Precursor mRNA

Definition

noun
plural: precursor mRNAs
An immature or incompletely processed mRNA molecule in eukaryotes that needs to be processed before it becomes a fully functional mature mRNA for transport into the cytoplasm

Details

Overview

The mRNA is a type of RNA involved in protein synthesis. It is synthesized based on DNA template on the nucleus. In eukaryotes, it needs to be processed before it can be fully functional, conveying the genetic code to the ribosomes (site of protein synthesis in the cytoplasm). The precursor mRNA (also called pre-mRNA) is the initial form of mRNA in eukaryotes. It is formed by transcribing the genetic information from DNA molecule. Before it is moved into the cytoplasm for translation at the ribosomal site, the pre-mRNA undergoes extensive processing within the nucleus.

Characteristics

The pre-mRNA is a single-stranded nucleic acid. It is the first form of RNA created through transcription prior to translation process. The pre-mRNA undergoes maturation to become mature mRNA. The pre-mRNA differs from a mature mRNA in including both exon and intron sequences. In mature mRNA, these sequences are removed. A mature mRNA also differs from pre-mRNA in having a Poly(A)tail added to the 3′ end of the RNA and a 5′ cap added to the 5′ end.
Pre-mRNA is a type of primary transcript. The primary transcript may also give rise to other types of RNAs (including non-coding RNAs such as tRNAs, rRNAs, and microRNAs). The primary transcript that becomes mRNA is one that which encodes at least one gene and later translated into a protein. Pre-mRNA is sometimes referred to as heterogenous nuclear RNA (hnRNA) because it forms the bulk of the latter. However, hnRNA may also include other nuclear RNA transcripts that do not end up as cytoplasmic mRNA.

Biological function

The pre-mRNA, as the name implies, is the precursor to mature mRNA. The pre-mRNA undergoes series of modifications after transcription to become a fully functional mature mRNA. The mature mRNA (or simply mRNA) is smaller than pre-mRNA since it excludes the introns present in the pre-mRNA. After which, the mRNA is transported from the nucleus to the cytosol where ribosomes are located. The ribosomes, together with tRNAs, decode the message that mRNA carries and create protein(s) accordingly.

Common biological reactions

Common biological reactions

A precursor mRNA is an immature mRNA molecule that is transcribed by the activity of the enzyme RNA polymerase. Then, this precursor undergoes further processing to become a functional mature mRNA. To initiate pre-mRNA synthesis (via transcription), the segment of the DNA that will be copied needs first to unwind from being a double helix. This results when the hydrogen bonds between complementary base pairs are broken. The antisense, one of the unwound two DNA strands, serves as a template for precursor mRNA synthesis.
To initiate transcription, the enzyme RNA polymerase traverses the DNA template strand from 3′ → 5′. However, the copying proceeds in the 5′ → 3′ direction, similar to DNA replication. Nonetheless, unlike DNA replication, transcription does not need a primer to start. Also, it utilizes uracil-adenine base pairing instead of thymine-adenine (as in DNA replication). In eukaryotes, there are distinct types of RNA polymerases that are involved depending on the type of RNA to be synthesized. In particular, RNA polymerase II is the type of RNA polymerase involved in the synthesis of pre-mRNA. In prokaryotes, only one type of RNA polymerase is involved in the synthesis of all RNAs. Furthermore, transcription in the prokaryotes occurs in the cytoplasm whereas in eukaryotes it takes place primarily in the nucleus before the mRNA is transported into the cytoplasm for translation.
In summary, the general steps of transcription (mRNA synthesis) are as follows:

1. RNA polymerase and certain transcription factors bind to the promoter region of the DNA.
2. A transcription bubble forms as the helical DNA unwinds and separates as a result of the breakage of hydrogen bonds between complementary nucleotides. Sigma factor binds to the unpaired nucleotides.
3. RNA polymerase adds RNA nucleotides that complement the nucleotides of one DNA strand. Take note that only one of the two DNA strands will serve as a template for transcription, i.e. the antisense strand of DNA of which the RNA polymerase will read from 3′ end to the 5′ end (3′ → 5′) while the mRNA is created in the opposite direction, i.e. 5′ → 3′. Four distinct ribonucleoside monophosphate residues are used to create pre-mRNA: adenosine monophosphate (AMP), cytidine monophosphate (CMP), guanosine monophosphate (GMP), and uridine monophosphate (UMP). These residues are added to the 3’hydroxyl group on the 3′-end of the growing mRNA. The result is mRNA transcript that matches the sequence of the non-template sense strand (also called the coding strand) except for thymine that is replaced by uracil.
4. Ribose sugar-phosphate backbone forms as the mRNA transcript forms.
5. The newly formed mRNA transcript is freed when the hydrogen bonds between the nucleotides of RNA and DNA are broken.
6. The pre-mRNA undergoes post-translational modifications, such as capping, splicing, and polyadenulation, to become fully functional mature mRNA that may be transported into the cytoplasm through the nuclear pore.

Regulators

A number of factors regulate the process of transcription. One of them is the transcription factors. These are proteins that bind to DNA, particularly to the so-called enhancers (i.e. DNA sequences that promote the transcription of genes and are located farther up or downstream from the genes they regulate). Some of the transcription factors could activate transcription while others could inhibit the process. They do so by recruiting particular enzymes that could either enhance or repress the access of RNA polymerase to DNA.
Silencers are DNA sequences that repress the transcription of genes. Similar to enhancers, the silencers are located father up or downstream from the genes they regulate. They inhibit transcription by binding to certain transcription factors, and then working together in destabilizing the initiation complex that activates RNA polymerase.

Supplementary

Also called

• pre-mRNA

Further reading

Compare

• mature mRNA

See also

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