Chapter 13_Translation Of mRNA Flashcards
One Gene One Enzyme Hypothesis
A single gene controls the synthesis of a single enzyme.
Polypeptide
A linear sequence of amino acids.
Protein
Composed of polypeptides.
Translation
Involves the translation of a nucleotide sequence (mRNA) into an amino acid sequence (polypeptide).
Sense Codon
The sequence of three bases in most codons specifies a particular amino acid.
Start and Stop Codons
- Start: AUG (Methionine)
- Stop: UAA, UAG, and UGA
Anticodons
3 nucleotide sequence that is complementary to codons in mRNA.
The genetic code is degenerate, which means…
…more than one codon can specify the same amino acid.
Reading Frame
A sequence of codons determined by reading bases in groups of three, beginning with the start codon.
Peptide Bond
Formed between the carboxyl group in the last amino acid of the polypeptide chain and the amino group in the amino acid being added.
N-Terminus
(Amino Terminal End) The first amino acid is located here. The 5’ end.
C-Terminus
(Carboxyl Terminal End) The last amino acid is located here. The 3’ end.
Side Chain
(R group) Each amino acid contains a unique side chain.
Nonpolar, Aliphatic Amino Acids
Glycine, Alanine, Valine, Leucine, Isoleucine, Proline, Cysteine, Methionine
Aromatic Amino Acids
Phenylalanine, Tyrosine, Tryptophan
Polar Neutral Amino Acids
Serine, Threonine, Asparagine, Glutamine
Polar Acidic Amino Acids
Aspartic Acid, Glutamic Acid
Polar Basic Amino Acids
Histidine, Lysine, Arginine
Nonstandard Amino Acids
- Selenocysteine
- Pyrrolysine
- Occasionally incorporated into polypeptides by the use of stop codons.
When does the folding process begin?
While the polypeptide is still being translated.
The progression from the primary structure of a polypeptide the the 3D structure of a protein is dictated by…
…the amino acid sequence within the polypeptide. In particular, the chemical properties of the amino acid side chains play a central role in determining the folding pattern of a protein.
Chaperones
Proteins that bind to polypeptides and facilitate their proper folding.
Secondary Structure
- The first stage of folding involves the formation of a regular, repeating shape (the secondary structure)
- Comes in the form alpha helix and the beta sheet.
- A single polypeptide may have some regions that fold into an alpha helix and other regions that fold into a beta sheet.
Tertiary Structure
Short regions of secondary structure within a polypeptide are folded relative to each other. This is a thermodynamically favorable process.
Quaternary Structure
Proteins composed of two or more polypeptides that associate with each other to make a functional protein.
Adaptor Hypthesis
The position of an amino acid within a polypeptide chain is determined by the binding between the mRNA and an adaptor molecule carrying a specific amino acid.
Aminoacyl-tRNA Synthetases
Catalyze the attachment of amino acids to tRNA molecules. Cells produce 20 different of these, 1 for each of the 20 distinct amino acids.
Charged tRNA
(Aminoacyl-tRNA) The amino acid is attached to the 3’ end of the tRNA by a covalent bond.
Catalytic function of aminoacyl-tRNA synthetase
- An amino acid and ATP bind to the enzyme. AMP is covalently bound to the amino acid, and pyrophosphate is released.
- The correct tRNA binds to the enzyme. The amino acid becomes covalently attached to the 3’ end of the tRNA. AMP is released
- The “charged” tRNA is released.
Isoacceptor tRNA
When two or more tRNAs that differ at the wobble base are able to recognize the same codon.
Ribosome
The macromolecular arena where translation takes place.
Bacterial and Eukaryotic Ribosomes are assembled from…
…rRNA and proteins
Small and Large Subunits
- Bacteria: 30s (Small) + 50s (Large) = 70s
- Eukaryoitc: 40s (Small) + 60s (Large) = 80s
Nucleolus
Where the assembly of the rRNAs and ribosomal proteins to make the 40s and 60s subunits.
Ribosomes contain discrete sites where tRNAs bind and the polypeptide is synthesized.
- The two sites for tRNA binding to the robosome are peptidyl and aminoacyl site.
- A third site is called the exit site.
Stages of Translation
- Initiation: The ribosomal subunits, mRNA, and the first tRNA assemble to form a complex.
- Elongation: The ribosome slides along the mRNA in the 5’ to 3’ direction, moving over the codons.
- Termination: A stop codon is reached. Disassembly occurs, and the newly made polypeptide is released.
Initiator tRNA
Recognizes the start codon in the mRNA.
Describe initiation
- IF1 and IF3 bind to the 30S subunit.
- IF2, which uses GTP, promotes the binding of the initiator tRNA to the start codon in the P site.
- IF1 and IF3 are released.
- IF2 hydrolyzes its GTP and is released.
- The 50S subunit associates.
Shine-Dalgarno Sequence
A nine nucleotide sequence within BACTERIAL mRNA. Facilitates the binding of mRNA to the ribosome.
Describe Elongation
- A charged tRNA binds to the A site. EF-Tu facilitates tRNA binding and hydrolyzes GTP.
- Peptidyltransferase, which is a component of the 50S subunit, catalyzes peptide bond formation between the polypeptide chain and the amino acid in the A site. The polypeptide is transferred to the A site.
- The ribosome translocates 1 codon to the right. This translocation is promoted by EF-G, which hydrolyzes GTP.
- An uncharged tRNA is released from the E site.
- This process is repeated, again and again, until a stop codon is reached.
Decoding function
16S rRNA can detect when an incorrect tRNA is bound at the A site and will prevent elongation until the mispaired tRNA is released from the A site. It is important in maintaining high fidelity of mRNA translation.
Peptidyl transfer
The polypeptide is removed from teh tRNA in the P site and transferred to the amino acid at the A site.
Release Factors
Molecular mimics that resemble the structure of tRNAs.
Describe Termination
- tRNA in P site carries completed polypeptide.
- A release factor (RF) binds to the A site.
- The polypeptide is cleaved from the tRNA in the P site. The tRNA is then released.
- The ribosomal subunits mRNA, and release factor dissociate.
