Ribosomes are structures within cells with a single critical function: to make proteins.
Ribosomes themselves consist of about one-third protein by mass; the other two-thirds consist of a specialized form of ribonucleic acid (RNA) called ribosomal RNA, or rRNA. (Soon, you'll meet the other two major members of the RNA family, mRNA and tRNA.)
Parts of a Cell
As noted, prokaryotes – single-celled organisms that make up the domains Bacteria and Archaea – possess the four structures common to all cells.
- DNA: This nucleic acid holds all of the genetic information about its parent organism, which is transmitted to subsequent generations. Its "code" is also used to make proteins through the sequential processes of transcription and translation.
- A Cell Membrane: This double plasma membrane, consisting of a phospholipid bilayer, is a selectively permeable membrane, allowing some molecules to pass in unimpeded while barring entry to others. It provides shape and protection to all cells.
- Cytoplasm: Also called cytosol, the cytoplasm is a gelatinous matrix of water and proteins that serves as the substance of the interior of the cell. A number of important reactions take place here, and this is where most ribosomes are found.
- Ribosomes: Found in the cytoplasm of all organisms and elsewhere in eukaryotes, these are the protein "factories" of cells, and consist of two subunits. They contain the sites upon where translation occurs.
Eukaryotes have more complex cells, containing organelles, which are surrounded by the same sort of double plasma membrane that surrounds the cell as a whole (the cell membrane). Some of these organelles, most notably the endoplasmic reticulum, host a great many ribosomes. Chloroplasts of plants have them, as do the mitochondria of all eukaryotes.
The endoplasmic reticulum (ER) is like a "highway" between the nucleus of the cell and the cytoplasm, and even the cell membrane itself. It shuttles protein products around, which is why it is advantageous for ribosomes, which make those proteins, to be neighbors with ER.
When ribosomes are seen bound to ER, the result is called rough ER (RER). ER untouched by ribosomes is called smooth ER (SER).
Translation is the final step in the process of the cell carrying out genetic instructions. It starts, in a sense, with DNA making messenger RNA (mRNA) in a process called transcription. The mRNA is a sort of "mirror image" of the DNA from which it was copied, but it contains the same information. The mRNA then attaches itself to ribosomes.
The mRNA is joined on the ribosome by specific molecules of transfer RNA (tRNA) that bind to one and only one of the 20 amino acids found in nature. Which amino acid residue is brought to the site – that is, which tRNA arrives – is determined by the nucleotide base sequence on the mRNA strand.
mRNA contains four bases (A, C, G and U), and the information for a given amino acid is contained in three consecutive bases, called a triplet codon (or sometimes just codon), such as ACG, CCU, etc. This means that there are 43, or 64, different codons. This is more than sufficient to code for 20 amino acids, and this is why some amino acids are coded for by more than one codon (redundancy).
Amino Acids and Proteins
Amino acids are the building blocks of proteins. Where proteins consist of polymers of amino acids, also called polypeptides, amino acids are the monomers of these chains.
(The distinction between a polypeptide and a protein is largely arbitrary.)
Amino acids include a central carbon atom joined to four distinct components: a hydrogen atom (H), an amino group (NH2), a carboxylic acid group (COOH) and an R-side chain that gives each amino acid its unique formula and distinctive chemical properties. Some of the side chains have an affinity for water and other electrically polar molecules, whereas the side chains of other amino acids behave in an opposite way.
The synthesis of proteins, which is simply the addition of amino acids end to end, involves the linkage of the amino group of one amino acid to the carboxyl group of the next. This is called a peptide linkage, and it results in the loss of a water molecule.
Ribosomes can be said to consist of ribonucleoprotein, since, as described above, they are assembled from an unequal blend of rRNA and proteins. They consist of two subunits that are classified in terms of their sedimentation behavior: a large, 50S subunit and a small, 30S subunit. ("S" here stands for Svedberg units.)
The large subunit contains 34 different proteins, along with two types of rRNA, a 23S kind and a 5S kind. The small subunit contains 21 different proteins and a type of rRNA that checks in at 16S. Only one protein is common to both subunits.
The components of the subunits are themselves made in the nucleolus inside the nuclei of prokaryotes. They're then transported through a pore in the nuclear envelope to the cytoplasm.
Ribosomes do not exist in their fully assembled form until they are called upon to do their jobs. That is, the subunits spend all of their "leisure time" alone. So when translation is getting underway in a particular part of a given cell, ribosome subunits in the vicinity are starting to become acquainted again.
Much of the function of the larger subunit relates to catalysis, or the speeding up of chemical reactions. This is normally the purview of proteins called enzymes, but other biomolecules occasionally act as catalysts, too, and portions of the large ribosomal subunit are an example. This makes the functional component a ribozyme.
The small subunit, in contrast, seems to have more of a decoder function, getting translation past the very beginning stages by locking onto the right large subunit at the right spot at the right time, carrying what the pair needs to the scene.
Steps of Translation
Translation has three main phases: Initiation, elongation and termination. To summarize each of these parts of transcription in brief:
Initiation: In this step, incoming mRNA binds to a spot on the small subunit of a ribosome. A specific mRNA codon triggers an initiation by tRNA-methionine. It is joined there by a specific tRNA-amino acid combination determined by the mRNA sequence of nitrogenous bases. This complex connects to the large ribosomal subunit.
Elongation: In this step, polypeptides are assembled. When each incoming amino acid-tRNA complex adds its amino acid to the binding site, this is transferred over to a nearby spot on the ribosome, a second binding site that holds the growing chain of amino acids (i.e., the polypeptide). Thus incoming amino acids are "handed off" from one spot to another on the ribosome.
Termination: When the mRNA is at the end of its message, it signals this with a particular base sequence that flags "stop." This causes the accumulation of "release factors" that prevent the binding of any more amino acids to the polypeptide. Protein synthesis at this ribosomal location is now complete.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.