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