Ribosomes - Basics

Basics only





Ribosome’s are a cell structure that makes protein (seat of protein synthesis).
• Ribosomes are often referred as PROTEIN FACTORY of the cell.
• Protein is needed for many cell functions such as repairing damage or directing
chemical processes.


































• The ribosome is a complex molecule made of ribosomal RNA molecules and
proteins that form a factory for protein synthesis in cells.

1. RIBOSOMES | 1

RIBOSOMES
• Ribosome’s are a cell structure that makes protein (seat of protein synthesis).
• Ribosomes are often referred as PROTEIN FACTORY of the cell.
• Protein is needed for many cell functions such as repairing damage or directing
chemical processes.
• The ribosome is a complex molecule made of ribosomal RNA molecules and
proteins that form a factory for protein synthesis in cells.

DISCOVERY OF RIBOSOMES
• Ribosomes were discovered by ROBINSON AND BROWN (1953) in plant cells and
by GEORGE E. PALADE (1955) in animal cells.
• G E Palade (1955) also coined the term of ribosome.
• G E Palade was awarded the Nobel Prize in 1974 for his work.
OCCURRENCE OF RIBOSOMES
• Ribosome’s can be found floating within the cytoplasm or attached to the
endoplasmic reticulum.
• The location of the ribosomes in a cell determines what kind of protein it
makes.
• If the ribosome’s are floating freely throughout the cell, it will make proteins
that will be utilized within the cell itself.
• When ribosomes are attached to endoplasmic reticulum , it is referred to as
rough endoplasmic reticulum or rough ER. Proteins made on the rough ER are
used for usage inside the cell or outside the cell.
• 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 may occur singly as monosomes or in rosettes and helical groups
called polyribosomes or polysomes.
ULTRASTRUCTURE AND COMPOSITION OF RIBOSOME
• Around 37 to 62% of RNA is comprised of ribosomal RNA (rRNA) molecules and
the rest is variety of proteins. Therefore, it is a RIBONUCLEOPROTEIN .
• Each ribosome has a large component and a small component that together
form a single unit composed of several ribosomal RNA molecules and dozens of
proteins.
• 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.

2. RIBOSOMES | 2

• The smaller subunit fits over the larger one at one end like a cap. Mg
2+
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 ).

• Prokaryotes have 70S ribosomes respectively subunits comprising the little -
small subunit of 30S with 16S RNAs and the large subunit of 50S with 23S + 5S
RNAs.
• Eukaryotes have 80S ribosomes respectively comprising of little -small 40S with
18S RNAs and substantial large 60S subunits with 28S+5S+5.8S RNAs.
• The ribosomes seen in the chl oroplasts of mitochondria of eukaryotes are
comprised of big and little subunits composed of proteins inside a 70S particle.
• The RNA is arranged in different tertiary structures.

3. RIBOSOMES | 3



• Fully assembled ribosomes have three tRNA binding sites: an A site for
incoming aminoacyl-tRNAs, a P site for peptidyl-tRNAs, and an E site where
empty tRNAs exit.

4. RIBOSOMES | 4

FUNCTIONS OF RIBOSOMES
1. They assemble amino acids to form specific proteins, proteins are essential to
carry out cellular activities.
2. The genetic message from the mRNA is translated into proteins during DNA
translation. The mRNA is synthesized in the nucleus and is transported to the
cytoplasm for further process of protein synthesis.
3. The sequences of protein assembly during protein synthesis are specified in
the mRNA.
4. In the cytoplasm, the two subunits of ribosomes are bound around the
polymers of mRNA; proteins are then synthesized with the help of transfer
RNA.
5. The proteins that are synthesized by the ribosomes present in the cytoplasm
are used in the cytoplasm itself. The proteins produced by the bound
ribosomes are transported outside the cell.