8. Mutation and Recombination

Question 1

mutations: their chemical basis and effects

Answer 1

stable, heritable changes in sequence of bases in DNA
point mutations are most common
-from alteration of single pairs of nucleotide
-from the addition or deletion of nucleotide pairs

large mutations are less common
-insertions, deletions, inversions, duplication, and translocations of nucleotide sequences

mutations can be spontaneous or induced

Question 2

spontaneous mutations

Answer 2

arise without exposure to external agents
may result from errors in DNA replication
-due to base tautomerization resulting in transition and transversion mutations
-due to insertion or deletion of nucleotides
may also result from the action of mobile genetic elements such as transposons

Question 3

induced mutations

Answer 3

caused by agents that DIRECTLY DAMAGE DNA
1) base analogs
-structurally similar to normal bases
-mistakes occur when they are incorporated into growing polynucleotide chain

2) DNA-modifying agents
-alter a base causing it to mispair

3) intercalating agents –> EtBr
-distort DNA to induce single nucleotide pair insertions and deletions

Question 4

examples of mutagens

Answer 4

acridine orange- intercalating agent
UV light- promotes pyrimidine dimer formation

Question 5

effects of mutations

Answer 5

1. wild type
   -most prevalent form of gene
2. forward mutation
   -wild type to mutant form
3. reversion mutation
   -mutant phenotype to wild type phenotype
4. suppressor mutation
   -wild-type phenotype is restored by a second mutation at a different site than the original mutaiton

Question 6

mutations in protein coding genes

Answer 6

point mutations
-in protein-coding genes can affect protein structure in a variety of ways

are named according to if and how they change the encoded protein

the most common types are : silent, missense, nonsense, and frameshift mutations

Question 7

silent mutation

Answer 7

change nucleoside sequence of codon, but not the encoded amino acid

Question 8

missense mutation

Answer 8

a single base substitution that changes codon for one amino acid into codon for another amino acid

Question 9

nonsense mutation

Answer 9

coverts a sense codon to a nonsense codon (for example, a stop codon)

Question 10

frameshift mutation

Answer 10

results from insertion or deletion of base pairs in the coding region of the gene

Question 11

replica plating technique

Answer 11

used to detect auxotrophic mutants

Question 12

excision repair

Answer 12

corrects damage that causes distortions in double helix

two types of repair systems are known
-nucleotide excision repair
-base excision repair

both remove the damaged portion of the DNA strand and use the intact complementary strand as a template to synthesize new DNA

Question 13

recombinational repair

Answer 13

corrects DNA with damage in both strands

involves recombination with an undamaged molecule
-in rapidly dividing cells, another copy of chromosome is often available

RecA protein catalyzes recombination events

Question 14

horizontal gene transfer (HGT)

Answer 14

differs from vertical gene transfer-transfer of genes from one independent, mature organism to another
-stable recombinant has characteristics of donor and recipient

important in evolution of many species
-expansion of ecological niche, increased virulence
-occurs in the three mechanisms evolved by bacteria to create recombinants
-genes can be transferred to the same or different species

Question 15

genetic recombination

Answer 15

molecular events in homologous recombination
-RecA is essential, identified in all Bacteria, in Archaea and most Eukarya

Question 16

RecA mechanism

Answer 16

Endonuclease nicks one strand of donor molecule
Helicase separates nicked from other strand
Single-stranded segment binds single-strand binding protein and RecA

Question 17

transposable elements

Answer 17

segments of DNA that move about the genome in a process called transposition
-can be integrated into different sites in the chromosome

are sometimes called “jumping genes”

simplest transposable elements= insertion sequences

transposable elements which contain “extra” genes are called composite transposons

Question 18

simple transposition

Answer 18

also called cut and paste transposition

transposase catalyzes excision of the transposable element, followed by cleavage of new target site and ligation into site

Question 19

F factors- bacterial plasmids

Answer 19

F factors contain the information for formation of sex pillus (allows for conjugation to occur)

-attach F+ cell to F- cell for DNA transfer during bacterial conjugation

F factors have insertion sequences (IS)
-assists in plasmid integration

F+ –> F- becomes F+ –> F- becomes F+
F- is still F-

Question 20

bacterial transformation

Answer 20

uptake of naked DNA by a competent cell followed by incorporation of the DNA into the recipient cell’s genome

Question 21

transduction

Answer 21

the transfer of bacterial genes by viruses

viruses (bacteriophages) can carry out the lytic cycle (host cell is destroyed) or viral DNA integrates into the host genome (becoming a latent prophage)

Question 22

generalized transduction

Answer 22

any part of bacterial genome can be transferred

occurs during lytic cycle of virulent phage

during viral assembly, fragments of host DNA mistakenly packaged into phage head
-generalized transducing particle