The structure of DNA and RNA

      Nucleotides consist of a(n) 5-carbon sugar, a phosphate group, and a nitrogenous bases connected by . The phosphate group is attached to the 5’ carbon of the sugar and the nitrogenous base is attached to to 1’ carbon of the sugar. The two classes of nitrogenous base found in nucleic acids are purines and pyrimidines. Purines have a structure and pyrimidines have a single-ring structure. Adenine (A) and guanine (G) are the purines common to both DNA and RNA. Cytosine (C) is the pyrimidine found in both DNA and RNA. The pyrimidine thymine (T) is found in DNA and the pyrimidine uracil (U) in RNA. In the DNA double helix, A only binds with T and G only binds with C (When RNA is being transcribed, U takes the place of C as the complement to G). This is referred to and promotes the conservation of nucleotide sequences in that make up the genetic code. In addition, that one purine always pairs with one pyrimidine is what maintains the width of the DNA double helix. Nucleic acids strands are formed by a reaction between nucleotides. An H from a sugar is removed and an OH from a phosphate group are removed. The product of this is a bond lining the nitrogenous bases along with the formation of a molecule of water. A molecule of DNA consists of a two nucleotide strands that take the shape of a double helix. Alternating sugar and phosphate groups for the backbone of each strand. The two strands are connected to one another by the hydrogen bonds joining nitrogenous bases. Three hydrogen bonds connect A with T and two hydrogen bonds connect G with C. The two DNA strands of a double helix are . One strand runs in the direction of the 5’ carbon of the sugar to the 3’ carbon of the sugar. The other stand of the helix runs in the 3’ to 5’ direction. There are three primary differences between DNA and RNA. As already noted, DNA contains thymine whereas RNA contains uracil. DNA contains a(n) sugar and RNA contains a(n) ribose sugar. In addition, DNA is double stranded and RNA is single stranded. There are four major types of RNA. which is transcribed from DNA and carries the genetic information to ribosomes. tRNA which transfer amino acids to ribosomes and which is a structural component of ribosomes. There is also RNAi which place a role in the regulation of gene expression. DNA replication occurs in three main stages. Unwinding, synthesis, and DNA ligation. Unwinding involves the enzymes . DNA helicase, which breaks the hydrogen bonds between nitrogen bases, and which synthesizes the RNA primer that is required for DNA polymerase synthesis of the new complementary DNA strand. Recall that this enzyme can only build a DNA strand in the 5’ to 3’ direction. This results in DNA replication having a leading strand in which DNA is replicated continuously and a lagging strand, the old 5’ to 3’ strand, that is replicated in small pieces referred to as . It is during DNA ligation that DNA fragments are connected by the action of . Since each of the new DNA double helices consists of one old and one new strand, the mechanism of DNA replication is referred to as .