DNA and chromosomes may seem like completely separate things. Well, they’re not. In fact, all chromosomes are individual DNA molecules coiled and twisted around, because DNA is huge. At least in eukaryotes it is. That’s one of the first differences between eukaryotes and prokaryotes in their DNA – prokaryotes have less DNA.
Eukaryotic DNA is stored within the nucleus of each cell (apart from cells without one, e.g. red blood cells). Because of its sheer size, it must be organised well. Proteins called histones help do just that:
In eukaryotes, mitochondria and chloroplasts have their own DNA. Similarly to prokaryotes, this organelle DNA is short, circular and not associated with proteins.
Having already covered the basics of DNA, let’s turn our attention to the principles which govern what actually happens to DNA and how this results in life being the way it is!
DNA is a large molecule made up of variable bases (adenine, thymine, cytosine, guanine). The precise sequence and location of these bases determines what structure a second molecule, mRNA (messenger RNA) has once it’s “read” the template DNA. In turn, the sequence and location of mRNA bases determines what amino acids will be chosen in the assembly of a given protein that the original DNA encoded for, once it reaches a ribosome and is constructed by tRNA (transfer RNA).
DNA can also code for ribosomal RNA which makes up ribosomes, and tRNA.
mRNA stands for messenger ribonucleic acid. DNA is deoxyribonucleic acid, and the only difference really is in the sugar in the backbone. A more important difference is that mRNA is single-stranded unlike double-stranded DNA. Additionally, instead of the base thymine, mRNA uses uracil. So while adenine pairs up with thymine in DNA, it pairs up with uracil in mRNA. Knowing that, the mRNA derived from this DNA (looking at the top strand) would be as follows:
DNA: ATGGGTACAAATGC (top strand)
TACCCATGTTTACG (bottom strand)
mRNA: AUGGGUACAAAUGC (single strand)
As you can see, both the top DNA strand and the mRNA are complementary to the bottom DNA strand (in reality either top or bottom may be read, but for simplicity we only look at the top strand whenever it’s given – we assume that is the gene of interest). Therefore the top strand may be called the coding strand while the bottom is the template strand. It’s called template because it’s the bit of DNA used to actually build up the mRNA according to. The result? The coding strand of DNA except that T is replaced by U!
How is mRNA read? An amino acid is coded for by 3 bases in a row. These are called triplets. AUG codes for methionine (Met) which happens to be the amino acid which signals that a new gene starts, if at a certain position within the overall code. Therefore it’s known as a start codon.
The 3 Secrets of mRNA/DNA
There are 3 key properties of the genetic code which regulate its activity.
1. The genetic code is universal. That’s right, the 4 bases are the same in all living things – humans, apples, worms, swans, oak trees, etc.! Moreover, the amino acids coded for by these bases are also completely the same, so AUG codes for the amino acid methionine in all living organisms.
2. The genetic code is non-overlapping, so if you have an mRNA AUGCGA it would be read “AUG”, “CGA” and not “AUG”, “UGC”. The amino acids obtained would be methionine and arginine (Arg).
Tables and diagrams showing you what codes corresponds to what amino acids are widely available and you won’t be expected to memorise them.
3. The genetic code is degenerate. That might sound slightly offensive, but bear with! Look above, what do the triplet codes UGU and UGC code for (start reading from the inside out by picking each letter)? They both code for cysteine (Cys). How about CUU, CUA, CUC and CUG? They all code for leucine (Leu). This property of different triplet codes coding for the same amino acid is why the genetic code is termed degenerate.
tRNA (Transfer RNA)
We know DNA is double-stranded and uses A, G, C and T bases, while mRNA is single-stranded and uses U instead of T. What about tRNA? Well, tRNA is a very different soup indeed.
It’s clover-shaped and uses the same bases as mRNA. It is single-stranded, and where one part of the strand meets another there are hydrogen bonds between bases just like in DNA except that in DNA there are 2 strands bonded rather than 2 parts of the same strand).
At the top of tRNA as seen above there is an amino acid binding site (P is seen as attached), while at the bottom there is an anticodon – in this case it’s GAA. The anticodon is complementary to an mRNA codon (triplet code – in this case it would have to be CUU).
Eukaryotic DNA is mostly non-coding. Many regions of it are simply repeats of the same sequence. Within genes there are also regions of genetic code that do not code for amino acids and hence do not become part of a polypeptide chain.
The regions inside a gene that are not used, are cut out before translation into the polypeptide. They are called introns while the regions that make it to translation are called exons.