Types of RNA: Structure, Function and Classification

Introduction

𝗧𝘆𝗽𝗲𝘀 𝗼𝗳 𝗥𝗡𝗔 are the molecules that turn the genetic information stored in 𝗗𝗡𝗔 into working 𝗽𝗿𝗼𝘁𝗲𝗶𝗻𝘀. understanding 𝗿𝗥𝗡𝗔, 𝘁𝗥𝗡𝗔, 𝗺𝗥𝗡𝗔 and 𝘀𝗻𝗥𝗡𝗔 is essential, because they explain how cells carry out 𝗽𝗿𝗼𝘁𝗲𝗶𝗻 𝘀𝘆𝗻𝘁𝗵𝗲𝘀𝗶𝘀, 𝘁𝗿𝗮𝗻𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻 and 𝘁𝗿𝗮𝗻𝘀𝗹𝗮𝘁𝗶𝗼𝗻.
𝗥𝗶𝗯𝗼𝗻𝘂𝗰𝗹𝗲𝗶𝗰 𝗮𝗰𝗶𝗱 (𝗥𝗡𝗔) is a single-stranded nucleic acid made of ribonucleotides. Each type has a specific role: 𝗿𝗥𝗡𝗔 builds the ribosome, 𝘁𝗥𝗡𝗔 carries amino acids, 𝗺𝗥𝗡𝗔 carries the genetic code, and 𝘀𝗻𝗥𝗡𝗔 helps remove introns during 𝗦𝗽𝗹𝗶𝗰𝗶𝗻𝗴.

What is RNA (Ribonucleic Acid)?

𝗥𝗡𝗔 is a long, 𝘂𝗻𝗯𝗿𝗮𝗻𝗰𝗵𝗲𝗱, polymeric nucleic acid made of 𝗿𝗶𝗯𝗼𝗻𝘂𝗰𝗹𝗲𝗼𝘁𝗶𝗱𝗲𝘀.
It is usually 𝘀𝗶𝗻𝗴𝗹𝗲-𝘀𝘁𝗿𝗮𝗻𝗱𝗲𝗱 but can fold into double-stranded regions.
Each nucleotide has 𝗿𝗶𝗯𝗼𝘀𝗲 𝘀𝘂𝗴𝗮𝗿, a 𝗽𝗵𝗼𝘀𝗽𝗵𝗮𝘁𝗲 group and a base (𝗔, 𝗨, 𝗚 or 𝗖).
𝗨𝗿𝗮𝗰𝗶𝗹 replaces thymine.
The three major types are 𝗿𝗥𝗡𝗔 (~𝟴𝟬%), 𝘁𝗥𝗡𝗔 (~𝟭𝟬–𝟭𝟱%) and 𝗺𝗥𝗡𝗔 (~𝟯–𝟱%).

Classification of RNA

𝗚𝗲𝗻𝗲𝘁𝗶𝗰 𝗥𝗡𝗔: genetic material in some viruses (e.g., 𝗧𝗠𝗩, 𝗛𝗜𝗩).
𝗡𝗼𝗻-𝗴𝗲𝗻𝗲𝘁𝗶𝗰 𝗥𝗡𝗔: involved in protein synthesis and gene regulation.

𝗿𝗥𝗡𝗔: ribosomal RNA
𝘁𝗥𝗡𝗔: transfer RNA
𝗺𝗥𝗡𝗔: messenger RNA
𝘀𝗻𝗥𝗡𝗔: small nuclear RNA

1. Ribosomal RNA (rRNA)

Structure and Origin

It is extracted from 𝗿𝗶𝗯𝗼𝘀𝗼𝗺𝗲𝘀 and is a long, single-stranded chain.
It folds and combines with proteins to form 𝗿𝗶𝗯𝗼𝗻𝘂𝗰𝗹𝗲𝗼𝗽𝗿𝗼𝘁𝗲𝗶𝗻 surfaces.
It is the 𝗺𝗼𝘀𝘁 𝗮𝗯𝘂𝗻𝗱𝗮𝗻𝘁 RNA, about 𝟴𝟬% of total cellular RNA.
Its molecular weight is about 𝟬.𝟱–𝟮.𝟬 × 𝟭𝟬⁶ 𝗗𝗮𝗹𝘁𝗼𝗻𝘀.
Ribosomes measure about 𝟮𝟬𝟬–𝟮𝟱𝟬 Å.

Location

Ribosomes lie in the 𝗰𝘆𝘁𝗼𝗽𝗹𝗮𝘀𝗺, either 𝗳𝗿𝗲𝗲 or attached to the 𝗿𝗼𝘂𝗴𝗵 𝗘𝗥 (𝗥𝗘𝗥).
rRNA is made in the 𝗻𝘂𝗰𝗹𝗲𝗼𝗹𝘂𝘀, where ribosomal subunits are assembled.

