Your body is made up of tiny units called cells – as many as 100 trillion of them, according to some estimates. Within the nucleus of every one of these cells is a set of instructions which tell the cell what role it will play in your body. These instructions, essentially a blueprint or recipe for building different parts of the cell, come in the form of a molecule called DNA. Short for deoxyribonucleic acid, DNA consists of two thread-like strands that are linked together in the shape of a double helix.
What is DNA?
DNA is made up of four chemical bases: Adenine (A), Cytosine (C), Thymine (T), and Guanine (G). These bases are combined into pairs – adenine with thymine and cytosine with guanine – to make up the “rungs” of the DNA ladder (see Figure 21.1). Each “rung,” more accurately called a base pair, is one of three billion such pairs which work together to provide the instructions for building and maintaining a human being – the human genome. The exact order in which these base pairs are combined is called the DNA sequence. Much in the way letters of the alphabet are combined to form words and sentences, the sequence of these bases are the “letters” which spell out the genetic code.
What is a Chromosome?
Within the nucleus of each cell, the DNA molecules are coiled around proteins into tiny structures called chromosomes. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. One chromosome in each pair is inherited from the mother, and the other from the father. Twenty-two of these pairs, sometimes called autosomes, look the same in both males and females. The 23rd pair, called the sex chromosome because it determines gender, is the one which differentiates males and females. Females have two copies of the X chromosome, one from each parent, while males have one X chromosome from their mother, and one Y chromosome from their father. It is the father who determines the sex of his child.
What is a Gene?
Genes are sections or segments of DNA that form the individual units of heredity. They are carried on the chromosomes and contain instructions for making molecules called proteins. Each protein enables a cell to perform its own special function. The hemoglobin in red blood cells, for example, is responsible for transporting oxygen throughout your body. Another protein, insulin, helps you metabolize your food. The keratin protein is what helps your hair and nails to grow. If you look at DNA as a recipe for creating a living thing, then genes and proteins are the ingredients which work together to build, repair, and run your body.
The traits which make us each unique are also inherited from our ancestors. Physical characteristics such as curly hair, blue eyes, and a tendency for acne are all determined by our genes. Scientists also believe that many emotional and behavioral traits, at least in part, are influenced by an individual’s genetic makeup. Eating habits, intelligence, a penchant for aggressiveness, and even sleeping patterns all have their roots in our DNA.
Because genes are carried on the chromosomes, humans have two copies of each gene, one inherited from the mother and one from the father. The two copies aren’t necessarily the same, however. Just like snowflakes, genes come in variant forms. These variations are known as alleles. Different alleles are what produce variations in inherited traits. This is why your individual traits such as hair color or blood type may not match those traits in either of your parents.