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Molecular Genetics 1. Introduction a. We have studied “classical” or “Mendelian” genetics up to this point. - It leaves a lot of questions unanswered. i. . What really is a gene? ii. How do different alleles actually differ? iii. Is there a “genetic code” and how is it decoded to make an organism? b. Now we will answer these questions i. They were answered in a series of experiments done through the 1930's to 1950's c. 2. The big question is how can living things make so many different proteins Concept of Gene a. Coined by W. Johannsen in 1909 - referred to hereditary factors of Mendel i. Indivisible units on chromosomes ii. Really not so indivisible now iii. Genes carry the code to make an organism b. One Gene One Enzyme hypothesis i. Beadle and Tatum 1. Studied the mold Neurospora crassa 2. Created mutants using radiation 3. Found each mutant was defective in only 1 enzyme 4. Concluded that one gene produces one enzymge ii. New definition of gene - a nucleic acid sequence that encodes a functional polypeptide or RNA sequence c. DNA i. Made of repeated units called nucleotides 1. sugar 2. nitrogenous base a. Adenine 1 Molecular Genetics b. Thymine c. Guanine d. Cytosine ii. Watson and Crick found that DNA is a double helix 1. Like a spiral staircase a. Bases are steps b. Sugars are railings c. Bases match with one another - hydrogen bonds between each i. A-T ii. G-C d. Thus they are anti-parallel i. 5' end on one is matched with 3' end of other e. Complementary i. If you know the sequence on one strand - you can tell the sequence on the other 1. Remember the chromatids? 2. Each time a cell divides the old strand is a template for a new strand 2. How does DNA code for a protein? a. 4 bases -20 amino acids b. Codon i. 3 bases in a row that specify a particular amino acid 1. It turns out that many amino acids have more than 1 codon 2. 3. There are also stop codons How does system code for a protein a. Codons are organized in a line - just like amino acids on a protein 2 Molecular Genetics b. Transcription i. Base pair sequence is transcribed onto messenger RNA (mRNA) 1. RNA is like DNA a. It is single stranded b. Uracil replaces thymine c. RNA polymerase makes a complementary copy of the DNA d. e. A-T-G-C-T-A-C-T DNA U-A-C-G-A-U-G-U mRNA Only 1 strand of DNA is used i. Called the Coding Strand' c. Translation i. mRNA goes out into cytoplasm and attaches to ribosome ii. A bunch of ribosomes line up on mRNA iii. There are 20 transfer RNA's in the cytoplasm of the cell 1. These are shaped like a cloverleaf a. One end attaches to a specific amino acid b. The other end is an anticodom that is the exact opposite match for a particular codom c. The tRNA attached briefly to the mRNA on the ribosome and brings in a particular amino acid at just the right place in the protein chain 3 Molecular Genetics 2. 4. 2. Thus exactly the right protein gets made Regulation of gene expression a. Very necessary b. Development of embryo c. Timing of events in life of organism d. Won't talk about it Recombinant DNA a. It turns out that DNA is very durable and can be manipulated i. Restriction endonucleases - scissors ii. Ligaases - paste iii. Allow us to manipulate DNA 1. Make our own genes 2. Transfer genes from one organism to another a. Splice genes into bacteria so that they make chemicals that we want b. b. Natural pesticides in plants - BT cotton Polymerase chain reaction i. Allows us to amplify DNA ii. Copy from small amount 1. Forensic science a. DNA is unique for an individual b. If you can sequence DNA, you can identify the individual i. DNA fingerprinting c. Genomics i. If we knew the sequence of DNA in a representative human we could determine the genes to make a human 1. Important in detecting errors - mutations 2. Human DNA has been sequenced - at Wash U in St. Louis 4 Molecular Genetics a. 3. d. Some of our graduates helped Tests to detect cystic fibrosis or Huntington's chorea Molecular genetics of cancer i. Neoplastic growth - tumors ii. Cancer caused by mutation of oncogenes 1. Normal form is proto-oncogenes iii. Complex system to control cell division 1. Conversion from proto-oncogene to oncogene breaks down this system 5