation • Actual unwinding of DNA to start RNA synthesis. • Requires Initiation Factors.
Elongation • RNA Polymerase untwists DNA 1 turn at a time and adds complimentary bases. • Exposes 10 DNA bases for pairing with RNA nucleotides.
Elongation • Enzyme moves 5’ 3’. • Rate is about 60 nucleotides per second.
Comment • Each gene can be read by sequential RNA Polymerases giving several copies of RNA. • Result - several copies of the protein can be made.
Termination • DNA sequence that tells RNA Polymerase to stop. • Ex: AATAAA • RNA Polymerase detaches from DNA after closing the helix.
Final Product • Pre-mRNA • This is a “raw” RNA that will need processing.
Modifications of RNA 1. 5’ Cap 2. Poly-A Tail 3. Splicing
5' Cap • Modified Guanine nucleotide added to the 5' end. • Protects mRNA from digestive enzymes. • Recognition sign for ribosome attachment.
Poly-A Tail • 150-200 Adenine nucleotides added to the 3' tail • Protects mRNA from digestive enzymes. • Aids in mRNA transport from nucleus.
RNA Splicing • Removal of non-protein coding regions of RNA. • Coding regions are then spliced back together.
Introns • Intervening sequences. • Removed from RNA. • Some contain sequences that regulate gene expression and many affect gene products
Exons • Expressed sequences of RNA. • Translated into AAs.
Introns - Function • Left-over DNA (?) • Way to lengthen genetic message to protect coding regions. • Old virus inserts (?)
Introns- Function • Way to create new proteins with exon shuffling • New combinations of exons= new proteins for evolution
Translation • Process by which a cell interprets a genetic message and builds a polypeptide.
Materials Required • tRNA • Ribosomes • mRNA
Transfer RNA = tRNA • Made by transcription. • About 80 nucleotides long. • Carries AA for polypeptide synthesis.
Structure of tRNAYou have a diagram of this • Has double stranded regions and 3 loops. • AA attachment site at the 3' end. • 1 loop serves as the Anticodon.
Anticodon • Region of tRNA that base pairs to mRNA codon. • Is a compliment to the mRNA bases, so reads the same as the DNA codon.
Example • DNA - GAC • mRNA - CUG • tRNA anticodon - GAC
Ribosomes • Two subunits made in the nucleolus. • Made of rRNA (60%)and protein (40%). • rRNA is the most abundant type of RNA in a cell.