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The Structure of Cells and DNA

• Most cells in the body contain a nucleus.

• The nucleus contains DNA.

• DNA is a long molecule made of units called bases. 

• Bases come in four types: adenine, thymine, cytosine, guanine (A, T, G, C). 

• The bases pair together to form a double-strand of DNA.

• The double strand twists into a helix.

• The DNA helix is folded up and packaged into structures called chromosomes.

Your Chromosomes

• Humans have 46 chromosomes, which come in 23 pairs. One of each pair has been inherited from your mother, one from your father.

• Males and females have the same basic set of chromosomes, except for one pair known as the sex chromosomes. Females usually have two X chromosomes, while males usually have one X chromosome and one Y chromosome.

• Each chromosome contains many genes.

The Gene's Role

• A gene is a single genetic instruction that can be passed from parent to child. 

• Every person has two copies of each gene, except for genes found on the sex chromosomes. 

• Some genes tell the body how to make proteins. However many genes do not code for proteins. 

• Some genes are turned on or off at different times in life. 

• Different genes are active 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

• DNA changes happen throughout a person's life.  A change in the usual base pair order is known as a genetic variant or "mutation". A genetic variant can be compared to a spelling difference in the DNA code.

• A genetic variant that changes or stops the way a gene works can have a significant impact and may cause disease.  This is called a "pathogenic" or disease-causing variant. 

• Not all genetic variants cause problems. Sometimes a variant doesn't affect the way a gene works at all. This is called a benign variant

• A genetic variant can happen in one or both copies of a gene. 

• A variant present in one copy of a gene is called "heterozygous". This means the two copies of the gene are NOT the same.  

• When both copies of a gene have the same variant it is called "homozygous".   
  

Last reviewed: April 2020
Reviewed by: Kaiser Permanente Genetics