DNA is a molecule found in the nucleus of cells in all living organisms that hold the information necessary to make proteins. The information is held in the sequence of nucleotides that the DNA is made of (adenosine, cytosine, thymine, and guanine). Sequences that hold the information on how to make a specific protein are called genes. The sequence of a gene is read in triplets of nucleotides and copied to messenger RNA which later is transcribed into a sequence of amino acids that make a protein. When a gene is expressed, it means that the information it held has been used to make a protein. However, the DNA alone can not express itself, so the whole process of making a protein from the information from the DNA sequence is orchestrated by different proteins that participate in different parts of the process of expressing a gene. So, DNA makes proteins, but proteins also take part in replication, translation, and transcription of DNA. Different cells in different tissues express different genes and make different proteins. So, for example, the muscle cells express the genes for proteins needed to make muscle tissue like myosin and the cells in the lens of the eye express the protein crystalline necessary for forming this part of the eye.
