New News Biology #48

DNA (Part 4): How Are Proteins Made? Transcription & Translation Explained

Protein Synthesis = the process of making proteins.

The nucleus of a cell contains all the genetic material of that cell, which we call

DNA. The reason DNA is known as the fundamental to life is because of ‘genes’, which are a specific sequence of bases that makes a specific sequence of amino acids which combine to make a particular protein. However, to make a protein, these specific sequence of bases have to be read by ‘ribosomes’, which are outside the nucleus.

That’s when the problem occurs. If we want to use a gene, we have to use the ribosomes to read it, however, the DNA is too big to leave the nucleus. So we have to make a copy of that gene before we do anything else. Because we only copied a small gene, and not the entire DNA strand, it is small enough to leave the nucleus and get to the ribosome. This copy is known as mRNA (messenger RNA), and it’s just a copy of a single gene.

Although mRNA is similar to DNA, there are some important differences worth noting. Firstly, it’s much shorter than DNA, as it’s only a single gene long. Secondly, it’s only as single strand, not a double helix. Thirdly, the base thymine is replaced by ‘uracil’.

Transcription : How mRNA is Made

Say we have a DNA, and within it there is this gene we want to copy. An enzyme called ‘RNA polymerase’ is placed at the end of the gene. What happens next is that a section of the two strands of DNA, the section closest to the enzyme, separates, exposing the bases within. Then the enzyme sorts of ‘goes along’ the gene, separating the entire gene and creating the mRNA as well. What we should remember here is that the strand of mRNA will always be complementary to the DNA.

For example, a ‘C’ on DNA will correspond to a ‘G’ on the mRNA, a ‘G’ to a ‘C’, a ‘T’ to an ‘A’. But the special part is that an ‘A’ doesn’t correspond to a ‘T’, instead, the mRNA will give a ‘U’.

Another thing to notice here is that the DNA strands will open up just ahead of the enzyme, and close just after the enzyme has gone past. When the enzyme has finished its job, it detaches from the DNA, leaving a mRNA that can go out of the nucleus and to the ribosome.

Translation: How the mRNA is read by the ribosome

Before we get into translation, here’s a quick recap: A DNA strand is made up of ‘triplets (each triplet is made up of three bases)’, which are also sometimes known as ‘codons’. Each triplet helps code for a specific amino acid, with 20 amino acids in total. For example, the triplet ‘A,G,U’ codes for the amino acid ‘serine’, while ‘C,C,A’ codes for ‘proline’.

The first step of translation is the ribosome and mRNA strand bind together. Here another structure comes into play, the ‘tRNA’, which stands for transfer RNA. The tRNA have an amino acid at the top, and an ‘anti-codon’ at the bottom. The anti-codon contains a triplet of bases which is complementary.

The tRNA with the complementary codon that the ribosome is reading right now will be attracted to the ribosome and the corresponding triplet. So the tRNA brings with it the amino acid for the triplet. As more and more tRNA with the correct amino acids get attracted to their corresponding triplets on the mRNA strand, the ribosome is able to build up an amino acid chain. Once it has joined two tRNA together, it moves along the mRNA strand and attracts another tRNA to it, building the chain triplet by triplet, tRNA by tRNA. Most importantly, the addition of this third tRNA means that the first tRNA can detach, and leaving its amino acid behind, so no more than two tRNA are in the chain at once.

When the ribosome has finished building an entire amino acid chain that perfectly matches the mRNA, the amino acid chain will detach from the ribosome. Finally, it can fold up on itself to form a protein.