Ch 17 part 1 Protein Synthesis (Translation)

How does an mRNA molecule produce a protein?

Things you will need to know in order to understand translation: mRNA codon chart Redundancy of the code Structure of tRNA Ribosome structure

mRNA Codons:  codon = group of 3 mRNA nucleotides  **1 codon codes for 1 amino acid  During translation, proteins are synthesized according to the genetic message of sequential codons along the mRNA

Figure 17.5 shows the “dictionary” for the mRNA codons and their corresponding amino acids

Redundant but not Ambiguous Codons GAA and GAG both specify glutamic acid (redundancy) Neither of them specifies any other amino acid (no ambiguity) *FYI: Redundancy can also be called “degeneracy”

 Transfer RNA (tRNA) is the interpreter between the 2 forms of information: base sequence in mRNA and amino acid sequence in polypeptides tRNA Structure

Amino acid location (CCA) 3 exposed bases -an enzyme links a specific amino acid from the cytosol/cytoplasm to each tRNA molecule on the 3’ end (CCA) using ATP for energy

LE 17-14b Hydrogen bonds Amino acid attachment site Anticodon Symbol used in this book Anticodon Three-dimensional structure

LE Amino acid Aminoacyl-tRNA synthetase (enzyme) Pyrophosphate Phosphates tRNA AMP Aminoacyl tRNA (an “activated amino acid”)

 Molecules of tRNA are specific for only 1 amino acid -one end of tRNA attaches to a specific amino acid -the other end Hydrogen bonds to mRNA codon by base pairing (anticodon = a sequence of 3 bases on tRNA) Amino acid

Ribosome Structure large ribosomal subunit (50s) small ribosomal subunit (30s) –each subunit = ribosome/protein complex an mRNA binding site 3 tRNA binding sites EPA

LE 17-16b P site (Peptidyl-tRNA binding site) E site (Exit site) mRNA binding site A site (Aminoacyl- tRNA binding site) Large subunit Small subunit Schematic model showing binding sites EPA

LE 17-16a tRNA molecules Exit tunnel Growing polypeptide Large subunit mRNA 3 Computer model of functioning ribosome Small subunit 5 E P A

Processes of Translation Translation includes 4 processes: initiation, elongation, translocation, and termination

Initiation of Translation mRNA binds to small ribosomal subunit Initiator tRNA brings1 st amino acid, Methionine (Met) Large ribosomal subunit binds –Met is in the “P” site –“A” site is available for the next tRNA

LE Met GTP Initiator tRNA mRNA 5 3 mRNA binding site Small ribosomal subunit Start codon P site 5 3 Translation initiation complex E A Large ribosomal subunit GDP Met

Elongation The next tRNA anticodon complementary base pairs with the mRNA codon in the “A” site aligning the appropriate amino acid next to “Met.” Ribosome forms a peptide bond between “Met” and the 2 nd amino acid and it passes the elongating polypetide chain to the tRNA in the “A” site

Translocation Ribosome moves (translocates) the tRNA in the “A” site to the “P” site The empty tRNA moves to the “E” site where it is released mRNA moves along/through ribosome to expose the next mRNA codon to the “A” site Elongation continues

LE Ribosome ready for next aminoacyl tRNA mRNA 5 Amino end of polypeptide E P site A site GDP E PA GTP GDP E PA E PA

Termination When a “STOP” codon is reached, there isn’t a complementary tRNA A “release factor” binds to the “A” site –Causes the addition of a water molecule to the polypeptide which hydrolyzes the completed polypeptide from the tRNA Ribsomal subunits dissociate

LE Release factor Stop codon (UAG, UAA, or UGA) Free polypeptide 3 When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA. The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome. The two ribosomal subunits and the other components of the assembly dissociate.

Polyribsomes Typically, a single mRNA is used to make many copies of a protein simultaneously Also called “polysomes”

LE 17-20a Incoming ribosomal subunits Growing polypeptides Completed polypeptide Start of mRNA (5 end) End of mRNA (3 end) Polyribosome An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes.

LE Ribosomes mRNA Signal peptide Signal- recognition particle (SRP) SRP receptor protein CYTOSOL ER LUMEN Translocation complex Signal peptide removed ER membrane Protein SRP: Signal-Recognition Particle- if a protein is to be secreted from the cell (or used in a lysosome), an SRP will attach to the growing polypeptide chain and “drag” it and the ribosome to the ER (aka rough ER)

What is the DNA strand called that gets transcribed by RNA? A.Antisense B.Sense C.Leading D.lagging

Which RNA are involved in Translation? A.tRNA B.rRNA C.mRNA D.All of the above E.A and B

Where does tRNA pick up new amino acids? A.Nucleus B.Rough ER C.Golgi apparatus D.cytosol

When the 1st tRNA is finished, what is the name of the site where it will leave? A.A site B.P site C.E site D.Methionine site

What happens at the promoter region of DNA in Transcription? A.RNA polymerase reaches a specific sequence of DNA that causes transcription to end B.RNA polymerase promotes the synthesis of an RNA primer C.DNA polymerase reaches a specific sequence of DNA that causes transcription to end D.RNA polymerase binds to a specific DNA sequence to begin transcription

What is the name of the covalent bond created between amino acids? A.Glycosidic linkage B.Ester C.Peptide D.phosphodiester

What are the differences between introns & exons? A Introns are translated but exons are not Introns are excised from mRNA but exons remain part of it B Exons are translated but introns are not Exons are excised from mRNA but introns remain part of it C Introns are translated but exons are not Exons are excised from mRNA but introns remain part of it D Exons are translated but introns are not Introns are excised from mRNA but exons remain part of it