Ch 10: Protein Synthesis DNA to RNA to Proteins Same two steps produce all proteins: 1) DNA is transcribed (copied) to form RNA Occurs in the nucleus RNA moves into cytoplasm 2) RNA is translated (read) to form polypeptide chains, which fold to form proteins

Three Classes of RNAs Messenger RNA (mRNA) Ribosomal RNA (rRNA) Carries protein-building instruction Ribosomal RNA (rRNA) Major component of ribosomes Transfer RNA (tRNA) Reads mRNA and delivers amino acids to ribosomes

1. TRANSCRIPTION To start you must remember that DNA is made up of nucleotides (A,T,G, C) These letters are in a specific order because they are “instructions” for how to make proteins RNA will “decode” the instructions

Transcription Start: mRNA is produced by reading DNA inside the nucleus and using the base pairing rules. RNA nucleotides = A, U, C , G A pairs w/ U : C pairs w/ G DNA= AGGTTA mRNA= UCCAAU

Transcription Only small stretch is a template RNA polymerase catalyzes nucleotide addition Product is a single strand of RNA

How do we start to make mRNA? Promoter region is a base sequence in the DNA that signals the start of a gene to be read (to make mRNA) For transcription to occur, RNA polymerase (an enzyme!) must first bind to a promoter mRNA continues to build until it reaches the “code” for STOP. It is then released promoter region

When the DNA is transcribed it is called Precursor mRNA (or Pre mRNA) It is filled with introns and exons Introns: intervening nucleotide (space holders) Exons: expressed segments of a gene

mRNA is made up of introns and exons Introns are then excised (cut) from the premRNA to form Mature mRNA Exons stay and are expressed! Mature mRNA is what then moves onto step 2 (translation) Introns and Exons

Messenger RNA (mRNA) Each codon, is specific for an amino acid. Made up of codons: sequence of three bases: exp AUG - methionine. Each codon, is specific for an amino acid.

Genetic Code Set of 64 base triplets: Codons 61 specify amino acids Nucleotide bases read in blocks of three 61 specify amino acids 3 stop translation

Code Is Redundant Twenty kinds of amino acids are specified by 61 codons Most amino acids can be specified by more than one codon Exp: Six codons specify leucine UUA, UUG, CUU, CUC, CUA, CUG

Near-Universal Genetic Code

Near-Universal Genetic Code

Meet the other two players needed for Translation tRNA rRNA (Ribosome)

Three Stages of Translation Initiation: All three RNA molecules meet up Elongation: mRNA is read by the tRNA. tRNA delivers the amino acids to the growing protein (polypeptide chain) Termination: A stop codon is read and the process is complete

Elongation: how does the tRNA know what amino acid to bring? ~ Each tRNA contains a 3 base anticodon ~ In order to deliver the correct amino acid, the codon and the anticodon must match up using the complimentary base pair rules!

As the tRNA delivers the correct amino acids ~ the amino acids will begin to link together with a peptide bond ~ the tRNA will release the it and go off to find another amino acid to keep the process going.

Termination 1: STOP codon is reached 2: release the mRNA and polypeptide chain from the ribosome. (is free to join the pool of proteins in the cytoplasm or to enter rough ER) The two ribosomal subunits now separate

Polysome A cluster of many ribosomes translating one mRNA transcript Transcript threads through the multiple ribosomes like the thread of bead necklace Allows rapid synthesis of proteins

What Happens to the New Polypeptides? Some just enter the cytoplasm Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified

Gene Mutations Base-Pair Substitutions Silent mutation: no amino acid change; redundancy in code Missense: change amino acid Nonsense: change to stop codon

Gene Mutations B. Frameshift Mutations: Insertion: Extra base added into gene region Deletion: Base removed from gene region Both shift the reading frame Result in many wrong amino acids

Chromosome Mutations Transposons: DNA segments that move spontaneously about the genome (When they insert into a gene region, they usually inactivate that gene) 1.) Inversion 2.) Translocation 3.) Deletion 4.) Duplication

Prokaryote vs. Eukaryote genes Prokaryotes DNA in cytoplasm circular chromosome naked DNA no introns Transcription & translation are simultaneous in bacteria Eukaryotes DNA in nucleus linear chromosomes DNA wound on histone proteins RNA polymerase requires transcription factor introns vs. exons Walter Gilbert hypothesis: Maybe exons are functional units and introns make it easier for them to recombine, so as to produce new proteins with new properties through new combinations of domains. Introns give a large area for cutting genes and joining together the pieces without damaging the coding region of the gene…. patching genes together does not have to be so precise. intron = noncoding (inbetween) sequence eukaryotic DNA exon = coding (expressed) sequence