Unit 3: Chapter 16 Flashcards
Mutations
Heritable changes in DNA sequence
Point mutations
single nucleotide changes
Ex. Insertions or deletions
Spontaneous mutations
arise in absence of any stimulus
Where can spontaneous mutations result from?
- errors in DNA replication
- head on collisions between replisome and polymerase
- spontaneously occuring lesions in DNA
- action of mobile genetic elements
What are examples of spontaneous mutations?
Insertion, deletion, transititon, transversion
Tautomerization
Nitrogenouse base of nucleotide shifts to tautomeric form which allows for unique base pairing to occur (2 or more interconvertible structures)
Transition mutation
Stable change of nucleotide sequence from purine to purine or pyrimidine to pyrmidine
The shape of purine with purine is
Too wide
Transversion mutation
Changes of nucleotide sequence from purine to pyrmidine which causes steric problems
The shape of pyrimidine with pyrimidine is
Too narrow
Insertions
Occurs at short stretches of repeated nucleotides (AT) and slippage in synthesizing new daughter strand
What is the shape for purione with pyrimidine?
Normal base pairing
Deletions
Occurs at short stretches of repeated nucleotides (AT) and slippage in parental old strand
Spontaneous occuring lesions in DNA
- Purines lose their base (depurinated) while the phosphate sugar backbone remains intact
- Forms apurinic site which cannot base pair and may cause mutation after next round of replication
Induced mutations
Results of exposure to mutagen which can be physical/ chemical agents that damage DNA
Base analogs
- Example of chemical induced mutagen
- Structurally similiar to normal bases and mistakes occurs when they are incorporated into growing polynucleotide chain
This is an example of what: 5- Bromouracil is base analogue of thymine that undergoes tautomeric shift more frequently than normal base
Base Analogs
DNA modifying agents
Alter a base causing it to mispair
This is an example of what: methyl-nitrosoguanidine adds methyl groups to guanine causing it to mispair with thymine
DNA modifying agents
Intercalating agents
- distort DNA to induce single nucleotide pair insertions and deletions
- mutagens are planar and insert themselves between stacked bases of helix
This is an example of what: ethidium bromide intercalcates in DNA and use as stain
Intercalcating agents
Ultraviolet radiation
thymine dimers between 2 thymine bases on the same strand
Wildtype
The most prevalent form of gene and its associated phenotype
Forward mutation
Wild type to mutant form
Reverse mutation
Mutant phenotype to wild type phenotype
Supressor mutation
Wild type phenotype is restored at a different site than original mutation
Where are mutations?
in regulatory or coding sequences in tRNA and rRNA genes
Silent mutation
change nucleotide sequence of codon but not amino acid
(minimal effect)
Missense mutation
Single base substitution that changes codon for one amino acid into codon for another amino acid
Nonsense mutation
Converts sense codon to nonsense (STOP: TAG, TGA, TAA) codon
(early stop)
Frameshift mutation
Results from insertion or deletion of base piars in coding region of gene
(can be most detrimental)
Proofreading
- 1st defense of DNA repair
- Fix mistakes in base pairing by DNA polymerase
Mismatch repair
- Mismatch correction enzyme scans newly synthesized DNA for mismatched pairs
- Mismatched pairs are removed and replaced by DNA polymerase
DNA methylation
- Parental DNA is methylated and new DNA temporarily lacks methyl groups
- Repair system cuts out the mismatch from unmethylated strand
Excision Repair
corrects damage that distorts the DNA double helix by removing damaged DNA
Nucleotide excision repair
Removes thymine dimers or other injury that produces distorted DNA
Base excision repair
Removes damaged or unnatural bases yielding apurinic/ apyrimidic (AP) sites
Photoreactivation
- direct repair to directly split thymine dimers and light is required
- catalyzed by photolyase
Recombinational Repair
- Corrects DNA that has both bases of a pair missing or damaged
- Uses RecA
SOS response
Global control network for repair
