In this article we will discuss about:- 1. Discovery of Ribosomes 2. Occurrence of Ribosomes 3. Functions.
Discovery of Ribosomes:
Ribosomes were discovered by Robinson and Brown (1953) in plant cells and by Palade (1955) in animal cells. Palade (1955) also coined the term of ribosome. A large number of ribosomes occur in a cell. For example, a single cell of bacterium Escherichia coli contains 20000-30000 ribosomes. Their number in eukaryote cells is several times more.
Ribosomes are naked ribonucleoprotein protoplasmic particles (RNP) with a length of 200-340 A and diameter of 170-240A which function as the sites for protein or polypeptide synthesis. Ribosomes are popularly known as protein factories. They are sub-spherical in outline. A covering membrane is absent. Each ribosome consists of two unequal subunits, larger dome shaped and smaller oblate-ellipsoid.
The large subunit has a protuberance, a ridge and a stalk. The smaller subunit possesses a platform, cleft, head and base. It is about half the size of larger subunit.
The smaller subunit fits over the larger one at one end like a cap (Fig. 8.40). Mg2+ is required for binding the two subunits (Below 0.0001 M Mg2+ the two subunits dissociate while above this strength the ribosomes can come together to form dimers Fig. 8.39).
Occurrence of Ribosomes:
Ribosomes may occur singly as monosomes or in rosettes and helical groups called polyribosomes or polysomes (Gk. poly- many, soma- body).
The different ribosomes of a polyribosome are connected with a 10-20 A thick strand of messenger or mRNA (Fig. 8.41). The maintenance of polyribosome requires energy. Polyribosomes are formed during periods of active protein synthesis when a number of copies of the same polypeptide are required.
Ribosomes occur in all living cells with the exception of mammalian erythrocytes or red blood corpuscles. Depending upon the place of their occurrence, ribosomes are of two types, cytoplasmic and organelle. The organelle ribosomes are found in plastids (plastid ribosomes) and mitochondria (mitoribosomes).
The cytoplasmic ribosomes (cytoribosomes) may remain free in the cytoplasmic matrix or attached to the cytosolic surface of endoplasmic reticulum with the help of a special ribophorin or SRP protein.
Attachment occurs through larger or 60 S subunits. Different types of ribosomes may produce different types of proteins, e.g., structural proteins from free cytoplasmic ribosomes and globular proteins from ribosomes bound to ER.
The bound ribosomes generally transfer their proteins to cisternae of the endoplasmic reticulum for transport to other parts both inside and outside the cell. They are also sent to intracellular organelles like nucleus, mitochondria and chloroplasts. Newly synthesised proteins are assisted in their folding and transport by specific proteins called chaperones.
The size of the ribosomes is determined by sedimentation coefficient in the centrifuge. It is measured as Svedberg unit called S (S =1 x 10-13 sec). The cytoplasmic ribosomes of eukaryotes are 80 S.
They have a size of 300—340 Ax 200-240 A and mass of 4.0—4.5 million daltons. The cytoplasmic ribosomes of prokaryotes (PPLO, bacteria, and blue – green algae) are 70 S. The size is 200-290 A x 170-210 A and mass is 2.7-3.0 million daltons (Fig. 8.42).
The organelle ribosomes are also 70 S but in mammalian mitochondria they have sedimentation coefficient of 55 S. The two subunits of 80 S ribosomes are 60S and 40S while 70S ribosomes have 50S and 30 S subunits. A tunnel occurs between the two subunits for passage of mRNA. The larger subunit has a groove for pushing out the newly synthesized polypeptide.
A ribosome has four sites for specific attachments:
(i) mRNA binding site,
(ii) A or amminoacyl site for binding to newly arrived amino acid carrying tRNA.
(iii) P or peptidyl site with tRNA carrying growing polypeptide,
(iv) E or exit site for freed tRNA before it leaves the ribosome.
80S ribosomes are synthesized inside the nucleolus. Proteins come from cytoplasm. 5S RNA is synthesized separately while others are formed by the nucleolus. 80S ribosomes do not become functional inside the nucleolus.
Their subunits come out of the nucleus and become operational in cytoplasm. 70S ribosomes of prokaryotes are formed in the cytoplasm while those of semi-autonomous cell organelles are formed in their matrix.
Chemically a ribosome is made of two parts, proteins and rRNA. The ribosomes of liver cells may also contain lipids to the extent of 5-10%. Usually more rRNA is present in 70S ribosomes as compared to protein (60-65: 35-40) while the reverse is true for 80S ribosomes (40-44: 56-60). 40S subunit of 80S ribosome contains 33 protein molecules and a single 18S rRNA.
30S subunit of 70S ribosome possesses 21 protein molecules and 16S rRNA. 60S subunit of 80S ribosome has 40 protein molecules and three types of rRNAs- 28S, 5.8S and 5S. 50S subunit of 70S ribosome contains 34 protein molecules and two types of rRNAs— 23S and 5S. Proteins are both structural and enzymatic.
Functions of Ribosomes:
(i) Protein Factories:
Ribosomes are sites for polypeptide or protein synthesis.
(ii) Free and Attached Ribosomes:
Free ribosomes synthesise structural and enzymatic proteins for use inside the cell. The attached ribosomes synthesise proteins for transport,
(iii) Enzymes and Factors:
Ribosomes provide enzymes (e.g., Peptidyl transferees) and factors for condensation of amino acids to form polypeptide,
Ribosome contains rRNAs for providing attaching points to mRNA and tRNAs.
Ribosome has a tunnel for mRNA so that it can be translated properly,
Newly synthesized polypeptide is provided protection from cytoplasmic enzymes by enclosing it in the groove of larger subunit of ribosome till it attains secondary structure.