Biology 12

THINKING LAB

Transcription in Reverse

Background

Along with a gene, a coding stretch of DNA will contain
other sequences that help guide the process of protein
synthesis. In this activity, you will work backwards from a
polypeptide chain to construct a stretch of DNA that might
code for this product.

You Try It

1.The illustration at right shows an imaginary polypeptide

produced by a bacterial cell. Using Table 8.1 (on

page 254) and the information given about its amino

acid sequence, draw one possible nucleotide sequence

of the DNA molecule containing the gene for this

polypeptide.

2.Label the template DNA strand, the promoter

sequence, and the terminator sequence. How does

the structure of the promoter sequence ensure that

the right DNA nucleotide sequence is transcribed?

3.Write a paragraph or prepare a table contrasting the

process of DNA replication with that of mRNA

transcription.

4.Could DNA work as a messenger molecule instead

of mRNA? What would be the effect on transcription?

Write down some ideas and discuss them with a partner.

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Chapter 8 Protein Synthesis • MHR 261

As you saw in Chapter 7, the removal of introns
does not always follow the same pattern in any
given gene. As shown in Figure 8.12, a stretch of
nucleotides that is retained as an exon in one
pre-mRNA transcript may be excised from another.
As a result, a single eukaryotic gene can code for a
variety of different protein products. This process
helps to account for the fact that although your
genome contains an estimated 30 000 to 35 000
genes, your body can produce well over 100 000
different proteins.
After the processing steps are complete, the
finished mRNA molecule is transported from the

nucleus to the cytoplasm, where it begins the

process of translation. Figure 8.13 on the following

page summarizes the main steps involved in

transforming pre-mRNA into finished mRNA in a

eukaryotic cell. After the processing steps are

complete, the finished mRNA molecule is

transported from the nucleus to the cytoplasm.

In both prokaryotes and eukaryotes (as you have

learned in this section), the process of translation

from mRNA to protein begins when a strand of

mRNA reaches the cytoplasm of the cell. You will

examine the events of translation in more detail in

the next section.

exon 1 intron 1 exon 2 intron 2 exon 3 intron 3 exon 4

e 1 i 1 e 2 i 2 e 3 e 4

e 1 e 2 e 3 e 4

i 3

i 1 i 2 i 3

e 1

e 2

e 3 e 4

i 1 i 2

i 3

exon 1 exon 2 exon 3 exon 4

e 1 e 2 e 3 e 4

exon 1 exon 3 exon 4

e 1 e 3 e 4

Figure 8.12As shown here, alternative splicing patterns can allow a single gene to
code for more than one polypeptide.

The pre-mRNA transcript includes

four exons and three introns.

A

One splicing pattern leaves all four

exons intact.

B

A different splicing pattern could

remove one or more exons from

the final mRNA molecule.

C