Nucleic Acids- Properties of the DNA Double Helix

What do the terms denaturation and reannealing mean with respect to nucleic acids?

With respect to nucleic acids, denaturation refers to the separation of the strands, whereas reannealing refers to separated strands joining back together.

Level of denaturation can be measured by observing differences in light absorbance. Single-stranded DNA absorbs light more effectively than double-stranded DNA.

How can we denature DNA?

DNA can be denatured by:

• Increasing the temperature. This disrupts hydrogen bonds between the bases.
• Introducing certain chemicals. Some chemicals can form their own hydrogen bonds with the bases, disrupting hydrogen bonds between bases in the process.
• Increasing the pH. Increasing the pH will deprotonate some of the bases, reducing their ability to form hydrogen bonds with bases from the other strand.

DNA can be protected from denaturation if placed in a salt solution, as positive ions will interact with the negatively-charged phosphate groups, stabilising the molecule.

What is Tm?

Tm is the temperature at which half of the nucleic acid has been denatured. The lower the Tm, the more prone the nucleic acid is to denaturation.

What influences the Tm of any nucleic acid?

As mentioned above, temperature, pH and presence of certain chemicals are all factors that can influence the Tm of a nucleic acid.

Be able to define basic gene structure in eukaryotes and prokaryotes.

Eukaryotic genes are composed of exons and introns. Exons are the parts that code for amino acids, whereas introns are the bits in between that are usually spliced out during mRNA processing. There is a promoter region immediately upstream of the coding area (i.e. before the bits that get translated), as well as untranslated regions downstream that are involved in termination of gene transcription.

Prokaryotic genes, in contrast to eukaryotic genes, do not have introns. They do, however, also have upstream promoter regions and downstream untranslated regions.

Another difference between eukaryotes and prokaryotes is that, in eukaryotes, a translated mRNA will only code for one protein at a time, whereas a translated mRNA in prokaryotes may code for several proteins at a time. Eukaryotic genes can, however, code for multiple different proteins if the exons are spliced in a different order. All of this is a topic for another post, however.