Introduction

DNA TRANSCTION

Transcription is mechanism by which a template strand of DNA is utilized by specific RNA polymerase to generate one of four distinct classification of RNA.

These four RNA classes are

1.       Messenger RNA(mRNA) - This class of RNA is the genetic coding templates uses by the translational machinery to determine the order of amino acids incorporated into an elongating polypeptide in the process of translation.

2.       Transfer RNAs (tRNAs) - This class of small RNA form covalent attachments to individual amino acids and recognize the encoded sequences of the the mRNAs to allow correct insertion of amino acid into elongating polypeptide chain

3.       Ribosomal RNAs(rRNAs) - This class of RNA is assembled,together with numerous ribosomal proteins to form the ribosomes

4.       Small RNAs: this class of RNA includes the small nuclear RNAs(snRNAs) involved in RNA splicing and the microRNAs(miRNAs) involved in the modulation of genes expression through tthe alteration of target mRNA activity

Basic of Transcription:

Coding strand: The DNA strand of a DNA molecule which has the same base sequence as the RNA transcript (T is replaced with U in RNA strand)

Template strand: The DNA strand of a DNA molecule that is used as a template for RNA synthesis.

STEPS OF TRANSCRIPTION

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.

Elongation:

• Only one of the unmound DNA strands acts as a template for the RNA synthesis.
• RNA polymerase can only add nucleotids to the 3' end of the strand so like DNA, RNA must be synthesized in the 5' to 3' direction.
• Free ribonucleotides triphosphates from the cytoplasm are paired up with their commplementary 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.

Termination:

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

 

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