Question

What is translation, where does it occur? How many ribosomal subunits are there, and where are they constructed? What are the P and A sites? What occurs during initiation, elongation, and termination and what are the three steps of elongation. What is the function of the stop codon?

Answer 1

Ans 1: Translation: It is the process of synthesis of protein from the information available on m-RNA (messenger RNA).  As clear from the name only here m-RNA is messenger which brings message from DNA that is present in the nucleus.

Translation occur in cytoplasm with the help of ribosome ,protein and 3 different RNAs that are

• t-RNA( transfer-RNA): Transfers amino acids as per the information available on m-RNA.
• r-RNA ( Ribosomal-RNA): It makes ribosome.
• m-RNA ( messenger-RNA): Receive  information from DNA and then come to cytoplasm for synthesis of protein as per information received.

Ans2: Number of ribosomal units required for translation: 2 ribosomal sub units are required for translation but their types are different in prokaryotes and eukaryotes. Ribosomes are made up of r-RNA and proteins.

Prokaryotes: 70s ribosome made up of 2 subunits 50s and 30s

Eukaryotes: 80s ribosome made up of 2 subunits 60s and 40s

Ribosome construction in prokaryotes: In these cells ribosomes are synthesized in cytoplasm as they lack membrane bound cell organelles that are present in eukaryotes.

Ribosome construction in Eukaryotes: In eukaryotes r-RNA (except 5s r-RNA) and proteins joins in the nucleolus to form ribosomal subunits , for that ribosomal proteins comes from cytoplasm to nucleolus through nuclear pores. Later in the cytoplasm these two subunits assembles to form a functional 80s ribosome.

Ans 3: Both P-site and A-site are present on the ribosome.

P-site: It is the abbreviated form of peptidyl site. Growing polypeptide chain is present in this site.

A-site: It is the abbreviated form of acceptor site. This site can also be called as arrival site of incoming t-RNA having desired amino acid.

Ans 4: m-RNA acts as a template for the synthesis of protein.

• m-RNA comes from nucleus to cytoplasm carrying massage from DNA.
• With the help of amino acyl t-RNA synthetase enzyme first amino acid that is always methionine  loads on the t-RNA.

Initiation: It has following steps:

• small sub unit of ribosome binds to the m-RNA and scan it for the AUG codon that is the start codon.
• As soon as small sub unit of ribosome finds the AUG codon it attaches there and met-tRNA having methionine amino  acid attaches on m-RNA (with anticodon UAC that pairs with codon AUG) .
• At this point large sub unit of ribosome assembles with small sub unit thus forms complete ribosome.
• Large subunit has 3 sites A-site (acceptor site) , P-site (peptidyl site) and E site that is exit site.
• m-RNA along with met- t RNA and ribosome sub units forms the initiation complex. Now elongation phase will start.

Elongation: Following are the steps:

• New t-RNA having amino acid as per next available codon will come and bind with anti codon on  A-site
• Now peptide bond will form between methionine and new amino acid. In this way dipeptide will form.
• Ribosome will move one codon ahead and empty t-RNA will go to E-site and removed from ribosome.
• Then new A-site will become free for new t-RNA with amino acid
• same step repeat untill it reaches stop codon.

Termination:

Termination occurs once ribosome reaches stop codon that UAA, UAG, UGA. At this point popeptide chain will release from ribosome.

This is generalized translation of prokaryotes and eukaryotes.