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Biotechnology : Principles and Processes

Multiple Choice Questions

141.

A segment of DNA has 120 adenine and 120 cytosine bases. The total number of nucleotides present in the segment is

142.

Gel electrophoresis is used for

Cutting of DNA into fragments
Separation of DNA fragments according to their size
Construction of recombinant DNA by joining with cloning vectors
Isolation of DNA molecule

143.
Polysome is formed by

Several ribosomes attached to a single mRNA

Many ribosomes attached to a strand of endoplasmic reticulum

A ribosome with several sub-units

Ribosomes attached to each other in a linear arrangement

Several ribosomes attached to a single mRNA

In prokaryotes, ribosomes attach to the 5' end of mRNA as soon as transcription begins. A bunch of ribosome moves along a single mRNA molecule adding 15 amino acids/second to the polypeptide chain, almost the same speed at which RNA Polymerase transcribes the mRNA. The group of ribosomes together with the single mRNA molecules, they are translating is called polysome. The use of polysomes is advantageous to a cell since the overall rate of protein synthesis is increased compared to the rate that would occur if there were no polysomes.

In eukaryotic cells, the ribosomes are attached to rough endoplasmic reticulum by ribophorin protein. Electron microscopy reveals that membranes of homogenised endoplasmic reticulum disrupt to form closed vesicles called microsomes. Microsomes derived from rough endoplasmic reticulum are studied with ribosomes and are called rough ribosomes.