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Protein synthesis is a two-step process by which body cells make proteins in the cell nucleus and the cytoplasm. Gene expression, or the function whereby body cells read and utilize genetic code from DNA to create molecules, is a prerequisite to protein synthesis. Although gene expression occurs naturally, biotechnology advances like gene synthesis allow biologists to assemble gene sequences, producing high-efficiency protein molecules.
Protein molecules vary according to their functionalities; for example, structural proteins differ in structure from hormone proteins. However, protein synthesis is similar for all protein types,
as shown in the two-step process below.
- Transcription
Transcription involves copying the genetic code present in DNA into RNA (ribonucleic acid). It takes place in the membrane-enclosed nucleus of the cell. To better understand transcription, let’s start by exploring the basic structure of DNA.
A single DNA molecule contains smaller chemical molecules called nucleotides. Nucleotides are the building blocks in DNA bound together by chemical bonds to form a long chain. Afterward, two nucleotide chains intertwine and create a single double-helix DNA strand.
During transcription, the DNA strand partially unwinds to expose its gene sequence for copying onto the single RNA strand. The gene sequence refers to the specific order in which nucleotides appear -in a particular gene.
- Initiation
During initiation, an enzyme called RNA polymerase moves along the DNA strand in question while reading and scanning its gene sequences to establish a promoter sequence.
Promoter sequences are sections of the DNA that mark the beginning of a gene strand sequence. They indicate the sense/coding strand and define the direction of coding. Note that just like we cannot read a book in both directions, neither is a DNA sequence read.
The two directions lengthwise of a DNA promoter sequence are 5’ (five prime) and 3’ (three prime). Therefore, the RNA polymerase relies on the coding strand to establish the direction to read the code. (Find out more on DNA directionality later in this article).
While one half of the unwound DNA strand is the coding strand, the other half is known as the antisense or the non-coding strand. The two halves are complementary opposites akin to an object and its mirror image.
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- Elongation
The enzyme RNA polymerase moves along the DNA strand as it deciphers code and synthesizes mRNA. Regarding DNA strand directionality, each DNA strand end has a number, and one end is 3,’ and the other end is 5’. RNA polymerase moves while adding nucleotides matching the DNA sequence to the 3’ end of the RNA strand. After RNA polymerase deciphers the DNA code on the coded strand, it moves up the DNA strand from the 3’ end toward 5’.
- Termination
When RNA polymerase receives a signal that it has reached the end of the DNA, it stops reading the gene sequence. Instead, it begins reading the termination sequence, a signal present in DNA that tells the RNA polymerase when to stop coding the mRNA.
Consequently, the newly-coded pre-mRNA sequence curls into a hairpin-like loop structure, snaps away from the DNA coding strand and becomes a pre-mRNA. However, the pre-mRNA must undergo transitional processing before it becomes a mature mRNA ready for translation. Below are the transitional phases included in mRNA processing.
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mRNA processing
mRNA processing prepares the pre-mRNA for translation. Translation occurs in the nucleus, while the preceding stages of protein synthesis occur in the cytoplasm. The mRNA processing facilitates the pre-mRNA’s movement from the nucleus to the cytoplasm.
Capping
Capping involves adding protective caps on the pre-mRNA’s 5’ and 3’ ends. First, an A7-methylguanosine cap is set at the 5’ end and of the pre-mRNA during elongation. Second, a poly (A) also gets added to the 3’ end of the pre-mRNA.
Both the A 7-methylguanosine and the poly (A) tail protect the mRNA from degradation. Second, they also help initiate translation since the initiation factors for protein synthesis must recognize the A 7-methylguanosine cap before commencing translation. Third, the poly (A) tail helps bind the proteins that initiate translation to the mRNA.
Splicing
Splicing is an enzyme-led intermediate step in mRNA processing and entails two crucial changes to the pre-mRNA. First is the removal of non-coding regions of the gene sequence (introns) from the pre-mRNA. Second is the formation of splice bonds between the remaining coding regions of the gene sequence (exons).
- Translation
Translation occurs in the ribosomes situated in the cell’s cytoplasm. During translation, the ribosomes read out the genetic code written on the mRNA to synthesize proteins. The code is saved in the mRNA as codons or a sequence of three RNA nucleotides corresponding to a specific amino acid group. The codons also function as stop signals during translation.
Besides ribosomes and codons, the other crucial element during the translation stage is the transfer RNA (tRNA). The tRNA catalyzes the assembly of amino acids into protein chains held together by peptide bonds.
While the RNA carries codons, the tRNA carries anticodons complementary to the RNA codons. The tRNA also has amino acids that correspond to the codons. During translation, the anticodon sequence present in tRNA recognizes the codon sequence in mRNA as the ribosomes read out the code. Therefore, the tRNA loses each amino acid and bonds with the complementary codon.
As the amino acid becomes disengaged from the tRNA, they move to the ribosomes and bond-forming a polypeptide chain that grows until it reaches a stop codon. However, although proteins feature one or more polypeptide chains, most proteins undergo further processing after translation. The Golgi apparatus/ Golgi body helps package the protein molecules destined for specialized functions in the plasma.
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Conclusion
Protein synthesis is essential for life since proteins regulate almost all physiological functions. Therefore, expert knowledge of protein synthesis helps researchers and biologists make ground-breaking discoveries in fields like medicine, nutrition, health, and wellness.
