Biology 12

262 MHR • Unit 3 Molecular Genetics

SECTION REVIEW

1. What is the objective of transcription?


2. List the main steps in the transcription process
   in a eukaryotic cell.


3. A portion of an mRNA molecule has the
   sequence CCUAGGCUA. What is the sequence of
   the anti-sense strand of the DNA?


4. Given that gene expression involves transcribing
   information from only one strand of the DNA
   molecule, what are the biological advantages of
   double-stranded DNA?


5. Explain the difference between a pre-mRNA
   transcript and an mRNA molecule.


6. Write one or two sentences to describe the
   function(s) of each of the following:
   (a) 5 ′cap (c)spliceosome
   (b)poly-A tail (d)terminator sequence


7. If a gene has four exons and three introns, what
   is the total number of different polypeptide products
   that the gene could code for? Explain your answer.


8. Your research partner develops a form of RNA
   polymerase, poly-X, that recognizes the promoter
   sequence CCACC. She then decides to compare
   the action of this enzyme with that of the RNA
   polymerase normally synthesized by a bacterial cell.
   She transcribes the bacterial genome first with
   regular RNA polymerase, and then with poly-X.

(a)What results do you think she will find?

(b)Suppose the results do not conform to your

expectations. What should her next steps be in

trying to determine the properties of poly-X?

9. You are to draw a flowchart for a magazine article
   that describes some evolutionary adaptations found
   in living cells. The flowchart will accompany a
   discussion about reasons why accuracy is less
   important in transcription than in DNA replication.
   Include a caption with your chart.


10. In an experiment, you transcribe mRNA from
    E. coliDNA. You discover that when you put the
    mRNA in solution with a copy of the template DNA,
    the DNA and mRNA base-pair with each other. Then
    you transcribe all the E. colimRNAs from every gene
    in the E. colichromosome. When you put all these
    mRNA molecules together in solution with copies of
    the E. colichromosome, how much of the DNA will
    remain unpaired? Explain your reasoning.


11. The action of the spliceosome is the first known
    example of enzymatic activity by a molecule other
    than a protein. For this reason, some RNA molecules
    are called ribozymes (ribo{some} +{en}zyme). Some
    scientists consider the properties of these RNA
    molecules to be important clues about the origins of
    life on Earth. In small groups, discuss the role such
    molecules might have played in this scenario. As a
    hint, consider the role of DNA and proteins in DNA
    replication and gene expression.

MC

I

C

I

K/U

K/U

K/U

MC

K/U

K/U

K/U

5 ′ 3 ′

exon 1 exon 2 exon 3 exon 4

intron 1 intron 2 intron 3

leader

leader

leader

trailer

trailer

trailer (includes

termination sequence)

start codon stop codon

transcription

splicing

5 ′ cap

5 ′ cap

AAA ... AAA

AAA ... AAA

poly-A tail

poly-A tail

Processing of pre-mRNA in eukaryotic cells

Figure 8.13During the processing of a pre-mRNA transcript in a eukaryotic cell,

only the nucleotide sequence from the start codon to the stop codon is translated

into a polypeptide. Both prokaryotic and eukaryotic mRNA contain non-coding

leader and trailer sequences.

The entire gene, including

introns, exons, and leading and

trailing segments, is transcribed

from the DNA template.

A

The ends of the pre-mRNA

molecule are modified. One

modified G nucleotide is added

to the 5 ′end, while a series of

A nucleotides (anywhere from

several dozen to a few hundred)

is added to the 3 ′end.

B

The introns are removed and the

exons are spliced together. The

finished mRNA molecule is now

ready for translation.

C