Microbiology: Test Answers on Micro Bio – Flashcards
Unlock all answers in this set
Unlock answersFrequency
|
o Spontaneous mutations occur every 1x109 bases o One every 1 million replications for 1000 base pair gene o Beneficial because generates diversity and drives evolution o More common with mutagens |
Mutagens
|
o Substances that increase mutation rate § Increase rate 10 to 1000 fold in DNA § 1 mutation for every 1000 to 100,000 replication of 1000 base pare gene o Chemicals and some types of radiation are mutagens § Ionizing radiation § Low wavelength ultraviolet light |
Ionizing radiation
|
o X-rays, gamma rays, and high speed subatomic particles § X rays and gamma rays – high energy electromagnetic waves 1nm or less in length · Produced from decomposing atoms and stars like sun § High speed subatomic particles like electrons are released by some natural elements and cathode ray tubes AKA particulate radiation o Ionizing radiation causes OH- in cytoplasm to lose electrons § Form hydroxyl free radicals § Cause mutations in DNA by altering nucleotide bases § Also causes breaks in backbone |
Mutagenic Low Wavelength Ultraviolet Light |
o Electromagnetic energy with wavelengths between 40 and 310 nm § 260nm most efficient § Adjacent pyrimidine bases in DNA bond together covalently · Eg Thymine Dimer § Pyrmidine dimmers interfere with DNA replication and cause cell death and mutations |
Chemical mutations
|
o 3 groups § Nucleotide-altering chemicals § Frameshift mutagens § Nucleotide analogs |
Nucleotide Analogues
|
o Structurally similar to nucleotide bases o 5-bromouracil similar to thymine § Incorporated into replicating DNA instead of thymine § Pairs with guanin not adenine § When replicated, causes base pair mutation in daughter DNA |
Nucleotide altering Chemicals
|
o Alter nucleotide bases o Nitrous acid § Reacts with amino group of adenine and converts it to carbonyl group § Altered adenine (hypoxanthine) pairs with cytosine when replicated § Base pair change in daughter |
Frameshift Mutagens
|
o Chemicals that insert themselves between bases and cause insertions or deletions when DNA replicated § Benzopyrene, coal tars, acridine |
DNA Repair
|
o Light repair enzymes repair pyrimidine dimers o Dark repair enzymes repair pyrimidine dimers o Base excision repair enzymes o Mismatch repair enzymes o SOS Response § Drastic repair mechanism only used when DNA damage is so severe |
Light and Dark enzyme repair of pyrimidine dimmers
|
o Light repair use light energy to break bond o Dark repair enzymes excise and replace dimer nucleotides § Can be use for any damaged base |
Base excision repair enzymes
|
o Mutated DNA cut out, polymerase repairs the gap with ligase
|
Mismatch repair |
| just removes individual nucleotide by recognizing methylated bases |
Identifying Mutants
|
o Wild types - Commonly found phenotypes o Mutants can be detected by testing for altered phenotypes § Positive selection § Negative (indirect) selection |
Positive selection
|
o Selects mutant cells and rejects non mutants § Growing cells on penicillin to find resistance mutants |
Negative (indirect) selection
|
· Replicate plating · More tedious · Often used to isolate auxotroph, mutant has a new nutritional requirement not seen in wild type o Eg. Isolate mutant that cannot synthesize tryptophan o Use two medias, one with and without tryptophan o Grow on tryptophan containing media o Replicate plate with a membrane on both two new agars, one with and one without tryptophan Compare |
Ames Test
|
o Rapid inexpensive test that utilized histidine auxotroph of Salmonella typhimurium to identify mutagens and possible carcinogens o Expose Salmonella histidine auxotroph to § Test chemical and liver enzymes § Liver enzymes o Liver enzymes replicate changes of chemicals our body will produce o If mutagen it will increase number of mutated bacteria that can synthesize histidine when grown on a histidine deplete plate o 90% of mutagens are carcinogens o Mutagens then tested for carcinogenic properties in animals |
Genetic Recombination
|
o Exchange of genes between two DNA molecules o Eukaryotes use crossing over of two homologous chromosomes o Beneficial to eukaryotes because it produces diversity in gametes |
Gene transfer and recombination in Prokaryotes
|
o Horizontal Gene Transfer - Transfer of DNA between two different cells o 3 mechanisms § Transformation § Conjugation § Transduction o All methods have a donor cell giving part of genome to recipient cell o Recombinant cell – cell that incorporates part of another’s DNA into own chromosome o Create genetic diversity but also spread antibiotic resistance |
Transformation |
· Transfer of genes from 1 cell to another as naked DNA in solution · Discovered in 1928 by Frederick Griffith · Live non-encapsulated S. pneumoniae do not infect, encapsulated do, heat treated encapsulated don’t, however heat treated encapsulated and live non-encapsulated do kill o Non encapsulated take up dead encapsulated DNA · 1944 Avery, Macleod, and McCarty showed DNA was transformative agent o key experiment in showing DNA carried genetic information · Competent cells – cells that take up DNA in solution o Not all cells are Most of DNA taken up is degraded, some is recombined |
Transduction
|
o Transfer of genes from one cell to another by virus o 2 mechanisms in bacteria § Generalized transduction – DNA from degraded chromosome of bacteriophage infected bacterial cell is packaged into new phages. Any gene can transfer § Specialized transduction – specific bacterial genes are packaged in new phages o Mechanism § Bacteriophages infect § Breakdown DNA § Repackage some Donor DNA § Repackaged DNA used in new phage |
Conjugation
|
o Transfer of DNA from donor cell to recipient by direct contact o E coli and other gram-neg bacteria use conjugation with sex pili § Sex pili genes on Fertility plasmid § F+ cells contain F factor § F- cells do not. They are recipients o Mechanism § Sex pili reels another cell in § Cells contact § Plasmid replicates a strand § Transfers daughter strand to recipient § New cell is F+ o HFR cells have F factor in chromosome § During conjugation, only part of chromosome replicates and is transferred § New cell is not F+ but has recombinant DNA |
Clinical correlation. Conjugation and transformation can spread antibiotic resistant genes
|
o Staphylococcus aureus used to be penicillin sensitive o Causes 11% of nosocomial infections o 95% now resistant due to plasmid coding for penicillinase which hydrolyzes penicillin o 40% methicillin resistant o MRSA strains usually have several plasmids and are resistant to many antibiotics o Costly and hard to treat o Vancomycin treats them now § Also used to treat Enterococcus infections responsible for about 8% of nosocomial infections § 30% of enterococcus isolates vancomycin resistant due to a plasmid o Enterococci and Staphylococci can transmit plasmids Some vancomycin resistant MRSA found |
Transposons
|
· Small pieces of DNA 70-40,00 BP) that can move from one location in DNA to another · Cause frameshift mutations · Present in genome of all cells but usually inactive · Ends contain palindrome o Ex. GAATTC and CTTAAG · Simplest are called insertion sequences o Contain transposase enzyme and palindromes at each end o Recognizes its own repeat sequence o Duplicates target site o Inserts a copy of IS1 between target site and copy · Complex transposons have genes in between two IS1 transposons · Can cut itself out and bring gene with it · Another possible way of transferring antibiotic resistance · 50% of human genome composed of transposons · Normally inactive due to methylation · If activated for some reason (like viral infection) can insert itself Insertions can cause frameshifts, and inactivate an essential gene like a protooncogene and cause cancer |
