ECDAAugust 2009
Genetic disorders is now known to be a result of alterations of DNA sequences which correspondingly results in alterations of the amino acid sequence of the protein product. The consequence then is an altered conformation and function of the protein
DNA sequence
Amino acid sequence
Protein conformation
Protein function
DNA MUTATION
MUTATION
Any alteration in gene sequence constitute a mutation. Mutation involves a change in the shape, structure or nucleotide sequence of the DNA
Mutation may be spontaneous or induced by agents called mutagens
Mutation may be lethal to the organism or it may lead to the development of new species
MUTATION
Mutation in the gene may be small or submicroscopic affecting only one of a small number of nucleotides, hence are called point mutations
Mutation in the gene can be large and be seen under the light microscope in the case of gross chromosomal abnormalities
POINT MUTATIONS A point mutation can be a change in a
single base or addition or removal of one or more nucleotides in the DNA
Single base changes are of 2 types: Transition – involves a change of a
pyrimidine to another pyrimidine or a purine to another purine
T <--> C or G <--> A Transversion – involves a change of a
purine to a pyrimidine or a pyrimidine to a purine
T <--> A G <--> TC <--> G A <--> C
POINT MUTATIONS
Point mutations may have no effect on the amino acid sequence of the protein only if the substituted base is the third in the codon. Ex. CCA CCG
proline proline
Point mutation may result in the incorporation of a different amino acid in the protein, called a missense mutation Ex. CCA CGA
proline arginine
POINT MUTATIONS
The mutation may result in the premature appearance of a stop codon resulting in a shorter protein which is likely to be nonfunctional Ex. AAG UAG
Lysine Term
Ex. UGG UAG Tryptophan Term
Ex. UAU UAG Tyrosine Term
FRAMESHIFT MUTATIONS Addition and removal of one or more
nucleotides in DNA, called insertion and deletion, respectively, may result in frameshift mutations Ex. For normal strand shown
AUGCGGUCUUGCAAAGGC... met arg ser cys lys gly
A mutation causing deletion of base uracylAUGCGGCUUGCAAAGGC... met arg leu ala lys ala
FRAMESHIFT MUTATIONS
Ex. For normal strand shownAUGCGGUCUUGCAAAGGC... met arg ser cys lys gly
A mutation causing insertion of base adenine after the base in position 6
AUGCGGAUCUUGCAAAGGC... met arg ile leu gln
arg
PHYSICAL MUTATIONS
Physical agents include ultraviolet (UV) and ionizing radiations.
The base most commonly affected by UV radiation is thymine. When hit by high energy photons,
diradicals are formed and can pair up forming covalent bonds.
For thymine, the major product is the thymine-thymine cyclobutane dimer (T-T dimer)
T-T dimer
CHEMICAL MUTATIONS
Mutations caused by:
alkylating agents
deaminating agents
intercalating agents
CHEMICAL MUTATIONS
Alkylating agents The largest class of “potential” mutagens
present in the environment N-nitrosoamines found in cigarette smoke
is metabolized by liver enzymes to form alkylating agents
Guanine is the most reactive among the four bases toward nearly all alkylating agents
When guanine is alkylated, it may be ignored resulting in a deletion in the daughter strand
CHEMICAL MUTATIONS
Deaminating agents Sodium nitrite is used as a preservative,
color enhancer, and color fixative in bacon, smoked fish, tocino, etc.
When ingested, sodium nitrite is converted to nitrous acid in acidic conditions. Nitrous acid is a deaminating agent and
removes groups from adenine, guanine, and cytosine. Deamination of adenine results to hypoxanthine, a structural analogue of guanine, hence may base pair with cytosine resulting in transition
CHEMICAL MUTATIONS
Intercalating agents These substances bind to DNA by
becoming inserted between adjacent base pairs because of their flat ring structures Benzopyrene, found in automotive
exhaust and cigarette smoke Benzene, an organic solvent Aflatoxin, a metabolic product of molds in
peanuts, oils, and grains
VIRAL MUTAGENS Some viruses contain oncogenes which
can be activated once they insert their DNA in the host’s genome or DNA
The process of inserting these viral genes is called lysogeny. When viral DNAs are inserted into the
host’s DNA, the sequence of the bases of the host’s DNA may be altered resulting in altered protein product, or activation of certain destructive genes.
Oncogenes – cancer-causing genes/DNA
DNA REPAIR MECHANISM
S
DNA REPAIR
Cells are equipped with enzymes that can repair changes in the structure of their DNA Photolyase – a photoreactivating enzyme
that recognizes pyrimidine dimers and monomerizes it upon absorption of visible light
DNA REPAIR
Cells produce antioxidants such as glutathione and metallothione, a low molecular weight protein
Vitamins such as Vitamin C, A, and E can function as free radical scavenger By binding to free radical elements, the
resulting product can be simply excreted from the organism
DNA REPAIR
EXCISION REPAIR A UV-specific endonuclease defects
dimers and makes a cut near the dimer The segment containing the dimer peels
away and DNA polymerase synthesizes DNA from the 5’ to the 3’ direction
DNA polymerase which has a 5’ to 3’ exonuclease activity cuts the segment containing the dimer
A DNA ligase joins the newly synthesized DNA to the original DNA
DNA REPAIR
INHIBITORS OF
GENETIC MECHANISMS
Infection is a major worldwide problem. Like any other organisms, infectious agents must undergo cell division and thus genetic mechanisms are important for their growth and survival.
In the S phase of Cell Division process, DNA synthesis and duplication occur. Thus, to prevent microbial proliferation, inhibition of genetic mechanisms must happen. This is possible with the use of antibiotics.
ANTIBIOTICS
STREPTOMYCIN Cause cell death
by binding to the small subunit (30s) of the prokaryotic ribosomes preventing binding of mRNA
anti-TB drug
ANTIBIOTICS
CHLORAMPHENICOL Binds with the 50s
subunit of the prokaryotic ribosomes blocking the action of peptidyl transferase and preventing the attachment of mRNA-30s complex to this large subunit
Drug against typhoid fever
ANTIBIOTICS
ERYTHROMYCIN Binds to the
50s subunit preventing translocation
Drug against gram (+) bacteria
ANTIBIOTICS
Tetracycline Interacts primarily
with the small subunit to prevent the binding with amino acyl tRNA
In DNA, it intercalates between DNA bases resulting in change of the conformation of the DNA
QUESTIONS