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The Basics of Human DNA

Human DNA

DNA is short for deoxyribonucleic acid, a molecule present in nearly all living organisms. It contains the biological instructions that makes each species unique.

Human DNA is composed of approximately 3 billion individual components that appear in two long strands in the shape of a "double-helix" - it has the look of a twisted ladder or spiral staircase. These two long strands are joined and held together by two nitrogenous bases, often called the "base pairs", that are the steps on our twisted ladder or spiral staircase analogy.

The unique genetic instructions for each individual are determined by his or her base pairs. There are four different nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G). These 4 bases are usually represented as the As, Ts, Cs and Gs and are sometimes called the DNA Letters and the DNA Alphabet.

Each individual receives DNA from his or her mother and DNA from his or her father. This means that an individual's inherited DNA consists of 2 separate DNA molecules each of which contains approximately 3 billion base pairs.

DNA is Inherited

When a woman's body creates an egg cell it creates a new DNA molecule by randomly selecting and copying sections of the DNA molecule that the woman received from her mother and by randomly selecting and copying sections of the DNA molecule that the woman received from her father. So the egg cell will contain 1 new DNA molecule with approximately 3 billion base pairs.

Because this DNA selection process is random an egg cell will on average receive 50% of its DNA from the woman's mother and 50% from the woman's father.

When a man's body creates a sperm cell it creates a new DNA molecule by randomly selecting and copying sections of the DNA molecule that the man received from his mother and by randomly selecting and copying sections of the DNA molecule that the man received from his father. So the sperm cell will contain 1 new DNA molecule with approximately 3 billion base pairs.

Because the DNA selection process is random a sperm cell will on average receive 50% of its DNA from the man's mother and 50% from the man's father.

When an egg is fertilized by the sperm the sperm's DNA and the egg's DNA become located in the nucleus of the egg cell which will then contain one DNA molecule from the woman and one DNA molecule from the man.

The initial cell divides and makes a copy of itself and then the two cells make copies of themselves - this copying process continues and eventually there are over 35 trillion cells in a human being.

With the exception of the sperm, egg and red blood cells, a copy of the DNA molecule received from an individual's mother and a copy of the DNA molecule received from an individual's father are in the nucleus of each of the individual's 35+ trillion cells.

DNA and Chromosomes

Human DNA is packaged into thread-like structures called chromosomes. So humans receive DNA from their mothers packaged in 23 chromosomes and DNA from their fathers packaged in 23 chromosomes. One chromosome might contain 150 million base pairs, another 100 million base pairs and combined the 23 chromosomes will contain approximately 3 billion base pairs.

So a human's DNA is in 46 chromosomes (23 inherited from his or her mother and 23 inherited from his or her father) and and the 46 chromosomes contain approximately 6 billion base pairs, sometimes called the As, Ts, Cs and Gs.

Human DNA and Genes

An individual's genetic instructions are contained in his or her genes. And genes are small sections of DNA that contain the instructions for each individual's characteristics – like eye and hair color. Or more technically - genes code for proteins by providing the body with the recipes to make proteins.

One individual gene may be a couple hundred base pairs long and another may be a couple thousand base pairs long. Humans have more than 20,000 genes and current estimates are that about 1 1/2 to 2 percent of a DNA molecule is used for genes. Each DNA molecule is approximately 3 billion base pairs long and so about 45 to 60 million base pairs contain all of an individual's genes. Since an individual has 2 DNA molecules each individual has 2 copies of each gene.

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