STRUCTURE:
Ribosomes are tiny particles, about 200 A. It is composed of both
proteins and RNA; in fact it has approximately 37 - 62% RNA, and rest
are made up of proteins.
The RNA present in ribosomes are obviously called ribosomal RNA, and
they are produced in the nucleolus, which is a prominent globular
structure in the nucleus. Thus, the proteins are gene products of themselves, and one ribosome is made up of dozens of genes.
The ribosomes fall into two categories: Those that are free to roam in the
cytoplasm , and those that are bound to gigantic, cobwebby organelles made up of membranes, called the
endoplasmic reticulum; thus, causing a
rough surface.
Although, the two kinds of ribosomes play similar roles in translating
mRNA to produce proteins, they are very distinct in where its product
is located. The ribosomes in the cytoplasm allows its protein to roam
about freely, while the bound ribosomes transfer their functional
protein into the endoplasmic reticulum. In addition, ribosomes are also
located within the mitochondria, and the chloroplast, but are only few
in content.
Click Here This spherical particle of 23nm, is composed of two subunits; a
large and
small In Eukaryotes, the co-efficient of ribosomes are 80s, of which is
divided into 60s for the large, and 40s for the small subunit. The 60s
contain 28s rRNA, with a small fragment that is attached noncovalently
and can be released upon heating; a 5.8s, and a very small - 120
nucleated of 5sRNA. Whereas, the 40s subunit has only a single 18s rRNA
.
In prokaryotes, however, the large and small subunits are split into
50s and 30s, making a total of 70s respectively. The 50s has two types
of rRNA -
a 23s and a 5s . It also has 32 different proteins. On the other hand, the 30s contains a single 16s rRNA plus, 21 different types of proteins
Label.
To help better understand what the s stands for in rRNA, let us use the
prokaryotes as an example. The 50s and 30s refers to the sedimentation
coefficient of the two subunits. This coefficient is a measure of the
speed with which the particles sediment through a solution when spun in
an ultra centrifuge. Thus, the particles with larger coefficient would
centrifuge and settle much faster since it is has more mass than the
particle with the smaller coefficient.
50s + 30s =======> 70s
Note that the two subunits above make up the entire ribosomal molecule
which is 70s. The reason the coefficients do not add up is because they
are not proportional to the particle weight.
During protein synthesis, ribosomes line up along the mRNA and form a
polysome, also called the polyribosome. The mRNA is aligned in the gap
between the 2 ribosomal subunits. It is possible that the nascent
peptide chain grows through a channel or groove in the large ribosomal
subunit. This is predicted to be the case since ribosomes protect a
segment of 30-40 amino acids from degradation. Speaking of amino acids,
up to 30 ribosomes can attach on one strand of mRNA to form amino acid
chains thus leading to protein formation. Ribosomes act as the backbone
for many molecules during translation. It provides room for many
structures to situate itself thus enhancing
protein synthesis. For example, mRNA inserts itself between the two subunits; the
peptidyl transferase complex - the enzyme that allows for the tRNA to
break apart from the amino acid on P-site; this enzyme lays across the
molecule, between the subunits. It contains the P and the A-site for
tRNA binding. Last but not least, the ribosome molecule allows the
growing polypeptide chain, to emerge from the back of the structure,
thus it is situated perpendicular to the mRNA chain.
Ribosomes have a tertiary structure.
Ribosomes make up a large part of cells in many species, which leads to
protein manufacturing. For example, in E.Coli (bacteria), they make up
about 1/4 of the total cell mass. They are intensely basiphilic
(having high affinity for bases).
Due to its complex structures, with many proteins and different kinds
of RNA, researchers have found it very difficult to study the macro
molecular structure of ribosomes, especially for the fact it is quite
impossible to observe its crystal using an x-ray diffraction. Thus,
scientists have been forced to use other means of study to map the
proteins and RNA components in ribosome. Some of these are the
cross-linking, immunoelectron microscopy, and low-angle neutron
scattering methods. The cross-linking shows the protein arrangement and
the types of bonds it forms within itself. The neutron scattering
experiments forms horizontal lines that show the entire structure of
ribosome, with its two subunits, and shows where the proteins are
arranged in the molecule. The empty regions around the proteins is
where the rRNA is located. The immunoelectron microscopy, shows the
proposed location of the 16s rRNA molecule of the small subunit, in
prokaryotes.
