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668 Chapter 20 NEL

will see that the sequence of nucleotides in a gene determines the sequence of amino

acids in a polypeptide.

Transcription

During transcription, the DNA sequence of a gene is copied (transcribed) into the

sequence of a single-stranded mRNA molecule.

Transcription is divided into three processes: initiation, elongation, and termination.

During initiation, an enzyme called RNA polymerasebinds to the DNA at a specific

site near the beginning of the gene. During elongation, RNA polymerase uses the DNA

as a template to build the mRNA molecule. During termination, the RNA polymerase

passes the end of the gene and comes to a stop. The mRNA is then released from the

template strand of DNA.

Initiation

Transcription starts when the RNA polymerase enzyme binds to the segment of DNA

to be transcribed and opens the double helix.Figure 4shows an electron micrograph

of this process. The RNA polymerase binds to the DNA molecule in front of the gene

to be transcribed in a region called the promoter. In most genes, the promoter sequence

contains a string of adenine and thymine bases that serves as the recognition site for RNA

polymerase. The promoter indicates which DNA strand should be transcribed and

where the RNA polymerase should start transcribing the DNA. Since the binding site

of RNA polymerase only recognizes the promoter region, it can only bind in front of

a gene.

Elongation

Once the RNA polymerase binds to the promoter and opens the double helix, it starts

building the single-stranded mRNA in the 5
to 3
direction. The promoter is not tran-

scribed. The process of elongation of the mRNA molecule is similar to DNA replica-

tion. However, RNA polymerase does not require a primer and it copies only one of the

DNA strands. The transcribed DNA strand is called the template strand. The mRNA

sequence is complementary to the DNA template strand except that it contains the base

uracil in place of thymine.

Termination

Synthesis of the mRNA continues until RNA polymerase reaches the end of the gene. RNA

polymerase recognizes the end of the gene when it comes to a stop signal called a

termination sequence. Transcription stops and the newly synthesized mRNA discon-

nects from the DNA template strand. RNA polymerase is then free to bind to another pro-

moter region and transcribe another gene.Figure 5, on the next page, summarizes the

steps in transcription.

gene mRNA

nucleus Transcription nuclear

envelope

Translation cytoplasm

polypeptide

protein

DNA

Figure 3

The central dogma of molecular genetics

RNA polymeraseenzyme that
transcribes DNA

Figure 4
The RNA polymerase (dark circles)
binds to the DNA strand and
initiates transcription. Transcription
occurs simultaneously at numerous
locations along the DNA.

promotersequence of DNA that
binds RNA polymerase in front of a
gene

template strandthe strand of DNA
that the RNA polymerase uses as a
guide to build complementary
mRNA

termination sequencesequence
of bases at the end of a gene that
signals the RNA polymerase to stop
transcribing