FROM GENE TO PROTEIN. Chapter 17. Proteins make the Organism. Overview of the Gene to Protein Concept. The Triplet Code. The Dictionary of The Genetic Code. Transcription. Genes have nucleotide segments upstream of them called promoters
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Proteins make the Organism
An enzyme that adds amino acids to the 3’ end of the free tRNA
E, P and A sites
Each ribosome has a binding site for mRNA and three binding sites for tRNA molecules.The P site holds the tRNA carrying the growing polypeptide chain.The A site carries the tRNA with the next amino acid.Discharged tRNAs leave the ribosome at the E site
When the stop codon enters the ribosome, a protein called a release factor enters the A site and hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. This frees the polypeptide and breaks up the ribosomal subunits.
There are 64 possible codons that can be produced with 4 bases ATGC read in sets of 3
43 = 64
61 of these 64 codons code for amino acids
3 of the 64 codons are called STOP codons and terminate the translation process. The STOP codons do not code for amino acids.
Some polypeptides have an ER signal sequence. The signal sequence is recognized by a Recognition Particle, or SRP. This is then bound to a receptor. This complex guides the protein through a channel like region. It also consists of a docking site for the ribosome.
A single base, say an A, becomes replaced by another. Single base substitutions are also called point mutations.
EXAMPLE: sickle-cell disease The replacement of A by T at the 17th nucleotide of the gene for the beta chain of hemoglobin changes the codon GAG (for glutamic acid) to GTG (which encodes valine). Thus the 6th amino acid in the chain becomes valine instead of glutamic acid.
With a nonsense mutation, the new nucleotide changes a codon that specified an amino acid to one of the STOP codons (TAA, TAG, or TGA). Therefore, translation of the messenger RNA transcribed from this mutant gene will stop prematurely. The earlier in the gene that this occurs, the more truncated the protein product and the more likely that it will be unable to function.
Most amino acids are encoded by several different codons. For example, if the third base in the TCT codon for serine is changed to any one of the other three bases, serine will still be encoded. Such mutations are said to be silent because they cause no change in their product and cannot be detected without sequencing the gene (or its mRNA).
Extra base pairs may be added (insertions) or removed (deletions) from the DNA of a gene. This usually causes a shift in the reading frame and changes the amino acid sequence of the protein from that point onward.
Frameshifts often create new STOP codons and thus generate nonsense mutations. Perhaps that is just as well as the protein would probably be too garbled anyway to be useful to the cell.
Wobble Rules: Read as 5' position in anticodon pairs with 3' position in codon:
G pairs with C or U;
C pairs with G;
A pairs with U;
U pairs with A or G;
I pairs with A, U, or C
Note that the anticodon position in the tRNA can also have the base inosine (I), a purine that is not present in the messenger RNA (codon)