LECTURE                                      RNA TRANSCRIPTION

In this lecture you will learn:

THE MOVIE
 
 
Transcription is when an RNA copy is made using DNA as a template   MOVIE HERE

This happens inside the nucleus of eucaryotic cells. 

The transcription process involves: 

1. Binding of RNA polymerases to the initiation site called a promoter (TATA box).

RNA polymerase binds to the DNA template and recognizes the first base to be copied.

2. Unwinding (melting) of the DNA double helix
Of the two DNA nucleotide chains fully exposed by the unwinding, only one contains the correct promoter sequences and acts as a template. The opposite, non template chain does not contain encoded information.
 

3.Elongation (nucleotides are added to the DNA template), synthesis of RNA based on the sequence of the DNA template strand. Once the first base is added, elongation begins and RNA nucleotides add sequentially until the polymerase reaches the end of the template.

4. Termination of synthesis and release of both RNA polymerase and RNA from DNA template. Sequences signal termination.

The RNA Polymerases

RNA polymerases are among the largest and most complex protein enzymes of living organisms.

  each polymerase consist of 2 large polypeptide subunits and about 6 to 10 smaller ones.
-- bacteria or procaryotes have only one type of RNA polymerase
eucaryotes, or true cells have RNA polymerase I, II, and II.
 


 
AN OVERVIEW OF SIGNALS FROM THE DNA TO SIGNAL FOR TRANSCRIPTION - THE TATA BOX
"Transcription, or RNA synthesis, is the process of creating an equivalent RNA copy of a sequence of DNA.[1] The first of five stages of transcription is preinitiation. In eukaryotes, RNA polymerase, and therefore the initiation of transcription, requires the presence of a core promoter sequence in the DNA. Promoters are regions of DNA which promote transcription and in eukaryotes are found some -30 to -90 base pairs (bp) upstream from the transcription start site (TSS). Core promoters are sequences within the promoter which are essential for transcription initiation. RNA polymerase is able to bind to core promoters during preinitiation in the presence of various specific transcription factors.

A common type of core promoter in eukaryotes is a short DNA sequence known as a TATA box. Most human genes lack a TATA box and use an initiator element or downstream core promoter instead. As in other metazoans, for genes lacking a TATA box, the initiator element (Inr) is functionally analogous, with a consensus YYA+1NT/AYY, to direct transcription initiation.[2] The Inr is the only element in metazoan protein-encoding genes known to be a functional analog of the TATA box, in that it is sufficient for directing accurate transcription initiation in genes that lack TATA boxes.[3] An Inr for mammalian RNA polymerase II can be defined as a DNA sequence element that overlaps a TSS and is sufficient for
   1. determining the start site location in a promoter that lacks a TATA box and
   2. enhancing the strength of a promoter that contains a TATA box.[4]" cite: wiki

You do not need to memorize this. The TATA or core promoter sequence 
is in the promoter and the enhancer and promoter loop to form a binding site 
for the RNA polymerase to initiate transcription. 

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REVIEW QUESTIONS:
What are the four steps of transcription?
In what part of the cell does transcription occur?
What sequence on the DNA is the initiator for transcription?
What is unusual about RNA polymerase?