BIOL 2230 Unit 2 week 3
BIOL 2230 Unit 2 week 3 BIOL 2230
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This 7 page Class Notes was uploaded by Allison Collins on Friday March 11, 2016. The Class Notes belongs to BIOL 2230 at Middle Tennessee State University taught by Anthony L Newsome in Fall 2015. Since its upload, it has received 28 views. For similar materials see Microbiology in Biology at Middle Tennessee State University.
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Date Created: 03/11/16
3/1 Translation Translation: process in which the genetic message carried by mRNA directs the synthesis of polypeptide chains with the aid of ribosomes Summary: The copy strand (mRNA) leaves the chromosome à mRNA finds a ribosome à mRNA sticks to a ribosome à tRNA carries codons that connect with corresponding nucleotides on mRNA à tRNA also carries an amino acid à tRNA breaks off, leaving the amino acid à tRNA picks up another amino acid and waits for another complementary codon à process repeats and each additional amino acid adds to an amino acid chain • So tRNA’s reaction with mRNA: tRNA delivers amino acids to corresponding nucleotides on mRNA o tRNA attaches to ribosome, which moves along the mRNA • Each set of 3 nucleotide bases of mRNA forms a codon o A codon codes for a single amino acid (most of the time) o Multiple codons can code for the same amino acid o Anticodon – associated with tRNA – 3 complementary bases to codon § tRNA has anticodon on one end and an amino acid on the other • Each time the tRNA attaches to the mRNA it leaves behind an amino acid, and eventually forms a polypeptide chain o Polypeptide chain gives rise to a functional protein o When done making protein, mRNA has a stop codon that indicates completion of translation à mRNA dissociates with ribosome § Stop codon does not code for any amino acid § There must be at least one tRNA for each amino acid • There are 64 total codons o 61 sense codons – that is, they specify incorporation of an amino acid into a protein § Some different triplet codons specify the same amino acid – is redundant o 3 nonsense (stop) codons – signal for ribosome to dissociate from the mRNA § Stop codons: UGA, UAG, UAA • So: codons = mRNA , anticodons = tRNA o mRNA and tRNA pair only in the presence of a ribosome • Remember: protein synthesis takes place on the ribosome and mRNA acts as a blueprint Organization of the genetic code The DNA sequence has a direct relationship to the amino acid sequence • There are 20 amino acids in proteins • This means that there must be at least 20 different codes – how does this work? • Only nitrogenous bases are variable o Only 4 bases à need 20 codes (codons) o If each codon were comprised of only one base, there would be only 4 codons o If each codon were comprised of 2 bases: 4 bases total x first possible 2-‐base combination for a code x second possible 2-‐base combo for a code à 4 x 2 x 2 à 4 à 16 combinations • Reality: 3 bases per codon à 4 = 64 combinations o Some codons code for the same protein 2 o Ex: GCT, GCC, GGA, GCG all code for the amino acid Alanine Mutations Mutation: change in DNA base sequence Point mutation – base substitution • At one point in DNA, one base is substituted for another • May or may not be an important change • After substitution, the polypeptide chain folds back on itself in formation of the protein • Depending on place of substitution, mutation may affect function of protein • Ex: active site may be altered, may inadvertently generate a stop codon (thus halting protein production) • The mutation is passed on to later proteins Frameshift mutation • More problematic – end result is a totally inappropriate protein – always a bad consequence • One or more nucleotide base pairs is inserted or deleted from DNA à changes whole reading frame of mRNA à incorrect amino acid inserted into polypeptide chain à produces wrong protein • Genetic diseases often result of just one faulty protein – there are hundreds of types of proteins • Radiation can knock out nucleotides • Example of frameshift mutation operation: o Original sequence: ATA CCG CAG TTC o Deletion of first adenine: TAC CGC AGT TC_ Mutagens: anything that can cause a mutation 3 Base analog: chemical similar to nitrogenous bases that get inadvertently incorporated into DNA • Cause faulty base pairing • Can be used as antiviral and antitumor drugs – very important in treatment of disease • Example: 2-‐aminopurine is very similar in chemical composition to adenine and can pair with cytosine à this causes incorrect RNA and thus incorrect protein 4 3/3 Testing for mutation Base analogs can cause mutation • Testing base analogs will likely be worth it in order to fight a deadly disease • One ring or two ring structures are more lik ely to be incorporated into DNA and cause a mutation • Scientists are currently testing whether new chemicals will substitute into DNA as base analogs o Identify chemical in a product or drug and wait 20-‐30 years to observe effects o Use in mice and wait a few generations All chemicals are subject to the Ames test • Take particular bacteria and add a chemical • Humans and bacteria have the same nitrogenous bases • Bacteria divide every 20-‐30 minutes – quickly end up with millions of bacteria • Check bacteria for mutations • If so, significant chance it will affect human DNA • Uses strain of Salmonella or E. coli to test chemicals for their mutagenicity and thus their potential carcinogenicity • Medications and products have not necessarily been fully tested or understood before being put on the market! Physical mutagens • UV light à Causes formation of thymine dimers o Dimer – bonding of two thymine molecules so that a pair is read as only one unit 5 • Not all mutagens are carcinogens but some are • Mutations can occur spontaneously o Generally spontaneous mutation rate is low – 1 per 10 9 replicated base pairs o Average gene is 103 base pairs long o Humans have about 23k – 25k genes o So average mutation rate is one per 10 replicated genes (1 per million) • Vast majority of mutations are harmful in eukaryotic organisms – often carcinogenic Genetic transfer and recombination in bacteria – 3 types • Transformation o Naked pieces of DNA transferred from one bacterium to another o Single circular chromosome – when bacteria die, DNA pieces float around in environment – can be taken up by other bacteria • Conjugation o Transfer of plasmids (small extra-‐chromosomal pieces of DNA) • Transduction o Bacteriophage – virus that only affects/infects bacteria o Attaches to bacteria and injects DNA to produce new bacteriophages • All 3 occur naturally Biotechnology – use of living organisms to make useful objects • Technological application that uses biological systems, living organisms, or their derivatives to make or modify products 6 Genetic engineering – deliberate modification of organism’s genetic information by directly changing its nucleotide sequence in order to make new and different proteins • Original genetic engineering: selective breeding – used throughout human history • Variety of methods – collectively referred to as recombinant DNA technology • Bacteria have played a central role in biotechnology 7
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