RNA has only one chain of polynucleotides. RNA is composed of ribonucleotides that are linked by phosphodiester bonds. Unlike DNA, RNA molecules are only made up of a chain of polynucleotides (they are single-stranded). RNA polynucleotide chains are relatively short compared to DNA.
DNA is a double helix, made up of two chains of polynucleotides. RNA is composed of only one chain of polynucleotides. The sugar in the sugar and phosphate backbone of DNA is deoxyribose, while ribose is present in RNA. Both DNA and RNA contain the bases adenine, guanine and cytosine.
In DNA, adenine pairs with thymine, but in RNA it pairs with Uracil. Ribonucleic acid (RNA) is usually a single polynucleotide chain. There are many types of RNA, in particular microRNA, that play diverse roles in processes ranging from transcription to protein synthesis. As a result of the transcription of genes from DNA, messenger RNA (mRNA) is obtained, so that the bases A, C, G and T are respectively transcribed to their U, G, C, A complements, where U indicates uracil (which is a pyrimidine).
This mRNA, copied from DNA, contains introns and exons, and during the splicing process the introns move and a mature mRNA containing only exons is obtained. In the splicing process, exons can be joined together in different ways, providing various possibilities and, as a result, many proteins can be obtained from one gene. The ability to cause two chains of polynucleotides, either DNA or RNA, containing complementary base sequences, to hybridize to form a double helix has proven to be of great value in various areas of nucleic acid research and technology.
Polynucleotides
are made up of a long, unbranched chain with a main structure of sugar (ribose or deoxyribose) and phosphate units, from which heterocyclic bases protrude from the chain at regular intervals. This can be considered the primary structure of polynucleotides, but it gives little indication of the shape taken by the molecule shaped like long chain in the cell.Phosphodiester bonds link carbon 5 of a ribose sugar molecule to the phosphate group of the same nucleotide, which in turn is linked by another phosphodiester bond to carbon 3 of the ribose sugar molecule of the next nucleotide in the chain. In a polynucleotide chain, the nucleotides are linked together by a phosphodiester bond to form a long chain of nucleotides. Both DNA and RNA are polynucleotides that are linked together by phosphodiester bonds between the ribose moiety of the nucleotides. Watson and Crick proposed that a DNA molecule consists not of one but of two polynucleotide chains wrapped one around the other to form a regular double helix.
Although single-stranded, RNA molecules can fold in such a way as to give them a secondary structure. With the exception of the RNA genomes of a few viruses, RNAs generally do not have a complementary strand that pairs with each base. The two strings are complementary to each other and one string is called a direct chain and the other is called the reverse chain. Like DNA, RNA (ribonucleic acid) nucleic acid is a polynucleotide: it is made up of many linked nucleotides in a chain.
Type II topoisomerases cut both strands of DNA and use an ATP-driven conformational change (called a lock) to pass a strand of DNA through the cut before rejoining the ends of the DNA. The base sequence of a single DNA strand can be determined using the chain terminator method, the development of which led Fred Sanger to win a second Nobel Prize (see section 2.4). In the reverse strand, primase RNA will bind to the reverse strand; however, due to the way the replication fork is formed, it will not be able to replicate continuously. Then, DNA polymerase will add nucleotides to the 3' end of the primase RNA by making a copy (backbone) of the front strand.
By convention, the continuously replicated strand is the main strand and the opposite strand is the lagging strand.
