Getting down to bases

When we decided to conduct a molecular genealogy study on the surname Helm, we completed some basic research on genetics. While looking at the many resources available that describe deoxyribonucleic acid, or DNA, we found one in particular contained a model that helped us understand the basics better — it was a publication called "DNA Program Kit" published jointly by the National Genome Research Institute, United States Department of Energy, and the Science Museum of Minnesota. You can find this publication online at www.genome.gov/DNADay/DNA_Programming_Kit_ Manual.pdf. We use a modified version of that model to help you understand the components of DNA that are used in molecular genealogy.

Being a family historian, you most likely have taken a research trip to a particular town to find the burial location for an ancestor. When you reached that town, your first stop was the local library. Entering the library, you quickly made your way to the reference room. You leafed through the reference room collection, looking for a cemetery index for that area. Finding an index, you located the chapter containing a list of gravestones for the cemetery where your ancestor is buried. Thumbing through the chapter, you find your ancestor's name, which is typed with some combination of 26 letters (assuming that there are no special characters and the book is in English).

You can think about the components of DNA like the library example mentioned above (see Figure 10-1 for a model of the components of DNA). The basic building blocks of humans are cells that function like little towns. Within each cell is a nucleus, the structure that contains all of the DNA. This nucleus acts as the library for the cell. Within the nucleus is the genome. The genome is the complete set of instructions for how the cell will operate — you can think of it as the reference book collection of the library.

Base pairs

Nucleus Chromatid Chromatid

Telomere

Centromere

Figure 10-1:

A model of a cell nucleus, chromosome, and deoxyribonu cleic acid.

Nucleus Chromatid Chromatid

Base pairs

Centromere

Telomere

Courtesy of the National Human Genome Research Institute

Telomere

Courtesy of the National Human Genome Research Institute

The human genome is composed of 23 pairs of chromosomes. A chromosome is the container that holds the strands of DNA. Each type of chromosome has a different set of instructions and serves a different purpose, much like a reference collection has several types of reference books. Particular sections of a chromosome are called genes. Genes contain specific sequences of information that determine a particular inheritable characteristic of a human. So, if chromosomes are the reference book, genes are the chapters within the book.

A particular gene can come in different forms called alleles. For example, the gene for eye color might come in a blue eye allele or a brown eye allele. To use our book analogy, a particular chapter of a book can be laid out in different ways. Alleles would be different layouts that a particular chapter could have.

Genes are composed of bases, also called nucleotides, which form the "rungs" of the DNA "ladder" that hold the DNA molecule together. There are four types of bases, including adenine (A), guanine (G), cytosine (C), and thymine (T). When forming the rungs of the DNA molecule, bases only attach in one way. Adenine always pairs with thymine on the opposite strand and guanine always pairs with cytosine. The attachment of bases together is called base pairing. The bases are the language of DNA. You read the sequence of the base pairs to determine the coding of the allele — just like reading the sequence of letters in a book forms a recognizable sentence.

Please understand that our DNA-library analogy is a very simplistic explanation of the molecular parts that are considered in genetic testing. DNA plays a much more complicated role in genetics than what we just covered. However, for the purpose of this chapter, our basic presentation on genetic structure should be sufficient for understanding the broader implications in DNA testing for molecular genealogy.

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