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Review Questions
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1. Why is transcription necessary?
DNA, the recipe for making proteins, never leaves the nucleus
(nucleoid region in bacteria). Yet all the protein-making machinery is
located out in the cytoplasm. So how does the information get to the
cytoplasm? DNA is transcribed into messenger RNA.
2. What is a transcript?
A transcript is not a copy of the original but has the same information
but stored in a different form. Court reporters make a transcript of
courtroom proceedings. They type out what everyone says during a
trial. Rather than speech, the information has been transcribed into a
written form. Since DNA is locked inside the nucleus, enzymes
transcribe the DNA into messenger RNA. Like any good transcript,
mRNA has the same recipe as the gene but the information now is in
the form of an RNA molecule. Aptly named, mRNA, once formed,
moves out of the nucleus and into the cytoplasm where the proteinbuilding molecules are stationed.
3. Explain the process of transcription.
Transcription begins with a gene. Rather than transcribing the entire
DNA molecule, only one gene at a time is transcribed. RNA
polymerase is a group of enzymes that makes the RNA transcript. First
off, the RNA polymerase binds to a promoter site on the DNA molecule
(the promoter indicates the start of the gene). Next, RNA polymerase
unwinds and unzips the length of one gene on the double helix. The
RNA polymerase only transcribes one of the DNA strands by using the
other strand as a template. The mRNA elongates as complementary
bases are added until the RNA polymerase reaches a termination
point. The hydrogen bonds are broken separating the newly formed
mRNA from the DNA template strand. The mRNA leaves the nucleus
through tiny pores in the nuclear envelope and enters the cytoplasm to
be translated into a sequence of amino acids. The last act of RNA
polymerase is to zip-up and rewind the DNA.
Coding Strand
Template Strand
5’ A T C G A T C G 3’
3’ T A G C T A G C 5’
↓
Coding Strand
mRNA
Template Strand
5’ A T C G A T C G 3’
5’ A U C G A U C G 3’
3’ T A G C T A G C 5’
↓
Coding Strand
Template Strand
mRNA
5’ A T C G A T C G 3’
3’ T A G C T A G C 5’
5’ A U C G A U C G 3’ → leaves the nucleus
4. List three differences between RNA and DNA.
RNA is an abbreviation of ribonucleic acid. Instead of the sugar
deoxyribose, like in DNA, the nucleotide of an RNA molecule has
ribose. Deoxyribose has one less oxygen atom than ribose. DNA is
double-stranded whereas RNA is single-stranded. The complementary
bases in DNA are A=T and G=C. In RNA, G=C but A=U. U is uracil
and takes the place of thymine.