Wednesday, November 19, 2008

DNA - What it is and What it does

DNA is made up of four different molecules. It is easiest to think of these molecules as lego blocks. In the diagram below there are two towers of Lego, each making the same shape and of the same height, but using only the same four colours blocks. However, those four blocks are used in different amounts and different orders.

DNA is made up of the four blocks A, T, C and G. These letters are simply taken from the names of these four blocks:


The technical name for these molecules is nitrogenous bases, but that isn’t really that important to your understanding of DNA.

This is a simplified diagram of DNA, it is a ladder, where each of the rungs are made up of two of the four nitrogenous bases (A, T, C, G). The above is one stand of DNA, DNA live in pairs, twisted around each other (this is called a double helix shape).

You may have noticed that in the two above diagrams, I have only represented Adenine paired with Thymine and Cytosine with Guanine, this is not a coincidence.

These nitrogenous bases bond in pairs within DNA and only these pairs. No matter where Adenine is found within DNA, it will always be paired with Thymine. This is simply because of the different atoms within the bases and their position within the molecule of the base. In other words, due to their shape, the molecules must form the A-T and C-G pairs. This can be seen well in the video on this page:

Click on - Finding the Stucture>Pieces of the Puzzle>Watson’s Base Pairing

After watching that video, I recommend watching the video under

Putting it together>The DNA double helix

The website above contains wonderful videos, interviews and information about the discovery of DNA and DNA replication, which I recommend perusing if you are interested, at least to watch the videos.

The rule of this paired bonding means that if you take one stand of DNA and ‘unzip’ it – slice it down the middle – you know exactly what the other half of the strand looks like.

If the DNA of all life on earth is made up of ATCG, why do we look so different? This is because the order of the nucleic acids within DNA is very important. In the English language, we have 26 letters which combine to make hungered of thousands of words. These words can be used to write the instruction manual for Monopoly or for how to set up an Xbox. The same letters and words are used, but because of their order, the outcomes are very different. The same goes for DNA, the order of the nitrogenous bases in the ladder of DNA is very important.

In order for the cell to operate the instructions within the DNA need to be read. The problem with this is that the DNA needs to be read outside the nucleus, in the cytoplasm. However, the DNA needs to be protected and is therefore, not allowed outside of the nucleus. Imagine that the nucleus is a library and the DNA is a very important, very rare reference book, which everyone needs to use. Therefore, the DNA is not allowed outside of the library. However, you are allowed to photocopy whatever pages in the DNA book you need. This is how DNA works, it is not allowed out of the nucleus, but copies of sections of DNA are allowed.

So how do our cells photocopy DNA? This is where we introduce DNA’s cousin – RNA. RNA is different to DNA in three important ways.

1. RNA comes only in single strands, not double

2. Instead of having the nitrogenous base Thymine, RNA replaces T with U – Uracil

3. RNA is not as important as DNA, it is simply a copy and therefore, can be taken out of the nucleus, used and then disposed of.

In understanding how DNA is copied (or Transcribed, the technical term), I will explain it in the most simplified way that I can.

When the DNA needs to be copied, a piece of blank RNA comes into the nucleus of the cell from the cytoplasm. The DNA is then ‘unzipped’, this means that the DNA is unwound from its helix shape and then each strand is separated from its partner. The RNA then moves in between the two separated pieces. An enzyme (the worker ant of the cell, I’ll talk more about them in a later blog), then reads one strand of the DNA and copies the corresponding base onto the RNA. This is the best example of why it is important for the bases of DNA to only have one partner.

Once the RNA has been ‘written’ on by the enzyme, it leaves the cell whilst the DNA ‘zips’ back up and whines back into the double helix shape. Once the RNA is in the cytoplasm, another enzyme of a different kind attaches to the RNA. This enzyme is designed to read the RNA and follow its instructions, this process is known as translation. Every three bases are read as one ‘word’ (a codon). These words correspond to 20 different types of amino acids. Amino acids are the building blocks of proteins. Proteins can become many different things, messengers (for example, hormones such as testosterone), enzymes and structural formations inside of cells. Proteins can control cell growth, and ultimately how we appear, our hair colour, height etc. There are transporters within the cytoplasm which carry the different types of amino acids, they each match up with the ‘words’ on the RNA until a chain of amino acids is created, this chain becomes a protein.

A fantastic video of translation is found at:
Under - Reading the Code>Putting it Together>Translation

To sum up the important information which I have relayed to you:

DNA is the instruction manual
> RNA messenger
> Proteins leaders of action
> Enzymes and biological chemicals do the work
= growth and development

I hope that this has been informative, if you have any questions, corrections or comments, please either comment on this blog or email me at:



Anonymous said...

Thanks. This helps a lot in a study that me and friends will be doing in our Botany class. :D

Anonymous said...

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