Transcription and Translation
Transcription is the copying of the base sequence inside a DNA strand into the
complementary base sequence on a strand of RNA. In eukaryotes this occurs in
the nucleus.
There are three types of RNA:
(i) Messenger RNA or mRNA
(ii) Transfer RNA or tRNA
(iii) Ribosomal RNA or rRNA
The three types of RNA perform different roles in protein synthesis, but all are
made in the nucleus but act in the cytoplasm.
It is believed that life began through self replicating RNA but was later replaced by
DNA which is more resistant to alkaline hydrolysis.
RNA is similar to DNA but there are some differences.
In an A form helix the base
pairs arrange themselves
directly over the central
axis, but in the B form the
base pairs are displaced
away from the central axis.
Remember that the double helix of DNA contains a 3, 5 strand and a 5, 3 strand.
For transcription the 3, 5 strand is called the template strand the 5, 3 strand is
called the coding strand.
Transcription occurs when RNA polymerase, RNAP binds to the template strand of
DNA at a promoter site. A promoter site on a DNA strand has a specific base
sequence directing transcription to start. This step is called initiation.
The RNA polymerase then runs along the template strand and as it does so it
adds complementary RNA bases to the lengthening RNA molecule in a process
called elongation.
Elongation stops when a base sequence gives a stop order in a process called
termination.
In initiation the double strand of DNA
opens at a promoter site which has a
specific base sequence.
This allows it to bind with RNAP. It moves
inthe 5 to 3 direction adding RNA
nucleotides to an increasingly long chain.
It eventually reaches a stop signal where
transcription stops.
After the RNA has been copied it exists as pre-mRNA. In this form it consists of
exons and introns. This mRNA needs to be spliced, and the introns removed. This
requires the use of endonuclease enzymes.
Only the exons leave the
nucleus as messenger RNA.
Once in the cytoplasm RNA binds with ribosomes.
The mRNA binds with a ribosome where translation occurs.
Translation has the following steps:
1.
After binding with a ribosome complementary base pairing occurs between
the mRNA molecule and a tRNA molecule.
Note that a tRNA molecule has three unpaired
bases forming an anticodon. The anticodon
hydrogen bonds with a corresponding
codon on a mRNA molecule.
Each tRNA molecule can be identified by its
anticodon, and carries only one type of amino
acid.
In the tRNA ribosome complex there are three possible sites for attachment of the
tRNA molecule.
The tRNA enters at an A site.
The mRNA strand then moves
along the ribosome so that the
former A site is now in the P
Position. A new tRNA now occupies
the A site. The amino acid on this
tRNA molecule now binds with the
amino acid in the P position. Then the mRNA slides along the ribosome, so that
the tRNA in the P position now occupies the E site and the amino acid in the A site
now occupies the P position. It also holds the dipeptide.
The process continues so that a protein can be made.