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BSC 219 (22)
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Lecture 9

BSC 219 Lecture 9: Genetics Lec 9

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Biological Sciences
BSC 219

TRANSLATION A codon is specific for one amino acid The mRNA is read three ribonucleotides at a time Different codons can code for the same amino acid mRNA acts as an intermediate between the DNA and the protein à mRNA is the template for translation Three ribonucleotides make up a codon TRANSLATION Translation requires: 1) Template mRNA 2) Ribosomes 3) tRNA RIBOSOMES RIBOSOME STRUCTURE The ribosome is a large complex of rRNA and proteins à 40 nm diameter Consists of 2 subunits One “large subunit” One “small subunit” Both made of protein and rRNA RIBOSOME STRUCTURE Ribosomes sizes were originally determined by sedimentation rate in a sucrose gradient Svedberg units used for sedimentation rates à s heaviest/largest lightest/smallest RIBOSOME STRUCTURE Prokaryotic ribosomes Prokaryotes have a ribosome that has a total size of 70s The large subunit in prokaryotes is 50s 70s 50s 30s The small subunit in prokaryotes is 30s Eukaryotes have a ribosome that has a total size of 80s The large subunit in eukaryotes is 60s 80s 60s 40s The small subunit in eukaryotes is 40s THE EUKARYOTIC RIBOSOME In eukaryotes, ach subunit has its own rRNA The 60s subunit has a 28S rRNA, a 5S rRNA, and a 5.8S rRNA The 40s subunit has a 18S rRNA Total of 4 rRNAs make up the ribosome RIBOSOMAL DNA In eukaryotes, the rRNAs are encoded by rDNA gene repeats à there are many copies throughout the genome RIBOSOME STRUCTURE In humans The rRNA transcribed from tandem repeats of the rDNA genes on the p arms of acrocentic chromosomes (13, 14, 15, 21, and 22) RIBOSOMAL DNA The 18S, 5.8S, and 28S are transcribed on one transcript from each repeat unit called the 45S pre-rRNA 28 S RNA rDNA gene 1 rDNA gene 2 rDNA gene 3 rDNA gene 4 RIBOSOMAL DNA NTS, nontranscribed spacer à buffer between repeats à doesn’t get transcribed ETS, external transcribed spacer à region outside the rRNA that is transcribed but is not part of the mature rRNA ITS, internal transcribed spacers 1 and 2 à regions between the rRNAs that are transcribed but are not part of the mature rRNA RIBOSOME STRUCTURE In humans The 45S pre-rRNA is enzymatically cleaved into the smaller components RIBOSOME STRUCTURE In humans 5S component is transcribed from repeats on Chromosome 1 The eukaryotic 5S is unique to eukaryotes The eukaryotic 5.8S is homologous or similar to the prokaryotic 5S TRANSFER RNA TRANSFER RNA Transfer RNA (tRNA) 75 to 90 ribonucleotides long (4S) The E. coli tRNA for tyrosine starts as a 126 ribont pre-RNA Processed to a 77 ribont tRNA TRANSFER RNA tRNAs form a “cloverleaf” structure The tRNA is single stranded but has regions that are complementary to each other These regions will bind to each other, forming Intramolecular hydrogen bonds between the bases TRANSFER RNA This creates stem loop or hairpin structures The stems are the paired regions The loops are the unpaired regions at the end TRANSFER RNA The 3’ end is unpaired making an overhang The 3' end of the tRNA has the sequence CCA and a free 3’- OH for bonding to the amino acid The 5' end of the tRNA has a 5'-pG This is where the amino acid will bind to the tRNA TRANSFER RNA At the bottom of the tRNA is the anticodon sequence wobble base It is complementary to the codon in the mRNA The third base of the anticodon is called a wobble base The wobble base can pair with more than one kind of base TRANSFER RNA Often the wobble base is a modified base that allows for pairing with different bases TRANSFER RNA The wobble hypothesis à the third base of the codon is less discriminatory for the amino acid than the other two bases TRANSFER RNA 1) Activates an amino acid
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