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The Initiation Society is the oldest Anglo-Jewish organisation. Founded in 1745, it exists to ensure the highest medical and religious standards for Bris Milah (circumcision) are adhered to by our Mohelim (practitioners). The Initiation Society works closely with the London Beth Din, and all of our Mohelim have undergone formal training in the ...
- Overview
- Key points:
- Introduction
- Transcription overview
- RNA polymerase
- Transcription initiation
- Promoters in bacteria
- Promoters in humans
- Elongation
- Transcription termination
An in-depth looks at how transcription works. Initiation (promoters), elongation, and termination.
•Transcription is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule.
•RNA polymerase is the main transcription enzyme.
•Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins).
•RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule.
•Transcription ends in a process called termination. Termination depends on sequences in the RNA, which signal that the transcript is finished.
•Transcription is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule.
•RNA polymerase is the main transcription enzyme.
•Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins).
•RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule.
What makes death cap mushrooms deadly? These mushrooms get their lethal effects by producing one specific toxin, which attaches to a crucial enzyme in the human body: RNA polymerase.1
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 more short-lived molecule).
Transcription is an essential step in using the information from genes in our DNA to make proteins. Proteins are the key molecules that give cells structure and keep them running. Blocking transcription with mushroom toxin causes liver failure and death, because no new RNAs—and thus, no new proteins—can be made.2
Transcription is essential to life, and understanding how it works is important to human health. Let's take a closer look at what happens during 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 unwind near the gene that is getting transcribed. The region of opened-up DNA is called a transcription bubble.
Transcription uses one of the two exposed DNA strands as a template; this strand is called the template strand. The RNA product is complementary to the template strand and is almost identical to the other DNA strand, called the nontemplate (or coding) strand. However, there is one important difference: in the newly made RNA, all of the T nucleotides are replaced with U nucleotides.
The site on the DNA from which the first RNA nucleotide is transcribed is called the +1 site, or the initiation site. Nucleotides that come before the initiation site are given negative numbers and said to be upstream. Nucleotides that come after the initiation site are marked with positive numbers and said to be downstream.
If the gene that's transcribed encodes a protein (which many genes do), the RNA molecule will be read to make a protein in a process called translation.
[Are there steps between transcription and 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 instance, if there is a G in the DNA template, RNA polymerase will add a C to the new, growing RNA strand.
RNA polymerase always builds a new RNA strand in the 5’ to 3’ direction. That is, it can only add RNA nucleotides (A, U, C, or G) to the 3' end of the strand.
[What do 5' and 3' mean?]
RNA polymerases are large enzymes with multiple subunits, even in simple organisms like bacteria. Humans and other eukaryotes have three different kinds of RNA polymerase: I, II, and III. Each one specializes in transcribing certain classes of genes. Plants have an additional two kinds of RNA polymerase, IV and V, which are involved in the synthesis of certain small RNAs.
To begin transcribing a gene, RNA polymerase binds to the DNA of the gene at a region called the promoter. Basically, the promoter tells the polymerase where to "sit down" on the DNA and begin transcribing.
Each gene (or, in bacteria, each group of genes transcribed together) has its own promoter. A promoter contains DNA sequences that let RNA polymerase or its helper proteins attach to the DNA. Once the transcription bubble has formed, the polymerase can start transcribing.
To get a better sense of how a promoter works, let's look an example from bacteria. A typical bacterial promoter contains two important DNA sequences, the - 10 and - 35 elements.
RNA polymerase recognizes and binds directly to these sequences. The sequences position the polymerase in the right spot to start transcribing a target gene, and they also make sure it's pointing in the right direction.
[How?]
Once the RNA polymerase has bound, it can open up the DNA and get to work. DNA opening occurs at the - 10 element, where the strands are easy to separate due to the many As and Ts (which bind to each other using just two hydrogen bonds, rather than the three hydrogen bonds of Gs and Cs).
In eukaryotes like humans, the main RNA polymerase in your cells does not attach directly to promoters like bacterial RNA polymerase. Instead, helper proteins called basal (general) transcription factors bind to the promoter first, helping the RNA polymerase in your cells get a foothold on the DNA.
Many eukaryotic promoters have a sequence called a TATA box. The TATA box plays a role much like that of the - 10 element in bacteria. It's recognized by one of the general transcription factors, allowing other transcription factors and eventually RNA polymerase to bind. It also contains lots of As and Ts, which make it easy to pull the strands of DNA apart.
Once RNA polymerase is in position at the promoter, the next step of transcription—elongation—can begin. Basically, elongation is the stage when the RNA strand gets longer, thanks to the addition of new nucleotides.
During elongation, RNA polymerase "walks" along one strand of DNA, known as the template strand, in the 3' to 5' direction. For each nucleotide in the template, RNA polymerase adds a matching (complementary) RNA nucleotide to the 3' end of the RNA strand.
[See the chemical reaction]
The RNA transcript is nearly identical to the non-template, or coding, strand of DNA. However, RNA strands have the base uracil (U) in place of thymine (T), as well as a slightly different sugar in the nucleotide. So, as we can see in the diagram above, each T of the coding strand is replaced with a U in the RNA transcript.
[See a diagram of the bases]
The picture below shows DNA being transcribed by many RNA polymerases at the same time, each with an RNA "tail" trailing behind it. The polymerases near the start of the gene have short RNA tails, which get longer and longer as the polymerase transcribes more of the gene.
RNA polymerase will keep transcribing until it gets signals to stop. The process of ending transcription is called termination, and it happens once the polymerase transcribes a sequence of DNA known as a terminator.
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People also ask
What are initiation factors in molecular biology?
What is translation initiation?
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How do initiation factors interact with repressors?
In molecular biology, initiation factors are proteins that bind to the small subunit of the ribosome during the initiation of translation, a part of protein biosynthesis. Initiation factors can interact with repressors to slow down or prevent translation.
