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DNA structure

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DNA structure

DNA (deoxyribonucleic acid) is a large molecule which carries the genetic information, or blueprint, of all life on Earth. Mutations arising in the DNA code account for the diversity upon which evolution by natural selection can work. Therefore, it is not far-fetched to say that DNA is one of the central, most important molecules in living organisms.

For such an important molecule, it sure looks beautiful:




DNA is a double helix i.e. two individual strands running along each other in an anti-parallel way, connected to one another by relatively weak hydrogen bonds. DNA's structure can be learned easily by thinking about the strands and the "stuff in-between" separately.


What are the strands made of?

The strands are made of repeating units consisting of a deoxyribose (sugar) molecule with a phosphate molecule attached to it; hence, it is called a sugar-phosphate backbone.





Phosphodiester bonds between nucleotides (above) create the backbone:




What is the centre made of?

Attached to the sugar molecules in the backbone are a different type of molecule called nitrogenous base. There are 4 bases in DNA: adenine, thymine, cytosine and guanine. These are abbreviated by their initials: A, T, C and G.

The hydrogen bonds are formed between these bases. Due to their complementary shapes, A always pairs with T, and C always pairs with G. A-T is linked by 2 H bonds, while C-G is linked by 3.

Here is a diagram of this arrangement:




And another:




The bases can be sorted into two categories: purines and pyrimidines depending on their ring structure:




As you can see, adenine and guanine are bigger and have two rings, while thymine and cytosine only have one ring. Uracil is similar to thymine and also pairs with adenine. The presence of uracil instead of thymine occurs in RNA rather than DNA.

DNA and RNA are key carriers of biological information. For example, DNA may store a gene coding for haemoglobin or insulin, which is then processed by RNA and ribosomes (which are sophisticated machines themselves made of RNA and proteins) to manufacture those proteins.

Both DNA and RNA are nucleic acids (that's the "NA" part of their acronym). They also contain a sugar group of a 5-carbon ring called a pentose. In DNA this is deoxyribose, while in RNA it's ribose. This completes their respective names: deoxyribonucleic acid and ribonucleic acid.

The monomers of these nucleic acid compounds are nucleotides. Aside from the nitrogen-containing base and the pentose, they contain a phosphate group.

As you can see, the nitrogenous base in the DNA nucleotide is one of four options: adenine, guanine, cytosine or thymine.

RNA has thymine switched for uracil, making its base options adenine, guanine, cytosine or uracil.

DNA is a very stable molecule, as its purpose of carrying genetic information is very important. Features of this are:

1. DNA is very temperature-resistant, and the H bonds only break at temperatures of about 92 degrees Celsius
2. The sugar-phosphate backbone acts as a shield to the bases, preventing interference from outside chemical reactions
3. The double helix gives stability
4. Many H bonds contribute to the stability
5. The structure of the sugar-phosphate backbone itself confers strength.

Therefore, the function of DNA can be summarised by replication and protein synthesis.

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