refers to a heritable change in genetic material
What are the 3 main categories of mutations?
1. gene mutations, 2. chromosome mutations,3. genome mutations
small change in DNA .. Affects a single or a few genes.
changes in chromosome structure
changes in chromosome number
Mutations provide for _________ ____________, a component that necessary for evolution to occur?
likelihood that a gene will be altered by a new mutation; (# new mutations in a gene)/(cell generation)
What factors can influence mutation rate?
lack of this gene = no repair made
What 2 gene characteristic factors can affect the rate of mutation?
1. size of gene (...larger genes have a greater chance for mutation);2. hot spots ( locations on chromosome that are more susceptible to mutation)
Somatic hypermutation (SHM)
mechanism that involves a programmed process of mutation affecting the variable regions of immunoglobulin genes which diversifies B cell receptors to expand their recognition range
the number of mutant alleles divided by the total number of that allele in a population
What is the mutation frequency if: 1 million bacteria were plated and 10 of them were mutant?`
1 in 100,000 or 10^-5
What 3 things does mutation frequency depend on?
1- mutation rate; 2- timing of the mutation; 3- likelihood that the mutation will be passed on to future generations
Different types of DNA mutations:
-point mutation,-INDEL; -silent mutations; -missense mutations; -nonsense mutations; -frameshift mutations
change in a single base pair
What are the 2 types of point mutations?
1. transition; 2. transversions
What is a Transition point mutation?
a change of a pyrimidine (C,T) to another pyrimidine (C,T) OR a purine (A,G) to another purine (A,G)
What is a Transversion point mutation?
a change of a pyrimidine to a purine or vice versa
Which is the more common type of point mutation?
*in*sertion or *del*etion of base pairs
do not alter the amino acid sequence of the polypeptide ( due to degeneracy and wobble)
base substitutions in which an amino acid change occurs
base substitutions that change a normal codon to a stop codon
the addition or deletion of bases not divisible by three
Explain the effect of a nonsense mutation on translation
(figure slide #12) altered codon= a nonsense codon and therefore premature termination of translation
Different types of gene mutations:
-forward mutation; -reverse mutation; -suppressor mutation
changes the wild-type genotype into a new variation
changes a mutant allele back to the wild-type allele
Mutations described based on their effects on the wild-type phenotype:
--deleterious mutations;--beneficial mutations;-- conditional mutations;--neutral mutations
decrease the chances of survival
enhance the survival or reproductive success of an organism
affect the phenotype only under a defined set of conditions.;-temperature-sensitive mutation
describes how mutations can be altered: a second mutation that counteracts the effects of a first mutation
2 types of suppressor mutations:
1- intragenic suppressors; 2-intergenic suppressors
second site is within the same gene as the first mutation
second site is in a different gene from the first mutation; (figure on slide #15); -tRNA suppressor gene mechanism for nonsense mutation:
The tRNA suppressor is an example of which type of suppressor mutation?
What is the result of the tRNA suppressor gene mechanism for nonsense mutation?
transcription and translation of mRNA with nonsense codon. The altered codon = nonsense codon. The altered anticodon in mutant tRNA gene. There is no premature termination of translation so a complete polypeptide is formed with one incorrect amino acid !
occurrence of this within a gene is affected by chromosomal rearrangement
a gene is left intact but the expression is altered bc of its new location
What are 2 common reasons for position effects?
(figure on slide # 16);1-movement to a position next to regulatory sequences;2-movement to a heterochromatic region
Germ-line mutations VS somatic cell mutations
can be passed to future generations.... cannot be passed to future generations
Where can somatic cell mutations occur?
-all body cells (no gametes); -autosome or sex chromosome
-result from abnormalties in cellular/biological processes ; -underlying cause originates within the cell
-caused by environmental agents
agents that can alter DNA structure (can be chemical or physical)
Salvadore Luria and Max Delbruck: what did they study & what was their question?
-studied resistance of E. coli to infection by bacteriophage T1. ; -Is ton^r due to spontaneous mutations or to a physiological adaptation that occurs at a low rate?; -fluctuation test
Luria-Delbruck: What does the physiological adaptation theory predict?
the number of ton^r bacteria is an essentially constant proportion of the total population. (in different bacterial populations this proportion should be very similar)
Luria-Delbruck: What does the random mutation theory predict?
the number of ton^r bacteria will fluctuate in different bacterial populations based upon the timing of the mutation
figure on slide #22
Joshua and Ester Lederberg
looked at relationship bw mutations and environmental conditions that select for mutation.
What were the two theories of the J&E Lederberg experiment?
