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CISN Summary of Genetic Structure:

The DNA Code - is made up of very long chains of four chemical bases or ‘letters’ - A (adenine), G (guanine), T (thymine) and C (cytosine.)

  1. In the DNA information, each 'word' is a combination of three of these base pair letters.

  2. Each three-letter word (codon) tells the cell to produce a particular amino acid that forms proteins.

  3. The sequence of those three-letter words in the gene enables the cells to assemble the amino acids in the correct order to make up a protein.
Image courtesy of the National Cancer Institute


  1. DNA stores all the information needed for our cells to function, it provides a "blueprint"

  2. There are 4 bases in DNA

  3. A always pairs with T (A-T or T-A)

  4. C always pairs with G (C-G or G-C)

  5. DNA is double stranded

  6. DNA replication is how cells produce exact copies of themselves

  7. Three base pairs called codons, form various amino acids which when combined form proteins.

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  1. Genes are made up of DNA.

  2. Every cell in our body contains the same genes but different genes are active in different cell types, tissues and organs, producing the necessary specific proteins.

  3. Some genes are turned on and others are turned off.

  4. Each chromosome contains many genes but each has a different number and type.

  5. Genes instruct our cells how to function by making mRNA and then proteins by reading the DNA blueprint.



Chromosomes are large DNA molecules composed of two chemical strands that are twisted around each other to form a "double helix." We each have 23 pairs of chromosomes - 22 pairs are identical or autosomal and the 23rd pair is a set of sex chromosomes (XX=female / XY=male).



RNA is one of the two types of nucleic acids found in all cells. RNA uses a different base pairing rule than DNA:

  • A matches only with U (uracil) instead of T

  • C still matches with G.

In the cell, RNA is made from DNA (the other type of nucleic acid), and proteins are made from RNA.



Proteins are assembled from amino acids using information encoded in genes. Proteins are the basis of body structures such as skin and hair and of substances such as enzymes, cytokines, and antibodies.


Genes to mRNA to Proteins - Explanation of the image below

When a gene expresses itself, it "switches on" to produce a protein. The gene does so by first directing the synthesis of an intermediary molecule called messenger ribonucleic acid (mRNA).

Image courtesy of 1996 To Know Ourselves, The U.S.Department of Energy and the Human Genome Project



Every organism, including humans, has a genome that contains all of the biological information needed to build and maintain a living example of that organism.

The biological information contained in a genome is encoded in its deoxyribonucleic acid (DNA) and is divided into discrete units called genes. Genes code for proteins that attach to the genome at the appropriate positions and switch on a series of reactions called gene expression.


The "Central Dogma"

"DNA makes RNA, RNA makes protein, and proteins make us." Francis Crick

  All individuals are 99.9 percent the same with respect to their DNA sequence.
Image courtesy of Genome Management Information System, Oak Ridge National Laboratory

We have provided background information about cells, the cell cycle and genetic structure to help you understand the normal process of each of these components. We have a little more basic information to provide and then we will move to the disruption of the normal process that leads to cancer.



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