Lecture 9 February 10 2003 R.Jones Chapters 10,11, 12
Transcription, Translation and the Genetic Code (Ch. 10), DNA, the Gene, Genetic Engineering (Ch. 12)
1.When mRNA is translated into protein two other types of RNA are involved, ribosomal RNA (rRNA found in the ribosome) and transfer RNA (tRNA).
2. Whereas the mRNA carries information about the order of amino acids in proteins in the form of the triplet code (known as the codon), tRNA is specific for each amino acid and recognizes the triplet code with a three base anticodon. Each tRNA has an amino acid attachment site as well as an anticodon (Fig. 10.11B).
3.The triplet codon of the mRNA and the anticodon of the tRNA base pair thus ensuring that the correct amino acid is inserted into the groing protein chain.
3. Translation occurs in the cytoplasm. tRNAs bring amino acids to the correct codon in the mRNA and the ribosome causes a chemical bond to form between each adjacent amino acid, causing the protein chain to elongate (see Figure 10.6A).
4. A large fraction of the eukaryotic genome consists of non-coding DNA often referred to as junk DNA. In the human genome more that 95% of the genome is non-coding (see 12.13).
5.The Trascriptional Unit in which genetic information is found also contains non-coding DNA. This DNA can serve regulatory functions (e.g in regions called promoters), but large introns that are non-coding are found between EXONS. It is the exons that carry the information specifiying the order of amino acids in preoteins (see Figs. 10.9B and 10.10).
6. The trnascriptional unit gives rise to pre-mRNA by transcription. Pre-mRNA is converted to mature mRNA by a process called RNA processing where introns are removed. Mature mRNA is transported to the cytoplasm where translation occurs (see Fig. 10.10).
7. Mutations are now understood in the context of the information present in the sequence of bases in DNA. Errors introduced into DNA during chromos0me duplication can lead to mutations. For example the loss of a nucleotide can result in a frame shift so that the genetic code is completely altered. Changes in a base can cause a change in the amino acid that is spcified in the protein. For example GUU encodes valine but GCU encodes alanine. So a change from U to C in the second base of the triplet code would cause alanine to be inserted into the protein instead of valine.
8. Genetic Engineering: Chapter 12: The basis for the technology lies in the discovery of interesting tools but relies on the faithful copying of DNA to DNA or RNA to DNA by the rules of base pairing A to T/U and G to C.
9. Bacterial plasmids and restriction enzymes allow DNA to be cut and pasted and the cloning, i.e. multiplication of DNA to be carried out in bacteria or by PCR. Cloning of DNA simply means making multiple exact copies of the DNA. By inserting a modified plasmid carrying a foreign gene into a bacterium it is also possible to translate that gene in bacteria and accumulate the protien product. Porcine insulin is made this way in bacteria and is used to treat diabetes.
10. PCR-polymerase chain reaction allows multiple copies of DNA to be made in the test tube. PCR requires primers (naked strands of DNA cannot be duplicated by DNA polymerase without a primer, i.e., the replication process must first be initiated before replication can continue). Heat resistant DNA polymerase (Taq polymerase) allows DNA to be heated, uncoiling the DNA which is then replicated by Taq polymerase and primers, this is then heated to unwind DNA that is then replicated etc (see Fig. 12.12 of the textbbok).