As described in the DNA section, DNA is like the code which explains which order amino acids go in, when making up a protein. However, like any code, it needs a translator if it's going to make sense. What happens, then, if the translator is somewhere else? A copy of the original code will be written down, and taken to wherever the translator is. This is pretty much what happens with DNA.
Messenger RNA, rather unsurprisingly, is the messenger. The original code is transcribed from the DNA 'template'. Since the DNA template has the code for every single protein the body is going to make, it's far too big to move around. However, the mRNA that is made can slip happily out of the nucleus, and find its way to where the code can be translated. Though still big, it is much smaller than a DNA molecule, and unlike the double-stranded DNA, RNA tends to be single stranded, which means it's just one long strand.
In humans, it's a little bit more complicated. Instead of just producing mRNA, which then shoots off and is translated into a protein, the cell modifies the molecule a bit so that it can make it more effective. The first thing produced from the DNA template is called heteronuclear RNA or hnRNA. This is a huge molecule - much bigger than the final mRNA that is to be produced. However, it's got some junk bits in it. Part of it is the gene - the thing that will be translated. These 'expressed' bits, made up of 'codons' that will be used, are called exons - there are about 10-20 in each gene. However, these codons are split up by 'intervening' codons, or introns. This is just junk code, and makes up about 90% of the original gene, but the purpose of it isn't really known! Lots of guessing has been made, but nothing widely confirmed. When the hnRNA is modified, the introns are taken out, leaving bits that will code for the protein.
There are also parts that make the molecule more stable. Nucleic acids - that is, RNA and DNA - are easily attacked, which wouldn't be so much of a problem, if it weren't for the fact that any damage to them can lead to the wrong proteins being produced, and only a slight difference can lead to huge consequences. This is called mutation. To try and avoid this, the hnRNA molecule has certain things added to it. At the 5' end, it is capped. This is where a special nucleotide (something called 7-methyl-guanosine or 7mG) is added to the beginning of the molecule, and it is also added in a special way. Usually all the nucleotides face the same direction, so the fifth carbon of one nucleotide is joined through to the third carbon of the next (5' to 3'). However, at the 5' end of the hnRNA molecule, the extra nucleotide being added is turned round, so that the two 5' ends are joined to each other - the fifth carbons are joined, and the third carbon of the new base is 'sticking out'.
At the other end - the 3' end - something called a poly-A tail is added. This is where somewhere between 150 and 200 adenosine nucleotides are added to the 3' end of the molecule. It may seem a bit excessive, but this helps for a variety of reasons. Firstly, it makes the mRNA more stable - so when the message is on its way to translation, it's less likely to get damaged. It also helps the mRNA get out of the nucleus (the part of a cell where the DNA is kept) and into the cytoplasm (the part of the cell where the proteins are produced), and makes any further changes that need to be made to the mRNA easier.
All in all it's quite complicated, but if you don't understand the structure, it's not a huge problem. Just remember that mRNA is a sequence of nucleotides which carries a copy of the code to where it can be translated. This message is really important, so certain things happen to stabilise it and make sure it's kept safe, but ultimately the most important point is in the name - it's the messenger.
When the DNA code is translated, amino acids have to be brought together so they can bind together and form a protein. If the amino acids just happily floated about, the chances of them coming together in the right order would be very slim indeed, but fortunately the DNA system has been developed to make sure they come together appropriately. Each amino acid is bound to a transfer RNA or tRNA molecule.
Like mRNA, the tRNA is made up of a series of nucleotides. Also, the tRNA molecule is made in much the same way as the mRNA molecule. The process of transcription means that the RNA molecule uses the sequence of nucleotides in the DNA molecule to determine its own structure. So you end up with a string of nucleotides - but this time much smaller. A mRNA molecule could be as big as 3000 nucleotides long, as there may be many amino acids to code for. However, a tRNA molecule isn't coding for loads of amino acids - only one amino acid is going to be bound to it. For this reason, it's much smaller - perhaps as much as 90 bases long.
