Proteins are made up of long chains of amino acids. In humans, there are 20 different naturally occurring amino acids. A sequence of three bases codes for a single specific amino acid.
So, the order of bases in the DNA determines which proteins are produced. When the amino acids join together, they form a unique sequence, which then folds to create a unique shape.
Every protein has a particular number and order of amino acids, which is why each protein has a different, specific shape and function. For example, some proteins are:
Enzymes – Biological catalysts – An example is amylase
Hormones – Send messages to different parts of the body – An example is insulin
Structural proteins – Provide mechanical support – An example is collagen
There are many other types, and these proteins are responsible for all the cellular functions in the body.
Protein synthesis occurs in two stages:
1. The two strands of the DNA helix unwind, as the weak hydrogen bonds between the complementary base pairs break.
2. An enzyme known as RNA polymerase binds to the DNA in a non-coding region located near the beginning of a gene.
3. RNA polymerase moves along the DNA strand and then builds a complementary strand, known as mRNA (messenger RNA).
4. The mRNA strand leaves the nucleus through the nuclear pore, to travel to the ribosome.
An RNA molecule has nucleotides similar to those in a DNA molecule; however, in RNA, thymine (T) is replaced by uracil (U). During transcription, uracil in the newly formed mRNA strand pairs with the adenine (A) base of the DNA template strand.
1. After the mRNA leaves the nucleus, it travels through the cytoplasm, and then attaches to a ribosome.
2. Within the cytoplasm, there are carrier molecules called transfer RNA (tRNA). They transport specific amino acids. Each tRNA molecule has a region known as the anticodon, which is a sequence of three unpaired bases that can pair with corresponding bases on the mRNA molecule. After delivering its amino acid, the tRNA molecule exits the ribosome.
3. A peptide bond forms between the two amino acids.
4. This process will continue until a long amino acid chain forms, which is called a polypeptide.
After translation, the polypeptide chain will fold into a specific three-dimensional shape to form a functional protein.
In the diagram below, you can see the whole process of protein synthesis.