1. Outline
Review the central principle in the flow of genetic information
Revisit the idea that building a protein is like baking a cake
Focus on translation: the “who, where, and how” of protein building
Apply what we’ve learned

2. Review
DNA stores instructions for the synthesis of proteins using RNA as an intermediate
Proteins = polymeric chains of amino acids; vital roles in cells include speeding up chemical reactions, transport, communication, structural support, movement & defense
Flow of genetic
information:
DNA
transcription
RNA
translation
Protein
Deceptively simple, but let’s recap the finer points

3. Last time we discussed…
Protein synthesis is similar to cake baking
Both require:
instructions (recipes = genes)
disposable copy (recipe card = mRNA)
interpreter (chef = tRNA)
Equipment to associate ingredients (oven = ribosome (rRNA))

4. Cake Baking Protein building
Start with a cookbook
Start with DNA
(which contains)
(which contains)
Collection of recipes
Collection of genes
(one is chosen to make)
(one is chosen to make)
Recipe card
(a disposable copy)
mRNA
(a disposable copy)
(which is read by)
(which is read by)
Chef(s)
tRNAs
(interpret(s) words
on card to)
(interpret codons
on mRNA to)
Assemble(s) ingredients
Assemble amino acids
(baked together by)
(joined/bonded by)
Oven
Ribosome (rRNA)
(yielding)
(yielding)
Cake
Protein

5. Transcription vs. Translation
The first half (transcription) of each process is dedicated to the copying of information
The second half (translation) is dedicated to the deciphering of coded instructions and synthesis of final product
It is the latter on which we now will focus

6. Translation
Players: mRNA (recipe card copy), tRNAs (chefs)*, amino acids (ingredients), and ribosome (oven)
Location: cytoplasm
Occurs in Three Steps:
1) Initiation  2) Elongation  3) Termination
* - tRNAs bring in correct amino acids by “reading” triplet clusters of ribonucleotide bases on mRNA molecule, called codons, through base-pairing with tRNA’s anticodon region
mRNA

7. Initiation Small ribosomal subunit binds to mRNA
Initiator tRNA binds to start codon
Large ribosomal subunit binds
Initiation complex is complete

8. Elongation Codon recognition (in “A” site) Peptide bond formation
Translocation (tRNA in “A” is now in “P” site)
Process repeats and polypeptide chain grows

9. Termination Stop codon encountered (in “A” site)
Release factor protein binds
Completed polypeptide is freed from last tRNA
Ribosomal complex falls apart

10. Cracking the “nucleotide code”
Purpose: to know what three-letter “words” (codons) code for particular amino acids
When: By 1966 scientists deciphered the 64 RNA codons for all 20 amino acid building blocks
Result: a (nearly universal) cipher (pictured below)
Importance: allows us to predict a resulting polypeptide chain’s amino acid sequence from any gene’s nucleotide sequence

11. Practicing What We’ve Learned
Let’s find the amino acid sequence (in 3-letter abbreviation) of the polypeptide product that would result from the following mRNA sequence and review translation:
5'-GAGGUAUGUUGGACCCCUGACAUG -3'
Met
Leu
Asp
Pro
Stop

12. Practicing Independently
Using the chart provided, give the amino acid sequence (in 3-letter abbreviation) of the polypeptide product that would result from the following mRNA sequence; underline the start codon and circle the stop codon:
5'-GAGGUAUGAAUGUAUGGUCACAUGAGUUAUAGCAA -3'

13. Objectives Achieved
Know DNA directs synthesis of proteins using RNA as an intermediate
Differentiate between the steps involved in transcription vs. translation
Identify the players involved in the process of translation, where this process take place, and the steps of protein synthesis
Predict a protein’s a.a. sequence using the genetic code