It is the process in which the DNA sequence of a gene is copied (transcribed) to form an RNA molecule.
RNA polymerase is the main transcription enzyme.
Transcription begins when the RNA polymerase binds to a promoter sequence near the start of a gene (directly or through auxiliary proteins).
The RNA polymerase uses one of the DNA strands (the template strand) as a template to form a new complementary RNA molecule.
The transcription ends in a process called termination. The termination depends on the sequences in the RNA, indicating that the transcription is complete.
RNA polymerase is crucial because it carries out transcription, the process of copying DNA (deoxyribonucleic acid, the genetic material) into RNA (ribonucleic acid, a similar but shorter-lived molecule).
Transcription is an essential step in the use of gene information in our DNA to make proteins. Proteins are the key molecules that give structure to cells and keep them functioning.
Blocking transcription with fungal toxin causes liver failure and death, since new RNAs can not be produced, and therefore, no new proteins.
Transcription is essential for life, and understanding how it works is important for human health. Let’s take a closer look at what happens during transcription.
General description of RNA transcription
Transcription is the first step of gene expression. During this process, the DNA sequence of a gene is copied into RNA.
Before transcription can take place, the DNA double helix must relax near the gene that is transcribed. The region of open DNA is called a transcription bubble.
The transcript uses one of the two DNA strands exposed as a template; this thread is called a template thread. The RNA product is complementary to the template strand and is almost identical to the other strand of DNA, called the non-component (or coding) strand.
However, there is one important difference: in the newly created RNA, all T nucleotides are replaced by U nucleotides.
The site in the DNA from which the first RNA nucleotide is transcribed is called the + 1 + 1 plus site, 1 or the start site. The nucleotides that come before the initiation site receive negative numbers.
The nucleotides that come after the initiation site are marked with positive numbers.
If the gene that is transcribed encodes a protein (which many genes do), the RNA molecule will be read to form a protein in a process called translation.
RNA polymerases are enzymes that transcribe DNA into RNA. Using a DNA template, RNA polymerase builds a new RNA molecule through base pairing. For example, if there is a G in the DNA template, the RNA polymerase will add a C to the new growing RNA chain.
RNA polymerases are large enzymes with multiple subunits, even in simple organisms such as bacteria. In addition, humans and other eukaryotes have three different types of RNA polymerases: I, II, and III. Each one specializes in transcribing certain kinds of genes.
During the transcription process, the information encoded within the DNA sequence of one or more genes is transcribed into an RNA chain, also called RNA transcription.
The resulting single-stranded RNA molecule, composed of ribonucleotides containing the bases adenine (A), cytosine (C), guanine (G) and uracil (U), acts as a mobile molecular copy of the original DNA sequence.
Transcription in prokaryotes and in eukaryotes requires that the DNA double helix partially unwind in the region of RNA synthesis.
The unrolled region is called a transcription bubble. The transcription of a particular gene always comes from one of the two DNA chains that acts as a template, the so-called antisense chain.
The RNA product is complementary to the DNA template strand and is almost identical to the non-terminal DNA strand, or the sense strand.
The only difference is that in RNA, all T nucleotides are replaced by U nucleotides; during RNA synthesis, U is incorporated when there is an A in the complementary antisense strand.
After formation of the pre-initiation complex, the polymerase is released from the other transcription factors, and the elongation is allowed to proceed with the RNA that synthesizes the polymerase in the 5 ‘to 3’ direction.
RNA polymerase II (RNAPII) transcribes most of the eukaryotic genes, so this section will focus mainly on how this specific polymerase achieves elongation and termination.
Although the enzymatic elongation process is essentially the same in eukaryotes and prokaryotes, the eukaryotic DNA template is more complex. When eukaryotic cells are not dividing, their genes exist as a diffuse, but still widely compressed, mass of DNA and proteins called chromatin.
The DNA is hermetically packaged around charged histone proteins at repeated intervals.
These DNA-histone complexes, collectively referred to as nucleosomes, are regularly spaced and include 146 DNA nucleotides coiled twice around the eight histones in a nucleosome like a thread around a spool.
For the synthesis of polynucleotides to occur, the transcription machinery needs to move the histones away each time they find a nucleosome. This is achieved by a special protein dimer called FACT, which means ” facilitates the transcription of chromatin .”