lecture 5 for BGYA01

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University of Toronto Scarborough
Biological Sciences
Clare Hasenkampf

Lecture 5 BGYA01 SEPT. 25, 2007 Now I want to continue our discussion of how DNA directs cellular activity. DNA does not DIRECTLY control cellular activity, instead it acts INDIRECTLY through other molecules. DNA>>>>>>>>>>RNA>>>>>>>>protein. Proteins directly affect cellular activity. transcription translation Last class we looked at how DNA is used as a template to make RNA in a process known as transcription. There are 3 major types of RNA produced by the process of transcription: transfer RNA (tRNA) - transfer RNA genes serve as the template for these ribosomal RNA (rRNA) ribosomal RNA genes serves as the template for these and messenger RNA (mRNA) genes encoding proteins serve as the temple for these. All three types of RNA are needed to make protein, but only mRNA encode the information for the sequence of the amino acids. Now that we have looked at transcription it is time to turn begin to think about how mRNA encodes the information to make protein. TRANSLATION The process of making protein is called translation. We already know that proteins are polymers, and that the monomer units are the amino acids. We already know that there are twenty different amino acids, each different amino acid with its on unique R group, (as illustrated in Table 3.2, page 43). Every different protein has its own shape, size and biological function. The size, shape, and function of each protein is determined by the amino acids that make it up. The precise order of the amino acids in the protein determines the proteins shape and function. How are the correct amino acids brought together for each specific protein? We will discovertoday that it is the order of the bases of mRNA that encodes the information for the order of amino acids of the protein. We have to see how the RNA codes for protein. Lets think a little about math and code. www.notesolution.comHow many different one nucleotide codes do we have if there are only four different bases? A U G C THERE ARE ONLY 4, one ribonucleotide codes. How many different two ribonucleotide codes can we come up with if there are only four different ribonucleotides? AA, AU, AG, AC, UA, UU, UG, UC, GA, GU, GG, GC, CA, CU, CG, CC = 16 How many different three nucleotide codes can we come up with? There are 64 different three ribonucleotide combinations. Clicker Question If the sequence of ribonucletides in a mRNA encodes the information to determine the order of the amino acids in a polypeptide, what is the simplest number of ribonucleotides that could determine the code? a) one ribonucleotide b) two ribonucleotides in a row c) three ribonucleotides in a row. ********** correct answer d) Four ribonucleotides in a row. THE G ENETIC CODE What is the genetic code? It is the order of the bases along the mature messenger RNA. And very importantly the unit of code is a TRIPLET; that is the sequence of three consecutive bases in a mRNA, constitutes one unit of code. The sequence of three consecutive bases in a mRNA molecule, coding for a specific amino acid, is called a codon. Through careful experiments, done by earlier scientists we now know exactly what three bases encode each of the different amino acids. To examine the genetic code lets look at figure 12.6, page 264. As you can see from a careful look at the table, there are 64 different possible triplet combinations of the four bases. Since there are 64 different codons and only 20 different amino acids, This means there is some redundancy in the code . In other words most amino acids are encoded by more than just one codon. This is also sometimes the degeneracy in the genetic code. The term degeneracy here means that more than one different codon is used to specify the same amino acid. So from Table 12.6 we see what the genetic code is. Please notice that most triplets encode an amino acid, but three triplets encode encode stop translation. One important codon does double duty. It encodes start translation and the aminoacid methionine. So there are 64-3= 61 codons that encode an amino acid. www.notesolution.com
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