1. In contrast to prokaryotic mRNAs, eukaryotic mRNAs are extensively modified. Modifications include
* Addition of a 5'-5' cap of a methyl guanosine to protect from degradation * Addition of a poly-A tail at the 3' end under the control of the sequence AAUAAA near the end of the mRNA (also to protect from degradation) * Editing - modification of bases chemically, such as was described in class for the Apo-B proteins * Splicing - removal of introns between exons.
2. Splicing is a modification to eukaryotic mRNAs that occurs in the middle of the mRNA. Splicing also occurs to tRNAs and rRNAs in eukaryotes.
3. Splicing involves removal of internal sequences from RNA followed by joining of ends. The removed sequences are called introns. The segments that make it into the final RNA are called exons.
4. The only sequences common to all spliced RNAs are a GU sequence at the 5' end of the intron and an AG at the 3' end of the intron. A third sequence - an A residue surrounded by pyrimidines also is common.
5. Protein/RNA complexes called snRNPs mediate the splicing process in higher eukaryotes. snRNPs contain small nuclear RNAs (snRNAs) and proteins.
6. In splicing, the hydroxyl of the A residue attacks the phosphate of the phosphodiester bond at the 5' end of the intron, creating a 5'-2' bond (part of the lariat structure). Attack by the released 3' end of the exon on the 3' end of the intron joins the two exon ends and releases the intron as a lariat.
7. Exon shuffling (occurs in splicing in different tissues) allows cells to make many versions of protein from a single sequence. This is important in immunology and in fine-tuning cellular needs that are tissue specific.
8. Lower eukaryotes are able to excise introns by an autocatalytic mechanism. At least one prokaryotic gene is spliced autocatalytically.
9. In splicing, the U1 snRNA forms base pairing with the 5' end of the intron sequence.
10. In splicing, the U2 snRNA froms base pairs with the pyrimidine-rich region in the intron and with the snRNA of U6. Pairing with the intron forces outwards the 'A' residue that attacks the phosphate, as noted in class.
1. Translation is performed by ribosomes on mRNA and occurs in the 5' to 3' direction. The rate of translation in bacteria is about the same as the rate of transcription (45-50 bases or 15-17 amino acids per second). The 5' end of the coding region corresponds to the amino end of the protein. The 3' terminus of the coding region corresponds to the carboxyl end of the protein.
2. Translation is performed by ribosomes on mRNA and occurs in the 5' to 3' direction. The rate of translation in bacteria is about the same as the rate of transcription (45-60 bases or 15-20 amino acids per second). The 5' end of the coding region corresponds to the amino end of the protein. The 3' terminus of the coding region corresponds to the carboxyl end of the protein.
3. As polypeptides are being synthesized, the previously synthesized chain is attached to the free amine of the incoming (new) amino acid and the entire complex is, as a result, attached to the 'new' tRNA. Thus, polypeptides are synthesized in the amino to carboxyl terminus.
4. Transcription and translation are coupled together in bacteria, but not in eukaryotes.
5. Translational accuracy is about one error per thousand to ten thousand amino acids. Greater accuracy would slow translation down, so a balance is struck between the need for accuracy and the need to synthesize proteins reasonably rapidly.
This course in general biochemistry is intended to integrate information about metabolic pathways with respiration (respiratory control) and initiate the student into a microscopic world where blueprints are made of deoxyribonucleic acids, factories operate using enzymes, and the exchange rate is in ATPs rather than Yens or Euros. Beyond explaining terms, and iterating reactions and metabolic pathways, this course strives to establish that the same principles that govern the behavior of the world around us also govern the transactions inside this microscopic world of the living cell. And by studying and applying these principles, we begin to understand cellular and bodily processes that include sensory mechanisms.
Topics include: 1. Lipids, Membranes and Transport 2. Electron Transport, Oxidative Phosphorylation and Mitochondrial 3. Transport Systems 3. Lipid Metabolism 4. Nucleotide Metabolism 5. DNA Replication 6. Transcription 7. Translation