Translation and
Transcription
By George Googasian
Transcription
Transcription is the process of making a specialized
RNA molecule.
Transcription occurs in the nucleus of eukaryotes and
in the cytosol of prokaryotes.
There are three stages in Transcription of RNA from a
DNA template; initiation, elongation, and
termination.
The Three Steps of
Transcription
Initiation
Initiation occurs when the enzyme RNA polymerase
attaches to a specific region of the DNA. This
attachment site is called the promoter region
because it promotes transcription. It is located just
before the segment of the DNA coding strand that
will be transcribed. In eukaryotic cells, proteins
known as initiation factors must be present for the
RNA polymerase to attach to the promoter region.
Elongation
The second stage is Elongation of the RNA. RNA
polymerase partially unwinds the DNA, exposing the
strand of the gene. The enzyme moves along the
DNA away from the promoter site as it builds an RNA
molecule. The sequence of DNA nucleotides
determines the sequence of the RNA chain. A single
complementary strand of RNA, called a primary
transcript, is made.
Termination
The third stage of transcription is Termination. When
RNA polymerase reaches the terminator region, or
the end of the DNA to be transcribed, the enzyme
and primary transcript are released from the DNA.
This ends transcription.
RNA Processing
RNA Processing
All three types of RNA are processed in the nucleus of
eukaryotes before they leave the nucleus. Enzymes add
additional nucleotides and chemically modify of remove others.
First, enzymes attach a cap of chemically modified guanine
nucleotides to the starting end of the mRNA molecule. Next,
other enzymes replace part of the opposite end with a tail of
100-200 adenine nucleotides. This addition is called a poly-A tail.
The cap and the tail help protect mRNA from enzymes that
break down nucleic acids. Generally, the longer the poly-A tail,
the longer the life span of a particular mRNA. The tail also helps
transport RNA out of the nucleus. The cap helps the mRNA
attach to a ribosome and begin translation.
RNA Processing Cont.
The final step in mRNA processing involves removal
of some internal segments of the RNA that do not
code for protein. These are called introns. The parts
of the transcript that remain and code for protein are
called exons.
RNA Splicing
The process of removing introns and rejoining cut ends is
called splicing. Splicing requires precise recognition of the
site to be cut. Splicing enzymes recognize the sequence
GU at one end of an intron and AG at the other end. This
can happen in several ways. Protein enzymes catalyze the
splicing of tRNA in yeast and many other eukaryotes.
Catalytic RNAs splice RNAs produced by mitochondria,
chloroplasts, and many unicellular eukaryotes. If introns
are left in RNA, the consequences can be serious. In
addition to splicing, an important step in the processing of
tRNA is the chemical modification of several nucleotides
and folding into a cloverleaf shape.
rRNA Processing
Ribosomal RNA is not involved in coding. The primary
rRNA transcript is spliced and modified to produce
mature rRNA molecules. These molecules bind to
proteins to form the large subunits of ribosomes.
Often several ribosomes bind to the same mRNA
molecule, forming a polyribosome.
Translation
Translation
Protein synthesis translates the codon sequence of mRNA
into the amino-acid sequence of a protein. This happens
on ribosomes, where tRNA acts as a molecular adapter.
One end of a tRNA molecule carries a specific amino acid.
The corresponding anticodon is the opposite end of the
tRNA molecule. In turn, the anticodon pairs with the
mRNA codon that encodes this particular amino acid.
Attachment of the correct amino acid to its tRNA molecule
is called tRNA charging. Twenty different enzymes carry
out the tRNA charging reactions. Each enzyme bonds a
different amino acid to its matching tRNA.
Translation Cont.
Charged tRNA, mRNA, and the growing polypeptide
chain come together at specific binding sites on a
ribosome. At these sites, tRNA anticodons base pair
with mRNA codons. This positions the amino acids
they carry so that they can bond to the growing
polypeptide chain. Thus the sequence of codons
dictates the amino-acid sequence.
Translation Cont. Cont.
One of the binding sites, the P site, holds the tRNA
carrying the growing polypeptide chain. The A site
holds the tRNA carrying the next amino acid to be
added to the chain. Next to the P site is the exit site,
or E site. An uncharged tRNA leaves the E site after
its amino acid is added to the growing chain. During
translation, the ribosome moves along the mRNA
strand one codon at a time. Translation involves the
three same stages; initiation, elongation, and
termination, as transcription does. All three stages
require enzymes that are part of the ribosome.
Initiation
During initiation of transcription, the ribosome
attaches at a specific site on the mRNA. This site is
the start codon.
Elongation
During elongation, peptide bonds join each amino
acid with the next in sequence. A charged tRNA
whose anticodon matches the next codon on the
message enters the A site of the ribosome. This
positions the amino acid it carries to form a peptide
bond with the amino acid attached to the tRNA at the
P site. When the bond forms, the polypeptide chain
transfers to the tRNA at the A site. The entire
ribosome moves down the mRNA to position the
next codon at the A site. The uncharged tRNA leaves
the E site. Now the tRNA that holds the growing
polypeptide is at the P site. The A site is open and
available for the next matching tRNa to bring in an
amino acid.
Termination
Translation terminates when a stop codon reaches
the A site of the ribosome. Three codons can signal
the end of translation; UAG, UAA, or UGA. No tRNA
has an anticodon to complement these stop signals.
Instead of a tRNA, a special protein known as a
release factor binds to the stop codon in the A site.
At this point translation stops and the tRNA releases
the polypeptide. The ribosome lets go of the mRNA,
the tRNA, and the release factor; then the two
ribosomal units separate.
Translation Errors
Errors sometimes occur during translation. The most common
translation error results from misreading the nucleotide
sequence. Initiation determines exactly where translation will
begin. Starting from this point, the grouping of bases into
codons is called the reading frame. If the start is shifted by one
or two nucleotides in either direction, the frame changes. A
different sequence of codons and amino acids will result. Some
errors are due to splicing mistakes or changes in the DNA. If a
nucleotide changes so that a codon becomes a stop codon,
translation can terminate partway through the message. The
result is a partial polypeptide. Insufficient amounts of a
particular amino acid also can disrupt translation.