Used when damage is SO GREAT that normal repair mechanisms wonât work
SOS response activation
RecA protein initiates recombindation repair and acts as protease to destroy LexA to increase production of excision repair enzymes
RecA protein initiates recombination pair by activating over 50 genes
Recombination
Process in which one or more nucleic acids are rearranged or combined to produce new nucleotide sequences
(INCREASE IN GENETIC VARIANCE)
Vertical gene transfer
Transfer of genes from parents to progeny
Examples of vertical gene transfer
Asexual reproduction by microorganisms
Sexual reproduction by eukaryotes
Horizontal Gene Transfer
Genes from one independent mature organism to another
Mechanisms of Horizontal Gene Transfer
Transformation, Conjugation, Transduction
Transformation
DNA aquired directly from environment
Conjugation
DNA transfered from a donor cell
Transduction
DNA transported in a bacteriophage
4 Fates of DNA in recepient
- Integration
- Separate existence of DNA
- Remains in cytoplasm
- Degradation
Homologous recombination
carried out by RecA proteins and double strand break occurs for crossing over
Site specific recombination
Does not require long sequence homology
Recombination at specific target sites and uses recombinase
Transposable elements
Genetic elements that move within or between genomes
(JUMPING GENES)
Simple Transposition
Cut and Paste
Transposase catalyzes
Genes jump into different place
Replicative transposition
Copy gene and 2 places with same gene
Mobile genetic element remains at og site
Conjugative plasmids
Independent from chromosomes and has own genes for conjugation
F Factor
Fertility fator
Have genes for cell attachment and plasmid transfer between E. Coli cells
Episome
Can exist outside chromosome or be integrated
J. Lederberg and E. Tatum
Incubated 2 auxotrophs bacteria together and noticed recombination
B. Davis
U tube experiment that kept cells separate showed as gene transfer
(Showed cell to cell contact must be required)
W. Hayes
Gene transfer during conjugation was unidirectional transfer (donor to recepient)
F+ x F- =
F+
Sex Pilus
Used to establish contact between F+ and F- cells
During F+ and F- mating what happens?
Direct cell to cell required
Plasmid only transfered not chromosomal DNA
Rolling circle replication
Mode of DNA replication in which replication fork moves around a circular DNA molecule, displacing a strand to give a 5â tail that is also copied to produce new double strand DNA
Hfr strain
Donor and contains F factor integrated into their chromosome
Hfr x F- =
F-
F- x F- =
Fâ
F. Griffith
Discovered transformation
Natural transformation
Bacteria lyse and release DNA into environmet
During natural transformation, DNA has to come in contact with what to be imported
Competent cells
Natural transformation of Streptococus pheomoniae
Becomes competent during exponential phase of bacterial growth
Natural transformation of Bacillus subtillus
Becomes competent during stationary phase of bacterial growth
Natural transformation of Haemophilus Influenzae
Takes up DNA only from closely related species
Artificial Transformation
Lab technique that induces cels to take up DNA and used for cells not naturally competent (Ex. E. Coli)
Bacteriophages
Bacterial viruses
Virulent bacteriophages
Can carry out lytic cycle
Temperant bacteriophages
Lysogen which is insertion of viral genome into bacterial chromosome
Generalized transduction
Any part of bacterial genome can be transferred
Specialized transduction
Errors in lysogenic cycle insert genomes into specific site in host chromosome
Origin of drug resistance from
Immunity genes and Horizontal Gene Transfer
Immunity Genes
Resistance genes in nature
Horizontal gene transfer drug resistance
Transfer immunity genes from antiobiotic producers to nonproducing microbes
Where can resistance genes be found?
Bacterial chromosome, plasmids, transposons, other mobile genetic elements
R (resistance) plasmid
can be transferred by horizontal gene transfer
genes code for enzymes that destroy or modify drugs
Movement of antibiotic resistance genes through
Conjugative transposons
