Answer:
Take a moment to look at your hands. The bone, skin, and muscle you see are made up of cells. And each of those cells contains many millions of proteins^1
As a matter of fact, proteins are key molecular "building blocks" for every organism on Earth!
How are these proteins made in a cell? For starters, the instructions for making proteins are "written" in a cell’s DNA in the form of genes. If that idea is new to you, you may want to check out the section on DNA to RNA to protein (central dogma) before getting into the nitty-gritty of building proteins.
Basically, a gene is used to build a protein in a two-step process:
Step 1: transcription! Here, the DNA sequence of a gene is "rewritten" in the form of RNA. In eukaryotes like you and me, the RNA is processed (and often has a few bits snipped out of it) to make the final product, called a messenger RNA or mRNA.
Step 2: translation! In this stage, the mRNA is "decoded" to build a protein (or a chunk/subunit of a protein) that contains a specific series of amino acids. [What exactly is an "amino acid"?]
The central dogma of molecular biology states that information flows from DNA (genes) to mRNA through the process of transcription, and then to proteins through the process of translation.
The central dogma of molecular biology states that information flows from DNA (genes) to mRNA through the process of transcription, and then to proteins through the process of translation.
_Image modified from "Central dogma of molecular biochemistry with enzymes," by Daniel Horspool (CC BY-SA 3.0). The modified image is licensed under a CC BY-SA 3.0 license._
In this article, we'll zoom in on translation, getting an overview of the process and the molecules that carry it out.
The genetic code
During translation, a cell “reads” the information in a messenger RNA (mRNA) and uses it to build a protein. Actually, to be a little more techical, an mRNA doesn’t always encode—provide instructions for—a whole protein. Instead, what we can confidently say is that it always encodes a polypeptide, or chain of amino acids.
[Wait, what is the difference?]
Genetic code table. Each three-letter sequence of mRNA nucleotides corresponds to a specific amino acid, or to a stop codon. UGA, UAA, and UAG are stop codons. AUG is the codon for methionine, and is also the start codon.
Genetic code table. Each three-letter sequence of mRNA nucleotides corresponds to a specific amino acid, or to a stop codon. UGA, UAA, and UAG are stop codons. AUG is the codon for methionine, and is also the start codon.
In an mRNA, the instructions for building a polypeptide are RNA nucleotides (As, Us, Cs, and Gs) read in groups of three. These groups of three are called codons.
There are 616161 codons for amino acids, and each of them is "read" to specify a certain amino acid out of the 202020 commonly found in proteins. One codon, AUG, specifies the amino acid methionine and also acts as a start codon to signal the start of protein construction.
There are three more codons that do not specify amino acids. These stop codons, UAA, UAG, and UGA, tell the cell when a polypeptide is complete. All together, this collection of codon-amino acid relationships is called the genetic code, because it lets cells “decode” an mRNA into a chain of amino acids.
Each mRNA contains a series of codons (nucleotide triplets) that each specifies an amino acid. The correspondence between mRNA codons and amino acids is called the genetic code.
5'
AUG - Methionine
ACG - Threonine
GAG - Glutamate
CUU - Leucine
CGG - Arginine
AGC - Serine
UAG - Stop
3'
To see how cells make proteins, let's divide translation into three stages: initiation (starting off), elongation (adding on to the protein chain), and termination (finishing up).
Getting started: Initiation