Transcription: Copying DNA
Just as we have the ability of copying text from a book, our cells can copy information stored within the cell’s nucleus. This information is genetic (DNA) and contains the instructions to create any protein the cell might need. However, DNA cannot leave the cell’s nucleus and therefore a copy (message RNA) is required to send that message other organelles within the cell. This process of copying DNA into mRNA is called transcription.
TL; DR
- Transcription is the process of copying part of DNA into a new messenger molecule (mRNA).
- Transcription is a three-step process of: initiation, elongation and termination.
- Transcription is the first step towards creating new proteins.
The Process of Transcription
The transcription of DNA into mRNA is necessary for all protein synthesis. The DNA contains instructions for all the proteins a cell might want to produce. However, if you recall the Central Dogma of Biology in order to use these instructions, they first need to be copied into a format (RNA) that the protein machinery (ribosome) is able to read. Since cells rely on proteins to functio normally, the process of transcription is fundamental to all cellular life.
Why is transcription necessary?
Why doesn’t DNA translate directly into protein? This is a fantastic question that I hear a lot. You might think it’s because DNA cannot leave the nucleus, but bacteria (prokaryotes) lack a nucleus and also use transcription.
Perhaps the straightforward answer is that nature evolved this way. We know that RNA existed prior to DNA and that protein synthesis reads and translates RNA nucleotides. Since RNA is not very stable and is easily destroyed by enzymes, it is hypothesized that DNA evolved as a more method to store genetic information. Though DNA and RNA are similar at the molecular level, their very different molecules structurally. These structural differences are another reason why we can’t use DNA is not able to be directly translated into protein.
The Process of Transcription
The transcription of DNA to RNA requires DNA, transcription factors, RNA polymerase and nucleotides. Initially transcription factors assemble a complex on DNA that allow RNA polymerase to bind a promoter sequence. Together, The procress is somewhat straightforward, with one nucleotide being added to the mRNA strand for every nucleotide read in the DNA strand. that results in an genetic copy (either DNA or RNA) of the original DNA contain within the genome. In this process, a portion of the cell’s DNA is read and copied by a protein-enzyme called polymerase. RNA polymerase is a specific type of polymerase that transcribes the DNA molecule into an RNA molecule. Since this molecule is designed to serve as a message between DNA information and a resulting protein, this RNA molecule is called messenger RNA – or mRNA. But the road to protein doesn’t end there. At this point, the mRNA will exit the nucleus and need to be ‘read’ by a ribosome – we’ll cover that in the next section on translation.
This all happens a sequence of three steps:
- Initiation
- Elongation
- Termination
Key Terms
- Transcription Factors – proteins that regulates the transcription
- Promoter – a sequence of DNA where the proteins bind to initiate the process of transcription
- RNA polymerase – protein enzyme that synthesizes an RNA polymer from DNA.
- Coding Strand – the strand of DNA that will be replicated in RNA during transcription
- Template Strand – the strand of DNA that is used to add complementary nucleotides to an elongating RNA molecule.
- mRNA – messenager RNA molecule that contains the information from the coding strand of DNA.
Step One: Initiation
The promoter is a DNA sequence where proteins (known as transcription factors) and RNA polymerase will interact with the to DNA to initiate transcription. In most cases, promoters exist upstream of the genes they regulate. The sequence of a promoter is particular important because it determines how a particular gene is regulated
Transcription requires the DNA double helix to partially unwind in the region where mRNA is to be synthesized. The region of unwinding is called a transcription bubble.
Step Two: Elongation
During transcription just one of the two DNA strands is copied. The template strand is the DNA strand read by RNA polymerase. While reading the template strand, RNA polymerase will the mRNA strand that is created is a copy of the non-template strand which is also called the coding strand. In other words, the mRNA product is complementary to the template strand and is almost identical to nontemplate DNA strand (with the exception that RNA contains a uracil (U) in place of the thymine (T) found in DNA).
During elongation, RNA polymerase proceeds along the DNA template adding nucleotides by base pairing with the DNA template in a manner similar to DNA replication, with the difference that an RNA strand is being synthesized that does not remain bound to the DNA template. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it.
Step Three: Termination
The termination of transcription happens at a DNA sequence known as a terminator. This instructs the RNA polymerase to dissociate from the DNA template and liberate the newly synthesized mRNA. Depending on the gene being transcribed, there are two kinds of termination signals, but both involve repeated nucleotide sequences in the DNA template that cause the RNA polymerase to stall, leave the DNA template, and free the mRNA transcript. On termination, the process of transcription is complete. At this point, the mRNA is almost ready to exit the nucleus. However, a couple of post-transcriptional modifications take place. You can ready about post-transcriptional modifications here.
Transcription is the First Step towards Creating Proteins
References
- yourgenome. “What is the ‘Central Dogma’?”. Cambridge, UK. Wellcome Genome Campus, https://www.yourgenome.org/facts/what-is-the-central-dogma. License: CC BY 4.0
- Fowler, S, Roush R, and Wise J. “Molecular Biology.” Concepts of Biology. Houston, TX: OpenStax, https://openstax.org/books/concepts-biology/pages/9-3-transcription. License: CC BY 4.0 License Terms: Edited & Adapted | Access for free at https://openstax.org/books/biology-2e/pages/1-introduction..
- Zedalis, J, and Eggebrecht, J. “3.5 Nucleic Acids.” Biology for AP® Courses, p. OpenStax, https://openstax.org/books/biology-ap-courses/pages/15-1-the-genetic-code. License Terms: Edited & Adapted | Access for free at https://openstax.org/books/biology-ap-courses/pages/1-introduction