Transcription of DNA

Definitions & Key takeaways

Transcription is the process by which genetic information in DNA is copied into RNA. The process occurs in the nucleus and is critical for gene expression. The three main steps involved in transcription are initiation, elongation, and termination. During initiation, RNA polymerase binds to a specific region of DNA called the promoter. During elongation, RNA polymerase adds complementary RNA nucleotides to the growing RNA chain. Termination occurs when the RNA polymerase reaches a sequence of DNA called the terminator, and the RNA molecule is released. Finally, RNA molecules undergo post-transcriptional processing to prepare them for translation, the process by which proteins are synthesized from RNA. The resulting RNA molecule is called messenger RNA (mRNA) and is transported out of the nucleus and into the cytoplasm, where it serves as a template for protein synthesis.

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Deep within the cell’s nucleus, there’s our DNA. DNA is made up of genes, and each gene is basically a specific part of the DNA that codes for a protein.
And genes become proteins in two steps: transcription and translation. Transcription is the first step in creating a protein, during which a specific gene is “read” and copied on an individual mRNA, or messenger RNA molecule - which is like a blueprint with instructions on what protein to build.
Now, DNA has two strands, which wrap one around the other to form the characteristic “double helix”. Each single strand of DNA is composed of four types of nucleotides - which are the individual “letters” or “building blocks” of DNA.
Nucleotides of DNA are made out of a sugar - deoxyribose, a phosphate, and one of the four nucleobases - adenine, cytosine, guanine, and thymine - or, commonly, A, C, G, T for short.
The nucleotides on one strand pair up through hydrogen bonds with nucleotides on the opposing strand, to create the double-stranded DNA : specifically, A bonds with T, and C bonds with G, so they’re called complementary bases.
Now, with these two strands - one strand is called the coding, or the sense strand, and the other strand is called the template, or the non-sense anti-sense strand.
The coding strand has a coding sequence of nucleotides that serves as a master blueprint for our protein. It’s a what-you-see-is-what-you-get kind of thing.
The template strand, on the other hand, has a sequence of nucleotides that is complementary to the sequence on the coding strand.
In addition, the two DNA strands also have a “direction” - the coding strand runs from the 5’ end towards the 3’ end, while the template strand runs from the 3’ to the 5’ end.
A bit like two snakes coiled up together but facing different directions. So, if the coding strand looks like this: 5’ end - A A T C C A G T A - 3’ end The template strand will look like this: 3’ end - T T A G G T C A T - 5’ end *Disclaimer: no cats were harmed in the making of this strand.
I think i would have drawn the template strand inverted over the coding strand so that it is crystal clear that they are complementary.
Not sure it needs to be changed, though. Now, transcription starts with the unpacking of DNA from chromatin and de-helicization - meaning that the double helix unwinds a bit so that individual genes are exposed.
The starting point of a gene is determined by a promoter region, which is a repetitive non-coding sequence of nucleotides - for example, T A T A T A T A sequence is one very famous promoter, called the TATA box - that marks where to begin transcribing.
A few dozen proteins and enzymes come together to form what’s called a pre-initiation complex around the promoter, also featuring an enzyme called RNA polymerase.
Then, a process called elongation occurs, which is where RNA polymerase unzips the two strands by shearing the hydrogen bonds between the complementary nucleotides for the length of around 14 base pairs.
This open area is within the RNA polymerase, and is called the transcription bubble. The RNA polymerase follows the template strand and uses it to assemble an mRNA molecule, that is the mirrored image of the template strand.
Now, mRNA is slightly different from DNA. First off, it uses a slightly different set of nucleotides, where the T is replaced by uracil, or U.
The U will normally pair with A, as T would. Also, mRNA runs in the opposite direction compared to the template strand - so from 5’ end to 3’ end.
So, when reading the template strand, RNA polymerase will move along from the 3’ end of the template strand towards the 5’ end, while creating the mRNA molecule in reverse - from 5’ end to 3’ end.
RNA polymerase assembles mRNA by attaching complementary nucleotides. For example, if we’re making a protein based on our coding strand from before: 5’ end - A A T C C A G T A - 3’ end The template strand looks like this: 3’ end - T T A G G T C A T - 5’ end Then the mRNA will look like this : 5’ end - A A U C C A G U A - 3’ end For reference, the coding strand will look like: 5’ end - A A T C C A G T A - 3’ end A striking similarity.
As RNA polymerase goes down the DNA, the hydrogen bonds between the nucleotides that have already been transcribed reform - zipping the DNA back together.
Eventually, RNA polymerase reaches a specific sequence called the terminator sequence. The terminator sequence contains two complementary sequences in a row, on a single strand.
When they get transcribed into the mRNA, the newly formed sequences “bond” with each other, creating a “hairpin loop”, and that makes the RNA polymerase detach from the DNA strand, by simply yanking it off.
That causes termination of transcription - mRNA detaches from the enzyme, DNA closes back up, and RNA polymerase goes off to transcribe another gene.
But the mRNA is not done yet, and requires some modifying. So an enzyme called polyadenylate polymerase adds For example, a 7-methylguanosine cap is added at the 5’ end of RNA, kinda like a nucleotide “cap”, and at its 3’ end, the enzyme polyadenylate polymerase adds a poly-adenine tail - meaning, a long sequence of only adenine nucleotides.
Think of these as those plastic bits on shoelace ends - their purpose is to make the mRNA more sturdy. But the story isn’t over, because the mRNA has to undergo some editing.
mRNA can be divided into sections of exons and introns - the exons code for the protein, while the introns don’t. So to trim down the unnecessary bits, a molecular machine called the spliceosome comes in and splices the introns out.
This is kinda like video editing - when you cut material from the initial filming to make a much cleaner final cut. In the end, we get a sequence of exons only, with a cap and a poly-adenine tail, which is ready to code for a protein.
The freshly made mRNA floats out of the nucleus and hooks up with an idle ribosome in the cytoplasm, where it can create a protein.
I’d show the intron getting excised to make it clear that it’s cut out of the DNA. Alright, as a quick recap!
The DNA is made out of genes, which can be converted into mRNA protein blueprints through a process of transcription. The promoter is the beginning of the gene.
The RNA polymerase binds to it, and starts assembling mRNA from it. In the end the mRNA detaches, and proceeds outside of the nucleus.