The Structure of DNA

Molecular Genetics

DNA is the genetic material passed from parent to offspring for all life on Earth. The technology of molecular genetics developed in the last half century has enabled us to see deep into the history of life to deduce the relationships between living things in ways never thought possible. It also allows us to understand the workings of evolution in populations of organisms. Over a thousand species have their entire genome sequenced, and there have been thousands of individual human genome sequences completed. These will allow us to understand human disease and the relationship of humans to the rest of the tree of life.

Two Types of Nucleic Acids

Deoxyribonucleic Acid (DNA) Ribonucleic Acid (RNA)

DNA Chromosomes in Eukaryotes

Eukaryotes, whose chromosomes each consist of a linear DNA molecule, employ a different type of packing strategy to fit their DNA inside the nucleus. At the most basic level, DNA is wrapped around proteins known as histones to form structures called nucleosomes. The DNA is wrapped tightly around the histone core. This nucleosome is linked to the next one by a short strand of DNA that is free of histones. This is also known as the "beads on a string" structure.

Interphase Stage of Mitosis

In interphase, the phase of the cell cycle between mitoses at which the chromosomes are decondensed, eukaryotic chromosomes have two distinct regions that can be distinguished by staining. There is a tightly packaged region that stains darkly, and a less dense region. The darkly staining regions usually contain genes that are not active, and are found in the regions of the centromere and telomeres. The lightly staining regions usually contain genes that are active, with DNA packaged around nucleosomes but not further compacted.

The Structure of RNA

Like DNA, RNA is a polymer of nucleotides. Each of the nucleotides in RNA is made up of a nitrogenous base, a five-carbon sugar, and a phosphate group. In the case of RNA, the five-carbon sugar is ribose, not deoxyribose. Ribose has a hydroxyl group at the 2' carbon, unlike deoxyribose, which has only a hydrogen atom.

Plant and Animal Breeding

Molecular genetic techniques have revolutionized plant and animal breeding for human agricultural needs. All these advances in biotechnology depended on basic research leading to the discovery of the structure of DNA in 1953, and the research since then that has uncovered the details of DNA replication and the complex process leading to the expression of DNA in the form of proteins in the cell.

Building Blocks of DNA

Nucleotides which are made up of three parts: a deoxyribose (5-carbon sugar), a phosphate group, and a nitrogenous base.

RNA Nucleotides

RNA nucleotides contain the nitrogenous bases adenine, cytosine, and guanine. However, they do not contain thymine, which is instead replaced by uracil, symbolized by a "U". RNA exists as a single-stranded molecule rather than a double-stranded helix. Molecular biologists have named several kinds of RNA on the basis of their function. These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)--molecules that are involved in the production of proteins from the DNA code.

DNA Chromosomes in Prokaryotes

The chromosomes of prokaryotes are much simpler than those of eukaryotes in many of their features. Most prokaryotes contain a single, circular chromosome that is found in an area in the cytoplasm called the nucleotide. The DNA is twisted beyond the double helix in what is known as supercoiling. Some proteins are known to be involved in the supercoiling; other proteins and enzymes help in maintaining the supercoiled structure.

"Beads on a String" Structure

The nucleosomes are the "beads" and the short lengths of DNA between them are the "string". The nucleosomes, with their DNA could around them, stack compactly onto each other to form a 30-nm--wide fiber. The fiber is further coiled into a thicker and more compact structure.

Sugar-phosphate Groups Line up in a Backbone

The phosphate group of one nucleotide bonds covalently with the sugar molecules of the next nucleotide, and so on, forming a long polymer of nucleotide monomers. The sugar-phosphate groups line up in a "backbone" for each single strand of DNA, and the nucleotide bases, stick out from this backbone. The carbon atoms of the five-carbon sugar are numbered clockwise from the oxygen as 1',2',3',4', and 5' carbon of one nucleotide and the 3' carbon of the next nucleotide. In its natural state, each DNA molecule is actually composed of two single strands held together along their length with hydrogen bonds between the bases.

Double Helix

Watson and Crick proposed that the DNA is made up of two strands that are twisted around each other to form a right-handed helix, called a double helix. Base-pairing takes place between a purine and pyrimidine: namely, A pairs with T, and G pairs with C. In other words, adenine and thymine are complementary pairs, and cytosine and guanine are also complementary base pairs. This is the basis for Chargaff's rule.

DNA Sequence

With the exception of identical twins, each person's DNA is unique and it is possible to detect differences between human beings on the basis other unique DNA sequence.

Four types of Nitrogenous Bases in DNA

Adenine (A) and Guanine (G) are double-ringed purines. Cytosine (C) and Thymine (T) are smaller, single-ringed pyrimidines. The nucleotide is named according to the nitrogenous base it contains.

Metaphase Stage of Mitosis

At the metaphase stage of mitosis, when the chromosomes are lined up in the center of the cell, the chromosomes are at their most compacted. They are approximately 700 nm in width, and are found in association with scaffold proteins.

Chargaff's Rule

Because of their complementarity, there is as much adenine as thymine in a DNA molecule and as much guanine as cytosine. Adenine and cytosine are connected by two hydrogen bonds, and cytosine and guanine are connected by three hydrogen bonds. The two strands are anti-parallel in nature; that is, one strand will have the 3' carbon of the sugar in the "upward" position, whereas the other strand will have the 5' carbon in the upward position. The diameter of the DNA double helix is uniform throughout because a purine (two rings) always pairs with a pyrimidine (one ring) and their combined lengths are always equal.

DNA Analysis

DNA Analysis has many practical applications beyond forensics and paternity testing. DNA testing is used for tracing genealogy and identifying pathogens. In the medical field, DNA is used in diagnostics, new vaccine development, and cancer therapy. It is now possible to determine predisposition to many diseases by analyzing genes.

DNA is Packaged in a Ordered Way

DNA is a working molecule; it must be replicated when a cell is ready to divide, and it must be "read" to produce the molecules, such as proteins, to carry out the functions of the cell. For this reason, the DNA is protected and packaged in very specific ways. In addition, DNA molecules can be very long. Stretched end-to-end, the DNA molecules in a single human cell would come to a length of about 2 meters. Thus, the DNA for a cell must be packaged in a very ordered way to fit and function within a structure (the cell) that is not visible to the naked eye.