RNA Transcript - Formation and Types

Introduction:

Ribonucleic acid (RNA) is a type of nucleic acid that is present in the cells, and it is structurally similar to deoxyribonucleic acid (DNA), which is double-stranded, but RNA is single-stranded. RNA has a backbone of a ribose sugar and a phosphate group, and each sugar is attached to one of the bases, which are adenine (A), cytosine (C), uracil (U), or guanine (G). There are different types of RNA, such as ribosomal RNA (rRNA), messenger RNA (mRNA), and transfer RNA (tRNA). Other types of RNA that are involved in gene regulation, RNA interference, and RNA editing are small nuclear RNA (snRNA), microRNA (miRNA), and small interfering RNAs (siRNA).

What Is RNA Transcript?

The RNA strand which is formed when a gene is transcribed is called RNA transcript. Precursor mRNA is a form of RNA transcript that matures in mRNA and translates into protein. RNA transcripts not only code proteins but also help in DNA replication, post-transcriptional modification, regulate genes, and are also involved in protein synthesis.

How Is RNA Transcript Formed?

RNA transcription takes place in the living cell. Transcription is a process where RNA copy is made from a piece of DNA.

The steps involved in it are:

1. The first step is initiation:

• RNA polymerase (RNA Pol or RNAP) is a type of enzyme that helps in the formation of RNA from a DNA template strand.

• This RNA polymerase binds to a region of the gene called a promotor. This enzyme causes the DNA helix to unwind, which gets transcribed; the unwinded DNA is called a transcription bubble.

• In transcription, any one strand of DNA is used as a template strand, and the other strand is called a nontemplate strand or coding strand.

• The RNA that is formed is complementary to the template strand and similar to the nontemplate strand.

2. The second step is elongation:

• The RNA polymerase reads the template strand, and the RNA molecule is built using complementary base pairs.

3. The final step is termination:

• The transcription process ends when the enzyme crosses the termination sequence.

• The RNA transcript is formed and detached from the DNA strand.

What Are the Types of RNA Transcripts Formed?

The RNA transcript that is formed is different for different types of RNAs, which are:

Pre-mRNA, which gets processed into messenger RNA. The steps involved in this are:

1. Addition of a 5' Cap At the Beginning: A5' cap is a modified guanine (G) base, which protects the RNA transcript from breaking down and also helps in getting attached to the ribosome.

2. Addition of 3' Poly-A Tail At the End: A 3' end of the RNA transcript undergoes a polyadenylation signal during transcription, and an enzyme cuts the RNA and adds 100 to 200 adenine (A) base making it poly-A tail, which helps in keeping the RNA transcript stable and helps in moving out from nucleus to cytoplasm.

3. RNA Splicing: In this process, a gene's non-coding regions (introns) from the primary messenger RNA transcript are removed, and the coding regions (exons) are joined together to form a mature messenger RNA.

Primary transcript of tRNA, which gets processed into transfer RNA. The steps involved in this are:

1. The primary transcript of tRNA is synthesized from the tRNA gene with the help of an enzyme RNA polymerase.

2. The 5′ end and 3′ end of pre-tRNA are removed with the help of nucleases.

3. To the 3′ ends of the pre-tRNA, the CCA sequence of nucleotides is added after the removal of the 3′ end. Amino acids are added to this site.

4. Over a hundred modifications of nucleotides occur, such as changing adenine (A) to pseudouridine, uridine to dihydrouridine (D), and uridine to dihydrouridine (D). This helps in the stabilization of the structure, expansion, enhancement, alteration, or restriction of codon-anticodon interactions and recognition by aminoacyl-tRNA synthetase.

5. The introns (the non-coding regions) are then spliced.

6. After these steps, mature tRNA is formed.

Pre-rRNAs get processed into ribosomal RNA (rRNA). The steps involved in this are:

1. Pre-RNA is synthesized by RNA polymerase one from a locus that contains a highly repeated ribosomal DNA (rDNA) gene.

2. Four rRNAs are transcribed as two long molecules (precursor). The one containing pre-RNA is processed into 5S rRNA, and the remaining into 28S, 5.8S, and 18S rRNA.

3. With the help of enzymes, the precursors are cleaved into a large subunit (the 60S) and a small subunit (40S), and some nucleotides are methylated.

4. These two subunits form together and help in protein synthesis.

What Are the Types of RNA Formed From RNA Transcript?

There are three types of RNA, which are:

1. Messenger RNA (mRNA): It carries the genetic information to make proteins. They transfer the information that is stored in DNA from the nucleus to the cytoplasm, where protein synthesis is carried out.

2. Transfer RNA (tRNA): It is a link that connects the chain of amino acids that forms a protein and messenger RNA (mRNA). Therefore, this helps in protein synthesis from mRNA.

3. Ribosomal RNA (rRNA): This also helps in protein synthesis by attaching to tRNA and rRNA.

Other types of RNA called small regulatory RNA, which involve gene regulation, RNA interference, and RNA editing, are:

1. Small Nuclear RNA (snRNA): These help in the spicing of introns (the non-coding regions) from primary genomic transcripts.

2. MicroRNA (miRNA): MicroRNAs are small single-stranded RNA molecules, which are non-coding and help in regulating gene expression. These microRNAs bind to target mRNA (which helps in protein synthesis) to negatively regulate their function by preventing translation into proteins by a mechanism.

3. Small Interfering RNAs (siRNA): They help in regulating the expression of genes.

Conclusion:

RNA is a type of nucleic acid whose major function is to carry out protein synthesis. Types of RNA such as messenger RNA, transfer RNA, and ribosomal RNA are formed by the transcription process, which forms RNA transcripts such as pre-mRNA, primary transcript of tRNA, and pre-rRNAs, which undergo various steps to become mature RNAs. RNA transcripts are not only involved in protein synthesis but also in regulating genes, DNA replication, and post-transcriptional modification.