Transcription (Synthesis of RNA) – A General Account

In this article, we will discuss about the synthesis of RNA. The process of RNA synthesis is known as transcription.

DNA contains information for the synthesis of cell’s specific proteins. DNA is located in the nucleoid (prokaryotes) or nucleus (eukaryotes); and protein synthesis occurs in the cytoplasm.

DNA does not move to the site of protein synthesis (ribosomes) to directly guide the process. Instead, it transfers its information to mRNA molecules which move to the ribosomes to direct protein synthesis.

The process of the formation of RNA from DNA template is called transcription (written across). It involves rewriting the genetic message coded in DNA into an RNA molecule. It occurs in the nucleus during the G₁ and G₂ phases of cell cycle. DNA has promoter and terminator sites. Transcription starts at the promoter site and stops at the terminator site. The mechanism of RNA synthesis was worked out in the late 1950s by the American investigators Jerard Hurwitz, Samuel B. Weiss, and Audrey Stevens by independent in vitro experiments.

RNA transcription requires:

(i) The enzyme RNA polymerase or transcriptase

(ii) A DNA template

(iii) All four types of ribonucleoside triphosphates (ATP, CTP, OTP and UTP).

(iv) Divalent metal ions Mg²⁺ or Mn²⁺ as a cofactor.

No primer in needed for RNA synthesis. RNA transcription takes place as follows. As already written, formation of RNA (Ribonucleic Acid) over DNA template is called transcription. It is meant for taking the coded information from DNA to the site where it is required for protein synthesis. The process of transcription is catalyzed by RNA polymerase or transcriptase.

All RNA, with some exceptions in the case of RNA viruses, is generated through transcription. Whether RNA is synthesized from only one or from both of the DNA strands? It seems reasonable that only one strand would be used, because transcription of RNA from both strands would produce two complementary RNA strands from the same stretch of DNA, and these presumably would produce two different kinds of protein (with different amino acid sequences).

In fact, a great deal of chemical evidence confirms that transcription takes place on only one of the two DNA strands (though not necessarily the same strand throughout the entire chromosome), i.e., only one strand serves as template; this strand is called the antisense strand.

As a result, the base sequence of an RNA molecule is complementary to that of antisense strand which served as its template, while it is the same as that of the other strand, known as the sense strand, of the concerned DNA double-stranded helix.

It may be emphasized that the sense strand is not copied, i.e., does not serve as template during transcription, is complementary to the strand that is copied, and, as a result, has the same base sequence as that of the RNA produced. In contrast, it is the antisense strand which serves as the template for transcription.

A molecule of mRNA is synthesized by using a length (called transcription unit) (about 10⁵— 10⁶ nucleotides long) of a single strand of DNA as a template. RNA consists of a single strand of nucleotides and is very similar in chemical structure to one strand of the double helix of a DNA molecule, except that uracil (U) replaces thymine (T) as the base complementary to adenine (A), and the sugar in the backbone of the RNA molecule is ribose instead of deoxyribose.

The first step in mRNA synthesis is that a length of the double helix of a molecule of DNA unwinds by breaking the hydrogen bonds between the corresponding bases in the paired strands. The sequence of the bases A (adenine), T(thymine), C (cytosine) and G (guanine) in one of the DNA strands is copied or transcribed into the complementary sequence of the bases U (uracil), A, G and C in mRNA (Fig. 7.4 & 7.5).

To elaborate it further, if following is the sequence of bases in DNA— the resulting mRNA will have the following sequence of bases (nucleotides):

UU CC AG AC UG UG AA CC

It is, therefore, important as to which of the two strands of DNA is transcribed, because the two strands will give rise to different polynucleotide chains. The genetic information encoded in the sequence of nucleotides in the nuclear DNA is transcribed into mRNA, which then carries this information into the cytoplasm of the cell. However, this “messenger” role of mRNA is not the only one it plays in proteins synthesis; its message must be again copied, or translated, into a sequence of amino acids in a protein.

RNA is transcribed from a single strand of DNA at a time. However, the same strand is not necessarily transcribed throughout the entire chromosome or through all stages of the life cycle. The RNA produced at different stages in the life cycle hybridizes to different parts of the chromosome, showing the different genes that are activated at each stage.

In λ phage, each of the two DNA strands is partially transcribed at a different stage. In phage T7, however, the same strand is transcribed for both early-acting and late-acting genes. In any case, the RNA is always synthesized in the 5′ → 3′ direction.

Transcription requires enzyme RNA polymerase. Eukaryotes have three RNA polymerases, I (for ribosomal or rRNAs 28S, 18S and 5.8S), II (for hn RNA for messenger or mRNA, snRNAs) and III (for transfer or tRNA, 5SRNA and scRNAs). Different parts of DNA are involved in transcription of various ribonucleic acids. Prokaryotes have only one RNA- polymerase which synthesizes all types of RNAs. RNA polymerase of Escherichia coli has five polypeptide chains β, β’, σ, α. and ω.

The holoenzyme has a molecular weight of 4, 50,000. Sigma or a factor recognizes the start signal or promoter region of DNA. The remaining part of the polymerase enzyme is called core enzyme. Transcribing segment of DNA has promoter and terminator regions (Fig. 7.6).

A termination factor called Rho (p) factor is required for termination of transcription. A number of other factors are also required for unwinding of DNA duplex, stabilization of unwound DNA strand, base pairing, and separation and processing of transcribed RNA.