The Structure of Cells and DNA
Most body cells contain a nucleus
The nucleus contains DNA
DNA is a long molecule made of units called bases
Bases come in four types, labled A, T, G, and C
The bases pair together to form a double-strand of DNA
The double strand twists into what is called a helix
The helix is folded up and packaged inside what are called chromosomes
Humans have 46 chromosomes, which come in 23 pairs. One of each pair has been inherited from your mother, one from your father.
22 of the 23 pairs are identical in both males and females, the 23rd pair is known as the sex chromosomes. Females have two X chromosomes, while males have one X chromosome and one Y chromosome.
The Gene's Role
All the cells in the body contain the same set of 46 chromosomes. Each chromosome contains thousands of genes that direct the structure and function of each cell in the body.
The function of genes is to make proteins.
Each gene makes a protein that has a specific function in the cell.
Different genes are activated in different cells. For example, genes that code for brain function are active in brain cells, and not active in liver cells. Genes that code for liver function are active in liver, not brain cells.
How Genes Change or Mutate
Genes are constantly undergoing change. A change in the order of the base pairs is known as a "mutation". A mutation can be compared to a spelling error in the DNA code.
A gene produces proteins. A genetic mutation that alters the genes ability to produce a functioning protein, or a mutation that reduces the amount of the protein being made can have a significant impact on the body, and may cause disease.
Not all mutations cause problems. Sometimes a mutation doesn't effect the function of the gene at all.
Since we have two copies of every gene, a mutation can occur in one, the other or both copies of the gene.
A mutation that occurs in one copy of a pair of genes while the other copy has no mutation is called being "heterozygous". Heterozygous means 2 (or more) things that are NOT like each other.
A mutation that occurs in both copies of a pair a genes is called being "homozygous". Homozygous means that 2 (or more) things that ARE like each others.
Last reviewed: 1/11/13
Reviewed by: Kaiser Permanente Genetics