Chapter 7: Anatomy and Function of a Gene: Dissection Through Mutation Learning Objectives

Question 1

o Describe complementation testing and how its results distinguish mutations in a single gene from mutations in different genes

Answer 1

 Complementation testing: process where heterozygosity for loss-of function mutant recessive alleles for two different genes that function in same pathway produced normal phenotype
 A heterozygote has one mutation on one chromosome and a different mutation on its homolog; if mutations are in different genes, heterozygote will be wild-type; if both mutations affect same gene phenotype will be mutant

Question 2

o Distinguish between the effects of mutation in somatic and germ-line cells

Answer 2

 Mutation in germline: because they give rise to gametes, they will be passed down to the next generation
 Mutations in somatic: not passed on; to be maintained in individual, mutation needs to be cloned

Question 3

o Summarize the factors associated with difference in mutation rate

Answer 3

 Different genes, different mutation rates
 Higher mutation rate in multicellular organism than in bacteria
 Gene function: easy to disrupt, hard to restore – many different mutations can disrupt a gene’s function, while only a few mutations can restore function to a previously inactivated gene
 Higher mutation rate in human sperm that in human eggs
 More cell divides, more likely it is that mutation will accumulate in genomes

Question 4

o Summarize the consequences of mutation for individuals and for the evolution of species

Answer 4

 Mutations are the raw material for evolution. Although many mutations are harmful, rare mutations may confer a selective advantage.

Question 5

o List mechanisms by which cells can repair damaged DNA and correct replication errors

Answer 5

 Reversal of DNA base alteration
 Removal of damaged bases or nucleotides (enzymes remove aberrant bases leaving AP site, AP endonucleases cut sugar-phosphate backbone creating a gap; DNA exonucleases make gap bigger with is filled in with correct information by DNA polymerase; DNA ligase seals gap)
 Nucleotide excision repair corrects damaged nucleotides (exposure to UV light; thymine dimer form, UvrB and C endonucleases nick strand containing dimer; damaged fragment is released from DNA, DNA polymerase fills in gap with new DNA; DNA ligase seals preaired strand)
 Double-strand break repair via homologous recombination
 Repair of double-strand breaks by non-homologous end joining – mechanism for stitching back together ends formed by double-strand breaks; it relies on proteins that bind to ends of broken DNA strands and bring them close together
 In bacteria, methyl-directed mismatch repair corrects mistakes in replication
 Error-prone systems: a last resort
 Health consequences of mutations in genes encoding DNA repair proteins

Question 6

o Define mutagen and describe how mutagens are used in genetic research

Answer 6

 Mutagen: any physical or chemical agent that raises the frequency of mutation above spontaneous rate

Question 7

o Explain how errors in DNA replication can cause mutations

Answer 7

 DNA replication errors can cause nucleotide substitution resulting from base tautomerization or expansions/contractions of trinucleotide repeats)

Question 8

o Describe natural processes that can produce mutations by damaging DNA

Answer 8

 Depurination (hydrolysis of A or G bases leaves a DNA strand with unspecified base)
 Deamination (removal of amino group from C initiates process that causes a transition after DNA replication)
 X-rays (break sugar-phosphate backbone that splits DNA molecule into smaller pieces which may be put back improperly)
 UV radiation (adjacent Ts to form dimers which can disrupt readout of genetic information)
 Radioactivity (formation of free radicals that can alter individual bases)

Question 9

o Explain how the fluctuation test and replica plating have shown that mutations arise randomly and spontaneously

Answer 9

 Results of the fluctuation test and replica plating experiments showed that resistance mutations arise randomly in bacterial cells prior to bactericide exposure

Question 10

o Describe four types of point mutations: transitions, transversion, deletions, and additions

Answer 10

 Deletion: occurs when block is lost from DNA molecule
 Transition (substitution): one purine (A or G) replaces other/ one pyrimidine (C or T) replaces other
 Transversions (substation): purine changes to pyrimidine/vice versa
 Insertion: addition to a DNA molecule or more nucleotide pairs