Ribosomes || MANISH MEVADA

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CELL : STRUCTURE AND FUNCTIONS

 Ribosomes (Palade Paericles)

• 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 eucaryotic cells is several times more.
• Ribosomes are naked ribonucleoprotein protoplasmic particles (RNP) with a length of 200-340 Å and diameter of 170-240Ả which function as the sites for protein or polypeptide synthesis.
• Ribosomes are popularly known as protein factories.
• They are subspherical 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.
• Mg2 + is required for binding the two subunits (Below 0.0003 M or 0.3 mM Mg2 + the two subunits dissociate while above this strength the ribosomes can come together to form dimers.

• Ribosomes may occur singly as monosomes or in rosettes and helical groups called polyribosomes (Rich, 1963) 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.
• 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 ( plastidoribosomes ) 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 3 sec ).
• The cytoplasmic ribosomes of eucaryotes are 80 S. They have a size of 300-340 Åx 200-240 Å and mass of 4.0 - 4.5 million daltons.
• The cytoplasmic ribosomes of procaryotes ( PPLO , bacteria, blue - green algae) are 70 S.
• The size is 200-290 8 x 170-210 Å and mass is 2.7-3.0 million daltons.
• 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 mRNA.
• The larger subunit has a groove for pushing out the newly synthesised polypeptide.
• A ribosome has four sites for specific attachments.

1. mRNA binding site.
2. A or amminoacyl site for binding to newly arrived aminoacid carrying tRNA.
3. Por peptidyl site with tRNA carrying growing polypeptide.
4. E or exit tRNA before it leaves the ribosome

• 80S ribosomes synthesised inside the nucleolus.
• Proteins come from cytoplasm.
• 5S RNA is synthesised 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 cyto plasm.
• 70S ribosomes of procaryotes are formed 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 705 ribosomes as compared to protein (60-65 : 35–40) while the reverse is true for 80S ribosomes (40-44 : 56-60).
• 405 subunit of 80S ribosome contains 33 protein molecules and a single 18S rRNA.
• 30S subunit of 70S ribosome possesses 21 protein molecules and 16S TRNA.
• 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, 235 and 5S.
• Proteins are both structural and enzymatic .

Functions

1. Protein Factories . Ribosomes are sites for polypeptide or protein synthe sis.
2. Free and Attached Ribosomes . Free ribosomes synthesise structural and enzymatic proteins for use inside the cell . The attached ribosomes synthesise proteins for transport.
3. Enzymes and Factors . Ribosomes provide enzymes ( e.g. , Peptidyl transferase ) and factors for condensation of amino acids to form polypeptide.
4. rRNA . Ribosome contains rRNAs for providing attaching points to mRNA and tRNAs.
5. mRNA . Ribosome has a tunnel for mRNA so that it can be translated properly.
6. Protection . Newly synthesised polypeptide is provided protection from cytoplasmic enzymes by enclosing it in the groove of larger subunit of ribosome till it attains secondary structure .

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Manish Mevada

M.Sc, M.Phil, B.Ed

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