chapter 8

vertical gene transfer does NOT cause genetic change

simply movement of genetic information from parent to daughter cells

what do geneticists compare mutants to?

the wild type (cell without mutation)

contrast transposons vs. genomic islands

transposons cause mutations and provide a mechanism for transferring genes -insertion sequence encodes only the enzyme responsible for transposition

importance of transposons (example)

transposons yield vancomycin resistant staphlococcus aureus strain -patients infected with S.aureus are suceptible to vancomycin Also had vancomycin resistant strain of Enterococcus faecalis Transferred transposon- containing plasmid to S. aureus Transposon jumped to plasmid in S. aureus

what is reversion?

when spontaneous mutations change back to original state

consequences of adding or removing nucelotides

adding or subtracting one or two nucleotide pairs causes a frameshift mutation -this changes the reading frame of the corresponding mRNA molecule so -that an ENTIRELY DIFFERENT set of codons is translated -likely results in a shortened, non-functional protein

mutagen

agent that induces change in DNA

repair of damaged DNA

if not repaired, can lead to celldeath; cancer in animals several different repair methods

base substiution

incorrect nucleotide is incorporated during DNA synthesis point mutation: -change of a single base pair

the mobile gene pool

perhaps 75% of E.coli genes found in all strains -called core genome remaining genes make up mobile gene pool -includes plasmids, transposons, genomic islands, phage DNA

the mobile gene pool- plasmids

plasmids found in many bacteria and archea -some eukarya -usually double stranded DNA -generally nonessential; cells survive loss

deletion or addition of nucleotides

-impact depends on number of nucleotides -3 pairs changes one codon - one or two pair yields frame shift mutation: changes the reading frame of the corresponding mRNA molecule so that an entirely different set of codons is translated different set of codons is translated

radiation

1. UV radiation forms thymine dimers -covalent bonds between adjacent thymines 2. xrays cause single and double stranded breaks in DNA - can also alter nucleobases

3 groups of chemical mutagens

1. modify nucleobases in DNA -change their base pairing properties 2. base analogs resemble nucleobases -can mistakenly be incorporated by DNA 3. intercalating agents cause frame shift mutations -flat molecules that insert between base pairs in DNA strand

what are 2 was in which genetic changes occur?

1. mutations -change an organisms phenotype 2. horizontal gene transfer

3 main classes of chemicals

1. some modify nucleobases - change base-pairing properties -increase change of incorrect nucleotide incorporation 2. base analogs- resemble nucleobases -have dif. hydrogen bonding properties -can be mistakenly incorporated by DNA polymerase 3. Intercalating agents: cause frame shift mutations -flat molecules that insert between adjacent base pairs in DNA strand

spontaneous mutations

caused by normal processes -occur randomly, infrequently 1. base substitutions 2. additions/deletions 3. transpons

recombiants

cells that acquire new properties as a result of the transfer of genetic material

what is the mobile gene pool?

conserved genes- can move from one DNA molecule to another carried on mobile genetic elements include plasmids, transpons, genomic islands, phage DNA

conjugation

DNA transfer between bacterial cells involving DIRECT cell to cell contact 1. plasmids (most frequent) 2. chromosomal DNA (less common) -involves spacial cells -transfers some but not all of the donar cell chromosome

horizontal gene transfer

DNA transfer from one bacteria to another by conjugation, DNA mediated transformation

3 outcomes of horizontal gene transfer

1. transformation -naked DNA is taken up by bacteria 2. transduction -bacterial DNA is transferred by viruses 3. conjugation -DNA transfer during cell to cell contact

3 mechanisms genes are transferred by

1. transformation: naked DNA is taken up by bacteria 2. transduction: bacterial DNA transfer by viruses 3. conjugation: DNA transfer during cell to cell contact

3 possible outcomes of base substitutions

Silent mutations: -produces the same amino acid even though DNA is different Missense mutations: -leads to a different amino acid Nonsense mutation: -stop codon is inserted

how do xrays and uv light damage DNA?

UV radiation forms thymine dimers xrays cause single and double stranded breaks in DNA and can alter nucleobases

transposons

also called jumping genes -can move from one gene to another -process is called transposition

plasmid

an extrachromosomal DNA molecule that replicates independently of the chromosome often the location of antibiotic resistance in bacteria

direct selection vs indirect selection

direct -bacteria cells are innoculated onto a medium that supports the growth of the mutant but not the parent indirect -isolates mutants that have lost a function and need a growth factor -uses technique of replica plating

process of DNA-mediated transformation and role of competent cells

involves the uptake of naked DNA by recipient cells naked DNA is DNA that is free in the cell's surroundings- not contained within a cell or virus recipient cell must be competent- physiologial state that allows the cell to take up DNA

transformation

involves uptake of naked DNA; free floating recipient cell must be competent- physiological state that allows it to take up DNA

mobile gene pool- genomic islands

large segments of DNA that originated in other species pathogenicity islands: -encode the ability to cause disease

chemical mutagens

may cause base substitution or frame shift mutations

transduction

mechanism of horizontal gene transfer in which bacterial DNA is transferred inside a phage coat

describe generalized transduction

mechanism of horizontal gene transfer in which bacterial DNA is transferred inside a phage coat a fraction of DNA- produced when phage encoded enzyme cuts the bacterial genome, mistakenly enters the phage protein coat. the product is called a transducing particle; it carries no phage DNA it is NOT a phage- contains bacterial DNA

horitzontal gene transfer

movement of DNA from one cell to another

what is an agent that induces change?

mutagen

mutant selection

mutants rare, difficult to isolate- 2 approaches 1. direct selection -bacteria cells are innoculated onto a medium that supports the growth of the mutant but not the parent 2. indirect selection -isolates mutants that have lost a function and need a growth factor

difference between mutation and horizontal gene transfer

mutation changes the existing nucleotide sequence of a cell's DNA, and then is passed on to the progeny (daughter cells) through vertical gene transfer; can change phenotype horizontal gene transfer- transfer of DNA from one organism to another through transformation, transduction or conjugation

mobile gene pool- transposons

not only cause mutations- also move genes chromosome--> plasmid

induced mutations

result from outside influence 1. chemical mutagen 2. radiation mutagen

3 possible outcomes of base substitutions

silent -base substitution has occurred but no new amino acid is made missense -altered codon codes for a different amino acid non-sense- -base substitution creates a stop codon, resulting in a non functional protein

how do transposons cause mutations?

the gene into which a transposon jumps is insertionally activated by the event -the inserted DNA disrupts the function of the gene so that it is no longer expressed

transduction

transfer of genes by bacteria called bacteriophages: phages -consist of DNA and RNA surrounded by a protein coat -attaches to bacterial cell -injects nucleic acid

vertical gene transfer

transfer of genes from parents to offspring