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Lecture

LEc 32

7 Pages
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Department
Biology
Course Code
BIOL 1010
Professor
Igor Kovalchuk

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Description
Translation Lec 32  Translation- synthesis of a polypeptide, which occurs under the direction of mRNA  During the process, the linear sequence of bases in mRNA is translated into the linear sequence of amino acids in a polypeptide  During translation proteins are synthesized according to a genetic message of sequential codons along mRNA  Transfer RNA (tRNA)- is an interpreter between the two forms of information- base sequence in mRNA and amino acid sequence in polypeptides tRNA aligns appropriate amino acids to forma a new polypeptide.  To perform this action tRNA must: o Transfer amino acids from the cytoplasm’s amino acid pool to a ribosome o Recognize the correct codons in the mRNA Transfer RNA (tRNA)  Function: o Carries amino acids to the ribosome for assembly into polypeptides  Therefore: translates the mRNA genetic code  Molecules of tRNA are specific for only one particular amino acid  Each type of tRNA associates a distinct mRNA codon with one of the 20 amino acids used to make proteins o One end of tRNA molecule attaches to a specific amino acids o The other end attaches to an MRNA codon by base-pairing with its anticodon  Anticodon- a nucleotide triple in tRNA that base pairs with a complementary codon in mRNA  tRNAs decode the genetic message, codon by codon The structure of tRNA  Mechanism o Complementary and antiparallel nucleotides of the anticodon on the tRNA molecule pairs with the mRNA codon o Determines which amino acid is added by the ribosome to the growing polypeptide  Structure o Single strand of RNA, 80bp long o Folded into cloverleaf configuration, driven by complementary base pairing o Anticodon loop o Amino acid-binding site (3’-end) tRNA  In eukaryotes tRNA must travel from the nucleus to the cytoplasm, where translation occurs  In prokaryotes and eukaryotes each tRNA molecule can be used repeatedly  There are only about 45 distinct types of tRNA. This is enough to translate 64 codons, as some tRNA recognize two or three codons specifying the same amino acid  This is possible because the rules are relived between the third base of an mRNA codon and the corresponding base of a tRNA anticodon  This exception to the base-paring rules is called wobble Wobble  Wobble- the ability of one tRNA to recognize two or three different mRNA codons;  Occurs when the third base (5’end) of the tRNA anticodon has some play or wobble, so tat it can hydrogen bond with more then one kind of base in the third positions (3’ end) of the codon  EXAMPLE- the base U in the wobble position of a tRNA anticodon can pair with either A or G in the third position of an mRNA codon o Some tRNAs contain a modified base called inosine (l), which is in the anticodon’s wobble position and can pair with U,C, or A in the third position of an mRNA codon o This, a single tRNA with the anticodon CCI will recognize three mRNA codons: GGU, GGC, or GGA- all coding for glycine Aminoacyl-tRNA synthetases  The correct linkage between tRNA and its designated amino acid must occurs with the help of aminoacyl-tRNA synthetase  Each of the 20 amino acids has a specific aminoacyl-RNA synthase  The aminoacyl-tRNA complex releases from the enzyme and transfers its amino acid to a growing polypeptide on the ribosome. An aminoacyl-tRNA synthase joins a specific amino acid to a tRNA  Linkage of the tRNA and amino acid is an endergonic process that occurs at the expense of ATP o The active site of the enzyme binds to the amino acid and an ATP molecule o The ATP loses two phosphate groups and joins to the amino acid as AMP (adenosine monophosphate) o The appropriate tRNA covalently bonds to eh amino acid, displacing the aMP from the enzyme’s active site o The enzyme releases the aminoacyl tRNA also called an “activated amino acid” Ribosomes  Ribosomes coordinate the pairing of tRNA anticodons to mRNA codons  Ribosomes have two subunits-large and small- separated when not involved in protein synthesis  Ribosomes are composed of about 60% ribosomal RNA (rRNA) and 40% protein  Large and small subunits of the ribosomes are o Constructed in the nucleolus o Dispatched through nuclear pores to cytoplasm o Once in the cytoplasm, are assembled into functional ribosomes only when attached to an mRNA  Compared to eukaryotic chromosomes, prokaryotic chromosomes are smaller and have different molecular composition  Selection of effective drug therapies against bacterial pathogens capitalizes on the difference. o Tetracycline and streptomycin can be used to combat bacterial infections, because they inhibit bacterial growth w/o affecting the ribosomes of the eukaryotic host The ananatomy of a ribosome  (a) A functional ribosome consist of two subunits each an aggregate of ribosomal RNA and many proteins o This is a model of a bacterial ribosome  o The eukaryotic ribosome is roughly similar, but larger with more proteins and rRNA molecules  (b) A ribosome has an mRNA-binding site and three tRNA binding sites, known as the P, A and E sites  (c) A tRNA fits in to a binding site when its anticodon base-pairs with an mRNA codon. o The P Site holds the tRNA attached to the growing polypeptide o The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain o Discharged tRNA leaves via the E site Translation is an RNA-directed synthesis of a polypeptide  In the process of translation a cell interprets a genetic message and builds a protein accordingly.  As a molecule of mRNA slides through aribosome, codons are translated into amino acids, one by one  The interpreters are tRNA molecuels, eac
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