Gene Expression: Transcription and Translation
The Central Dogma
Protein synthesis requires two steps: transcription and translation.
simplistic diagram below illustrates the concept that three bases in
DNA code for one amino acid. The DNA code is copied to produce mRNA.
Later in this chapter, we will learn that RNA may be modified. The
order of amino acids in the polypeptide is determined by the sequence
of 3-letter codes in mRNA.
Bonds with Adenine:
# of Strands:
RNA has a variety of different functions in the cell. Three of these are listed below.
Messenger RNA (mRNA)
Messenger RNA contains genetic information. It is a copy of a portion of the DNA.
carries genetic information from the gene (DNA) out of the nucleus,
into the cytoplasm of the cell where it is translated to produce
Ribosomal RNA (rRNA)
This type of RNA is a structural component of the ribosomes. It does not contain a genetic message.
Transfer RNA (tRNA)
Transfer RNA functions to transport amino acids to the ribosomes during protein synthesis.
Small nuclear RNA (snRNA)
strands of RNA are complexed with protein producing small nuclear
ribonucleoproteins (snRNP). One function, described later in this
chapter, is the modification of the RNA transcript.
Transcription is the synthesis of RNA from a DNA template.
Only one strand of DNA is copied.
A single gene may be transcribed thousands of times.
After transcription, the DNA strands rejoin.
of the RNA produced by transcription is not used for protein synthesis.
These RNA molecules have other functions in the cell.
The enzyme RNA polymerase is responsible for creating RNA by copying the template strand of DNA.
Before transcription can begin in eukaryotes, proteins called transcription factors must bind to a region of the DNA called the promoter. The promoter identifies the start of a gene, which strand is to be copied, and the direction that it is to be copied.
RNA polymerase binds to the transcription factors and the promoter.
In bacteria, RNA polymerase binds directly to the promoter without the assistance of transcription factors.
RNA polymerase unwinds the DNA.
polymerase arranges nucleotides that are complimentary to the DNA
strand being copied. RNA contains uracil instead of thymine.
The direction of synthesis is 5' to 3'.
gene can be transcribed many times by multiple RNA polymerase molecules
all transcribing at the same time. One RNA polymerase molecule follows
another as transcription proceeds.
In bacteria and in eukaryotes, transcription ends after a specific code is transcribed. In bacteria, a termination sequence
in the DNA indicates where transcription will stop. In eukaryotes,
transcription stops shortly after a sequence of bases called the
The strand of RNA that is initially produced by transcription is called a primary transcript.
Some primary transcripts are never translated into protein. These RNA molecules have other functions in the cell.
eukaryotic cells, primary transcripts that are to be translated into
protein are modified. These transcripts are called precursor mRNA (or pre-mRNA).
modified guanine nucleotide "cap" is added to the 5’ end and a poly-A
tail (50 to 250 adenines) is added to the 3’end of the molecule. These
modifications are thought to 1) enhance the movement of mRNA through
the nuclear pores into the cytoplasm, 2) prevent the destruction of
mRNA by hydrolytic enzymes, and 3) help ribosomes attach during
The 5' end and the 3' end each contain nucleotides
that are not translated into protein. These two regions are called the
5' UTR (untranslated region) and the 3' UTR.
Eukaryotic genes contain regions that are not translated into proteins. These regions of DNA are called introns (intervening sequences) and must be removed from mRNA by a process called RNA splicing. Their function is not well understood.
The remaining portions of DNA that are translated into protein are called exons
(expressed). After intron-derived regions are removed from mRNA, the
remaining fragments- derived from exons- are spliced together to form a
mature mRNA transcript.
process of RNA splicing is carried out by complexes of proteins and
small RNA molecules called spliceosomes. The RNA component of
spliceosomes is called small nuclear RNA or snRNA. The snRNA is
joined together with protein to form small nuclear ribonuclearprotein
(snRNP). Small ribonuclearproteins and other proteins together form
Some introns have catalytic (enzyme) capabilities
and they are able to catalyze their own removal from the primary
Transcription and mRNA processing occur in the nucleus.
