DNA does not 'make proteins', it provides the information which defines which proteins of what kind are made by the cell. The actual process of protein synthesis is bit complicated - https://www.youtube.com/watch? read more
Because our DNA does not make proteins, but just transmits the instructions to do this. Proteins are made up of amino acids. A few of these - the essential amino acids - have to be consumed in the diet, as they cannot be made in our bodies. read more
Now our bodies can make many amino acids. There are 20 total amino acids. The ones we can make are called "Non-essential" amino acids. The essential ones are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. We cannot make these ones, so we have to eat proteins and foods with them in it. read more
On average we eat about 13% of our energy (calories) from protein. That is already well above what we actually need for good health (8-9%) but it allows for having a relatively poor amino acid profile. read more