Genetic Testing What Is Dna

What is DNA? Most people understand that DNA carries information we inherit from our parents. The corkscrew shape of DNA is seen in DNA models in science classrooms across the world. But few people really know the reason behind DNA’s peculiar structure or how DNA encodes our hereditary traits.

DNA is short for deoxyribonucleic acid. Each human egg and sperm carries within it three billion DNA ‘bases’ that must be copied billions of times before a newly fertilized egg develops into a human adult. We share this amazing ability with almost every living creature in existence.

DNA Structure

Look at the DNA models on this page. Chances are you recognize them from school, magazines or movies. The ‘twisted ladder’ DNA structure is one of science’s most famous images. DNA models are called double helixes, describing the two twisting lengths of DNA.

The two long lengths of a DNA model are composed of nucleotides. Each nucleotide contains a sugar molecule, a phosphate molecule and one of four hydrocarbon DNA bases:

  • adenine (A)
  • cytosine (C)
  • guanine (G)
  • thymine (T).

The four DNA bases are usually referred to simply as A, C, G and T. 99 percent of all DNA bases are identical for every human on the planet. The remaining one percent account for all the differences that make us unique.

The DNA bases on one side of a DNA model attach to DNA bases on the other side, but only in very specific ways. A can only bind to T, and C only pairs up with G. There are only four different combinations of DNA bases:

  • A-T
  • C-G
  • G-C
  • T-A.

Despite this apparent simplicity, different combinations of paired bases account for the vast range of human features and traits.

Sequencing DNA

DNA structure is set up so each length of DNA is divided into three nucleotides. A set of three nucleotides is called a codon. There are 64 possible codons, based on different combinations of the A, T, G and C bases.

At this point it’s helpful to think of a length of DNA as an instruction book for reproducing cells, with each codon representing one word. For instance, a length of DNA code might look like this:

AATTCGGATTACCAT

Broken down into codons, the line of DNA reads like this:

AAT TCG GAT TAC CAT

That’s only half the story, as there are two lengths of DNA for every double helix. But DNA testing only requires half the story, because each DNA base only connects with one other type of base. So if one length of DNA reads:

AAT TCG TAC CAT,

the other side reads:

TTA AGC ATG GTA.

However that’s not exactly accurate, as the two lengths of DNA are laid out ‘head to toe.’ In other words, we’ve got the second sequence backwards, and have to reverse the order of the bases, leaving us with a codon sequence that looks like this:

ATG GTA CGA ATT.

DNA Models and Genes

A DNA sequence that contains instructions for producing a protein is called a gene. According to the Human Genome Project, humans possess approximately 20,500 genes. Each human gene is composed of anywhere from 1,000 to 1 million bases.

DNA and Cells

The majority of our DNA is stored in tightly wound packages called chromosomes. Humans have 23 different chromosomes.

A strand of DNA is very thin, so thin it even appears blurred when viewed through an electron microscope. A DNA strand’s thinness is the only way it can be bundled within a cell: Some strands are two inches long.

In addition to the DNA found in chromosomes, a small portion of DNA is found in the mitochondria, the cells’ energy producers. You inherit your mitochondrial DNA from your mother because only the egg retains mitochondria after fertilization.

DNA Reproduction

For a cell to reproduce, each new cell must carry a complete set of DNA, identical to its mother cell. Here is the way this is accomplished:

  • Cells reproduce by dividing in two
  • Before the cell divides the double helix ‘unzips’, dividing into two single strands of DNA.
  • These two strands connect with ‘free-floating’ DNA bases produced by the cell during division.
  • Because the DNA bases only pair up in four different ways, each single strand of DNA pairs up with the correct DNA bases and two copies of the double helix are then available: one for each cell.

DNA Structure and Mutations

The division and replication of double helixes is very effective, but not perfect. Occasionally bases are added, deleted, or simply copied wrong. When this occurs the cell has mutated. While many mutations are harmless, others produce cancerous cells or other health conditions. Some harmless genetic mutations are considered hereditary markers. DNA testing for these markers can help us determine our ancestry.

Resources

Genetics Home Reference. (2008). What is DNA? Retrieved August 26, 2008, from the Genetics Home Reference Web site: ghr.nlm.nih.gov/handbook/basics/dna.

National Human Genome Research Institute. (2008). Deoxyribonucleic acid (DNA). Retrieved August 26, 2008, from the National Human Genome Research Institute Web site: www.genome.gov/25520880.

Riley, D. (2005). DNA testing: An introduction for non-scientists an illustrated explanation. Retrieved August 26, 2008, from the Scientific Testimony Web site: www.scientific.org/tutorials/articles/riley/riley.html.

Transgalactic Ltd. (2005). What is DNA? Retrieved August 26, 2008, from the Bio News Online Web site: www.bionewsonline.com/j/what_is_dna.htm.