Class Notes (837,490)
Canada (510,274)
Biology (2,229)
BIO130H1 (434)
Lecture 9

Bio130- Lecture 9.docx

8 Pages
Unlock Document


Bio130- Lecture 9 (pg. 366-390) From RNA to Protein: Most genes in cell produce mRNA molecules that serve as intermediaries on the pathway to proteins. A mRNA Sequence is decoded in sets of three Nucleotides: Once mRNA has been produced by transcription and processing, the information present in its nucleotide is used to synthesize a protein. Transcription information transfer. DNA RNA Translation The conversion of information in RNA to proteins. The nucleotide sequence of a gene, through the intermediary of mRNA, is translated into the amino acid sequence of a protein by rules that are known as the genetic code. The sequence of the nucleotides in the mRNA molecule is read in consecutive groups of three. RNA is a linear polymer of four different nucleotides, therefore there are 64 different possible combinations of three nucleotides. Since there are only 20 amino acids, the code is redundant. Each group of three nucleotides is called a codon. Used in all present-day organisms There are three possible reading frames: In the process of translating a nucleotide sequence into an amino acid sequence, the sequence of nucleotides in an mRNA molecule is read in the 5 to 3 direction, and in consecutive sets of three nucleotides. In principle, therefore, the same RNA sequence can specify three completely different amino acids, depending on the reading frame. In reality however only one of these reading frames contains the actual message. tRNA Molecules Match Amino Acid to Codons in mRNA: The translation of mRNA into protein depends on adaptor molecules that can recognize and bind both to the codon and the amino acid. These adaptors cosist of a set of small RNA molecules known as transfer RNAs (tRNAs), whicha re about 80 nucleotides in length. Two regions of unpaired nucleotides situated at either end of the L-shaped molecule are crucial to the function of tRNA in protein synthesis. One of these regions forms the anitcodon. Anticodon: A set of three nucleotides that pairs with the complementary codon in an mRNA molecule. The other region is a short single stranded region at the 3 end of the molecule; this is the site where the amino acid that matches the codon is attached to the tRNA. Since the genetic code is redundant; there are more than one tRNA for many of the amino acids, or some tRNA molecules can base-pair with more than one tRNA and some tRNA are constructed so that they require accurate base-pairing only at the first two positions of the codon and can tolerate a mismatch at the third position. tRNAs Are Covalently Modified Before They Exit from the Nucleus: Both bacterial and eukaryotic tRNAs are synthesized by RNA polymerase III. Both bacterial and eukaryotic tRNAs are typically synthesized as larger precursor tRNAs, which are then trimmed to produce the mature tRNA. Some tRNA precursors contain introns that must be spliced out. tRNA splicing uses a cut-and-paste mechanism that is catalyzed by proteins. Specific Enzymes Couple Each Amino Acid to its Appropriate tRNA Molecule: Recognition and attachment of the correct amino acid depends on enzymes called aminoacyl- tRNA synthetases. Which covalently couple each amino acid to its appropriate set of tRNA molecules. Most cells have different synthetase enzymes for each amino acid. One of the many reactions coupled to the energy-releasing hydrolysis of ATP, and it produces high energy bond between the tRNA and the amino acid. Editing by tRNA Synthetases Ensures Accuracy: The synthetase must first select the correct amino acid, and most synthetases so so by a two- step mechanism. First, the correct amino acid has the highest affinity for the active site pocket of its synthetase and is therefore favored over the other 19. A second discrimination step occurs after the amino acid has been covalently linked to AMP Amino acid activationAn amino acid is activated for protein synthesis by an aminoacyl-tRNA synthetase enzyme in two steps. The energy of ATP hydrolysis is used to attach each amino acid to its tRNA molecule in a high-energy linkage. The amino acid is first activated through the linkage of its carboxyl group directly to an AMP moiety, forming an adenylated amino acid; the linkage of the AMP, normally an unfavorable reaction, is driven by the hydrolysis of the ATP molecule that donates the AMP. Without leaving the synthetase enzyme, the AMP linked carboxyl group on the amino acid is then transferred to a hydroxyl group on the sugar at the 3 end of the tRNA molecule. This transfer joins the amino acid by an activated ester linkage to the tRNA and forms the final aminoacyl-tRNA molecule. Hydrolytic editing: tRNA synthetases remove their own coupling errors through hydrolytic editing of incorrectly attached amino acids. The correct amino acid is rejected by the editing site. The error-correction process performed by DNA polymerase shows some similarities; however, it differs in so far as the removal process depends strongly on a mispairing with the template. This hydrolytic editing, which is analogous to the exonucleolytic proofreading by DNA polymerases, raises the overall accuracy of tRNA charging to approximately one mistake in 40,000 couplings. Amino acids are added to the C-terminal end of a growing polypeptide chain: The fundamental reaction of protein synthesis is the formation of a peptide bond between the carboxyl group at the end of a growing polypeptide chain and a free amino group on an incoming amino acid. A protein is synthesized step-wise from its N-terminal end to its C-terminal end. Throughout the entire process the growing carboxyl end of the poly peptide chain remains activated by its covalent attachment to a tRNA molecule. Each addition disrupts this high-energy covalent linkage, but immediately replaces it with an identical linkage on the most recently added amino acid. The RNA Message is decoded in Ribosomes: The synthesis in proteins is guided by information carried by mRNA in molecules. TO maintain the correct reading frame and to ensure accuracy, protein synthesis is performed in the RIBOSOME, a complex catalytic machine made from more than 50 different proteins, and several RNA molecules, the RIBOSOMAL RNAs (rRNAs). Eukaryotic ribosome subunits are assembled at the nucleolus, when newly transcribed and modified rRNAs associate with ribosomal proteins, which have been transported into the nucleus after their synthesis in the cytoplasm. The two ribosomal subunits are then exported to the cytoplasm, where they join together to synthesize protein. Both Eukaryotic and prokaryotic ribosomes have similar designs and functions, and are composed of one large and one small subunit that if together to form a complete ribosome with a mass of several million Daltons. When not actively s
More Less

Related notes for BIO130H1

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.