Protein Translation From Gene to Protein Honors Biology Ms. Kim

Central Dogma DNA  RNA  Protein Protein synthesis consists of 2 mains parts: – Transcription – DNA is copied in the nucleus, the result is the formation of mRNA – Translation – mRNA travels to the cytoplasm and attaches to rRNA and with the help of tRNA a protein is made

Translation mRNA  polypeptide (protein) Converting mRNA code (from DNA) to protein! – Protein = polypeptide How is mRNA used to make protein? The genetic code is read 3 letters (nucleotides) at a time 3 nucleotides= codon One codon codes for an amino acid

tRNA A cell translates mRNA message into protein with help from transfer RNA (tRNA) – Type of RNA – ~80 nucleotides – “t” shape – Carries amino acids – Matches codons to anticodons

Transfer RNA Transfer RNA (tRNA) Reads the mRNA code Carries the amino acid that will be added to the growing protein chain Notice the 3 bases at the bottom of the tRNA make up the anticodon The anticodon base pairs with the mRNA codon to make sure that each AA is delivered to the correct place on mRNA At the top of tRNA is an amino acid

tRNA Molecules of tRNA are not all identical – Each carries a specific amino acid – Each has an specific anticodon on the other end

Ribosomes Help bind tRNA anticodons with mRNA codons during translation Found on ROUGH Endoplasmic Reticulum RER (“ bound ”) or in cytoplasm (“ free ”) An t icodon ( t RNA) = codon (mRNA)

tRNA (with anticodons) mRNA (with codons)

Ribosomes There are 2 ribosomal subunits Constructed of proteins and ribosomal RNA or rRNA

Ribosomal RNA Ribosomal RNA (rRNA) Ribosome attaches to mRNA Contains the enzymes necessary for protein synthesis E P A Ribosome: 3 tRNA binding spots: E – exit P – current amino acid A – on deck amino acid Has a large & small subunit

Binding Sites in a Ribosome The ribosome has three binding sites for tRNA – The P site – The A site – The E site EPA P site (Peptide Bond-tRNA binding site) E site (Exit site) mRNA binding site A site (Amino acid- tRNA binding site) Large subunit Small subunit

\ Amino acid Growing polypeptide Next amino acid to be added to polypeptide chain tRNA mRNA Codons 3 5

What is Translation? Process of building a protein chain by reading the mRNA code Occurs in ribosomes Uses codons

Building a Polypeptide Translation can be divided into 3 phases – Initiation – Elongation – Termination

Initiation of Translation initiation stage: 1.mRNA binds to small subunit of ribosome 2.tRNA bearing 1 st amino acid (“start”) called MET (codon AUG) comes 3.tRNA binds to start codon (AUG)  anticodon (UAC) 4.Large ribosomal subunit binds  working ribosome 5.Initiator tRNA fits into the P site and holds the growing protein 6.The A site is empty and ready for the next A.A 1.2 subunits of a ribosome come together

 Every codon has a complementary anticodon which pairs with it  AUG of mRNA pairs with UAC of tRNA

Translation: Initiation mRNA is organized into a reading frame The reading frame is made of codons that specify specific amino acids – Every 3 bases (triplet) make up a codon – Codons are arranged following the initiation codon AUG

Large ribosomal subunit Initiator tRNA mRNA mRNA binding site Small ribosomal subunit Translation initiation complex P site GDP GTP Start codon Met U A C A U G E A Figure mRNA codons are what create the amino acids (aka – use the chart) NOT tRNA

Elongation Amino acids are added one by one to the preceding amino acid Peptide bonds are formed Amino acid

Elongation After initiation, A.A. are added to the first A.A… 3 step process: 1) Incoming tRNA anticodon pairs with mRNA codon 2) A new polypeptide bond is formed 3) tRNA shift (P leaves, A moves to the P spot)

Translation: Elongation A chain of amino acids is called a polypeptide chain This chain continues to form as long as tRNA is added to the mRNA strand

Termination of Translation When the ribosome reaches a stop codon on the mRNA – There are 3 stop codons UAA, UAG, and UGA Release factor (protein) binds to stop codon in A site (NOT tRNA)  polypeptide (protein) released At this point the amino acid or polypeptide will fold into a specific structure forming a working protein

Termination of Translation Release factor Free polypeptide Stop codon (UAG, UAA, or UGA)

Figure TRANSCRIPTION TRANSLATION DNA mRNA Ribosome Polypeptide Amino acids tRNA with amino acid attached Ribosome tRNA Anticodon mRNA Trp Phe Gly A G C A AA C C G U G GU U U GG C Codons 5 3 “EMPTY” tRNA

Translation (Protein Synthesis) Animation online.com/objects/index_tj.asp?objID=AP online.com/objects/index_tj.asp?objID=AP hill.com/sites/ /student_view0/ch apter15/animations.html# hill.com/sites/ /student_view0/ch apter15/animations.html# ogyI/course%20files/multimedia/lesson13/les sonp.html ogyI/course%20files/multimedia/lesson13/les sonp.html

Summary of Protein Synthesis Replication → DNA to DNA, occurs in nucleus Transcription → DNA to RNA, occurs in nucleus Translation → RNA to Protein, occurs in ribosome DNA  RNA  PROTEIN!

Example DNA sequence (template): 3’ TACGCTAGTACGATG 5’ mRNA sequence: 5’ AUGCGAUCAUGCUAC 3’ Codons: AUG CGA UCA UGC UAC Amino Acids: met (start)-arg-ser-cys-tyr Codons determine which of the 20 possible amino acids is needed