Cellular Transcription

By Ian Robinasn and Thomas WIlks

What is Transcription?

Transcription is the process of copying genetic information stored in a DNA strand into a transportable complimentary strand of RNA. Eukaryotic transcription takes place in the nucleus of the cell and proceeds in three sequential stages: initiation, elongation, and termination.
Eukaryotic transcription

The Three Stages of Transcription


In Eukariotic cells Initiation starts with a collection of proteins called transcription factors. These proteins mediate the binding of RNA polymerase and the initiation of transcription.

All of this happens at the promoter region which begins with a groups of Adenine and Thymine pair known as the TATA box.


In Elongation one strand of the DNA, the template strand (or noncoding strand), is used as a template for RNA synthesis this templates are known as transcription units. As transcription proceeds, RNA polymerase traverses the template strand and uses base pairing complimentary with the DNA template to create an RNA copy.


Transcription termination in eukaryotes is less understood but involves cleavage of the new transcript followed by template-independent addition of adenines at its new 3' end, in a process called polyadenylation.

mRNA Processing

Introns and Exons

Once the pre-mRNA is made it needs to be processed. The first part of RNA processing takes out bits of non-coding RNA. These bits of RNA are called introns and dont leave the nucleus. The part of the RNA that does code are called exons and do leave the nucleus. They're eventually translated into amino acids. Exceptions to this are the UTR's, they are not translated into proteins at all they stay with the mRNA.


UTR's regulate mRNA stability, cytoplasmic transport, subcellular localization and translation efficiency. They are also involved in the post transcriptional regulation of gene expression.


RNA Splicing is like cutting and pasting, you remove large portions of the RNA molecule that is initially synthesized and use them somewhere else. You only need 1,200 nucleotides yet the primary RNA transcript is made of 27,000 so you have large stretches of nonvoting regions that the.

Alternative RNA Splicing is when genes give rise to two or more polypeptides, depending on which segments are treated as exons during RNA processing.

An important component of these two processes are Spliceosomes. Spliceosomes Reese the introns and join other the l exons that flanked he intros.

RNA Splicing

5' Cap

The 5' Cap helps mature mRNA leaving the nucleus, protecting against degradation by hydrolytic enzymes, and help ribosomes tech 5' end when the mRNA reaches the cytoplasm. This process, known as mRNA capping, is highly regulated and vital in the creation of stable and mature messenger RNA ability to undergo translation during protein synthesis.

Poly-A Tail

The Poly-A Tail is a sequence of 50 o 250 adenine nucleotides added onto the 3' end of a pre-mRNA molecule. The Poly-A Tail has the same functions as e 5' Cap. It is also believed to protect the mRNA from degradation by exonucleases, and is essential for transcription termination, for export of the mRNA from the nucleus, and for translation.

*Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end (exo) of a polynucleotide chain.


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