Does RNA Have Thymine : 7 Facts You Need To Know


Ever wondered about the fact that whether or not the nitrogenous base pairs in RNA are the same as DNA? This post will discuss “does RNA have thymine” in great detail.

The nitrogenous base pairs of DNA are adenine, guanine, cytosine and thymine. RNA also has all of these but not thymine. RNA has another nitrogenous base pair that is uracil in place of thymine, which gives RNA structure its many features.

The RNA structure has a few distinctions compared to the structure of DNA, and one difference is the presence of ribose sugar in RNA while DNA has deoxyribose sugar. Another compositional difference is that RNA contains uracil base pairs instead of thymine. This uracil pairs via 2 H-bonds with adenine residues.

This single-stranded polymer contains many residues linked via 3′ to 5′ phosphodiester bonds. An interesting thing for you to know before we take a deep dive into the topic is that chemically thymine is equivalent to 5-methyl uracil. Add methyl group to the 5th carbon of uracil, and now you have got a thymine residue. Just a little structural difference but vast molecular roles.

Does tRNA have thymine?

After transcription of tRNA polypeptide, it undergoes various post-translational modifications, which are responsible for various changes in the structure. Among these modifications is the 5’-methylation of uracil residue of the T loop, which results in the formation of a thymine residue. So mature tRNA does contain a thymine residue. 

These post-translational modifications are indispensable for proper 3D folding and functional aspects of tRNA. So, all tRNA contains one thymine residue in the T loop.

tRNA stands for transfer RNA, and functionally it is associated with the task of transferring cognate amino acids to the growing polypeptide chain during the translation of mRNA. Among the three types of RNAs, this one has only 75-90 base pairs, making it the smallest.

Chemically speaking, the tRNA has various modified bases, but first, we must know a little about the structure of tRNA. Structurally, tRNA resembles a cloverleaf structure in 2D, but in reality (3D), it adopts an L-like structure with various loops. tRNA has various loops: D loop, anticodon loop, variable loop, and T loop.

does rna have thymine
Clover leaf structure of tRNA from Wikipedia

Is thymine present in ribosomal RNA?

Usually, thymine is absent in RNA, but in certain cases, various post-transcriptional modifications might alter the chemical composition, and that is what happens with the 23S subunit of rRNA in which two uracil residues (position 74 and 1939) have been reported to have 5′-methylation resulting in the formation of thymine residues. Apart from these modifications, thymine has not been reported in the structure of rRNA.

Ribosomal RNA (rRNA) constitutes the majority of the RNA content in a cell. The structure usually has two subunits – one large and one small. These subunits further contain various rRNA molecules and proteins, making it a complex of rRNAs and proteins.

Which RNA has thymine?

We have learned the basics of two types of RNAs: tRNA and rRNA. mRNA does not have a thymine residue, but the other two do have this residue, making them an exception to the general rule that thymine is absent from the structure of RNA. 

What base replaces thymine in RNA

Thymine is essentially replaced by uracil in ribonucleic acid. Uracil is a nitrogenous base pair belonging to the pyrimidine group. It has a six-membered heterocyclic ring structure with the chemical formula C4H4N2O2 and IUPAC name – Pyrimidine-2,4(1H,3H)-dione. As discussed earlier, if this uracil is methylated (addition of -CH3 group) at the 5′ position, it will be known as thymine base pair.

So thymine and uracil are structurally quite similar to one another to a certain degree, but their functional aspects are very much different, as we will see in upcoming sections.

Uracil has two tautomeric forms: amide and imide. It is called uridine (a ribonucleoside) when bound to a ribose sugar. The addition of phosphate groups will lead to forming a ribonucleotide named uridine 5′ triphosphate if three phosphate groups are added.

Why rna has uracil instead of thymine?

Production of thymine in a cell requires uridine monophosphate (UMP) as a starting material, two different enzymes and various cofactors, making it an energy invested process. Now, cells do not have to produce just one or two thymine molecules. They have to synthesize thousands of these base pairs, so by employing this discrimination, the cell conserves some energy as replacing thymine with uracil has no effect on adenine interaction and coding functions.

Another reason that needs separate addressing is that cytosine is chemically less stable and deamination of cytosine, which produces uracil, is a frequent process in cells. Now, cytosine base pairs with guanine (C: G), and this deamination will result in the formation of uracil in DNA, which will base pair with adenine in the daughter strand, and this leads to a C: A base pair mutation in the DNA and this mutation will be carried in all the progenies. 

The methyl group difference between thymine and uracil has no role in affecting their Watson-crick pairing as both of them pair with adenine via two hydrogen bonds. However, this discrimination between these two base pairs is quite stringent in a cell, so one might ask why that. 

So by discriminating between thymine and uracil, a cell conserves some energy and minimizes the transition type of mutations. 

Does dna does have uracil?

Due to the cytosine’s chemical instability, uracil does not appear in DNA as that would make it more prone to single base-pair mutations. That is why uracil is not found in DNA.

Why dna does not have uracil?

The mismatch repair enzymes are quite conserved throughout all the life forms, further signifying the necessity of uracil-thymine discrimination. 

The fundamental reason for the lack of uracil in DNA structure is the evolution of specific machinery within the cell that recognizes uracil in DNA and then excises it. Cells have a potent uracil-excision repair system initiated with UDG enzymes’ activation. 

Uses of uracil instead of thymine?

Apart from being an energy-conserving strategy, uracil in the RNA makes it prone to the attack of various enzymes. Still, its exclusion from DNA makes DNA more chemically resistant. This effect is desired because the various mRNAs are used to express genes, and sometimes this expression is context-dependent.

So, after an mRNA molecule has fulfilled its role and is no longer needed, the presence of uracil makes it easy for the cell to degrade it, so one RNA is degraded, and numerous others will be synthesized. They will be degraded then, and other RNA molecules will be synthesized, continuing a cycle of creation and degradation of RNA molecules.

Conclusion

tRNA does have a thymine residue in its T loop. Some post-translation modifications might result in the incorporation of thymine in the 23S subunit of rRNA, but other than these cases, RNA lacks thymine residues. I hope you can answer the question “does RNA have thymine” now.

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