Question

Explain the process of transcription. Be sure to include (these are in no particular order): DNA, mRNA, template strand, non-template strand, antiparallel, 5' and 3', RNA polymerase, promoter region (TATA Box), elongation, termination, 5' cap, poly-A tail, where the process takes place, primary RNA transcript versus finished mRNA strand, exons, and introns.

Answer 1

The process of copying Genetic information from one strand of DNA into RNA is termed as transcription.

- Here also the principles of complementary governs the process of transcription. Adenosine forms base pairs with uracil and cytosine with guanine.

- unlike DNA replication, which once sets in the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA.

- A transcription unit in the DNA is defined primarily by three regions in the DNA:

- A promoter

- A structural gene

- A terminator

- Since the two strands have opposite polarity and the DNA dependant RNA polymerase also catalyses the polymerisation in only one direction , that is 5' -> 3' , the strand which has polarity 3'->5' acts as a template and is also referred as template strand.

- The other strand which has polarity 5'->3' and the sequence same as RNA except thymine at place of uracil, is displaced during transcription. This strand is referred as Coding strand or non template strand.

- The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards the 5' end (upstream) of the structural gene( the reference is made with respect to the polarity of the coding strand).

- The terminator is located towards the 3' end (downstream)of the coding strand. And is usually defines the end of the process of transcription.

- there additional regulatory sequences that may be present upstream and downstream to the promoter.

- process of transcription:-

- RNA Polymerase binds to the promoter and initiates transcription and the process is called initiation. It uses nucleoside triphosphate as substrate and polymerases in a template dependant fashion following the rules of complementary.

It is somehow also facilitate opening of the helix and continues elongation.

- Only a short stretch of RNA remain bound to the enzyme . Once the polymerase reaches the terminator region , the nascent RNA falls off and so also the RNA polymerase. This results in termination of the transcription.

- the transcribed precursor of m-RNA is called hnRNA or heterogeneous RNA.

- the primary transcripts contains both Exons and Introns. Exons are the expressing unit and Introns are non functional. So Introns need to be spliced off from the RNA . Hence the primary transcripts subjected to a process called Splicing where Introns are removed and Exons are joined in a defined order.

- the hnRNA undergoes additional processing called the Capping and Tailing . In Capping unusual nucleotide - methyl guanosine triphosphate is added to the 5' end of the hnRNA

In Tailing Adenylate residues (200- 300) are added at 3' end in a template dependant manner. This is also called poly-A tail.

- it is fully processed hnRNA now called mRNA, that is transported out of the nucleus for translation.

The whole process takes place inside the nucleus. After transcription the mRNA is transported out of the nucleus and come to the cytoplasm.

The primary RNA transcripts contains Exons and Introns which are not present in finished mRNA.

Also finished mRNA has a CAP and a poly-A tail which primary RNA transcripts don't have.

Hope you got your answer. Thanks.