RNA Synthesis: Transcription Process in Detail
The Synthesis of RNA (Transcription)
The synthesis of RNA, or transcription, occurs in the nucleus and requires:
- A DNA strand serving as a template: Of the two chains of nucleotides that make up the gene, only one, the template, is actually transcribed, while the other, called the coding strand, is not.
- Enzymes: The process is catalyzed by RNA polymerases. In prokaryotes, only one exists, whereas in eukaryotes there are three: RNA polymerase I, II, and III. RNA polymerase I is involved in the formation of rRNA, RNA polymerase II in the synthesis of all mRNAs, and RNA polymerase III in tRNA and small rRNA.
- Ribonucleoside triphosphates (A, G, C, U): These are joined by an ester bond between the phosphoric acid, located in the 5′ position of a ribonucleoside triphosphate, and the -OH group located in the 3′ position of the last ribonucleotide of the forming RNA chain.
Transcription consists of three stages: initiation, elongation, and termination. Following this, there is a maturation of the RNA.
Initiation
RNA polymerase recognizes a start signal in the DNA to be transcribed. These signals, called promoter sites, are specific short sequences of nitrogenous bases that bind to RNA polymerase. The promoter indicates which of the two DNA strands is used as a template. The RNA polymerase causes the DNA double helix to open, exposing the base sequence of the DNA, and allowing ribonucleotides to be incorporated.
Elongation
Elongation is the addition of successive ribonucleotides to form RNA. The RNA polymerase moves along the template strand of DNA, “reading” it in the 3′-5′ direction. However, the direction of RNA synthesis is 5′-3′.
The enzyme selects the ribonucleoside triphosphate that has a complementary base to the DNA strand that acts as a template, and attaches it by an ester bond to the next nucleotide, releasing a pyrophosphate group. In eukaryotes, after the union of the first 30 ribonucleotides, a cap formed by methyl-guanosine triphosphate is added to the 5′ end. This cap will be a signal for the recognition of the start of translation.
Termination
RNA polymerase recognizes termination signals that indicate the end of transcription. This involves the closure of the bubble formed in the separation of DNA and RNA, and the release of the RNA transcript and RNA polymerase.
- In prokaryotes, the termination signal is a palindromic base sequence consisting of G and C followed by several Ts, causing a loop at the end of the RNA. This promotes separation from the DNA. The loop is formed by the self-complementarity of G and C bases located in the tail of the RNA.
- In eukaryotes, RNA polymerase transcribes long regions of DNA that exceed the length of the sequence encoding the protein. At certain points, an enzyme cuts the fragment of RNA that carries information to synthesize the protein. The DNA continues to be transcribed for hundreds or thousands of bases, but this transcript is degraded because it lacks a cap. The cut is signaled by a polyadenylation signal sequence, which appears on the RNA a few nucleotides before the cut-off point. After RNA separation, an enzyme (poly(A) polymerase) adds a sequence of about 200 adenine nucleotides to the 3′ end, called the poly(A) tail, which is involved in the maturation and transport of RNA out of the nucleus.