Protein Synthesis –
Proteins are made of a long chain of amino acids, which has been coded for by DNA.
The order of the bases of DNA determines which protein is made, by coding for a specific order of amino acids. This is protein synthesis, and in this article we are going to look at how it works.
You need to know ‘what is DNA‘ to remind yourself all about DNA.
- DNA (deoxyribose nucleic acid) is extremely important. All living things have DNA, or in the case of some viruses they have RNA instead.
- In eukaryotes, it is found within the nucleus and is arranged into chromosomes.
- DNA determines the characteristics of an organism. Specific codes of DNA are called genes.
- DNA is made up of nucleotides.
- Nucleotides are repeating units, that are made up of a phosphate group, a sugar group and a nitrogen base.
- There are 4 nitrogen bases: adenine, guanine, cytosine and thymine.
- The bases pair up and the DNA twists into a double helix shape.
A set of three bases in the DNA molecule codes for a particular amino acid.
GGT for example codes for the amino acid called Glycine.
The 3 base codes are known as codons. So GGT is a “codon” for Glycine.
20 Amino Acids :
There are 20 amino acids that make up our body’s proteins.
9 of these we need to take in through our diet as our body cannot manufacture them. These are known as essential amino acids.
Our body can however manufacture non-essential amino acids.
Whilst 20 amino acids build proteins, there are actually many others that do not form proteins, possibly over 250. They may form sugar.
For Example :
Before we start, you need to know ‘what RNA‘.
We have both DNA and RNA in our bodies.
We need them both DNA is the blueprint; it contains all of the instructions for the cell to grow, function and replicate.
The RNA carries out these instructions; it copies and transfers the genetic code from the DNA to ensure the relevant proteins are made. So just think of it as “DNA makes RNA make proteins”.
Whereas DNA stands for deoxyribonucleic acid, RNA stands for ribonucleic acid.
Whilst DNA is double stranded, RNA is single stranded.
Like DNA, RNA is made up of a long chain of nucleotides.
Each nucleotide consists of a ribose sugar, phosphate group and a nucleotide base.
RNA has a sugar called ribose, whereas DNA has a sugar called deoxyribose.
RNA has a base uracil, or U, whereas DNA has the base thymine or T. So in RNA C and G still pair, but now A and U pair.
We need to know about RNA because we will see it when learning about protein synthesis.
We will see two special types of RNA:
- Messenger RNA which is known as mRNA.
- Transfer RNA which is known as tRNA.
- We will see the mRNA being synthesised inside the nucleus, copied from the DNA code.
- The tRNA is found in the cytoplasm.
- For many years, we just thought RNA was a DNA photocopier as mRNA, the protein builder as tRNA and found in ribosomes as rRNA.
- However, RNA can also act as enzymes to speed up chemical reactions. And in many viruses, they have RNA instead of DNA.
- The RNA carries the genetic code in these viruses.
The DNA is in the nucleus and cannot move, but the ribosomes in the cytoplasm are where the proteins are made.
This means the code from the DNA needs to be copied and carried across to the ribosome by a molecule called messenger RNA or mRNA before the protein can be made.
Let’s have a look at how this all works.
In the nucleus, the enzyme RNA polymerase unwinds and unzips the two strands of DNA that contain the protein-making gene.
Only one of these strands is going to be replicated.
Complementary RNA nucleotides base pair with the chosen strand.
RNA polymerase also binds the RNA nucleotides together making a new RNA strand. This is the messenger RNA or mRNA.
This process inside the nucleus is called transcription.
The mRNA travels from the cell nucleus and out into the cytoplasm, until it reaches and attaches to a ribosome.
The ribosome then sticks amino acids together to make a polypeptide chain, following the order of amino acids as coded by the mRNA.Three base codons on the mRNA code for one amino acid.
This process is called translation.
Let’s look at translation in a little more detail.
Translation in Detail :
What is actually happening inside the ribosome ?
Transfer RNA or tRNA molecules, carrying specific amino acids, base pair with the mRNA inside the ribosome.
So the tRNA brings in the amino acids and the mRNA provides the information of the exact order that the amino acids need to be bonded together in, to ensure the correct protein is made.
As more tRNAs match up with the mRNA, the amino acid chain becomes longer. Eventually the polypeptide chain will form into the protein.
So there we have protein synthesis.
Inside the nucleus, the DNA molecule is unzipped from around the gene by DNA polymerase.
The DNA is replicated and mRNA is formed in the process of transcription. The mRNA travels out of the nucleus and into the cytoplasm, where it binds with a ribosome.
In the ribosome, the nucleotide sequence of the mRNA is translated by tRNA molecules which carry related amino acids.
The polypeptide chain is formed, and will eventually fold into the required protein.