rna: ribonucleic acid
RNA, as previously mentioned, is an acronym for ribonucleic acid. There are many forms of RNA which are quite similar to DNA. All types of RNA are transcribed from DNA in a process called transcription which is examined in detail in the Transcription and Translation section.
A quick comparison between the two effectively explains RNA generally.
information that mRNA carries is written in genetic code - a sequence of
bases. The code is not complicated - it's like a sentance - a series of
words. Each code word is called a codon, a sequence of three adjacent
nucleotides that specifies one of twentry amino acids.
are 64 possible codons (4 x 4 x 4), and each codon codes for an amino acid.
The table below shows which codons code for which amino acids.
There are 64 possible codons (4 x 4 x 4), and each codon codes for an amino acid. The table below shows which codons code for which amino acids.
You might notice that some codons code for the same amino acid. These are known as synonomous codons. For example, GGU, GGC, GGA, and GGG all code for the amino acid glycine.
The codon AUG is special. It can either specify the amino acid methionine in the middle of a protein, or it can act as an initiation or start signal, which tells the ribosome, "Start translating here." All proteins proteins start out with methionine as their first amino acid, but it is enzymatically removed from some proteins after synthesis.
Likewise,there are codons which specify the end of a protein. These codons are UAA, UAG, and UGA.
There is a collinear relationship between DNA, mRNA, and proteins. The beginning of mRNA is DNAs 5' end. The 5' end of mRNA corresponds to the beginning of the amino terminus of the resulting protein.
the structure of mrnamRNA in both prokaryotic and eukaryotic cells is divided into three sections:
The second section, the Coding Region, is the part of mRNA that actually codes for the protein - the codons.
Finally, there is the Trailer, which is simply a section that comes after the stop codons.
The diagram below shows the parts of mRNA.
In eukaryotes, mRNA that has just been transcribed from DNA must undergo two post-transcriptional modifications before it can be translated.
Eukaryotic mRNA has regions which don't code for proteins. These regions are called Intervening Sequences or introns. Regions which do code for something are called exons. After the primary mRNA transcript is produced, introns must be identified and removed so that only exons remain.
Splicing is the process by which introns are removed to produce mature mRNA. It is accomplished in the nucleus of a cell with the aid of splicesomes, large RNA-protein complexes that contain many different enzymes and several kinds of RNA. Splicesomes contain a short piece of RNA that complements base sequences found at either end of just about every intron.
Once a piece of eukaryotic mRNA has been produced, capped, had a poly-a-tail added to it, and once the introns have been removed, it can be translated. Prokaryotic mRNA lacks the methylated cap and the poly-a-tail, and there are no introns so it can be translated right after being transcribed.
One final note, mRNA can be polycistronic. That is, it can contain coding regions for producing more than one polypeptide. Each region contains its own start and stop codons so that seperate proteins are produced.
Transfer RNA, or tRNA for short, translates the language of nucleotides into the language of amino acids. It carries amino acids and places them in a protein that is being produced according to the instructions of mRNA.
Each tRNA molecule consists of approximately 90 nucleotides, but when it's longer when it's first produced. To reach its final state, introns are removed, and special enzymes remove segments from each end of it and change some of the bases so that it has more than four types of nucleotides. Finally, three nucleotides are added to the 3' end of every tRNA produced: CCA. In its mature form, the structure of tRNA is quite complex, but to simplify it, imagine a 3-leafed clover. That is the approximate shape since tRNA has 3 loops (the leafs) and one stem.
tRNA has two key features.
In eukaryotes, rRNA is transcribed exclusively within the nucleolus while other types of RNA are synthesized throughout the nucleus. After being produced, long primary rRNA strands are processed at once by a special enzyme to yield the specific shorter strands of rRNA that are needed for ribosome assembly.
In eukaryotes, there are three forms of rRNA:
rRNA forms the skeleton of ribosomes. The remainder of the ribosomes is comprised of proteins made in the cytoplasm. They enter the nucleus and then the nucleolus and then join rRNA. The assembly of ribosomes is completed in the cytoplasm.
Completed ribosomes have two parts:
Ribosomes have specific attachment sites that allow tRNA molecules and mRNA to be in the proper close contact that they require to synthesize proteins. Two of these sites are tRNA pockets called the P site and the A Site. The other sites are mRNA grooves. There is also a site where an enzyme called peptidyl transferase works to form bonds between adjacent amino acids. If you are a bit confused about this last bit, don't worry. It's all covered in Transcription and Translation.
|1998 ThinkQuest Team#18617, George Ma, Justin Wong, Liam Stewart|