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Where Does Transcription Occur In Cells?

RNA transcription in eukaryotic organisms occurs in the nucleus of the cell where the cell’s DNA is located. In prokaryotic organisms that lack a membrane-bound nucleus, transcription occurs directly in the cytoplasm of the cell. During transcription, a sequence of DNA is copied into mRNA transcript, which is then shuttled elsewhere in the organism to guide protein construction.

“DNA was the first three-dimensional Xerox machine.” — Kenneth Ewart Boulding

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Transcription in biology refers to the first step in gene expression when a segment of DNA is copied into RNA.  Transcription form an integral part of the 3 step-chain from DNA, RNA to protein. The product of transcription is called messenger RNA (mRNA) as it carries the genetic message from information in the DNA to the protein constructing mechanisms of the body. Proteins are a key building block of eukaryotic life. Without transcription occurring, the body would not be able to access the information stored in the genetic code and would be unable to construct the proteins necessary for survival and development.

Mechanism Of Transcription

The process of transcription is initiated when RNA polymerase, the enzyme that physically assembles the RNA molecule, binds to the specific DNA sequence to be transcribed. After binding to the site, RNA polymerase “unwinds” the strand of DNA to access the nucleotide sequence of bases. The region of the DNA strand that is opened up is called the transcription bubble and contains the DNA segment to be transcribed.

During transcription, RNA uses only one strand of DNA to copy information. This strand is called the template strand. The other strand is called the coding strand and assists in the regulation of the transcription process. As RNA proceeds down the sequence of DNA nucleotides in the template strands, it constructs the mRNA molecule by base-pairing. The resulting sequence of bases in the mRNA is not identical to the sequence in the template strand but consists of complementary pairs. For example, if there is a G base in the DNA strand, RNA polymerase will add a C base to the new mRNA molecule. Therefore, the mRNA is nearly identical to the coding strand, just with all the T nucleotides replaced with U nucleotides.

During this process, the main transcription may halt as transcription factors check for errors during the copying of code into mRNA. Once the process of copying is complete, the transcription has to stop. In bacteria, there are two main mechanisms for transcription termination called Rho-dependent and Rho-independent mechanisms. Rho-dependent termination involves the activity of a Rho-protein factor that destabilizes the bond between the mRNA strand and the template strand. During rho-independent termination, the transcription bubble is bent so that mechanical stress breaks off the completed mRNA molecule from the template strand. The process of transcription termination is less understood in eukaryotic organisms, but it is known it involves the addition of large chains of adenine bases to the mRNA strand, a process called polyadenylation. The polyadenylation of mRNA strands acts as a signal for RNA polymerase to remove itself from the template strand.

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“DNA is like a computer program, but far, far more advanced than any software ever created.” — Bill Gates

In bacteria, once the transcription is finished, the complete mRNA is ready for protein synthesis. In fact, in a number of bacterial species, the cell begins to construct the protein while transcription is still occurring. Bacteria can simultaneous transcribe and translate genetic code because the genetic material of the cell where transcription occurs floats freely in the cytoplasm. In eukaryotic organisms, there are still a few steps left in the transcription process before mRNA can be used to make proteins.

Post-Transcription Modification In Eukaryotes

After eukaryotic pre-mRNA is transcribed it goes through various post-transcription modifications. Pre-mRNA strands are coated in proteins that protect from degradation and allow it to survive the trip from the nucleus to the cytoplasm of the cell. The pre-mRNA is capped by a chain of guanosine bases which stabilizes the strand and also signals to the translation mechanisms where to start the translation.

Eukaryotic genes are composed of two types of sequences known as exons and introns. Exons are the portions that code for protein expression and are the active sections that contain the blueprint for proteins. Introns consist of the segments of genetic code interspersed between the exons. It is still not known exactly what function introns play in eukaryotic gene expression, but it is thought they play a role in gene regulation. During transcription, the RNA polymerase copies both exon and intron sequences into the pre-mRNA. During post-transcription modification, transcription factors go through the pre-mRNA and cut out the copied intron sequences. If intron sequences are not correctly removed from the pre-mRNA transcript, then the resulting protein will fail to function properly.

Once the pre-mRNA is modified appropriately, it is shuttled to the cytoplasm of the cell where tRNA and rRNA molecules begin to assemble the proteins specified by the nucleotide sequence in the mRNA.

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Viruses And Reverse Transcription

“The more I looked at DNA, the more I realized it was nature and nurture. It’s how genes and your environment work together to produce the person you are.” — Sam Keane

Although transcription in biology typically refers to the process by which mRNA is synthesized from DNA, transcription can also refer to “reverse transcription”, the process by which some viruses create complementary strands of DNA from existing RNA templates. Viruses like HIV are able to take existing strands of RNA and reverse engineer a complementary DNA strand. which is then integrated into the DNA of the host organism. Such viruses are called “retroviruses”

In fact, it is thought that large sequences of human DNA contain endogenous retroviruses incorporated into our genetic code over millions of years. Some estimates hold that 5-8% of human DNA consists of these endogenous retroviruses. The ability of viruses to modify and rewrite portions of their host’s genetic code via reverse transcription have made viruses a subject of interest to bioengineers. Genetically modified viruses are being investigated for therapeutic uses, including correcting genetic errors, treating cancer, and fighting other viruses like herpes or viral meningitis.

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