What is RNA?
RNA has the same primary structure as DNA. It consists of a sugar-phosphate backbone, with nucleosides attached to the 1' carbon of the sugar.
Differences between DNA & RNA
There are three significant differences:
1. RNA has a hydroxyl group on the 2' carbon of the sugar. This is the primary difference between deoxyribonucleic acid and ribonucleic acid.
2. DNA uses the nucleoside thymine while RNA uses a nucleoside called uracil:
A comparison between RNA & DNA.
Note the difference in pattern. RNA is less rigid.
3. RNA molecules are not restricted to a rigid double helix and can thus form many different tertiary structures. Each RNA molecule, depending on the sequence of its bases, can fold into a stable three-dimensional structure.
A typical RNA pattern.
Different types of RNA
mRNA - messenger RNA
This is a copy of a gene. It acts as a photocopy by having a sequence
complementary to one strand of the DNA and identical to the other strand. The mRNA acts as
a bus-boy (busybody) to carry the information stored in the DNA in the nucleus to the
cytoplasm where the ribosomes can make it into protein.
tRNA - transfer RNA
This is a small RNA. It has a very specific secondary and tertiary structure such
that it can bind an amino acid at one end, and mRNA at the other end. It acts as an
adaptor to carry the amino acid elements of a protein to the appropriate place coded by
the mRNA.
rRNA - ribosomal RNA
This is one of the structural components of the ribosome. It has sequence complementary to regions of the mRNA so that the ribosome knows where to bind to an mRNA it needs to make protein from.
snRNA - small nuclear RNA
This is involved in the machinery that processes RNAs. They travel between the nucleus and the cytoplasm. It acts as a power source to run the whole RNA cell and power the other RNA strands around the cell.
Transcription of DNA to RNA
How does the sequenced information from a DNA get transferred so that it can be carried to the ribosomes (sites of chemical reactions, where RNA is translated into protein), in the cytoplasm? The process is called Transcription. It is highly analogous (simlilar) to DNA replication.
Of course, there are different effectors, or proteins, that direct transcription. Primary among these is the RNA polymerase holoenzyme, plus an agglomeration (combination) of many different factors that together direct the synthesis of mRNA on a DNA template.
The transcription process can be simplified into three main sections:
1. The Initiation of Transcription.
2. Elongation of RNA Chains.
3. The Termination of Transcription.
Initiation of Transcription
RNA polymerase (refer to section on DNA and Chromatin), must be able to recognize the beginning of a gene so that it knows where to start synthesizing an mRNA. It is directed to the start site of transcription by one of its sub-units' affinity to a particular DNA sequence that appears at the beginning of genes. This sequence is called a promoter. It is a unidirectional sequence on one strand of the DNA that tells the RNA polymerase where to start and in which direction (that is, on which strand) to continue synthesis. The bacterial promoter almost always contains some version of the following elements:
This is a diagram of the Transcription process.
The RNA polymerase then stretches open the double helix at that point in the DNA and begins synthesis of an RNA strand complementary to one of the strands of DNA.
We call the strand from which it copies the antisense
or template strand, and the other strand, to which it is identical, the sense or coding strand.
The RNA polymerase recruits rNTPs (ribonucleic nucleotides
triphosphates) in the same way that DNA polymerase recruits dNTPs. This
kind synthesis is single stranded as the rNTPs and dNTPs synthesize to transcribe sequence
information from the DNA strand into mRNA, so that they are transmittable and convertible
into proteins.
Elongation of RNA Chains
| Elongation of RNA chains is catalyzed by the RNA polymerase core
enzyme. The RNA polymerase molecule contains both DNA unwinding and DNA rewinding
activities. In other words, the RNA polymerase will continually unwind the DNA double
helix ahead of the polymerization site, and at the same time, rewinds the complementary
DNA Strands behind the polymerization site as the polymerase moves along the DNA double
helix. The region of transient base-pairing between the growing RNA chain and the DNA template strand is very short, perhaps only about 3 base-pairs in length. The stability of the whole transcription process is maintained primarily by the binding of the DNA and the RNA Chain to the RNA polymerase. |
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Termination of Transcription
How does RNA polymerase know when to stop transcribing a gene?
If the gene carries transcribing endlessly, the mRNA will not be formed correctly, and as a result, production of the protein (which is its main purpose), is impossible. The answer to the termination procedure has to do with prokaryotes (during synthesis, it is important to know that a prokaryote is basic type of cell that does not have a nucleus or organelles, unlike eukaryotes). The DNA in prokaryotes just float freely in the cell. Thus, rNTPs can begin making protein from an mRNA immediately upon its synthesis, bypassing the nuclei stage.
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Refer to diagram on the left:
At the end of a gene, the sequence of the mRNA allows it to form a hairpin loop, which blocks the ribosome. The ribosome falls off the mRNA, and that is the termination signal recognized by the RNA polymerase. As soon as the ribosome falls off the mRNA, the RNA polymerase falls off the DNA and transcription ceases automatically.
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The diagram on the right shows what happens when DNA transforms into a protein.
The DNA strand is transcribed into pre-mRNA. |
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Beginning of Translation
For translation to begin, the introns ("junk" DNA) must first to be removed. Below we give you a pictorial representation of such a process.
