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Genes and Nucleotides
Written by Tim Sheppard MBBS BSc. Last updated 30/3/12

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What is a gene?

You've probably heard that DNA is the blueprint for life. But how is it built up? If the DNA is a big plan, how are the lines drawn? If the DNA is a factory producing everything that a cell needs to work, what gives the instructions for each bit? The answer is genes.

Each gene is one set of instructions for how to build one particular product. It's made up of nucleotides, each of which tells the cell about one particular amino acid; the sequence together will make up a code. This is translated into a protein. For most proteins, this will involve hundreds of nucleotides, as there are many amino acids in each protein. One gene will code for one protein, and contain all of the nucleotides in the right order for that protein.


What is a nucleotide?

A nucleotide is the building block of DNA and RNA molecules. A nucleotide is made up of a sugar with five carbons in it, a phosphate group, and a nitrogenous organic base. It has a 5' and 3' end, which determines the order of assembly during DNA replication and transcription, and thousands of nucleotides joined together will form a DNA or RNA molecule.

In RNA, the sugar is ribose. In DNA, the sugar is deoxyribose, which means it is much the same, but has one less hydroxyl group on carbon 2.


What is a nitrogenous organic base?

A nitrogenous organic base is one of a set of compounds which is attached to the backbone of DNA and RNA molecules. Coming from carbon 1 of the sugar, the base is part of the nucleotide building block of DNA molecules, and can be found in a number of different forms. It is nitrogenous because each different base used by DNA and RNA contains nitrogen. In DNA, the bases are adenine, cytosine, guanine and thymine, and in RNA they are adenine, cytosine, guanine and uracil. The bases could also be separated into purines and pyrimidines. It is the bases that link two DNA chains together, by forming hydrogen bonds with each other, but each base will only bond with one specific other base - a process called base pairing.


What is a purine?

In DNA and RNA nucleotides, as well as other important biological molecules, a purine is one of two nitrogenous organic bases.

Each purine is made up of a double ring - one hexagonal ring, one pentagonal ring, joined at one side - containing nitrogen. The two purines in DNA and RNA are adenine and guanine.


What is a pyrimidine?

In DNA and RNA nucleotides, as well as other important biological molecules, a pyrimidine is one of three nitrogenous organic bases.

Each pyrimidine is made up of a single ring containing nitrogen. The two pyrimidines in DNA are cytosine and thymine; the two pyrimidines in RNA are cytosine and uracil.


What is adenine?

Adenine is a nitrogenous organic base found in DNA and RNA nucleotides, as well as other important biological molecules.

It is a purine and will bond specifically to thymine in DNA molecules, and to uracil in RNA molecules.


What is cytosine?

Cytosine is a nitrogenous organic base found in DNA and RNA nucleotides, as well as other important biological molecules.

It is a pyrimidine and will bond specifically to guanine in both DNA and RNA molecules.


What is guanine?

Guanine is a nitrogenous organic base found in DNA and RNA nucleotides, as well as other important biological molecules.

It is a purine and will bond specifically to cytosine in both DNA and RNA molecules.


What is thymine?

Thymine is a nitrogenous organic base found in DNA nucleotides, as well as other important biological molecules.

It is a pyrimidine and will bond specifically to adenine in DNA molecules. It is not present in RNA molecules.


What is uracil?

Uracil is a nitrogenous organic base found in RNA nucleotides, as well as other important biological molecules.

It is a pyrimidine and will bond specifically to adenine in RNA molecules. It is not present in DNA molecules.


What is a base pair?

Base pairing is where two complimentary bases form hydrogen bonds with each other. In DNA, the complimentary bases are A and T, and C and G. This means that adenine will always form hydrogen bonds (two, in fact) with thymine, and cytosine will always form three hydrogen bonds with guanine. In RNA, the same occurs, but with adenine bonding with uracil instead of bonding with thymine. Unless the structure of the base is changed, a base will only pair up with its specific complimentary base - it will only form hydrogen bonds with the base that has a complimentary structure.

You might notice that in each pair, a purine bonds with a pyrimidine . This means that although some are G-C and others are A-T or A-U, they will be similar shapes, ensuring the stable ladder-like structure of the two strands running parallel.

The hydrogen bonds are individually weak, and so that bases can easily come apart. This enables DNA and RNA to function as it does. When a base is said to be 'exposed', it is not forming hydrogen bonds with any other base, and has electronegative atoms and delta-positive hydrogens available for bonding with any local base. This is why, when the hydrogen bonds do break and the chains come apart, an exposed base can pair up with any free-floating complimentary nucleotide.


Further Reading