DNA and RNA are the two types of polynucleotide chains found in nature. DNA and RNA differ from each other in both structural and functional aspects. Think of DNA as two long strings wrapped around each other to form a spiral, or a double helix, as it's called. Each of the two shoelaces is called a “strand” and is made of something called “polynucleotides”.
Poly means many, and a nucleotide is simply a few different components linked together. Therefore, a polynucleotide means many nucleotides linked together. The single-cell human genome is estimated to have approximately 6.4 billion nucleotides. With so much genetic information stored in a single cell, it needs to be packaged and stored efficiently.
In humans, DNA is packaged in two steps. First, two chains of polynucleotides are linked together by hydrogen bonds, forming a structure that we call a double helix of DNA. The two strings are complementary to each other and one string is designated as a direct chain and the other as the reverse chain. A chain is complementary to another chain when the sequence of one chain is joined to a specific sequence of another chain. Specifically, an adenine in one chain will only be linked to a thymine in the other chain by two hydrogen bonds and the cytosine in one chain will only be linked to a guanine in the other chain by three hydrogen bonds.
During the second step of DNA packaging, double-helical DNA is wrapped around histone proteins, and these proteins are packed close together until they form a chromosome. Each normal human cell contains a total of 46 chromosomes that are made up of two sets of 22 chromosomes and a pair of sex chromosomes. Since a normal human cell has two sets of chromosomes, this is called a diploid. An abnormal human cell can have different multiples of chromosomes, which is called aneuploidy (e.g., DNA is mostly double-stranded), two chains of polynucleotides intertwine together in the form of a double helix (fig.
Each strand of the helix consists of alternate sugar and phosphate residues, with the bases projecting inward. The two chains are held together, in an antiparallel way (that is, with opposite polarity), through weak and non-covalent interactions (which are analyzed in chapter 1 between base pairs). A can form base pairs with T, while C can form base pairs with G, this “Watson-Crick (WC)” base pairing is the result of the complementarity of form and hydrogen bonding (see Fig. Therefore, a DNA strand is a molecule.
Two chains (two molecules) of DNA give us the natural and stable helical shape. A preparation of identical single-stranded DNA fragments is labeled with 32P at its 5' ends and divided into four samples. Nucleotides are more complex than peptides; therefore, polynucleotides, when single-stranded, can be expected to be even more polymorphic than polypeptides. The thing is that, until now, I had always thought that a DNA molecule was composed of two chains, which were polynucleotides, and both were linked together.
Although hydrogen bonds between specific base pairs are largely responsible for the stability of the double helix, hydrophobic forces generate 1 between stacked purines and pyrimidines also help keep the double-stranded structure relatively stiff. In DNA there are two spiral chains of polynucleotides arranged in a helical shape, while RNA is a single-stranded molecule. The normal, stable double-helix form of DNA can be converted into a denatured form in which the chains unwind, separate and assume the properties of random coils. This is important, since if you were performing a PCR reaction (for example), you would use single-stranded DNA as a primer, so it would be a DNA molecule.
Although hydrogen bonds between unpaired bases in single-stranded DNA and water are enthalpically less favorable than those between paired bases in double-stranded DNA, hybridization between two chains essentially replaces one set of hydrogen bonds with another; the overall contribution of hydrogen bonds to the stability of the double helix is only modest. So each strand is made up of many nucleotides linked together, think of this as if there were a lot of knots one after the other inside the shoelace, where each knot represents a nucleotide. The rigor with which base pairing occurs means that there is a complementary relationship between the two chains of each DNA molecule. Although single-stranded, RNA molecules can fold so that they have a secondary structure. An important concept to understand is that DNA has directionality, where one end of the DNA chains is the 5′ end (five primes) and the other end is the 3′ end (three primes).
Once the RNA chain of the gene of interest is synthesized, the hydrogen bonds in the twisted helix of RNA and DNA are broken, which will free the RNA chain from the DNA. A single double-stranded DNA molecule is composed of two helically shaped polynucleotides and they are connected to each other by hydrogen bonds.
