DNA is primarily known for its double helical structure.
However, there are various unusual DNA structures that can exist.
Understanding Unusual DNA Structures
To comprehend these structures, it is crucial to understand the sequences that lead to their formation.
Palindromic Sequences
Definition of Palindrome: A palindrome refers to a word, phrase, or sequence that reads the same forward and backward. An example is "race car."
DNA Context of Palindrome: In DNA, a palindrome is indicative of an inverted repeat, where:
One strand of DNA contains a particular sequence that is mirrored in the complementary strand.
Mirror Repeats
Definition: A mirror repeat occurs when a palindromic sequence exists within a single strand of DNA.
Orientation: This involves the sequence running from the 5' to 3' direction in a single strand, and requires:
A 180-degree rotation around the horizontal axis.
Another 180-degree rotation around the vertical axis.
Resulting Structures: This allows for the formation of structures such as hairpins or cruciforms.
Hairpin Structures
Formation: A hairpin structure forms from a single DNA or RNA strand, where:
The mirror repeat within the strand leads to a folding over, creating a hairpin loop.
Cruciform Structures
Formation: A cruciform structure arises when double-stranded DNA forms:
Two hairpins that create a cross shape, hence the name "cruciform."
Triplex DNA Structures
Definition: Triplex DNA consists of three strands coming together, which allows for:
An alternative hydrogen bonding configuration not found in regular double helical DNA.
Base Pairing in Triplex DNA:
Normal base pairs in double helical DNA (AT and CG) still form, but a third strand allows for additional hydrogen bonds utilizing:
Hoogsteen Positions: Additional positions on purines that allow for non-Watson-Crick base pairing, known as Hoogsteen pairing.
pH and Stability: Triplexes are most stable at low pH levels, where:
Example: Cytosine pairs normally with guanine, but also engages in Hoogsteen pairing with a protonated cytosine.
pKa Values: The pKa of protonated cytosine in triplex DNA rises from the normal value of 4.2 to over 7.5.
Formation Conditions:
Triplexes favor long sequences consisting of only pyrimidines or only purines, such as:
Guanosine triplexes (guanazine triplex) abundantly formed with all same base pairings.
CGC triad structures formed with two pyrimidine strands and one purine strand or vice versa.
Tetraplex Structures
Definition: A tetraplex is formed when four DNA strands come together.
Stability: This arrangement is prevalent in DNA with high proportions of guanosine residues.
Directionality: Tetraplexes can assemble in either:
Parallel Manner: All strands point in the same direction.
Antiparallel Manner: Strands point in opposite directions.
Formation: Example of guanosine quadriplex or guanosine tetrad structure, which is stable over a wide range of conditions due to guanosine's specific properties.