Protein Synthesis : Different RNAs & Steps

By the 1950s and 1960s, research pointed to the fact that DNA was required for protein synthesis. Proteins can act as enzymes and structural components in cells, among other things. Protein, for instance, makes up the hormone insulin and muscle fibres in humans. 

The protein synthesis process requires numerous critical components, including a supply of the 20 amino acids that make up most proteins. A set of enzymes will also be required to operate in the process. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are required for protein synthesis.

What is Protein Synthesis?

Before we move ahead, it is important to understand what protein synthesis is. It is the process through which protein molecules are created. It includes amino acid synthesis, transcription, translation, and post-translational events in living organisms.

Prerequisites for Protein Synthesis Process

How the amino acids are connected is crucial to the structure of a protein molecule. Amino acid sequencing is a form of coding that differentiates the protein molecule from others. This amino acid code is determined by a piece of genetic information in the DNA. The sequence of nitrogenous nucleotides in DNA constitutes the genetic code. The main concept of protein synthesis is based on converting the nitrogenous base into an amino acid sequence.

In the cytoplasm, the protein synthesis process takes place. The RNA transports information from DNA into the cytoplasm. RNA is comparable to DNA with a few differences. The sugar in RNA is ribose instead of deoxyribose, and the pyrimidine uracil is present in RNA nucleotides instead of thymine.

Different RNAs

Three variants of RNA are involved in protein synthesis. They are: 

1. Ribosomal RNA (rRNA) 
2. Messenger RNA (mRNA)
3. Transfer RNA (tRNA)

Ribosomal RNA (rRNA) is employed in the production of ribosomes. Ribosomes are microscopic molecules of rRNA and protein. The steps of bonding amino acids to form proteins take place within ribosomes. Ribosomes are abundant along the endoplasmic reticulum as well as the cytoplasm of the cell.

Transfer RNA is another significant form of RNA (tRNA). Transfer RNA is found in the cytoplasm of cells and transports amino acids to ribosomes for protein production. Enzymes bind tRNA units to amino acids in a very precise way during protein synthesis. For instance, molecule X of tRNA will only connect to amino acid X, whereas molecule Y of tRNA will only bind to amino acid Y. 

mRNA, often known as messenger RNA, is a single-stranded RNA molecule. It is analogous to DNA and contains the same genetic data as DNA. The process of translation results in the formation of proteins. The three-base codon contained in mRNA is translated into amino acids, resulting in the formation of proteins. In eukaryotes, mRNA synthesis occurs in the nucleus and then is transferred to the cell’s cytoplasm, wherein translation or protein synthesis occurs.

mRNA, tRNA and rRNA synthesis occur in the process of the enzyme RNA polymerase.

Protein Synthesis Steps 

The steps of protein synthesis are two-fold: transcription and translation. Each one will be discussed in detail below. 

Transcription

Transcription is the first of the protein synthesis steps. It is a fundamental postulate of molecular biology (known as the central dogma): DNA -> RNA. It is the process through which genetic information in DNA is transferred to mRNA. Transcription occurs in the nucleus of eukaryotic cells. During transcription, an mRNA strand is formed that is complementary to a stranded DNA molecule known as a gene. The DNA sequence may readily identify a gene. A gene has three fundamental areas: a promoter, a coding sequence (reading frame), and a terminator.

 Steps involved in transcription

There are three steps involved in transcription: initiation, elongation, and termination.

1. The commencement of transcription is called initiation. It happens when the RNA polymerase enzyme attaches to the promoter of a gene. This causes the DNA to unravel, allowing the enzyme to “read” the nitrogen bases in a strand of DNA. RNA polymerase is now ready to construct an mRNA strand with an equivalent base sequence. The promoter is not included in the resultant mRNA.
2. Elongation is adding nucleotides to the messenger RNA (mRNA).
3. Termination refers to the conclusion of transcription. The enzyme dissociates from DNA as it transcribes the terminator. After this step, the mRNA strand is produced.

The mRNA synthesis ends when the “stop” codon is attained. The mRNA separates from the DNA molecule, which resets to create a double helix. Meanwhile, the mRNA molecule travels through a gap in the nucleus and into the cell’s cytosol, where it goes toward the cytoplasmic or rough endoplasmic reticulum and attaches to the ribosomes.

Processing 

The freshly generated mRNA or primary transcript is known as hnRNA and must be processed before it can be transformed into mature mRNA. This is called Post-transcriptional processing of hnRNA. 

