The CHOP PediSeq Project

The Pediatric Clinical Sequencing Project

DNA and Genes

What is DNA?

DNA (deoxyribonucleic acid) is the basic material of heredity. It is a chemical made of four kinds of building blocks called bases. The four types of bases are: adenine (A), cytosine (C), guanine (G), and thymidine (T). These four bases are strung together in different combinations or sequences. DNA sequences are mostly the same in everyone. In fact, about 99% of the DNA sequence is identical in all humans. The 1% of the DNA that differs between people is what makes us each unique. The differences in the DNA between people are called variants.



What are genes?

Genes are segments of DNA that we inherit from our parents and pass on to our children. Most genes tell our body how to make products called proteins. Proteins are components of our body that perform a specific function so that our bodies work properly. We have about 23,000 genes that instruct our body on everything from how our heart will form and beat, to what color our eyes will be.

You can think of a gene as a book that contains instructions for how to create a specific protein. The order of the bases in DNA is like the letters that make up a sentence in a book. If there is a “typo” or a change in a letter in a word, it can alter the meaning of the sentence. For example, changing the letter “T” in the word tag to the letter “G” would result in the word gag, changing our understanding of the sentence. Similarly, a variant or mutation in a gene can alter the instructions a gene provides to the body, causing the body not to make a working protein. Such variants may lead to problems with a person’s growth and development, or cause disease. These variants are called mutations.



What are chromosomes?

Chromosomes are the structures in our cells that are made up of DNA. Our chromosomes are contained in the nucleus of the millions of cells that make up our body. We have 23 pairs of chromosomes (46 total). For every pair of chromosomes, one chromosome comes from our mother and one from our father. One pair of chromosomes is the sex chromosomes called the X and Y chromosomes. These two chromosomes determine if we will be male or female. Females have two X chromosomes and males have an X and a Y. Except for genes carried on the sex chromosomes, we inherit two copies of every gene, one from each parent.

The diagram below shows how our bodies are made of millions of cells that contain DNA packaged in chromosomes.




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