rRNA, tRNA and mRNA

rRNA, tRNA, and mRNA types

As you are just learning about biology, you may often confuse DNA and RNA. So, let's take a look at their definitions.

Figure 1 showcases the differences between DNA and RNA. All RNA is derived from DNA during a process known as Transcription.

Your DNA codes for many different types of RNA such as rRNA, tRNA, and mRNA.

rRNA stands for ribosomal RNA. This type of RNA will eventually be converted into Ribosomes and accounts for 80% of the total RNA found in a given cell. Ribosomes are specialized proteins that work to convert mRNA into functional proteins during a process known as Translation.

Like rRNA, tRNAs are very important for Translation. tRNA stands for transfer RNA, which exactly describes their function. These specialized tRNAs transfer amino acids during the translation of mRNA into proteins. Each of the 20 amino acids binds to a specific tRNA and is deposited onto the growing peptide chain.

mRNA stands for messenger RNA. This type of RNA is made during transcription and holds the blueprint for a functional protein. mRNA is directly made from your Genes and creates proteins coded for by your DNA.

rRNA, tRNA, and mRNA transcription

rRNA, tRNA, and mRNA are all synthesized during transcription. Transcription is the process of reading genetic codes in the form of DNA and transcribing it into RNA.

As previously mentioned, transcription is the first step of gene expression. Prior to transcription, a series of events must occur in order to recruit the right proteins to the gene that needs to be transcribed. Specialized proteins that make transcription possible are called Transcription Factors.

Transcription Factors travel to an area upstream of the gene known as the promoter region.

As the promoter region becomes exposed, TFIID is the first transcription factor that binds to the promoter. TFIID stands for transcription factor II D.

This means that it is the transcription factor associated with RNA polymerase II, which is the enzyme that facilitates transcription and actually makes the RNA transcript from the DNA strand. RNA polymerase II and the associated transcription factors are known as the preinitiation complex.

Now let's discuss the series of events needed to form the preinitiation complex.

• TFIID

• TFIIA

• TFIIB

• TFIIF; with RNA pol2

• TFIIE

• TFIIH

TFIIH is a very important transcription factor because it is functions to unzip the DNA molecule which exposes the DNA template strand. Once the template strand is unzipped, the preinitiation complex interacts with other transcription factors known as activators and repressors which dictate the rate of transcription.

Activators serve to increase transcription rates while repressors decrease transcription rates. We will not discuss the in-depth mechanisms of activators or repressors as they are beyond the scope of this article. As the preinitiation complex interacts with an activator, it begins to transcribe the template strand. See figure 2 for a visualization of transcription.

Once the template strand is synthesized, it goes through a series of post-transcriptional modifications such as 5' prime capping, 3' prime poly-A tail addition, and splicing.

As RNA polymerase is transcribing RNA it does so from the 5' end to the 3' end and the 5' cap and poly-A tail are needed to stabilize the RNA molecule and prevent it from being degraded.

Once the RNA is completely formed and processed it will be exported out of the nucleus and into the cytoplasm. In the case of rRNA, it will remain in the nucleus where it will be used for ribosome synthesis. tRNA and mRNA will be exported to the cytoplasm for translation to take place.

rRNA, tRNA, and mRNA functions

As previously mentioned, rRNAs come together to make up ribosomes which function to translate mRNA into functional proteins. tRNAs assist ribosomes during the translation process. Let's discuss the process of translation in further detail.

Recall that translation is the process of synthesizing amino acids from mRNA. During translation, ribosomes match the bases of the mRNA in sets of three, called codons.

Again the base pair rule comes into play during translation. tRNA molecules function to deposit the right codons on the growing amino acid sequence. When ribosomes reach a stop codon, it releases the fully formed amino acid sequence and the mRNA!

rRNA, tRNA, and mRNA interaction

rRNA, tRNA, and mRNA interact during translation. The steps in translation are as followed:

1. The ribosome binds to the mRNA molecule

2. The ribosome begins matching tRNA-bound codons to the mRNA molecule

3. As new tRNA molecules bind to the ribosome, the tRNA-associated amino acid gets deposited onto the growing peptide chain

4. This process continues until a stop codon on the mRNA is encountered

5. Once a stop codon is reached, the ribosome releases the fully formed amino acid sequence

6. The amino acid sequence is released and will be folded into a functional protein

See figure 3 for a visual representation of translation.

rRNA, tRNA, and mRNA differences

Let's finish off by looking at the differences between rRNA, tRNA, and mRNA.

rRNA, tRNA, and mRNA are all RNA molecules, but each has different functions. rRNAs make up ribosomes.

Within the ribosome, there are two different types of rRNAs: small rRNAs and large rRNAs. These rRNAs make up the small and large subunits respectively.

These specialized tRNAs transfer amino acids during the translation of mRNA into proteins. Each of the 20 amino acids binds to a specific tRNA and is deposited onto the growing peptide chain during translation.

Lastly, mRNA houses the blueprint of your Genes that will be translated into functional proteins needed for cell signaling and overall metabolism.