Remember me. Log in or Request an account. Current Groups Reports About. Group's public email, repo and wiki activity over time. W3C Team Posted on: September 3, Tools for this group Learn about available Community Group tools and how to configure a group's site to include links to tools on w3. The hydrogen bonds between adenine and thymine are important for DNA to maintain a double helix structure. Since they are not very strong bonds, they can be broken at elevated temperature.
In DNA replication and transcription, the initiation of these reactions often starts at A-T rich sites because the breakage of two hydrogen bonds between A and T requires less energy than G-C rich sites which have three hydrogen bonds between G and C. These ratios can vary between organisms, but the actual concentrations of A are always essentially equal to T and same with G and C.
For example, in humans, there's approximately:. It has to do both with the hydrogen bonding that joins the complementary DNA strands along with the available space between the two strands. Two purines and two pyrimidines together would simply take up too much space to be able to fit in the space between the two strands. This is why A cannot bond with G and C cannot bond with T. But why can't you swap which purine bonds with which pyrimidine?
The answer has to do with hydrogen bonding that connects the bases and stabilizes the DNA molecule. The only pairs that can create hydrogen bonds in that space are adenine with thymine and cytosine with guanine. A and T form two hydrogen bonds while C and G form three. It's these hydrogen bonds that join the two strands and stabilize the molecule, which allows it to form the ladder-like double helix.
Knowing this rule, you can figure out the complementary strand to a single DNA strand based only on the base pair sequence. For example, let's say you know the sequence of one DNA strand that is as follows:.
GC content and melting temperature must also be taken into account when designing primers for PCR reactions. Base stacking interactions between the pi orbitals of the bases' aromatic rings also contribute to stability, and again GC stacking interactions with adjacent bases tend to be more favorable. Note, though, that a GC stacking interaction with the next base pair is geometrically different from a CG interaction. Base stacking effects are especially important in the secondary structure of RNA; for example, RNA stem-loop structures are stabilized by base stacking in the loop region.
Chemical analogs of nucleotides can take the place of proper nucleotides and establish non-canonical base-pairing, leading to errors mostly point mutations in DNA replication and DNA transcription.
One common mutagenic base analog is 5-bromouracil , which resembles thymine but can base-pair to guanine in its enol form. Other chemicals, known as DNA intercalators , fit into the gap between adjacent bases on a single strand and induce frameshift mutations by "masquerading" as a base, causing the DNA replication machinery to skip or insert additional nucleotides at the intercalated site.
Most intercalators are large polyaromatic compounds and are known or suspected carcinogens. Examples include ethidium bromide and acridine. Further information: [[: pi stacking ]].
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