Transcription and Translation 

The genetic information that is stored in nearly every single one of our cells is just that -- information. It will not do the body any good unless that information is read and used properly. In some ways, it is like making a cake. It does not matter how good the recipe you have is if you don't have the ingredients, mix them properly, and bake according to directions.

The processes of transcription and translation make the information stored in our genes usable. Through these two processes, numerous proteins are created, and those proteins carry out the functions of our bodies. Without transcription or translation, our genetic information would be useless -- a recipe on a page with no way to bake the cake.

Transcription

Transcription is the process of creating messenger RNA (mRNA) from the DNA of our chromosomes. mRNA needs to be made because it is a molecule that can travel across the cell quickly and easily. DNA is not able to do this since the chromosomes on which DNA is located must stay in the nucleus of the cell. The nucleus is like the brain of the cell and contains almost all of our genetic information. It is separated from the rest of the cell by a membrane -- a barrier that prevents the chromosomes, and thus the DNA, from leaving. mRNA is able to cross this barrier, and carry genetic information into the rest of the cell. That genetic information is received by the ribosomes, which are the organelles responsible for creating proteins in the cell. mRNA acts as an intermediate between the nucleus and the ribosomes; it makes sure the genetic information needed to make proteins gets from point A to point B.

In order for transcription to take place, the DNA must be in its single-strand state. Remember that DNA typically is a double-stranded molecule that looks something like this:

But in order for mRNA to be created, only one of the strands can be utilized:

This strand is used as a template to create the mRNA. mRNA is very similar to DNA in its structure, and thus can “copy� the DNA by essentially substituting itself for the now-absent bottom strand. mRNA follows the same base-pairing rules as DNA with one major exception: mRNA does not use the letter “T,� the nucleotide thymine. Instead, mRNA uses a nucleotide with a similar base, uracil (“U�). The mRNA is created using a special molecule that “reads� the DNA strand, then calls for the correct nucleotides to create a corresponding mRNA strand:

The blue strand matches what was originally the complementary DNA strand, except there is now a “U� in place of each of the three “T's.� The mRNA has now made a copy of the complementary DNA strand, which it can transport outside the nucleus to be used in the formation of proteins. The mRNA strand moves to the ribosomes, where protein synthesis will occur.

Translation

Once transcription has taken place and the mrNA strand has migrated to the ribosomes, the intricate process of translation can begin. Translation needs four things in order to function properly: 1) the mRNA strand, 2) a ribosome, 3) a molecule of transfer RNA (tRNA), and 4) amino acids. The mRNA strand carries the genetic information. The ribosome is the “factoryâ€� where all of the components come together to make a protein. tRNA is another type of RNA that is able to both match genetic information and shuttle amino acids across a cell. Amino acids are the “building blocksâ€� of proteins -- every single protein in the human body (and there are thousands upon thousands) is made up of a different combination of amino acids.   

Amino acids are coded for by different combinations of the bases (A,C,G, and U). Every single combination is three letters long. For example, one amino acid is coded for by the combination ACG, and a completely different amino acid is coded for by the combination GCU. This is called a triplet code , or a codon . There are 64 triplet codes, but only 22 amino acids. Three of the triplet codes are what are known as “stop codons.� Stop codons do exactly what it sounds like: they signal the translation to stop. A stop codon indicates that the entire gene has been translated, all amino acids have been coded for, and no further translation is necessary in order to create the protein. Subtracting these 3 stop codons, there are still 61 triplet codes that code for the 22 amino acids. It follows, therefore, that each amino acid has more than one triplet code. For example, the triplet codes CCU, CCC, CCA, and CCG all code for the amino acid proline. The triplet codes UUA and UUG both code for the amino acid leucine.

The amino acids are carried by tRNA molecules. A tRNA molecule has its own triplet code, and this determines which amino acid it carries. A tRNA molecule with the triplet code UUA, for example, would carry the amino acid leucine. A typical tRNA molecule would look something like this:

This tRNA molecule has a triplet code of CGG. This code will match up with an mRNA code of GCC. The tRNA brings with it an amino acid (pink structure). The amino acid coded for by the letters CGG will be attached to the tRNA. In this case, the amino acid would be arginine.

The mRNA strand is able to recognize the triplet codes of tRNA. The mRNA will base-pair with a corresponding tRNA molecule, and thus the amino acid is delivered according to the genetic code. The ribosome is the machinery that puts this all together. It helps join the mRNA strand and tRNA molecule once they are properly matched up, and it helps put the amino acids together. One by one, the tRNA molecules match with the mRNA strand and deliver an amino acid. A chain of amino acids begins to build:

As the tRNA molecules deliver the amino acids, the amino acids link together. The chain that will eventually fold into a protein is being formed. As the mRNA chain continues, more and more tRNA molecules deliver more and more amino acids. The process does not end until a stop codon is reached.

Once an amino acid has been delivered and connected to the chain, the tRNA molecule detaches and moves away. The final product of translation is a chain of amino acids:

A careful observer might notice that the tRNA triplet codes, when lined up in their correct sequence, match the original DNA strand exactly except that thymine (T) has been replaced with uracil (U). The original DNA sequence coded for a certain series of amino acids, and that is what was delivered by the tRNA -- thanks to that go-between molecule, mRNA.