DNA Mutations
Mutations are random changes that occur within the sequence of bases in DNA. They
can be large-scale, altering the structure of the chromosomes, or small scale where they only
alter a few or even a single base or nucleotide. Mutations can occur for many reasons. For
example, DNA mutations can be caused by mistakes made by the DNA polymerase during
replication. As noted in chapter 9, DNA polymerases are highly processive enzymes that
contain proofreading and editing functions.
DNA mutations can also result through the replication of DNA that has been damaged
by endogenous or exogenous agents. The next section will highlight common types of DNA
damage and their effects. If a DNA polymerase encounters a damaged DNA base in the
template DNA during replication it may place a random nucleotide base across from the
lesion. For example, an adenine-containing nucleotide will often be added across a lesion,
regardless of what the correct match should be. This can lead to the formation of transition or
transversion mutations.
Ultraviolet Radiation
Natural sunlight stimulates the production of vitamin D, an important nutrient for the
formation of healthy bones. However, sunlight is also a major source of UV radiation.
Individuals who get excessive UV exposure are at a great risk of developing skin cancers.
There are three types of UV rays: UVA, UVB and UVC.
•UVC rays (100-280 nm) are the most energetic and damaging of the three rays.
Fortunately, UVC is absorbed by the ozone layer before reaching the earth’s surface.
•UVA rays (315-400 nm) possess the lowest energy and is able to penetrate deep into
the skin. Prolonged exposure has been linked to ageing and wrinkling of the skin. UVA
is also the main cause of melanomas.
•UVB rays (280-315 nm) possess higher energy than UVA rays and affect the outer
layer of the skin leading to sunburns and tans. Basal cell carcinoma and squamous cell
carcinoma are caused by UVB radiation.
DNA Damage by UV Radiation
DNA is composed of two complementary strands that are wound into a double helix.
The hereditary message is chemically coded and made up of the four nucleotides adenine
(A), thymine (T), guanine (G) and cytosine (C).
UVB light interferes directly with the bonding between the nucleotides in the DNA. The
two main DNA lesions formed by exposure to UVB are cyclobutane pyrimidine dimers (CPD)
and 6-4 pyrimidine pyrimidone photoproducts (6-4PPs), and its Dewar isomers.
CPDs are formed when two adjacent pyrimidine bases (thymine –TT or cytosine – CC)
become covalently linked producing a cyclic ring structure. 6-4PPs result from a single
covalent bond formed between the 5’ end of C6 and 3’ end of C4 of adjacent pyrimidines. This
leads to the formation of an unstable oxetane or azetidine intermediate depending on whether
the 3’ end base is a thymine or cytosine.
Subsequent spontaneous rearrangement of these intermediates gives rise to 6-4PP.
The pyrimidine dimers cause a kink in the DNA backbone, halting transcription and protein
synthesis. 6-4 pyrimidine pyrimidone adducts undergo isomerization to their Dewar form upon
exposure to another photon of light from UVB or UVA radiation. The most
common mutation induced by UVB is C to T transversion. Double base substitutions (CC to
TT) also occur, albeit less frequently.
UVA (and also UVB) radiation cause indirect damage to DNA via absorption of photons
by non-DNA chromophores. This generates reactive oxygen species like singlet oxygen or
hydrogen peroxide that oxidize the DNA bases causing mutations. The most common
mutation is the G-T transversion wherein guanine gets oxidized into 8-oxo- 7,8-
dihydroguanine (8-oxoG) hindering its pairing with cytosine. During the replication process, 8-
oxoG pairs with adenine. When the second strand is synthesized, 8-oxoG is replaced with a
thymine leading to a G-T transversion.
DNA Repair
The genetic lesions produced by UV radiation are often repaired soon after they are
formed, through a process called nucleotide excision repair. A nuclease enzyme recognizes
and removes a segment of DNA containing the lesion. Then, the polymerase inserts the
correct bases and ligase seals the gap. However, if unrepaired lesions accumulate or the
repair mechanism is faulty, it can lead to cell death, mutagenesis and even cancer.
Base excision repair differs from nucleotide excision repair in the types substrates
recognized and in the initial cleavage event. Unlike NER, the base excision machinery
recognizes damaged bases that do not cause a significant distortion to the DNA helix, such as
the products of oxidizing agents. For example, base excision can remove uridines from DNA,
even though a G:U base pair does not distort the DNA. Base excision repair is versatile, and
this process also can remove some damaged bases that do distort the DNA, such as
methylated purines. In general, the initial recognition is a specific damaged base, not a helical
distortion in the DNA. A second major difference is that the initial cleavage is directed at the
glycosidic bond connecting the purine or pyrimidine base to a deoxyribose in DNA. This
contrasts with the initial cleavage of a phosphodiester bond in NER.