Sedimentation Coefficient (Svedberg Unit, S)

The 𝗦𝘃𝗲𝗱𝗯𝗲𝗿𝗴 𝘂𝗻𝗶𝘁 measures the 𝗿𝗮𝘁𝗲 𝗼𝗳 𝘀𝗲𝗱𝗶𝗺𝗲𝗻𝘁𝗮𝘁𝗶𝗼𝗻 in a centrifuge.
S values are 𝗻𝗼𝘁 𝗮𝗱𝗱𝗶𝘁𝗶𝘃𝗲, because they depend on shape as well as mass.

Comparison Table

Feature	Prokaryotes	Eukaryotes
Whole Ribosome	70s	80s
Large subunit	50s(23s + 5s rRNA)	60s (28s + 5.8s + 5s rRNA)
Small subunit	30s (16s rRNA)	40s (18s rRNA)

Special Features

It is rich in 𝗚–𝗖 sequences.
It has few modified nucleotides, mostly 𝗺𝗲𝘁𝗵𝘆𝗹𝗮𝘁𝗲𝗱 bases.
Ribosomes contain about 𝟯𝟱–𝟰𝟬% 𝗽𝗿𝗼𝘁𝗲𝗶𝗻𝘀.
𝗙𝘂𝗻𝗰𝘁𝗶𝗼𝗻: it forms the structural framework of the ribosome and helps form 𝗽𝗲𝗽𝘁𝗶𝗱𝗲 𝗯𝗼𝗻𝗱𝘀, so it acts as a 𝗿𝗶𝗯𝗼𝘇𝘆𝗺𝗲.

2. Transfer tRNA (tRNA / sRNA / Soluble RNA)

Function

It is an 𝗮𝗱𝗮𝗽𝘁𝗼𝗿 molecule that carries 𝗮𝗺𝗶𝗻𝗼 𝗮𝗰𝗶𝗱𝘀 to the ribosome during 𝗽𝗿𝗼𝘁𝗲𝗶𝗻 𝘀𝘆𝗻𝘁𝗵𝗲𝘀𝗶𝘀.
It reads the mRNA codon through its 𝗮𝗻𝘁𝗶𝗰𝗼𝗱𝗼𝗻.

Abundance and Distribution

It forms about 𝟭𝟬–𝟭𝟱% of total cellular RNA.
It is the 𝘀𝗺𝗮𝗹𝗹𝗲𝘀𝘁 major RNA, about 𝟳𝟬–𝟵𝟬 nucleotides long.
There is at least one tRNA for every amino acid.
Prokaryotes have about 𝟯𝟬–𝟰𝟬 tRNAs, and eukaryotes about 𝟱𝟬–𝟲𝟬.

Cloverleaf Model

It was proposed by 𝗥. 𝗪. 𝗛𝗼𝗹𝗹𝗲𝘆 (1965, 𝗡𝗼𝗯𝗲𝗹 𝗣𝗿𝗶𝘇𝗲 1968) from yeast alanine tRNA.
The 2D structure resembles a 𝗰𝗹𝗼𝘃𝗲𝗿 𝗹𝗲𝗮𝗳 because the chain folds into 𝘀𝘁𝗲𝗺-𝗹𝗼𝗼𝗽 regions.
In 3D it folds into an 𝗟-𝘀𝗵𝗮𝗽𝗲.

Arms of tRNA

1. 𝗔𝗰𝗰𝗲𝗽𝘁𝗼𝗿 𝗔𝗿𝗺: formed by the paired 5′ and 3′ ends. The 3′ end has an unpaired 𝗖𝗖𝗔 tail, and the amino acid attaches to the terminal 𝗔.
2. 𝗗 (𝗗𝗛𝗨) 𝗔𝗿𝗺: contains 𝗱𝗶𝗵𝘆𝗱𝗿𝗼𝘂𝗿𝗶𝗱𝗶𝗻𝗲 and is recognised by 𝗮𝗺𝗶𝗻𝗼𝗮𝗰𝘆𝗹-𝘁𝗥𝗡𝗔 𝘀𝘆𝗻𝘁𝗵𝗲𝘁𝗮𝘀𝗲.
3. 𝗔𝗻𝘁𝗶𝗰𝗼𝗱𝗼𝗻 𝗔𝗿𝗺: carries three bases complementary to the mRNA 𝗰𝗼𝗱𝗼𝗻.
4. 𝗩𝗮𝗿𝗶𝗮𝗯𝗹𝗲 𝗔𝗿𝗺: differs in length between tRNAs and affects the 3D shape.
5. 𝗧𝝍𝗖 𝗔𝗿𝗺: contains 𝗽𝘀𝗲𝘂𝗱𝗼𝘂𝗿𝗶𝗱𝗶𝗻𝗲 (𝝍) and helps the tRNA bind to the 𝗿𝗶𝗯𝗼𝘀𝗼𝗺𝗲.

3. Messenger RNA (mRNA)

It is a single-stranded copy of a gene, made by 𝘁𝗿𝗮𝗻𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻.
It carries the genetic code from 𝗗𝗡𝗔 to the ribosome and acts as the 𝘁𝗲𝗺𝗽𝗹𝗮𝘁𝗲 for protein synthesis.
It forms about 𝟯–𝟱% of cellular RNA and is the most variable in size.

