From DNA to Protein:
Transcription and Translation
Each gene is the information to build
one protein (or polypeptide chain)
that the organism needs.
The first step in producing the protein
is making a mRNA copy of the gene.
This process is called transcription.
Differences between DNA and
mRNA
• While DNA is a double helix, mRNA
is a single strand of nucleotides.
• mRNA nucleotides have the sugar
ribose instead of deoxyribose
• mRNA has the nitrogen base uracil
instead of thymine.
The Process of Transcription
The process of transcription is similar to DNA
replication in that the DNA is unwound and
complementary nucleotides are added.
Differences:
• Only a gene is copied, not the whole
chromosome.
• RNA nucleotides are added instead of DNA
nucleotides.
– Uracil is paired with adenine instead of
thymine.
• Transcription occurs on an ongoing basis
as proteins needed, replication only occurs prior
Transcription
The enzyme RNA polymerase unwinds the
DNA double helix and adds the
complementary RNA nucleotides.
If the sequence of a gene is CGGCAT
what mRNA transcript will be produced?
DNA
mRNA
C
G
C
A
T
A
G
C
G
U
A
U
In RNA, the nucleotide uracil is used in
place of thymine.
DNA Transcription Video
Click once on image to start
http://www.youtube.com/watch?v=ztPkv7wc3yU
Transcription Animation
http://www.stolaf.edu/people/giannini/flashan
imat/molgenetics/transcription.swf
Modification of mRNA in Eukaryotic
Cells
• Genes in eukaryotic organisms contain
non-coding segments called introns.
• The coding portions of the gene are called
exons.
• The noncoding introns need to be
removed before the mRNA leaves the
nucleus.
Prokaryotic Cells
In prokaryotic cells,
there is no nuclear
membrane.
So mRNA
transcripts can not
be modified before
being translated.
So in prokaryotes, transcription of mRNA
and translation to produce the protein can
occur simultaneously