Question

Explain the process of protein synthesis in detail and how it links to making mRNA

Answer 1

Translation (protein synthesis) involves following steps

A) Activation of amino acids an md formation of amino Acyl tRNA complex:

It occurs in presence of amino acyl tRNA synthetase enzyme. Amino acid molecules are activated and then get attached to 3' end of tRNA.

B) formation of polypeptide chain:

It involves three steps

Initiation: in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin.

Elongation: in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain.

Termination : in the last stage, the finished polypeptide is released to go and do its job in the cell.

Initiation:

-A ribosome (which comes in two pieces, large and small)

-An mRNA with instructions for the protein we'll build

-An "initiator" tRNA carrying the first amino acid in the protein, which is almost always methionine (Met)

During initiation, these pieces must come together in just the right way. Together, they form the initiation complex, the molecular setup needed to start making a new protein.

Inside cells (and the cells of other eukaryotes), translation initiation goes like this:

-first, the tRNA carrying methionine attaches to the small ribosomal subunit.

-Together, they bind to the 5' end of the mRNA by recognizing the 5' GTP cap (added during processing in the nucleus).

-Then, they "walk" along the mRNA in the 3' direction, stopping when they reach the start codon (often, but not always, the first AUG).

In bacteria, the situation is a little different. Here, the small ribosomal subunit doesn't start at the 5' end of the mRNA and travel toward the 3' end. Instead, it attaches directly to certain sequences in the mRNA. These Shine-Dalgarno sequences come just before start codons and "point them out" to the ribosome.

Bacterial genes are often transcribed in groups (called operons), so one bacterial mRNA can contain the coding sequences for several genes. A Shine-Dalgarno sequence marks the start of each coding sequence, letting the ribosome find the right start codon for each gene.

Elongation

-after the initiation complex has formed, but before any amino acids have been linked to make a chain.

Our first, methionine-carrying tRNA starts out in the middle slot of the ribosome, called the P site. Next to it, a fresh codon is exposed in another slot, called the A site. The A site will be the "landing site" for the next tRNA, one whose anticodon is a perfect (complementary) match for the exposed codon.

Once the matching tRNA has landed in the A site, it's time for the action: that is, the formation of the peptide bond that connects one amino acid to another. This step transfers the methionine from the first tRNA onto the amino acid of the second tRNA in the A site.

Not bad—we now have two amino acids, a (very tiny) polypeptide! The methionine forms the N-terminus of the polypeptide, and the other amino acid is the C-terminus.

But...odds are we may want a longer polypeptide than two amino acids. How does the chain continue to grow? Once the peptide bond is formed, the mRNA is pulled onward through the ribosome by exactly one codon. This shift allows the first, empty tRNA to drift out via the E ("exit") site. It also exposes a new codon in the A site, so the whole cycle can repeat.

And repeat it does...from a few times up to a mind-boggling 33,33,000000 times! The protein titin, which is found in your muscles and is the longest known polypeptide, can have up to 33,33,000000 amino acids.

Termination

Polypeptides, like all good things, must eventually come to an end. Translation ends in a process called termination. Termination happens when a stop codon in the mRNA (UAA, UAG, or UGA) enters the A site.

Stop codons are recognized by proteins called release factors, which fit neatly into the P site (though they aren't tRNAs). Release factors mess with the enzyme that normally forms peptide bonds: they make it add a water molecule to the last amino acid of the chain. This reaction separates the chain from the tRNA, and the newly made protein is released.