Transcription

INTRODUCTIONJ

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.${}^{\mathrm{}}$

Death cap is still the commonest cause of mushroom poisoning in Europe’ says Newcombe. A notorious fungus, it is part of the Amanita genus that contains around 600 species, some of which are the most highly toxic in the world. It is believed that this genus alone is responsible for approximately 95% of all mushroom poisonings, with 75% of fatal fungal poisonings attributed to death caps , Amanita phalloides.

The toxicity of the Amanita species is due to the presence of two groups of toxins known as amatoxins and phallotoxins, both multicyclic peptides. It is believed that the death cap contains six related phallotoxins and five or more amatoxins.

           In particular, a-amanitin has a high specificity for RNA polymerase II in the liver. By inhibiting this enzyme it prevents the formation of mRNA and stops protein synthesis, resulting in cell death and subsequent liver failure. When filtered through the kidneys, the toxin can then be reabsorbed into the bloodstream and re-circulated around the body, causing repeated liver and kidney damage.

WHAT IS 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.

DNA transcription is a process that involves transcribing genetic information from DNA to RNA.

 The transcribed DNA message, or RNA transcript, is used to produce proteins. DNA is housed within the nucleus of our cells. It controls cellular activity by coding for the production of proteins. The information in DNA is not directly converted into proteins, but must first be copied into RNA. This ensures that the information contained within the DNA does not become tainted.

TRANSCRIPTION PROCESS

 Transcription of a gene takes place in three stages:-

• Chain initiation, 

• Chain elongation

• Chain termination.  

CHAIN INITIATION

      RNA polymerase binds to a sequence of DNA called the promoter, found near the beginning of a gene. Each gene (or group of co-transcribed genes, in bacteria) has its own promoter. 

     Once bound, RNA polymerase separates the DNA strands, providing the single-stranded template needed for transcription. 

CHAIN ELONGATION

       One strand of DNA, the template strand, acts as a template for RNA polymerase. 

As it "reads" this template one base at a time, the polymerase builds an RNA molecule out of complementary nucleotides, making a chain that grows from 5' to 3'. 

The RNA transcript carries the same information as the non-template (coding) strand of DNA, but it contains the base uracil (U) instead of thymine (T).

CHAIN TERMINATION

     Sequences called terminators signal that the RNA transcript is complete. Once they are transcribed, they cause the transcript to be released from the RNA polymerase.

     An example of a termination mechanism involving formation of a hairpin in the RNA is shown below.