Transcription

What is transcription?

Transcription is the synthesis of any type of complimentary RNA from a DNA template: note, several types of RNA can be encoded by a DNA strand [see DNA vs. RNA list]. Here, we focus specifically on transcription that leads to pre-mRNA, mRNA and eventually proteins. 

In the process of gene expression, transcription involves the production of messenger RNA (mRNA) from a DNA template. It takes place in the nucleus of a cell and is catalyzed by the enzyme RNA polymerase II.

RNA polymerase  All eukaryotes have three different types of RNA polymerase

RNA polymerase I transcribes rRNA genes

RNA polymerase II transcribes mRNA, miRNA, snRNA, and snRNA genes

RNA polymerase III transcribes an array of RNA genes, including but not limited to tRNA and 5S rRNA gene

 

 

 

 

 

The steps of transcription

The process of transcription entails several steps: 

1. Initiation

The first step of transcription to form mRNA involves RNA polymerase II binding to a promoter region  just upstream of the gene that is to be transcribed. Promoters are often classified as strong or weak based on their effects on transcription rates and thus gene expression. Transcription factors are proteins that help to position RNA polymerase II and assist in the breaking of the hydrogen bonds in the DNA helix. ³

2. Elongation

RNA polymerase II breaks the hydrogen bonds connecting two strands of DNA in the double helix. The enzyme then uses the single DNA strand as a template to build an RNA strand in the 5' to 3' direction, adding each complementary nucleotide to the 3' end of the strand. In RNA, the nucleotide thymine is replaced by the nucleotide uracil.³ 

What do we mean by 5' and 3'? This refers to the carbon numbers in DNA and RNA's backbone. The 5' carbon ribose ring frequently has a phosphate group attached, and the 3' carbon end has a hydroxyl (-OH) group attached. The asymmetry gives the DNA and RNA strands a "direction

The DNA strand moves through the RNA polymerase II enzyme. In the region behind where the nucleotides are being added to form the pre-mRNA strand, the DNA helix re-forms. This means that the pre-mRNA produced is eventually released from the DNA template a single strand. 

3. Termination

Termination marks the end of RNA polymerase II adding nucleotides to the pre-mRNA strand and the release of the pre-mRNA. Despite extensive research, there is still ambiguity surrounding the precise physiological cause of termination - several mechanisms are outlined in this review paper .

From pre-mRNA to mRNA

Eukaryotic pre-mRNAs must go through several additional processing steps before translation can occur. Firstly, they have a 5' cap added and a 3' poly-A-tail added to protect against transcript degradation.⁴ 

Many eukaryotic pre-mRNAs are subject to splicing. Here, the non-coding sections of the pre-mRNA (introns) are cut out, and the coding sections (the exons) are effectively glued back together.

Schematic showing pre-mRNA undergoing splicing to form mature mRNA.  

Alternative splicing may also take place, whereby exons or noncoding regions within the pre-Mrna transcript are joined or skipped, resulting in multiple mRNAs being encoded by a single gene.

After these modifications have taken place, the resulting strand is known as mature mRNA. This mature mRNA is then able to leave the nucleus and enter the cell cytoplasm where translation takes place.