TRANSCRIPTION

Transcription is the synthesis of an RNA strand from a DNA template.

• A gene's protein building instructions are transcribed to messenger RNA (mRNA).

• mRNA carries the code from DNA to the ribosomes where translation into a protein occurs.

The Three Steps of Transcription

Step 1 : Initiation

• RNA polymerase binds to DNA at a specific sequence of nucleotides called the promoter.

• The promoter contains an initiation site where transcription of the gene begins.

• RNA polymerase than unwinds DNA at the beginning of the gene.

Step 2 : Elongation

• Only one of the unwound DNA strands acts as a template for the RNA synthesis.

• RNA polymerase can only add nucleotides to the 3' end of the strand so like DNA, RNA must be synthesized in the 5' to 3' direction.

• Free ribonucleosides triphosphates (ATP, CTP,GTP and UTP) from the cytoplasm are paired up with their complementary base on the exposed DNA template.

• RNA polymerase joins the ribonucleoside triphosphates to form an mRNA strand.

• As RNA polymerase advances, the process continues.

• The DNA that has been transcribed, re-winds to form a double helix.

Step 3 : Termination

• RNA polymerase continues to elongate until it reaches the terminator, a specific sequence of nucleotides that signals the end of transcription.

• When transcription stops, mRNA polymerase and the new mRNA transcript are released from DNA.

• The DNA double helix reforms.

• The termination sequence usually consists of a series of adjacent adenines preceded by a nucleotide palindrome.

• This gives an RNA molecule that assumes a stem-and loop configuration.

• This configuration stops RNA polymerase from transcribing any further.

• 2 type of termination mechanisme is Intrinsic termination and Termination involves rho (p) protein.

• Intrinsic termination is controlled by specific sequences .

• Termination involves rho (p) is rho-dependent termination sequences cause hairpin loop to form

Transcription : Initiation, Elongation and Termination Steps