Introns, and Exons, and Splicing … Oh My!

One of the first things students of biology learn is basic molecular theory or, the central dogma of biology. The central dogma of biology states that “DNA makes RNA and RNA makes protein.” We know that the process of converting DNA to protein is not one involving magic, but rather more intricate methods termed, transcription and translation. For a quick review, check out my post A Primer on DNA and the Consequences of Mutation.

As we take a closer look at transcription, we will investigate how DNA is converted into pre-messenger RNA and ultimately into messenger RNA. The manner in which this occurs is termed RNA Splicing. Following splicing, mRNA is transported out of the nucleus to indirectly assist with protein synthesis.

In most eukaryotic genes, coding regions are interrupted by non-coding regions. Coding regions are termed exons, where alternatively non-coding regions are called introns. The coding region made up of exons is also called the open reading frame. During the process of transcription, DNA of an entire gene is translated into RNA, more specifically termed pre-messenger RNA (pre-mRNA). Pre-mRNA includes both exons and introns. Through a process, called RNA splicing, the introns of the gene are removed allowing the exons to be joined forming a continuous coding sequence. The continuous coding sequence is referred to as messenger RNA. mRNA contains a 5’ cap consisting of 7 methyl guanosine, a 3’ polyA tail, and is ready for translation into protein.

EXONS–Coding regions of the DNA that make up the mature messenger RNA

INTRONS–Non-coding regions that are removed from the DNA sequence to form mature messenger RNA

PROMOTER–Found at the 5′ end of the DNA sequence and functions to initiate transcription

RNA Splicing Made Easy

DNA: 5′–xxxxxx11111111111222222222222222222233333–3′

Transcribed to…

Pre-mRNA:5′–11111111111222222222222222222233333–3′

Further processing/splicing …

Mature mRNA: 5′ 7 Me G–111122222222222222233333–AAAAA 3′

 

Test your Knowledge!

  1. What is the function of the 5’ 7-methyl guanosine cap?
  2. What enzyme is responsible for the addition of the polyA tail to the 3’ end of the mRNA?
  3. What structures of the mRNA are translated into protein?

 

Answers

  1. The 7-methyl guanosine cap is recognized as a site that signals the start of translation
  2. Polyadenylate polymerase adds the multiple adenines to the 3’ end of an mRNA sequence
  3. Introns are spliced out of the open reading frame and the exons are translated into protein

 

 

L Noll Image_small

-LeAnne Noll, BS, MB(ASCP)CM is a molecular technologist at Children’s Hospital of Wisconsin and was recognized as one of ASCP’s Top Five from the 40 Under Forty Program in 2015.

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