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NEL Molecular Genetics 669

Section20.2

5 
^3 

RNA polymerase

mRNA

5 
^3 

5 


mRNA

DNA has rewound

direction of

transcription

mRNA

template strand

DNA

RNA polymerase

promoter

(a) Initiation

RNA polymerase binds to

DNA at a promoter.

(b)DNA double helix is unwound,

exposing the template strand.

(c) Elongation

mRNA is synthesized using one

strand of DNA as a template.

mRNA is synthesized in the

5 
to 3
direction.

(d)As elongation proceeds, RNA

polymerase moves along DNA,

synthesizing mRNA. DNA that

has already been transcribed

rewinds into a double helix.

(e)RNA polymerase reaches the

termination sequence at end of

gene.

(f) Termination

Transcription stops. mRNA and

RNA polymerase are released.

Practice

1. A short fragment of a particular gene includes the following sequence of
   nucleotides:
   TACTACGGT
   Write out the corresponding mRNA transcript.
   2.A short fragment of another gene includes the following sequence of nucleotides:
   ACCATAATATTACCGACCT TCG
   (a) Explain the purpose of the promoter region in transcription.
   (b) Copy the sequence into your notebook and circle the promoter region. Explain
   the rationale for your selection.

Figure 5
A summary of the process of transcription

Regulation of Transcription

This Audio Clip discusses the

regulatory factors that control

when and how much mRNA is

transcribed from a given gene.

http://www.science.nelson.com GO

+ EXTENSION