All polypeptides are made by common evolutionarily conserved process, known as translation.

The translation machinery is programmed by a transcript, a molecule of messenger RNA or mRNA. RNA is related to DNA and will be discussed in greater detail in the next section.

An mRNA contains all of the information required to define the primary structure of a polypeptide.

The information it contains is translated by the ribosome into a polypeptide.

The ribosome is a large multisubunit complex composed of polypeptides and ribosomal RNAs or rRNAs.

The cytoplasm of most cells is packed with ribosomes. In rapidly growing bacterial cell ~25% of total cell mass is ribosomes.

The ribosome is ~30 nm in diameter and has a molecular weight of ~3,000,000 daltons.

There are some characteristic differences between the ribosomes of bacteria and eukaryotes.

This is important from a practical perspective. A number of antibiotics inhibit translation by bacterial ribosomes, but not by eukaryotic ribosomes.

Both chloroplasts and mitochondria also have ribosomes.

These are inhibited by the same drugs that inhibit bacterial ribosomes.

This is yet another piece of evidence that chloroplasts and mitochondria are descended from bacterial endosymbionts.

A David S. Goodsell image.

Ribosomes are composed of approximately equal amounts of RNA and protein. A catalytic RNA, a ribozyme, lies at the heart of the ribosome - it catalyzes the addition of amino acids to the growing polypeptide chain.

We will talk more about RNA structure and function later.

An active ribosome is composed of a small and a large ribosomal subunit.

In eukaryotes, the small subunit contains a single rRNA and 21 ribosomal polypeptides. The large subunit contains two rRNAs and 34 polypeptides.

In the cytoplasm large and small ribosomal subunits are separated until they find an mRNA.

Together with accessory factors, they associate with the mRNA and assemble into a functional ribosome and begin the translation of the mRNA.

When the ribosome reaches the end of the region of the RNA that encodes the polypeptide, it is released, disassembles and is ready to start another cycle.

Among these accessory factors are the transfer RNAs or tRNAs.

These are small RNAs with an L-shaped structure. There is a specific type of tRNA for each amino acid.

For example, a tRNA specific for phenylalanine would be written tRNA^Phe.

A specific set of enzymes, amino acyl tRNA synthetases, recognize specific tRNAs and catalyze the attachment of the appropriate amino acid to the tRNA's acceptor stem.

The information that specified the sequence of amino acids in a polypeptide is arranged as a sequence of codons in the mRNA.

At the other end of the tRNA from the acceptor stem is the anticodon which recognizes the coded information present along the mRNA.

The mRNA moves through the ribosome, bringing one codon after another into place.

These are recognized by the amino acid-charged tRNAs.

Once in place, the ribosome then catalyzes the formation of a peptide bond.

This results in the movement of the growing or nascent polypeptide to the newly arrived amino acid-charged tRNA and the release of the now uncharged tRNA.

Here is a nice translation tutorial

This process uses energy both to move the mRNA through the ribosome and to form the peptide bonds.

Initiating translation

The ribosome does not start translating an mRNA at its end, but requires an internal signal to determine where translation should start.

The initial amino acid of the polypeptide in almost always a methionine and is encoded by the start codon AUG.

Similarly, the end of the polypeptide is marked by a stop codon, either UGA, UAA or UAG.

Accessory factors are associated with translation initiation, elongation and termination.

A tRNA associated with the elongation factor TU (EF-TU)

For example, the polypeptide release factors involved in translational termination look very much like a tRNA.

Since there are no tRNAs that recognize the stop codons, when the ribosome reaches a stop codon it pauses.

During that pause, a release factor binds in the space normally occupied by a tRNA.

This leads to the release of the polypeptide from the ribosome, and the disassembly of translation complex.