Lecture 24Outline of Last Lecture I. Messenger RNA (mRNA)II. RNA PolymeraseIII. PromotersIV. Post-Transcriptional ModificationsA. tRNA ModificationsB. mRNA Modifications Outline of Current Lecture I. Amino Acids and CodonsII. TranslationA. tRNA ChargingB. Wobble PairingIII. Decoding mRNACurrent LectureThis lecture moves on from looking at RNA transcription to the process of making proteins via translation. All the proteins made up in our bodies are formed from combinations of the 20 amino acids we have. These amino acids are what is encoded by our DNA. The question that might arise is “how can the four nucleotide bases create all this?” This is done through the use of codons. Codons are sequences of three basesthat specify an amino acid. Using three nucleotide codes allows for a total of 64 different combinations. This shows us that the genetic code is degenerate, or redundant. That is, many amino acids have more than one codon that codes for that amino acid. For example: the codons UUU and UUC both code for phenylalanine.There are three main “players” in the translation process. Firstly, we need our mRNA strand created during transcription. This strand is what encodes the sequence of proteins to be synthesized. Next, we need tRNA. These molecules are what “crack” the mRNA genetic code and help to read it. They also act as a bridge to link the codon and amino acid together. Finally, we need a ribosome. The ribosome is the location of the protein synthesis. These ribosomes are composed of proteins and rRNA. If we remember the structure of tRNA from last lecture, we can picture the clover-leaf shape. tRNAs have an anticodon loop at the bottom of them. This is what pairs complementary and antiparallel to the bases of the mRNA strand. This allows the tRNA to decode the mRNA sequence. For example: if the mRNA strand reads GCC from 5’ to 3’, the anticodon loop would read CGG, looping 3’ to 5’. tRNAs are “charged” with an amino acid through aminoacyl-tRNA synthetases. There is one synthetase for each of the 20 amino acids. The synthetase has to recognize the structure of the amino acid and the structure of the tRNA. It will then “charge”,or link, the amino acid to the 3’ hydroxyl end of the tRNA. As mentioned above, the genetic code is degenerate. This means that there is flexibility in the 3rd base position of a codon. This is known as a “wobble.” This allows an amino acid to recognize multiple codons. Thus BCHM 307 1nd Editionthere are several tRNAs for each amino acid, with its own anticodon loop sequence. We can also remember that tRNAs contain bases not normally found in DNA, such as inosine. Inosine can decode multiple codons, through its pairing with either A, U, or C. This helps to accommodate the redundancy of the genetic code. Now it is time to decode our mRNA sequence. There is always an initiation codon, the first codon to be translated. It is always methionine, AUG, in mRNA. This starting codon establishes the reading frame of the sequence. The reading frame denotes how each codon is read, that is, which three bases are read together. The sequence must be broken up correctly, in order for the right protein to be synthesized. In DNA, the start codon is ATG. There are also three “stop” codons that tell the tRNA to stop reading the mRNA. These are UAA, UAG, or
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