Alternative RNA Splicing
single gene is capable of producing more than one different polypeptide
by removing different introns from the primary RNA transcript.
example, humans have an estimated 20,000 genes. These genes produce as
many as 100,000 different proteins due to alternative RNA
Translation is the process where ribosomes synthesize proteins using the mature mRNA transcript produced during transcription.
diagram below shows a ribosome attach to mRNA, and then move along the
mRNA adding amino acids to the growing polypeptide chain.
Translation - Details
A mature mRNA transcript, ribosomal subunits, several tRNA molecules and attached amino acids are shown.
three-letter code in the mRNA is a codon. The tRNA molecules have
anticodons that are complimentary to the codons in RNA.
translation begins, a ribosome will be assembled from two ribosomal
subunits. The ribosome contains three attachment sites for tRNA
A ribosome attaches to the 5' end of the mRNA transcript. In
eukaryotes, the small ribosomal subunit first binds to a tRNA carrying methionine and then to the 5'
end of the mRNA.
The ribosome moves along the mRNA until it reaches the start codon (AUG).
At this point, the tRNA becomes attached to the mRNA and the large ribosomal subunit attaches.
A tRNA molecule transports the next amino acid to the ribosome. Notice that the 3-letter anticodon on the tRNA molecule matches the 3-letter code (called a codon)
in the mRNA. The tRNA with the anticodon "ACC" bonds with tryptophan.
It always transports tryptophan. Transfer RNA molecules with different
anticodons transport other amino acids.
peptide bond forms between the amino acid in the P site and the amino
acid in the A site. The growing polypeptide chain is now attached to
the tRNA in the A site.
The ribosome moves along the mRNA to expose another codon (GAU) for another tRNA molecule.
the tRNA in the P site moves into the E site, it is released. This tRNA
can now become attached to another amino acid.
mRNA codon in the A site is able to bind with the corresponding tRNA
(CUA). The tRNA with the CUA anticodon always transports asparagine.
Asparagine is now added to the growing amino acid chain.
A release factor binds to a stop codon causing the polypeptide chain to be released and causing the ribosomal subunits and mRNA to dissociate.
Summary Animation of Translation on the Internet
The web pages linked below contain animations that summarize transcription and translation.
synthetase catylizes the covalent bonding between tRNA and amino acids.
There are 20 different aminoacyl-tRNA synthetases, one for each amino
acid. The process requires ATP as an energy source.
Initiation factors participate in binding the two ribosomal units to mRNA.
Folding and Modification
The sequence of amino acids in a protein (the primary
structure) determines how it will fold. Folding may also require the assistance
of other proteins called chaperones.
The completed protein may be chemically modified before it
becomes functional. For example, some proteins require the attachment of a
In some cases, amino acids may need to be removed or the
polypeptide may need to be cut into shorter segments in order to produce a
The table below can be used to determine what amino acid corresponds to any 3-letter codon.
pairing of bases between the tRNA and mRNA does not always follow the
standard base pairing rules (A-U and G-C) for the third base pair. For
example, in some cases, if the third letter is G, it could pair with U
or with C. This phenomenon, called wobble, enables a specific tRNA to
pair with more than one codon.
Mutations are changes in the DNA.
A frameshift mutation is usually severe, producing a completely nonfunctional protein.
priniciple of a frameshift can be explained using the sentence below.
If the letters are read three at a time and one is deleted, the second
sentence becomes meaningless.
|THE BIG RED ANT ATE ONE FAT BUG|
THB IGR EDA NTA TEO NEF ATB UG?
Point mutations involve a single nucleotide, thus a single amino acid.
the sentence below, eliminating one letter does not change in the
remaining three-letter words and therefore may not cause a significant
change in the meaning of the sentence.
|THE BIG RED ANT ATE ONE FAT BUG|
THA BIG RED ANT ATE ONE FAT BUG
Silent, Missense, and Nonsense Mutations
Silent mutations are those that do not change the amino acid sequence. This
happens because amino acids have more than one spelling. Silent mutations code
for functional proteins.
A mutation that results in an amino acid substitution is called a missense mutation.
A mutation that results in a stop codon so that incomplete proteins are produced, it is called a nonsense mutation.
Exercise - Understanding DNA, mRNA, tRNA, and protein.
column in the table below represents three nucleotides. Within each
column, fill in the cells that are blank by using information from the
cell that is not blank.
|Template (anti-sense) strand||
List several characteristics that a chemical ought to have if it is to be used as genetic material.