As shown in Figure 1, assembly of the RNA molecule is accomplished under the influence of an enzyme, RNA polymerase. This large protein enzyme has many functional properties necessary for formation of the RNA molecule. These properties are as follows:
1. In the DNA strand immediately ahead of the gene to be transcribed is a sequence of nucleotides called the promoter. The RNA polymerase has an appropriate complementary structure that recognizes this promoter and becomes attached to it, which is the essential step for initiating formation of the RNA molecule.
2. After the RNA polymerase attaches to the promoter, the polymerase causes unwinding of about two turns of the DNA helix and separation of the unwound portions of the two strands.
3. The polymerase then moves along the DNA strand, temporarily unwinding and separating the two DNA strands at each stage of its movement. As it moves along, at each stage it adds a new activated RNA nucleotide to the end of the newly forming RNA chain through the following steps:
a. First, it causes a hydrogen bond to form between the end base of the DNA strand and the base of an RNA nucleotide in the nucleoplasm.
b. Then, one at a time, the RNA polymerase breaks two of the three phosphate radicals away from each of these RNA nucleotides, liberating large amounts of energy from the broken high-energy phosphate bonds; this energy is used to cause covalent linkage of the remaining phosphate on the nucleotide with the ribose on the end of the growing RNA chain.
c. When the RNA polymerase reaches the end of the DNA gene, it encounters a new sequence of DNA nucleotides called the chain-terminating sequence, which causes the polymerase and the newly formed RNA chain to break away from the DNA strand. The polymerase then can be used again and again to form still more new RNA chains.
d. As the new RNA strand is formed, its weak hydrogen bonds with the DNA template break away, because the DNA has a high affinity for rebonding with its own complementary DNA strand. Thus, the RNA chain is forced away from the DNA and is released into the nucleoplasm.
Fig1. Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The RNA polymerase enzyme moves along the DNA strand and builds the RNA molecule.
Thus, the code that is present in the DNA strand is eventually transmitted in complementary form to the RNA chain. The ribose nucleotide bases always com bine with the deoxyribose bases in the following combinations:
There Are Several Different Types of RNA. As research on RNA has continued to advance, many different types of RNA have been discovered. Some types of RNA are involved in protein synthesis, whereas other types serve gene regulatory functions or are involved in post transcriptional modification of RNA. The functions of some types of RNA, especially those that do not appear to code for proteins, are still mysterious. The following six types of RNA play independent and different roles in protein synthesis: 1. Precursor messenger RNA (pre-mRNA) is a large immature single strand of RNA that is processed in the nucleus to form mature messenger RNA (mRNA). The pre-RNA includes two different types of segments called introns, which are removed by a process called splicing, and exons, which are retained in the final mRNA.
2. Small nuclear RNA (snRNA) directs the splicing of pre-mRNA to form mRNA.
3. Messenger RNA (mRNA) carries the genetic code to the cytoplasm for controlling the type of protein formed.
4. Transfer RNA (tRNA) transports activated amino acids to the ribosomes to be used in assembling the protein molecule.
5. Ribosomal RNA, along with about 75 different proteins, forms ribosomes, the physical and chemical structures on which protein molecules are actually assembled.
6. MicroRNA (miRNA) are single-stranded RNA molecules of 21 to 23 nucleotides that can regulate gene transcription and translation.
