Four different type of RNA can be transcribed from DNA:
1. Primase - primer
RNA used to initiate DNA replication.
2. Messenger RNA (mRNA) - used to carry the instructions about polypeptide
structure to the site of translation.
3. Ribosomal RNA (rRNA) - part of the ribosome, used in the construction of
4. Transfer RNA (tRNA) - carry amino acids to the site of translation.
Transcription begins when an RNA polymerase binds to a promoter site on the 5' to 3' DNA (blueprint) strand. A small polypeptide portion of the RNA polymerase molecule called the sigma factor recognizes the promotor site sequence. A short leader sequence of about 10 DNA nucleotides separates the promoter from the actual gene sequence.
The RNA polymerase molecule untwists and separates the hydrogen bonds between the two DNA nucleosides. Triphosphate ribonucleotides (rATP, rUTP, rGTP and rCTP) are joined to their complementary bases on the DNA blueprint strand (A-U, G-C). Energy to drive this process is supplied by the extra two phosphate bonds carried by each triphosphate ribonucleotide.
As the growing mRNA strand is synthesized, it detaches from the DNA blueprint strand. This process continues until RNA polymerase reaches a terminator sequence of guanines and cytosines, followed by a repeating sequence of adenines. Since guanine forms three hydrogen bonds with the cytosine on the DNA template, it takes longer for the RNA polymerase to join these. A loop forms in the guanine-rich portion of mRNA, putting strain on the adenine-rich portion behind it, causing the molecule to separate from the DNA blueprint and releasing the RNA polymerase.
Another way transcription is terminated is called Rho-dependent, since it depends on a protein called Rho, located near the end of the mRNA transcript. The Rho protein moves along the mRNA until the termination sequence is reached, separating the DNA blueprint from the mRNA transcript and the RNA polymerase enzyme.
Eukaryotic transcription differs from that of bacteria in several ways:
1. RNA synthesis in eukaryotes occurs primarily
in the nucleus, with the exception of
RNA produced in the mitochondrion or chloroplast.
2. Eukaryotes have three different types of nuclear
RNA polymerase, one for the
synthesis of mRNA, one for the major rRNA gene and one for tRNA and smaller
3. At least five different transcription
factor proteins assist the binding of eukaryote
RNA polymerase to promoter sequences (bacteria only use a single sigma factor).
These are removed following initiation of transcription and replaced by proteins
called elongation factors (not found in bacteria).
4. Eukaryote mRNA undergoes RNA processing (pre-translational modification) prior to translation:
a. The 5' end of the mRNA transcript has a guanine nucleotide added
elongation. This is called capping.
b. At termination, proteins remove the mRNA transcript and adds 200
to the 3' end. This is called polyadenylation.
c. Capped and polyadenylated mRNA is called premessenger
RNA, since it
contains intervening sequences of RNA that do not code for the placement
of amino acids. These "junk" sequences are called introns and they separate
usable portions called "expressed" sequences or exons. Introns are removed
from the mRNA by small rRNA molecules called spliceosomes or
Prokaryotes often exhibit concurrent RNA transcription. In this process (below) several RNA polymerase molecules attach to the initiation sequence one after another forming a polyribosome complex. In this way, multiple copies of the same mRNA molecule can be made from the same gene or set of genes.