Structural Regions (Eukaryotes)

𝟱′ 𝗰𝗮𝗽 (7-methylguanosine): protects the mRNA and helps ribosome binding.
𝟱′ 𝗨𝗧𝗥 (untranslated region): helps regulate translation.
𝗖𝗼𝗱𝗶𝗻𝗴 𝗿𝗲𝗴𝗶𝗼𝗻: a series of codons.
𝟯′ 𝗨𝗧𝗥: affects stability and localisation.
𝗣𝗼𝗹𝘆-𝗔 𝘁𝗮𝗶𝗹: protects against degradation.

Codons

𝗜𝗻𝗶𝘁𝗶𝗮𝘁𝗶𝗼𝗻 𝗰𝗼𝗱𝗼𝗻: 𝗔𝗨𝗚, which codes for 𝗺𝗲𝘁𝗵𝗶𝗼𝗻𝗶𝗻𝗲.

𝗦𝘁𝗼𝗽 𝗰𝗼𝗱𝗼𝗻𝘀: 𝗨𝗔𝗔, 𝗨𝗔𝗚 and 𝗨𝗚𝗔. They code for no amino acid.

4. Small Nuclear RNA (snRNA)

These are short RNAs of about 𝟭𝟬𝟬–𝟮𝟭𝟱 nucleotides, found in the 𝗻𝘂𝗰𝗹𝗲𝘂𝘀.
They combine with proteins to form 𝘀𝗻𝗥𝗡𝗣𝘀 (𝘀𝗻𝘂𝗿𝗽𝘀).
snRNPs assemble into the 𝘀𝗽𝗹𝗶𝗰𝗲𝗼𝘀𝗼𝗺𝗲, which carries out 𝗽𝗿𝗲-𝗺𝗥𝗡𝗔 𝘀𝗽𝗹𝗶𝗰𝗶𝗻𝗴 (post-transcriptional processing).

Introns vs Exons

𝗜𝗻𝘁𝗿𝗼𝗻𝘀 are 𝗻𝗼𝗻-𝗰𝗼𝗱𝗶𝗻𝗴 segments that are cut out.
𝗘𝘅𝗼𝗻𝘀 are 𝗰𝗼𝗱𝗶𝗻𝗴 segments that are joined together.

Spliceosome Components

It is made of 𝗨𝟭, 𝗨𝟮, 𝗨𝟰, 𝗨𝟱 and 𝗨𝟲 snRNPs.
𝗨𝟲 is the smallest, with about 𝟭𝟬𝟬 nucleotides.
𝗨𝟮 is the largest, with about 𝟮𝟭𝟱 nucleotides.

Other Important RNAs

𝗵𝗻𝗥𝗡𝗔 (heterogeneous nuclear RNA): the primary transcript that is processed into mRNA.
𝗺𝗶𝗥𝗡𝗔 and 𝘀𝗶𝗥𝗡𝗔: short regulatory RNAs involved in 𝗥𝗡𝗔 𝗶𝗻𝘁𝗲𝗿𝗳𝗲𝗿𝗲𝗻𝗰𝗲 and gene silencing.
𝗹𝗻𝗰𝗥𝗡𝗔: long non-coding RNA that regulates gene expression.
𝗥𝗶𝗯𝗼𝘇𝘆𝗺𝗲𝘀: RNA molecules with 𝗰𝗮𝘁𝗮𝗹𝘆𝘁𝗶𝗰 activity.

Conclusion

In summary, the 𝘁𝘆𝗽𝗲𝘀 𝗼𝗳 𝗥𝗡𝗔 work together as a team to express genes. 𝗺𝗥𝗡𝗔 copies the message from DNA, 𝘁𝗥𝗡𝗔 brings the correct 𝗮𝗺𝗶𝗻𝗼 𝗮𝗰𝗶𝗱𝘀, 𝗿𝗥𝗡𝗔 forms the ribosome where 𝗽𝗲𝗽𝘁𝗶𝗱𝗲 𝗯𝗼𝗻𝗱𝘀 are made, and 𝘀𝗻𝗥𝗡𝗔 prepares pre-mRNA through 𝘀𝗽𝗹𝗶𝗰𝗶𝗻𝗴.

Remember the key facts for exams: 𝗿𝗥𝗡𝗔 is the most abundant, 𝘁𝗥𝗡𝗔 is the smallest, 𝗺𝗥𝗡𝗔 is the most variable in size, the 𝗮𝗰𝗰𝗲𝗽𝘁𝗼𝗿 𝗮𝗿𝗺 binds the amino acid, and the 𝗮𝗻𝘁𝗶𝗰𝗼𝗱𝗼𝗻 𝗮𝗿𝗺 reads the codon. A clear understanding of these 𝗥𝗡𝗔 𝘁𝘆𝗽𝗲𝘀 forms the foundation for advanced topics in 𝗺𝗼𝗹𝗲𝗰𝘂𝗹𝗮𝗿 𝗯𝗶𝗼𝗹𝗼𝗴𝘆, 𝗴𝗲𝗻𝗲𝘁𝗶𝗰𝘀 and 𝗯𝗶𝗼𝘁𝗲𝗰𝗵𝗻𝗼𝗹𝗼𝗴𝘆.