OPTION 3

•            A ribosome functions as a micro-machine for making proteins. Ribosomes are composed of special proteins and nucleic acids.

•            The TRANSLATION of information and the Linking of AMINO ACIDS are at the heart of the protein production process.

•            A ribosome, formed from two subunits locking together, functions to:

(1)         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.)

(3)         Export the polypeptide produced to the cytoplasm where it will form a functional protein.

 

 

•            Ribosomes are found ‘free’ in the cytoplasm or bound to the endoplasmic reticulum (ER) to form rough ER. In a mammalian cell there can be as many as 10 million ribosomes.

•            Several ribosomes can be attached to the same mRNA strand, this structure is called a polysome.

•            Ribosomes have only a temporary existence. When they have synthesised a polypeptide the two sub-units separate and are either re-used or broken up.

•            Ribosomes can join up amino acids at a rate of 200 per minute.

•            Small proteins can therefore be made fairly quickly but two to three hours are needed for larger proteins such as the massive 30,000 amino acid muscle protein titin.

 

 

Key concept:

 

Synthesis Activities

 

A.           Three types of RNA are necessary for protein synthesis:

1.           Messenger RNA (mRNA) complementary copies of distinct DNA, the genetic code.

a.           mRNA can be compared with a tape that contains all the genetic information of proteins, but it must pass through the ribosomes attached to the endoplasmic reticulum (ER).

b.           As the tape passes through the ribosomes, transfer RNA (tRNA) adds the exact amino acid to the newly forming proteins.

 

 

2.           tRNA transporter of specific amino acids (building blocks of proteins).

3.           Ribosomal RNA floating freely in the cytoplasm (polyribosomes) or attached to the ER.

B.           Protein synthesis also can occur on polyribosomes floating freely in the cytoplasm; proteins synthesized on the free polyribosomes are used in cellular metabolic processes: proteins synthesized on the ribosomes attached to the ER are transported out of the cell.