Overview Ribosome (Ribosoma):
Pages with explanations are linked to the
text below the images when available
|Stereo image of ribosomes of
the Nissel substance (rat)
+ polyribosomes (guinea-pig)
|Ribosomes on RER
of a human plasma cell
|free and RER-bound ribosomes
(plasma cell rat)
|human plasma cell RER + free
ribosomes, pharyngeal tonsil
|human plasma cell 2
|detail 2: free + RER-
|detail 3:free + RER-
Ribosomes (Terminologia histologica: Ribosomae) are
very tiny cell organelles consisting of ribosomal RNA and globular proteins.
They have diameters of 10 - 25 nm, and they are present in all living
|human plasma cell
|human plasma cell: RER
and free ribosomes
|ribosomes, RER and
|RER of a human
|RER + primary Lysosomes
human plasma cell
Several milions of these spherical corpuscules that are not surrounded
by membranes are encountered in cells with high output of proteins. A basophilic
staining in light microscopy points to a high content of ribosomes since
their rRNAs bind to basic staining chemicals.
Ribosomes are seen singly but more often lie in groups along messenger
RNAs. These groups are called Polyribosomes,
Polysomes or Ergosomes. Further a great amount of ribosomes is embedded
in the outer membrane of the rough endoplasmic reticulum
(RER) and the
outer nuclear membrane
which is connected to the RER.
Ribosomes are assembled in the nucleoli.
In humans and mammals they possess a greater 60s and a smaller 40s subunit.
Both of them are combined to the 80s ribosom (s = band in which particles
are seen after ultracentrifugation). The 60s subunit is comprised of 50
different proteins, a "free" 5s rRNA (ribosomal ribonucleic acid) with
120 bases, a 160 base-long 5,8s rRNA, which is bound on a third 28s rRNA
with 4,800 bases. The 40s subunit has 33 different proteins and a 1,900
base-long 18s rRNAt.
Ribosomes serve for protein synthesis involving a process called
translation.Translation is the realisation
of the genetic information. The aminoacid sequence of the synthesised protein
corresponds to the copy of the genetic code present in the m-RNA attached
to the ribosome. Proteinbiosynthesis is present during the G1-phase of
the cell cyclus, the phase of proliferation that belongs to mitotic interphase.
There are three steps in protein synthesis:
1. Initiation: a m-RNA binds to
the 40s-subunit of a ribosome.
2. Elongation: tranfer-Ribonucleic
acids (t-RNAs) bind to the ribosome according to the copy of the genetic
code on the m-RNA beginning at the startcodon. The specific bound aminoacids
of the t-RNAs are linke to each other by means of the enzyme peptidyl-transferase
to form the coded protein. After delivery of its aminoacid a t-RNAs leaves
the ribosome to give space for the next one while the protein is elongating.
3. Termination end of the process:
when the stopcodon of the m-RNA is reached the ribosome dissolves into
its subunits and the protein splices from the last t-RNA.
Since many ribosomes are seen along one m-RNA many molecules of the
same protein are synchronously syntetised.
--> rough endoplasmic reticulum (RER), outer
nuclear membrane, nucleoli
--> Electron microscopic atlas Overview
--> Homepage of the workshop
Two pictures were kindly provided by Prof. Dr. H.
Wartenberg. Page & copyright H. Jastrow.