DNA is a polymer with a Sugar-Phosphate repeating backbone. Each sugar has a nitrogenous organic base attached (either Adenine, Thymine, Cytosine or Guanine).
DNA backbone. You’ve got a deoxyribose sugar attached to a phosphate group & your sugar attached to a base via a BETA-glycosidic bond (eg forms via condensation reaction). Next to look at the bases!
There are two types of bases – Pyrimidine and Purine. Purines are BIGGER with 2 rings (imagine the bigger work being a smaller molecule).
– Pyrimidines (single ring) – T and C & Purines (double ring) – A & G
- A and T form 2 hydrogen bonds while C and G form 3.
- The number of H bonds is important as this will determine the strength of the DNA. DNA with a high CG content will have a higher decomposition temperature than DNA with a high AT content because it has a larger number of H bonds. These H bonds hold the helix together better, requiring more energy to break the helix apart.
A little terminology:
Sugar + Base = Nucleoside
Nucleoside + Phosphate = Nucleotide
A deoxynucleoside is a deoxysugar + base.
An oglionucleotide is a polymer of repeating nucleotides in a chain with less than 20 repeat units.
A polynucleotide is a longer chain of repeating units.
– Structure of a DNA Helix
- Bases are hydrophobic and stack on top of each other in the helix.
- Phosphates are on the outside – so the helix is highly -vely charged.
- DNA helix has 2 strands – these run ANTI-PARALLEL. Eg.
- It’s a right handed helix. Imagine a screw – spirals look like they are going down clockwise.
Remember that direction! I’ve drawn an extra white arrow going up on the second picture to show what the 5′—->3′ does.
- This is also known as B-DNA.
– Other (more rare) helix structures
- A-DNA. Helix follows same direction but the bases are pulled away from the centre of the helix nearer to the backbone. This happens when the DNA is dehydrated.
- Z-DNA. A normal helix…but backwards! The DNA strands are wound the opposite way, meaning it appears mirrored to normal B-DNA. This occurs in some GC containing sequences at high salt concentrations.
– Packing down the DNA Helix to more condensed structures (eg. Chromatin)
- Although the DNA helix is a fairly compact structure, this can be further wound to reduce it’s size. Imagine a helix is like a coil, then winding that coil round and round a roll…this picture borrowed from Wikipedia explains visually.
(Click picture to enlarge in a new window/tab)
- Chromatin contains 5 main proteins (H1, H2A, H2B, H3, H4) which are very basic (AKA contain lots of +vely charged amino acids like arginine and lysine) and which interact with the -vely changed DNA (due to the phosphate groups as explained previously).
- 2x (H2A, H2B, H3) form the round structure you see in the second box of the image above. The DNA helix wraps around this 1.6 times before wrapping round another cylindrical structure.
- Essentially the DNA is arranged around the histone cores like beads on a string which further packs down to form higher order structures such as chromosomes.