All living things grow and reproduce, and there must be a blueprint, directions if you will, that enables the organism to carry out these processes and maintain the integrity of the species. That blueprint, in most organisms, is deoxyribonucleic acid, better known as DNA. There are some viruses that do not have DNA, and instead use RNA as their blueprint. HIV is one of them. But many scientists argue that a virus is not a living organism, a topic to be discussed at a later date.
The structure of DNA was elucidated in 1953 by James Watson and Francis Crick. I had the honor of meeting Dr. Watson, the only survivor from that era, and I must say it was like meeting Mic Jagger. Watson stands as a symbol to perhaps the greatest achievement known to science, but make no mistakes, there were others involved in the discovery who never reaped the honors bestowed upon Watson and Crick. Rosalind Franklin deduced the structure of DNA prior to Watson and Crick through her X-ray diffraction photograph. Sadly, she passed away in 1958 at the age of 38 before the Nobel prize was awarded.
So what exactly is DNA? All proteins in the body are coded from DNA through a multistep process. DNA is a polymer, a molecule comprised of repeating similar parts. It is a double-stranded helix containing a series of the 5-carbon sugar deoxyribose, a nitrogenous base and a phosphate group. The phosphate group is what makes DNA acidic.
Let’s take a closer look at the double-stranded structure on the right. The backbone of DNA consists of alternating sugar and phosphate groups. The two strands run antiparellel. The sugar is chemically bonded to a nitrogenous base, which in turn forms hydrogen bonds with its complimentary base, forming complimentary base pairs. There are four different nitrogenous bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). A always pairs with T, and G always pairs with C. This molecule can replicate itself through a complex series of steps involving enzymes that are coded using its DNA. It’s the replication of this molecule that maintains the integrity of the dividing cell, and ultimately the species. If it were replicated erroneously, then the proteins that are coded from it will be altered, and they will not perform their job very well. As a result, genetic disorders may occur, as well as cancers.
DNA is separated into genes, the unit of hereditary information that codes for one particular protein. It is the sequence of nitrogenous bases, commonly known as the nucleotide sequence, that denotes what protein will be translated from that particular gene. Only one strand of the DNA codes for a protein, and that nucleotide sequence must remain constant. For example, a section of the nucleotide sequence that partially codes for a particular DNA repair protein is 5′ – GGC AAT CCT GTC CCC ATC – 3′. 5 Altering just one of these bases (creating a point mutation) may decrease the activity of this protein, or render it nonfunctional. That could be devastating, considering its job is to repair damaged DNA, or it could be beneficial in cases where you want to destroy the cell by way of DNA modulation, as in cancer chemotherapy. If just one point mutation in one particular gene could cause such drastic consequences, it is a wonder the human species is sill thriving. But there are many safeguards in place to circumvent the consequences caused by DNA damage, and that is what makes the cell so fascinating.
The human body is very complex and each cell contains 20,000 – 25,000 genes, and roughly 50 – 250 million base pairs. This is a lot of DNA, but fear not, DNA is efficiently packaged in the nucleus of most cell types (the exception being prokaryotic cells, which include bacteria). Before cell replication, the genetic material condenses further to form chromosomes. Each chromosome is comprised of a single strand of DNA, measuring on average 1.5 x 108 base pairs, that is tightly coiled and packaged along with proteins called histones. There are 23 pairs of chromosomes in human somatic cells (any cell except a sperm or egg). That makes 46 chromosomes per cell. Lined up end to end, the DNA in those chromosomes would extend approximately 2 meters in length. Not bad for a string of molecules.
The understanding of DNA has come a long way from the days of Watson and Crick, but we still have miles to go until we fully unravel this marvelous structure.