Question

fill in the blanks.
In mammals, the mitochondrial ribosomes are ______.
A. 55S
B. 70S
C. 80S
D.100S

Answer 1

The ribosomes are minute cell organelles present in the cell cytoplasm as well as in some cell organelles like mitochondria and chloroplast. These help in protein synthesis or translation. The protein synthesis takes place in the cytoplasm with the help of mRNA that is synthesized in the nucleus under the guidance of genes.

Complete answer:
Ribosomes are made up of ribosomal proteins and ribosomal RNA (rRNA). These are found floating freely in the cytoplasm and they are also attached on the rough endoplasmic reticulum (RER). Some cellular organelles like mitochondria and chloroplast also contain ribosomes. They have their separate machinery for the synthesis of a protein with the help of these ribosomes.
The ribosomes may be of two types on the basis of their sedimentation coefficient expressed in Svedberg unit (S).
-70S ribosome: These ribosomes are found in prokaryotes. They are made up of two ribosomal subunits called 30S and 50S.
-80S ribosome: These ribosomes are found in eukaryotes. They are made up of two subunits 60S and 40S.The mitochondria and chloroplast are similar to the bacterial or prokaryotic cell, hence, they contain 70s type ribosome.
Thus, the correct option is option B - 70S.

Note: In prokaryotic and eukaryotic both, ribosomes are found free-floating in the cytoplasm. In the case of eukaryotic cells, some ribosomes are also found attached to the rough endoplasmic reticulum where protein synthesis takes place also. The rough endoplasmic reticulum (RER) is attached to the outer nuclear wall where protein modification takes place and transports proteins to the Golgi body and other cell organelles.