What is Transcription?
Transcription is the process of converting spoken words or music into written text. Transcription is the biological process by which a strand of DNA is copied into a strand of RNA, with the latter serving as the “written” form of DNA.
It’s the initial step in making proteins or passing on information within a cell. Genetic information is stored in DNA, which is transcribed into RNA and then used to drive protein synthesis during the translation process. Messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA) are the three forms of RNA that may be synthesized (rRNA).
The process of transcription consists of four distinct phases: pre-initiation, initiation, extension, and termination. DNA in eukaryotic cells is kept in the nucleus, whereas in prokaryotic cells it is located in the cytoplasm.
Eukaryotic cells store their DNA in a structure called chromatin, which must be uncoiled for transcription to take place. In eukaryotes, numerous more processing steps are required to convert RNA into messenger RNA, making the process more involved than in prokaryotes.
The RNA polymerase subunit binds to a promoter region at the 5′ end of a DNA strand to begin the pre-initiation, or template binding, step. Once this is done, the DNA strand is denatured, separating the two complementary strands and providing the enzyme access to the strand that serves as the template.
The strand that acts as its complement is called the partner strand. Transcription cannot begin normally without the presence of promoter sequences in the DNA strand. Many promoter sequences, or motifs, have been identified; in prokaryotes, these include TATAAT and TTGACA, and in eukaryotes, TATAAAA and GGCCAATCT.
These sequences are made up of the four ribonucleotide bases that make up the DNA strand: adenine, thymine, guanine, and cytosine. The term “cis-acting elements” describes these particular sequences. In eukaryotes, RNA polymerase needs an extra transcription factor to attach to the promoter region.
The first complementary 5′-ribonucleoside triphosphate is incorporated into the RNA strand via initiation catalyzed by RNA polymerase. Keep in mind that the DNA nucleotide bases adenine and thymine, guanine, and cytosine, each have an anticodon.
However, adenine’s complement in ribonucleotides is uracil rather than thymine since RNA lacks thymine. Subsequent complementary ribonucleotides are inserted from 5′ to 3′ after the first 5′-ribonucleotide has been introduced. At this point, the DNA and RNA molecules are still linked via the phosphodiester linkages that connect these ribonucleotides (see Figure 1).
The first steps of transcription are shown in Figure 1. RNA polymerase is what is meant by the term RNAP®.
When the component dissociates from the DNA strand, the developing RNA strand becomes free from the DNA template strand and the chain may continue to extend. It’s the main enzyme’s job to make this happen (see Figure 2).
Transcription elongation, as shown in Figure 2.
When the core enzyme reaches a certain sequence of nucleotides known as a termination sequence, transcription stops. The RNA transcript then folds back on itself, using hydrogen bonding to help produce a hairpin secondary structure. An additional termination factor called rho() can facilitate cell death in prokaryotes.
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At the point where the RNA molecule is unbound from the complementary DNA strand, the termination process is complete. In eukaryotes, the process of termination involves an extra step called polyadenylation, in which an RNA tail consisting of several adenosine monophosphates is added.
Figure 3: Critical steps in the transcription process
What is Reverse Transcription?
Transcribing DNA from RNA is referred to as reverse transcription. Retroviruses, like HIV, employ this type of replication to quickly reproduce by creating modified DNA that may be integrated into a host cell. The enzyme reverse transcriptase is responsible for this. Figure 4 depicts this.
Reverse transcription is seen in Figure 4.
The Meaning of Translation
To translate is to make over from one language or medium to another. Proteins are synthesized from messenger ribonucleic acid (mRNA) through the biological process known as translation. Producing a specific polypeptide chain, or chain of amino acids, from the chemical information encoded in a particular strand of mRNA is the means by which this is achieved.
Proteins are created when these polypeptides fold properly. Various genes produce distinct proteins, which are in turn encoded by different strands of messenger RNA. Gene expression depends on this.
Figure 5: Amino acids are the translated form of the triplet code; certain of these acids also serve as the start and stop codons for the translation process.
There are three primary phases of translation: beginning, middle, and end. Translation in prokaryotes takes place in the cytoplasm, whereas in eukaryotes it happens in the endoplasmic reticulum. The ribosome is a protein complex essential for translation that varies in structure between prokaryotes and eukaryotes, primarily in the pace at which its subunits migrate during centrifugation and the number of proteins it contains.
