Nucleic Acids

A nucleic acid is a large molecule made of nucleotides. A nucleotide is an organic compound that forms the basis of a genetic sequence, consisting of a sugar, a phosphate, and a nitrogenous (containing nitrogen) base. Nucleic acids contain and convey the information that directs the production of proteins in an organism. They therefore direct the life processes within the organism. The two types of nucleic acids are DNA (deoxyribonucleic acid), which contains the "instructions" for protein synthesis, and RNA (ribonucleic acid), which ferries the information from the cell nucleus to the parts of the cell where proteins are made. Nucleic acids store genetic information.

DNA and RNA have similar structures. They are composed of nucleotides with five nitrogenous bases. Three of the nitrogenous bases are found in both DNA and RNA: adenine, guanine, and cytosine. In addition, DNA contains the nitrogenous base thymine, while RNA contains the base uracil.

The nitrogenous bases fall into two categories: purines and pyrimidines. Purines contain two carbon rings, while pyrimidines contain only one. In order to maintain the correct length across the "rungs" in the "ladder" of DNA, a purine can only pair with a pyrimidine and vice versa. The purines in DNA are adenine and guanine. The pyrimidines are cytosine (C), thymine (T), and uracil (U). In DNA, adenine (A) always bonds with thymine (T), and guanine (G) always bonds with cytosine (C). In RNA, uracil (U) replaces thymine (T), and bonds with adenine (A). The DNA helix consists of two strands of DNA that are held together by the hydrogen bonds that form between nitrogenous bases facing each other on the two strands. Because of the shape of these bases, adenine pairs with guanine and thymine pairs with cytosine.

Organisms create new cells in order to grow or replace cells that have died. Creating new cells involves copying all the DNA in the existing cell, so that the cell can divide into two new daughter cells, each with an identical copy of DNA.

In DNA replication, each strand acts as a template for the new complementary strand that will be attached to it. First, the DNA "unzips" to make the strands available for copying. Then the enzyme DNA polymerase latches onto the DNA template strand and builds a new strand containing the same nitrogenous bases as the strand facing the one being copied. Eventually, other enzymes bind all the fragments of DNA together, creating two complete double helices. When describing DNA replication, it is useful to refer to the 3′ (three prime) and 5′ (five prime) ends, which indicate the numbered carbon atoms in the sugar backbone. The 3′ carbon atom has a hydroxyl ( ${{-}\rm{OH}}$) group attached to it, and the 5′ carbon atom has a phosphate group ( ${{-}\rm{PO_4}^{2-}}$) attached to it. When a cell needs to make a particular protein, it initiates a process called protein synthesis. Protein synthesis is known as the central dogma of biology: DNA codes for RNA, which codes for proteins. Protein synthesis is a complex process that can be briefly summarized by this series of biochemical steps:

1. The DNA double helix unwinds to expose the gene to be transcribed by RNA.

2. A strand of complementary RNA is created from the DNA template and sent to another portion of the cell.

3. The RNA strand is read in triplets, called codons. Each triplet corresponds to a particular amino acid. For example, UGG (uracil, guanine, guanine) is code for the amino acid tryptophan, while the following three triplet codons code for phenylalanine, glycine, and serine.

4. The corresponding amino acids are compiled into the primary structure of the protein.