What Is DNA?
DNA is short for DeoxyriboNucleic Acid. DNA is a double-stranded helical molecule found in the cells of all organisms. DNA contains the biological, genetic instructions to build an organism which are passed down from parent to child, but it also controls the day-to-day function of all cells as well. A gene is a specific section of the long, double-stranded helical molecule of DNA which contains specific instructions for some specific function. Thousands of genes make up a chromosome and 46 chromosomes arranged in 23 pairs define the human genome. The complete human genome contains billions of instructions and bits of information. The focus of this lecture is to learn how to use certain specific types of DNA information which is passed down from parent to child over generations to aide in solving genealogical puzzles.
Basic biology and genetics tells us that the 23rd chromosome pair, in the human genome, is the chromosome set that determines gender. Males have both an "X" and a "Y" in their 23rd chromosome pair and are thus “XY”, but females carry two X’s or an "XX" for their 23rd chromosome pair. The unfertilized human egg cell always has a single X chromosome obtained randomly from one of the mother’s two X chromosomes when the egg cell is produced. The human egg will become a female embryo if the male sperm that initially reaches the egg cell carries an X-chromosome. The egg will become a male embryo if the male sperm that initially reaches the egg cell carries a Y-chromosome. The male embryo thus gets its Y chromosome from the father who in turn got it from his father. Thus you can see the Y-chromosome is passed down from generation to generation only through the male line. In order to better understand how we arrived at this point, we need to reach for the next level.
The complete set of DNA instructions for making an organism is called its genome. Found in every nucleus of a person’s many cells, the human genome consists of tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules, organized into structures called chromosomes. In humans, as in other higher organisms, a DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules, are connected by rungs of nitrogen--containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. Four different bases are present in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). See Figure 1 on the next page. The particular order of the bases arranged along the sugar- phosphate backbone is called the DNA sequence. These sequences specify the exact genetic instructions required to create a particular organism with its own unique traits.
The two DNA strands are held together by weak bonds between the bases on each strand, forming base pairs (bp). The human genome contains over 3 billion base pairs (bp). The complete detailed and verified mapping of the entire human genome is expected to be completed in 2003.
These three billion base pairs (bp) in the human genome are organized into 23 distinct, physically separate microscopic units or packets called chromosomes. All genes are arranged linearly along the chromosomes. The nucleus of most human cells contains 2 sets of chromosomes. Each parent provides one set. Each set has 23 single chromosomes; 22 autosomes and an X or Y gender chromosome. A normal female will have a pair of X-chromosomes in this 23rd chromosome set; a normal male’s 23rd chromosome set will have an X and Y chromosome.