Function:
The ribosomes plays a very important role in protein synthesis, which
is the process by which proteins are made from individual amino acids.
Without the ribosomes the message would not be read, thus proteins could
not be produced. Therefore, ribosomes play a very important role in
role in protein synthesis.
The primary agent in the process of translating the mRNA into a
specific amino acid chain is the ribosome, which consists of two
subunits. These subunits are made up of a third and extremely abundant
type of RNA, ribosomal RNA (rRNA), and together contain up to eighty-two
specific proteins assembled in a precise sequence.The ribosomes constituents must be put together in an extremely
precise position and sequence. This assembled ribosome displays a
series of small groves, tunnels, and platforms, where the action of
protein synthesis occurs
.There are the active sites, each dedicated to one of the tasks
required for translation of mRNA into protein. Proteins being
synthesized for export out of the cell, are made by ribosomes attached
to the rough endoplasmic reticulum. In contrast, proteins for use by
the cell are generally made in the cytoplasm by free ribosomes. Several
of these free ribosomes may attach to a single mRNA molecule, giving
rise to the polyribosome or polysome.
Protein synthesis takes place on polyribosomes (or polysomes) where 80S
ribosomes associate with an mRNA coding for a given protein. The
number of ribosomes associated in the polysomal chains depends on the
size of the mRNA. This is also associated with the size of the protein
that is being synthesized. Outside the polyribosome, the ribosomes are
dissociated and form a pool of free subunits. Transfer RNAs are also
bound to the ribosome. There are quite a few factors involved in the
formation of the initiation complex. These include: GTP, methionine
tRNA, an initiation codon in mRNA, 80S ribosomes, and three protein
factors
.
The process of protein synthesis begins with the capture of the tRNA,
which is carrying an amino acid, by an initiation factor. This binds to
a small ribosomal subunit, which occupies one of the active sites in
the ribosomes, the P (protein) site. This initiation complex recognized
and binds to the 5' end of an mRNA molecule and slides down to the
initiation codon, which is always an AUG sequence of amino acids. The
large subunit of the ribosome now joins the complex. A second tRNA is
now brought into the ribosome by the elongation factor. If the
anticodon of the tRNA pairs with the next codon of the message, the tRNA
occupies the A (acceptor) site on the ribosome. This positions the
second amino acid adjacent to the initiation methionine. Then an
enzyme, peptidyl transferase, which is part of the large ribosomal
subunit mediates the separation of the first amino acid from its tRNA
and the formation of a peptide bond between the initial methionine and
the amino acid is formed. The P site is now occupied by an uncharged
tRNA molecule .
The ribosome will now move down the mRNA by one codon, a process known
as translocation. This movement shifts the growing polypeptide chain to
the P position, and results in an empty A site, where a new charged
tRNA can enter and pair, by forming a hydrogen bond between the codon
and the anticodon. This holds the tRNA into place long enough for an
even more stable binding to occur.
The uncharged tRNA that previously occupied the P site is booted out
of the ribosome and will be recharged and recycled by the cell. The
energy needed for this process is supplied by the hydrolysis of
guanosine triphosphate (GTP). The process then continues along the
length of the mRNA, until the first stop codon is encountered. At that
point the action of a termination factor releases the completed protein
from the last tRNA and the ribosome dissociates into its component
parts.
Another function of the ribosomes occurs in the relation to the neuron
and axons. The cell body of a typical large neuron contains vast
numbers of ribosomes. Although dendrites often contain some ribosomes,
there are no ribosomes in the axon, and its protein must therefore be
provided by the many ribosomes in the cell body.
To see the process of protein synthesis
