The ribosome is a tiny molecular machine universally found inside living cells. There are millions of them inside eukaryotic cells, and more than ten thousand inside prokaryotic cells. In eukaryotic cells, many ribosomes are found clustered along the endoplasmic reticulum, but also within the cytoplasm in general and in organelles like the mitochondrion and chloroplast. The ribosomes inside bacteria and archaea are smaller than inside eukaryotic cells except that those inside the mitochondria and chloroplasts of eukaryotic cells are closer to prokaryotic ribosomes in size. Many scientists have claimed that the size difference between ribosomes in eukaryotic organelles and in the cell proper support the endosymbiosis theory. There are however multiple reasons why the endosymbiosis theory is false. (See discussion below.)

The ribosome is a very tiny molecular machine. It consists of two parts, a larger component and a smaller one, made from ribosomal RNA and associated proteins. This machine threads a strand of messenger RNA or mRNA (coming from the nucleus) between its two component parts. A groove allows access to another RNA molecule called transfer RNA or tRNA. This is a small RNA molecule which has a triplet code for an amino acid at one end and, if all goes well, the correct amino acid (matching the code at the other end). An enzyme, transfer RNA synthetase, attaches the correct amino acid to a tRNA.  This is the step that makes sure that the information carried in the DNA for a particular protein, is indeed produced by the ribosome reading what is on the mRNA.  There is no physical connection between the DNA and the protein produced by the ribosome. It all comes from the transfer RNA synthetase correctly connecting the appropriate amino acid to the back end of a tRNA which has the matching code at the other end.  Neither DNA nor mRNA control this complex process. The skill and planning for correctly functioning tRNA were conferred on the cell by a mind. There is no other way.