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Lecture 10

BSC 219 Lecture 10: Genetics Lec 10

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Biological Sciences
BSC 219

THE GENETIC CODE 1) Made of triplets of nucleotides (codons) 2) Degenerate and unambiguous 3) Nonoverlapping and commaless Nearly universal Contains one start codon and three stop codons THE GENETIC CODE The genetic code is degenerate, which means that some amino acids are specified by more than one codon GGG, GGA, GGC, and GGU all code for alanine We have more codons than we need There are 61 different codons that specify for the 20 amino acids There are three stop codons that terminate translation THE GENETIC CODE Since 61 codons for amino acids + 3 stop codons, this suggests up to 61 tRNAs may be required to “read” mRNA But, most organisms do not have more than 45 unique tRNAs Wobble base pairing allows for one tRNA to bind with multiple codons à the third base is flexible THE GENETIC CODE Inosine is a ribonucleoside that is found in tRNAs and is similar to guanine Inosine can pair with either A, U or C THE GENETIC CODE The four wobble base pairs Inosine + Cytosine Inosine + Adenine Inosine + Uracil Guanine + Uracil THE GENETIC CODE tRNA with an anticodon of CCC à pairs with GGG You only need two tRNAs for alanine GGG, GGA, GGC, and GGU all code for alanine and only differ at the third base tRNA with an anticodon of CCI à pairs with GGA, GGC and GGU THE GENETIC CODE The genetic code is unambiguous, which means that each codon specifies only one amino acid (or stop signal) GGG, GGA, GGC, and GGU only code for alanine THE GENETIC CODE The genetic code is read three nucleotides at a time in a continuous, linear manner and is nonoverlapping A sequence contains the following codons The genetic code is nearly universal à most organisms use the same codons for the same amino acids/stops THE GENETIC CODE There is a codon bias à differences in the frequency of occurrence of synonymous codons (codons for the same amino acid) in coding DNA A certain codon is used more often in the genome as compared to other codons for the same amino acid THE GENETIC CODE The genetic code includes start and stop codons The initial amino acid incorporated into almost all proteins is a methionine AUG is the only codon for methionine The nonsense codons (UAG, UAA, and UGA) serve as termination codons or stop codons THE GENETIC CODE Historical names for the stop codons UAG à amber UAA à ochre UGA à umber or opal THE GENETIC CODE A single DNA molecule can be transcribed into multiple mRNAs (isoforms) Though the genetic code doesn’t overlap, genes can THE GENETIC CODE Genes can also be found within other genes Smaller genes can be
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