1. Ribosomes – Structure and Function (A Molecular Biology Presentation)
Submitted by
Roy Antony (DU2019MSC0096)
MSC Biotechnology – 2nd Sem.
ADBU, Tapesia

2. Ribosomes –General Outlook & Structure
What are Ribosomes?
Ribosomes are nano macines or cell organelles associated with protein synthesis .They are present in all the Prokaryotic & Eukaryotic cells (except RBCs).
Hoiw many?:
The number of ribosomes in the cell veries from in a prokaryotic cell to about 10 million in a typical mammalian (Eukaryotic) cell.
Discovery:
Ribosomes were first observed by Romanian-American cell biologist George Emil Palade, using an electron microscope, in the mid-1950s.
Structure & Molecular Composition
Structurally each ribosome consists of two sub units – the larger unit and the smaller unit, both of which have unique function.
Chemically each ribosome unit is a ribonucleoprotein complex consisting of rRNAs, ribosomal Proteins and divalent ions, mainly Mg2+, which are responsible for the association and dissociation of the two sub units of ribosomes during protein synthesis.
The structure also contain the three tRNA sites, namely, Peptidyl (P), Acceptor (A) and Exit (E) sites which have specific role during protein synthesis.

3. Types of Ribosomes
Based on Where they are found - Free and Membrane bound Ribosomes:
Free Ribosomes: They are floating freely in the cytoplasm/cytosol (called free ribosomes) . In the prokaryotes, ribosomes float freely in the cytosol.. Several ribosomes attached to the same mRNA strand, is called a polysome.
Membrane bound Ribosomes:
They are bound to the smooth endoplasmic reticulum( called membrane bound ribosomes) to form rough ER.
In eukaryotic cells both these types of ribosomes are present. They are made in the nucleolus, and later settled in the cell.
Based on the Sedimentation Coeficient - 70S & 80S Ribosomes:
70S: prokaryotic ribosomes are 70S ribosomes, composed of two subunits having Svedberg coefficients of 30S (smaller subunit) and 50S (larger subunit). 
80S: Eukaryotic ribosomes are 80S ribosomes composed of a 60S large subunit and a 40S small subunit.

4. Comparison between 70S & 80S Ribosomes
S.No
Features
70S
80S 1
Presence
Mostly present in Prokaryotic cells, autonomous organnelles like mitochondrea, plastids, etc.
Present in Eukaryotic cells 2
Origin
synthesized in the cytoplasm
synthesized in the nucleolus .
Size
Smaller compared to 80S
(about 20 nm (200 Å) in diameter)
Larger compared to 70S
(are about 25 and 30 nm (250–300 Å) in diameter. 3
Type
Mostly freely floating type
Freely floating or membrane bound 4
Sub units & RNAs
50S (23S + 5S + about 34 proteins) & 30S (16S +about 21 Proteins. Total RNAs -03
60S (28S + 5.8S + 5S + about 49 Protein) &
40S (18S + about 33 Proteins). Total RNAs - 04 5
Inhibition
Protein synthesis of 70S ribosomes is inhibited by cycloheximide.
Protein synthesis of 70S ribosomes is inhibited by Chloramphenicol.

5. Function:
Ribosomes play key role in the protein synthesis by translation process..It includes 1. Iniciation, elongation and termination.
(1) Decode and Translate encoded information from the cell nucleus provided by messenger ribonucleic acid (mRNA),
(2) Link together amino acids selected and collected from the cytoplasm by transfer ribonucleic acid (tRNA). (The order in which the amino acids are linked together is determined by the mRNA) and,
(3) Export the polypeptide produced to the cytoplasm where it will form a functional protein.

6. References: 1. Introduction to Ribosomes: http://webdoc. sub. gwdg
Thank You