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The binding of the small ribosomal subunit to the 5′ end of messenger RNA (mRNA), which is produced during transcription from DNA, marks the beginning of the process. A two-step process is involved: first, the small ribosomal subunit attaches to a group of proteinaceous initiation factors; next, the whole complex binds to messenger RNA.
The mRNA start codon is located in many ribonucleotides upstream from this binding site. Then, the tiny ribosomal subunit acquires a charge from a tRNA molecule. After the small ribosomal subunit, messenger RNA (mRNA), and transfer RNA (tRNA) create a complex, the big ribosomal subunit binds to it.
The guanosine-5′-triphosphate (GTP) that powers the bonds are hydrolyzed in this step. The initiation factors become available following the incorporation of the big ribosomal subunit into the complex.
The pairing of a tRNA molecule with an amino acid is called “charging” and is an essential step in the translation process. Aminoacyl-tRNA synthetases catalyze this process by combining the amino acid with ATP (adenosine triphosphate) to create a reactive version of the amino acid called an aminoacyladenylic acid.
This forms a complex with ATP that can react with a tRNA molecule to produce a covalent link. The amino acid has been successfully transferred to the mRNA molecule by the tRNA.
Once the small and big ribosomal subunits have joined together and attached to the mRNA, elongation may begin. For further interaction with tRNA, the mRNA molecule forms a peptidyl site and an aminoacyl site. First, the tRNA connects to the peptidyl (P) site, and then the second tRNA binds to the amino (A) site to initiate elongation (aminoacyl site).
These two tRNA molecules are acting as amino acid couriers. Two tRNA molecules transporting different amino acids pair together to produce a peptide bond. This enzymatic process is known as peptidyl transferase. For the transfer RNA (tRNA) to leave the mRNA molecule, the covalent link between the tRNA and the amino acid at the P site must be broken. Using the energy released from the GTP, the tRNA at the A site transfers to the P site.
Thus, the P site includes a tRNA molecule bound to an amino acid, whereas the A site remains unoccupied and ready for further binding. The structure of the polypeptide chain is built on this. Peptidyl transferase catalyzes the formation of a peptide bond between the amino acid linked to the tRNA at the P site and the amino acid bound to the tRNA at the A site.
The covalent link between the amino acid and tRNA at the P site is broken and the tRNA is liberated. Amino acids are continuously added in this way to form a polypeptide chain.
When the ribosome complex reaches a stop codon, translation stops (see figure 5). At this point, the A site of the polypeptide chain is still unattached while it is linked to a tRNA at the P site. The final tRNA and the last amino acid in a chain are released from their binding site by GTP-dependent release factors.
After the tRNA has been freed from the ribosome complex, the ribosome itself separates from the mRNA strand into the small and large ribosomal subunits. Proteins are formed when this polypeptide chain folds back on itself. Figures 6 and 7 illustrate this procedure.
651px-Ribosome mRNA translation en
Illustration 6: A High-Level View of the Translation Procedure
The key steps in the translation process are depicted in Figure 7.
What Makes Translation Different from Transcription?
The two steps of transcription and translation from DNA genes to proteins are essential to the proper functioning of a cell. Without the other, the first procedure won’t be able to finish. There are, however, a number of significant distinctions between these procedures.
DNA, the RNA polymerase core enzyme, and the subunit are the primary building blocks of transcription. Parts of the translation process that are essential to its completion are messenger RNA (mRNA), the small and large ribosomal subunits, initiation factors, elongation factors, and tRNA.
When an enzyme does transcription, it first denatures a DNA double helix to get access to the template strand. Given that the template for translation is a single strand of mRNA, denaturation is unnecessary. Transcription results in RNA, which can take the form of messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA), whereas translation yields a polypeptide amino acid chain, the building block of proteins.
In eukaryotes, transcription takes place in the nucleus, whereas translation takes place in the cytoplasm and endoplasmic reticulum. In prokaryotes, these procedures take place in the cell’s cytoplasm. RNA polymerase in transcription and ribosomes in the translation is the regulating factors. This polymerase goes along the DNA template strand during transcription, while the ribosome-tRNA complex travels along the mRNA strand during translation.
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In Conclusion, Translation, and Transcription
DNA is only as effective as the organisms that contain it. For this reason alone, the procedures of transcription and translation need our attention. Both the DNA sequences and the molecules that are synthesized from them must function properly for cellular functions to run smoothly. Transcription and translation play crucial roles in DNA function at this point.
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