1- physiological adaptation theory (Lamarck's viewpoint); 2- random mutation theory (Darwin's viewpoint)
Describe the Joshua and Ester Lederberg experiment:
figure on slide #25
Lamarck wasn't entirely wrong .... well, at least for the bacteria that carry this system: adaptive immune defense
3 chemical changes that lead to spontaneous mutations:
1- depurination; 2- deamination; 3- tautomeric shift
spontaneous mutations can occur by mispairings or "_________" during DNA replication : all of these fall under this category
-wobble pairing; -INDELs; -"slipping"-slippage
removal of a purine: Adenine or Guanine
How does depurination occur?
covalent bond bw deoxyribose and purine base is somewhat unstable and it undergoes hydrolysis (beta-N-glycocidic link) that releases the base from the sugar.
lack of a purine caused by depurination
removal of an amino group from cytocine resulting in uracil!
What happens if DNA repair enzymes do not recognize the faulty Uracil (from deamination)?
a C-G to A-T mutation will result during later rounds of DNA replication
deamination of 5-methyl cytosine results in...
What happens when the deamination of 5-methyl cytosine occurs?
DNA repairenzymes cannot determine which of the two bases on the DNA strands is the incorrect base
methylated cytosine bases tend to create _____ _____ for mutation
hot spots (CpG islands)
a temporary change in base structure
2 forms of bases:
1-the common (Keto or Amino); 2-the uncommon (Enol or Imino)
How do the uncommon forms of bases created by taotomeric shifts differ from the common forms?
Alternate forms differ by a single proton shift in the molecule
Stable form of Thymine and Guanine
Stable form of Adenine and Cytosine
T and G can interconvert to an ______ form
A and C can interconvert to an ______ form
Rare forms of bases can promote....
incorrect AC and GT base pairs
When must a tautomeric shift occur in order for it to cause a mutation?
just prior to DNA replication, when the DNA is not in double helix
Example of slippage
figure on slide # 36
DNA loops out on template strand
PolIII skips bases
DNA loops out on daughter strand
PolIII adds random bases
Why might slippage occur?
long run of same nucleotide
Slipped Strand Mispairing:
if a DNA strand loops out and becomes displaced or if a DNA polymerase slips small insertions and deletions can occur
Where is replication slippage common?
particularly poly-G tracts
trinucleotide repeat expansion (TNRE)
causes several genetic diseases... in people with these disorders the length of a trinucleotide repeat has increased above a certain critical size and then disease symptoms occur
If expansion due to TNRE occurs within the coding sequence of the gene....
proteins aggregate with each other. typically the expansion is a CAG codon for glutamine)
If the expansion due to TNRE is located in noncoding regions of genes expansions may:
-cause abnormal changes in RNA structure
2 unusual features in common within TNRE disorders:
1-Anticipation ; 2-anticipation usually depends on who the disease is inherited from
the severity of the disease (a TNRE disorder) tends to worsen in future generations
chemical mutagens are categorized as:
-base analogs; -base modifiers; -intercalating agents
(may change pairing) become incorporated into daughter strands during DNA replication
(change one base into another) covalently modify the structure of a nucleotide
base modifier examples:
nitrous acid : replaces amino groups with keto groups ; nirogen mustards and ethyl methanesulfonate (EMS)
(alter the shape of DNA and cause INDEL) contain flat planar structures that intercalate themselves into DNA... when inserted, this distorts the helical structure)
what happens when DNA containing intercalating agents are replicated?
the daughter strand may contain single-nucleotide additions and/or deletions resulting in frameshifts
intercalating agents examples
acridine dyes; proflavin;Ethidium Bromide
2 types of physical mutagens:
1- ionizing radiation; 2- nonionizing radiation
short wavelength and high frequency
free radicals can be created by
ionizing radiation can cause:
-base deletions; -oxidized bases; -single nicks in DNA strands;-cross-linking; -chromosomal breaks
lower energy than ionizing (includes UV light)
nonionizing radiation causes:
the formation of cross-linked pyrimidine dimers (C or T) called *cyclobutane pyrimidine dimer* or less common 6,4-photoproducts
when might pyrimidin dimers cause mutations?
when DNA is replicated
What is the purpose of an Ames test?
to determine if something is a mutagen
uses a strain of Salmonella typhimurium that cannot synthesize histidine (amino acid) [it has a point mutation (or a deletion) in a gene involved in histidine biosynthesis]. A second mutation (reversion) may occur restoring ability to synthesize histidine. *Ames test monitors the rate at which the second mutation occurs*
The restoration of the ability to synthesize histidine in the S. typhimurium strain used in an Ames test is.....
a Reversion (reverse mutation) --> second mutation
Describe an Ames test
figure on slide #52