Although the tRNA molecule is single stranded, parts of the molecule are made up of sequences of complimentary bases. Imagine that there are five adenosine nucleotides at one point in the molecule, and a bit further along, there are five uracil nucleotides. Well, in RNA, adenosine forms hydrogen bonds with uracil, so the molecule will loop round, with the complimentary bases binding to each other. This base pairing occurs extensively throughout the molecule, leading to a structure known as the 'three leafed clover' shape.
In a tRNA molecule, the extensive base pairing leads to this distinctive shape. The image shown on the right is obviously not what a real tRNA molecule would look like, but is a good way of representing its structure. The structure for a tRNA molecule is, in fact, very important, because it has two essential features. One has been implied already - the binding site of the amino acid which it is to bind to. The other is something called the anticodon.
The molecule transcribed from the DNA - that is, when the tRNA molecule is first 'born' it wouldn't bind an amino acid. It is modified first, and the sequence 'CCA' (i.e. two cytosine nucleotides and an adenosine nucleotide) is added to the 3' end of the molecule. The final adenosine nucleotide doesn't have another nucleotide binding to its third carbon. When there is nothing bonded to the third carbon (i.e. at the 3' end of a nucleic acid molecule) there is simple an -OH group (oxygen and hydrogen) which appears at the end. Due to the addition of this 'CCA', an amino acid is able to bind to the final nucleotide, making the transfer RNA like a transporter for the amino acid.
So the tRNA has an amino acid attached to it. How does that help? Well, because the tRNA is made up of nucleotides, it is much more able to relate to the other nucleic acids floating about. As already mentioned, and amino acid could end up anywhere - it has no brain, so it doesn't know that it's supposed to bind with other amino acids in any order. The thing about nucleic acids, though, is that complimentary bases bind to each other - adenosine binds to uracil (in RNA) and cytosine binds to guanine.
A tRNA molecule has a particularly important bit, which is found in the same part of each transfer RNA molecule: the anticodon loop. This is a sequence of three nucleotides (the exact function of which will be explained later!) which will correspond to the codon on the mRNA molecule. The mRNA is, as previously mentioned, made up of expressed codons - expressed sequences of three bases. These will eventually pair up with the anticodons on tRNA molecules - but this wil hopefully be made clearer later. The important thing to remember is the the amino acid which binds to the tRNA molecule depends on this anticodon loop. For instance, if the anticodon loop is made up of the nucleotides uracil, guanine and adenosine (in that order) then the amino acid which binds will be threonine. However, if the nucleotides are adenosine, guanine and uracil (in that order), the amino acid which binds to the tRNA will be serine.
So far we've had the original code, the messenger, and something to transport the final product into the right order. However, we seem to be missing potentially the most important part of all - the reader, or translator. In terms of this DNA system, the reader is a ribosome, and a ribosome is made up of proteins, and ribosomal RNA or rRNA.
When rRNA is made in human cells (again through transcription) the products are referred to as '5s', '5.8s', '18s' and '28s'. The numbers don't just refer to weight but also to shape - a 5s unit contains around 100 nucleotides, but a 28s unit has more like 4000! However, the number of bases are not so important - simply consider the four different products.
A ribosome is made up of two units - a 40s and a 60s unit. Logically, if these went together this would produce a 100s unit. However, the number refers to shape as well - so you can't simply add them up! In fact, a ribosome is an 80s unit in humans. So how are the two units made up?
The 60s subunit is made from the 5s rRNA, the 5.8s rRNA, and the 28s rRNA, combined with about 50 proteins. The 40s subunit is just made from the 18s rRNA and about 30 proteins.
Once again, the structure of a ribosome is not the most important thing - knowing that it is made of both rRNA and proteins made be helpful, and knowing that it is made of the two subunits helps to understand why it is always represented in diagrams the way that it is. However, realistically the most important thing to remember is that the ribosome reads the message, so that it can be translated.