Exons and introns, which are coding and non-coding sections, are both found in hnRNA. It passes through splicing, capping, and tailing.

• Splicing is the process by which introns from the main transcript are deleted, and exons are linked together.
• Capping entails adding a cap of an uncommon nucleotide to the 5′ positions of hnRNA (usually methyl guanosine triphosphate).
• Tailing is the addition of 200-300 adenylate residues at the 3′-end of the process. The tail shields RNA from exonuclease destruction and is important in transcription cessation, mRNA transportation, and translation.

Following hnRNA processing, mRNA is delivered to the cytoplasm and undergoes the process of translation. 

 Translation

Translational is the second component of molecular biology’s central dogma: RNA -> Protein. It is the process of reading the genetic information in mRNA to create a protein. After leaving the nucleus, mRNA goes to a ribosome made up of rRNA and proteins. The ribosome recognizes the codon pattern in mRNA, and tRNA molecules transport amino acids in the right order to the ribosome. 

Just like transcription, the translation process is divided into three stages: initiation, elongation, and termination.

• Initiation:

The mRNA departs the nucleus via a nuclear pore and reaches the cell’s cytoplasm following transcription. The ribosome’s components attach to the mRNA at the area containing the methylation cap and the START codon. These are then connected by a tRNA that includes anticodons that match the mRNA’s START codon. An initiation complex is a collection of these three molecules (mRNA, ribosome, tRNA).

• Elongation:

tRNA continues to provide amino acids to the developing polypeptide via complementary base pairing involving mRNA codons and tRNA anticodons. As a tRNA enters the ribosome, one amino acid is added to the developing polypeptide. When this exchange is finished, the tRNA exits the ribosome. The ribosome advances one codon distance along the mRNA, and a replacement tRNA with its appropriate amino acid enters. The polypeptide expands when this procedure is repeated.

• Termination:

A stop codon terminates the elongation step after the mRNA coding. The stop codon does not require a tRNA but rather a release factor, which causes the complete unit (mRNA, ribosome, tRNA, and polypeptide) to disassemble, liberating all of the contents.

Following the protein synthesis, it is taken from the ribosome for more synthesis and action. For instance, the protein may be held in the Golgi apparatus before the cell’s discharge, or it could be preserved as a digesting enzyme.

A protein can either be employed as a structural element in the cell or produced as a hormone, like insulin. After synthesis, the mRNA unit disintegrates, and the nucleotides return to the nuclei. The tRNA compounds retreat to the cytosol to join other amino acids, while the ribosome prepares for the coming of a fresh mRNA molecule.

 Conclusion

What is protein synthesis? Protein synthesis refers to the process through which cells produce proteins. Transcription and translation are the two steps.

Transcription is the process through which genetic instructions in DNA are transferred to mRNA in the nucleus. It is divided into three stages: initiation, elongation, and termination. After being transcribed, the mRNA transports the information to a ribosome.

The ribosome, made up of rRNA and peptides, is where translation occurs. The commands in mRNA are read during translation, and tRNA transports the correct order of amino acids to the ribosomal molecule. 

After synthesising a polypeptide chain, it may undergo further processing to generate the completed protein.

 Frequently Asked Questions 

 1. Mention the role of ribosomes.

A. Ribosomes have the following functions:

• Ribosomes are in charge of protein or polypeptide production.
• Ribosomes include ribosomal RNAs, which are attachment locations for mRNA and tRNAs.
• It shields freshly produced proteins from cytoplasmic enzymes by trapping them in the cleft of a bigger subunit. 

2. Why aren’t both the strands of DNA replicated during the process of transcription? Explain. 

A. I) If both strands encode for RNA, two molecules of RNA and two separate proteins would be generated, complicating genetic makeup.

1. II) As RNA molecules are homologous, they will coil together to produce double-stranded RNA without undergoing translation, implying that transcription is pointless.

 3. What are the three forms of RNA, and what function do they play in protein synthesis?

 A. There are three kinds of RNA:

• Messenger RNA (mRNA): mRNA converts DNA’s genetic data into a format that can be processed and utilised to produce proteins. mRNA transports genetic data from a cell’s nucleus to its cytoplasm.
• rRNA stands for Ribosomal RNA: rRNA is present in a cell’s cytoplasm, within which ribosomes are present. The synthesis of mRNA into proteins is directed by rRNA.
• tRNA stands for Transfer RNA: tRNA, like rRNA, is found in the cytoplasm and is essential in protein production. Transfer RNA transports amino acids to the ribosome. The amino acids could then be combined and assembled to produce proteins.