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What will you learn? Protein Synthesis PLO B7 B8 It is expected that students will… demonstrate an understanding of the process of protein synthesis explain how mutations in DNA affect protein synthesis What is the connection between DNA and protein? • Remember: DNA can’t leave the nucleus but protein synthesis occurs in the cytoplasm … How? • protein message must be carried from the nucleus cytoplasm ribosomes RNA Structure: •polymer of nucleotides •Ribose sugar unit •single stranded •linear (not helical) •A, C, G, U - uses uracil (U) rather than thymine (T) p. 510 • made in nucleus, but functions outside of the nucleus Function: • aids in protein synthesis • uses DNA strand as the template to make its strand using complimentary base pairing 3 Types of RNA all help in the protein synthesis • messenger RNA (mRNA) – carries transcribed message from DNA to ribosome (nucleus to cytoplasm) • Transfer RNA (tRNA) – carries the amino acid that will base pair with mRNA to make polypeptide • ribosomal RNA (rRNA) – makes up the ribosome (along with proteins) Overview of gene expression Two Steps of Protein Synthesis: Transcription & Translation Step 1:Transcription • DNA code (gene) has instructions to make a particular protein • Instructions are copied into mRNA in the nucleus DNA mRNA A U T A C G G C Steps of Transcription A. Helicase opens the section of DNA (the gene) – Begins at INITIATION site – Continues until TERMINATION site B. complementary RNA base pairs attach to form the mRNA strand (RNA polymerase forms the RNA sugar-phosphate backbone and checks for mistakes) C. complementary mRNA detaches & leaves the nucleus through the nuclear pore and goes to the cytoplasm Transcription Animation • http://www.johnkyrk.com/DNAtranscription.h tml Step 2: Translation The mRNA code is made up of groups of three nucleotide bases known as codons. Each codon codes for a specific amino acid. Eg. AGC = Serine (see mRNA codon table handout) Eg. UGC = Cysteine CAU? CUU? GGC? AUG? UGA? AAA? How many different aa’s are there? mRNA Codons CODONS – mRNA base triplets Translation (in cytoplasm only) • Occurs at ribosome (a large and a small subunit made up of rRNA and proteins) – rRNA is made in the nucleolus • tRNA is found free-floating in the cytoplasm tRNA structure tRNA structure: » tRNA molecule is a small piece of RNA that has a specific aa attached to one end » at the other end, an anticodon (3 bases that will complement mRNA codon) Please note that there is more than one codon for each amino acid: mRNA codons: U C U G C C tRNA anti-codons: A G A C G G SERINE ALANINE Why do you think that aa is? 3 STEPS in Translation A. Initiation (means “start”) – – ribosomal subunits come together and mRNA attaches an INTIATOR (start) codon (always AUG) turns on the process B. Elongation (what is getting bigger?) – tRNA anticodon (with specific aa) matches up with the mRNA codon – Each tRNA leaves to find another aa as mRNA over one codon & another tRNA brings the next aa – aa’s continually peptide bond building a polypeptide C. Termination – When ribosomal unit reaches any ‘Terminator’ codon, the message is to STOP translation • There are 3 stop codons (UAA, UAG, UGA) – None of these have matching tRNA anticodon – Ribosome subunits split in 2 again What does translation look like? still translation but not a step Polyribosome – often several ribosomes work together on one protein • Each ribosome makes a section of the protein • This is done for very large proteins. This enables these large proteins to be made faster. • Also called a polysome Polyribosome Central Dogma of Biology Replication DNA Translation RNA Transcription Nucleus Enzymes Protein Structural Components of the Cell Cytoplasm (ribosomes)