Proteins are polymers of amino acids, produced in the cell by structures called “ribosomes”. There are 20 different amino acids that are analogous to letters in the alphabet. We have only 26 letters in our alphabet, but they can be ordered and rearranged to produce thousands of words and a near-infinite number of sentences. Amino acids work similarly in making proteins, where the order and arrangement of these amino acids determines the function of the protein.
DNA is a polymer of four different nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). It is arranged in long chains that form a double helix. On each side of the helix is a complementary, matching, nucleotide. Adenine (A) always pairs with thymine (T), while cytosine (C) always pairs with guanine (G). This enables DNA to replicate, or copy itself, by “unzipping” and adding complementary nucleotides to each single strand. This allows new cells to be produced with identical copies of the DNA found in the original cell.
Transcription: DNA to mRNA
In plant and animal cells, DNA stays in the nucleus. The instructions in DNA need to be copied into another molecule, RNA, to be carried to the ribosomes where they can be read to make proteins. RNA is another polymer of four nucleotides very similar to DNA. The only nucleotide that is different from those in DNA is uracil (U), which replaces thymine and complements adenine. mRNA is only a single strand that can leave the nucleus and be used by ribosomes as a recipe for making the protein. mRNA is produced in the nucleus in a process called “transcription”. The DNA strand is separated and the enzyme RNA polymerase adds complementary nucleotides to build a matching mRNA strand.
After the mRNA molecule is transcribed, it can leave the nucleus and move out into the cell where the ribosomes can translate the RNA into a protein.
Translation is the production of a protein from an mRNA strand by a ribosome. During translation, a ribosome uses tRNA molecules to determine the order of amino acids. The tRNA reads sections of mRNA three nucleotides at a time. These three nucleotide sections are called “codons”, and they are complemented by an anticodon on the tRNA molecule. AUG is the “Start” codon. Translation is divided into three parts: initiation, elongation, and termination.
A tRNA molecule attaches to the “Start” codon, AUG. This allows a ribosome to attach to the mRNA.
The ribosome continues to match antocodons to codons and add amino acids to the protein.
Occurs when a “Stop” codon is reached. The mRNA, ribosome, tRNA, and protein are all released. The protein folds into its shape and starts to work in the cell. The other components can be reused to make the protein again.