DNA – Deoxyribonucleic Acid
Mononucleotides
The structure of DNA is made up of many single units called mononucleotides.
Mononucleotides have three components: a pentose sugar, a base and a phosphate group, as shown in the diagram below:
A pentose sugar has 5 carbon atoms and in DNA this pentose sugar is deoxyribose. Deoxyribose is a modified version of ribose. The difference is the replacement of a hydroxyl group with a hydrogen atom. Essentially, it has lost an oxygen atom – hence the ‘de-oxy’ part of the name.
There are four possible nitrogenous (a compound containing nitrogen atoms) bases in DNA. The names of these bases are listed below, and each mononucleotide has one:
- Adenine (A)
- Cytosine (C)
- Guanine (G)
- Thymine (T)
Polynucleotides
The mononucleotides are joined together in condensation reactions between the phosphate group of one mononucleotide and the deoxyribose sugar of another. This forms a polynucleotide – a single strand of DNA.
KEY DEFINITION: Condensation Reaction – a reaction that involves the release of a water molecule when two molecules combine.
This bonding forms what is called a sugar-phosphate backbone.
The diagram below shows the structure of part of a polynucleotide:
Double Helix
DNA is double stranded, which means that two complementary polynucleotides join together. The two strands join together with hydrogen bonds between the bases.
The bases pair in a specific way – this is called complementary base pairing:
- Adenine (A) pairs with Thymine (T)
- Guanine (G) pairs with Cytosine (C)
A and T join with two hydrogen bonds and G and C join with three hydrogen bonds. The diagram below shows part of two complementary strands bonding together:
The strands of DNA are antiparallel – meaning they run in opposite directions. Once all the pairs have matched up the strands twist to form a double helix. This gives the final structure of DNA:
RNA – Ribonucleic Acid
Like DNA, RNA is also composed of a sugar, base and phosphate group. The main difference is that the sugar is ribose. The mononucleotides of RNA join together in the same way as they do in DNA to form a polynucleotide strand.
Another key difference between RNA and DNA is the bases. The bases of RNA are:
- Adenine (A)
- Cytosine (C)
- Guanine (G)
- Uracil (U)
Three of these bases are exactly the same as in DNA, however Thymine has been replaced by Uracil.
Unlike DNA, RNA is usually a single stranded molecule and the strand is usually much shorter than DNA molecules.
Summary of the Differences Between DNA and RNA
- The mononucleotides of DNA contain deoxyribose sugar, RNA mononucleotides contain ribose sugar.
- DNA is usually double stranded, RNA is usually single stranded.
- DNA mononucleotides have the bases A, C, G and T, RNA mononucleotides have the bases A, C, G and U.
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