TRANSCRIPTION

Initiation Transcription factors bind to the promoter

region

RNA polymerase binds to the promoter region The enzyme’s active site only recognizes the

promoter region

The promoter regioncontains many Adenineand Thymine bases andis upstream from thegene to be translated

Initiation A’s and T’s have __ H-bonds, C’s and G’s have __

H-bonds _______ take less energy to break than ______

which makes it an ideal spot for the promoter region

RNA polymerase moves past the promoter region until it reaches the start sequence and unwinds the DNA

For transcription, DNA is only unwound when being transcribed, then it rewinds

The part of DNA that is unwound is a transcription bubble

Elongation Just like DNA, RNA is built in the 5’ to 3’

direction

A primer is not required by the RNA polymerase, as was the case for DNA polymerase in DNA replication

The promoter is not transcribed

The strand of the DNA that is transcribed is called the template strand

Elongation The complimentary strand of the DNA

that is not transcribed is called the coding strand This strand in identical to the mRNA except

that it has Thymine instead of Uracil

The DNA is transcribed into RNA

Termination There are two different ways

Transcription can be terminated1. Rho-dependant

This method of termination relies on the appearance of a Rho protein to ‘boot off’ the RNA polymerase and release the mRNA

Termination2. Rho-independent

This method does not rely on a Rho protein to terminate transcription. In this case, the RNA polymerase reaches the termination sequence.

The termination sequence is high in A’s and T’s just like the promoter region.

Both types of termination cause the mRNA transcript to fold back on itself and disassociate with the DNA template strand, freeing up RNA polymerase to transcribe another gene

Posttranscriptional Modifications In eukaryotic cells, the primary

transcript (unprocessed RNA) needs to undergo capping, tailing and base excision, before it can leave the nucleus

Capping is the addition of a 5’ cap to the start of the transcript, basically it is a modified guanine nucleotide.

Capping has two functions: to protect the mRNA from digestive

nucleases and phosphatases as it exits the nucleus and enters the cytoplasm

to initiate translation

Posttranscriptional Modifications Tailing: about 200 adenine ribonucleotides, are

added to the RNA. The poly-A-tail, is added to the 3’ end of the transcript by an enzyme called poly-A-polymerase

The entire mRNA transcript consists of two regions – a coding region called exons, and a non-coding region called introns (which are interspread among the exons)

The introns are removed by spliceosomes so that their translation is prevented (because they don’t code for anything!)

The introns stay inside the nucleus and are degraded into recycled nucleotides

Posttranscriptional Modifications The “primed” mRNA transcript then

moves out of the nucleus and into the cytoplasm where it will be translated

There is no “quality control” mechanism for the mRNA strand

An incorrectly-made mRNA transcript will amount to a defective protein

However, so long as the original DNA template strand is correct, the multiple copies of the mRNA transcripts will more than likely compensate for any mistake in one mRNA strand

Summary! Initiation

Transcription factors bind to promoter region followed by RNA polymerase

Elongation mRNA is synthesized throughout the coding

region Termination

The terminating sequence is reached and the mRNA and RNA polymerase are released

http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html