Chromosomes, DNA & Genes

The body is made up of over a trillion cells and includes different types of cells such as skin, liver and blood cells.[1] Typically, all of the cells in our bodies, except for our mature red blood cells, contain 23 pairs of chromosomes. We inherit one of each chromosome from our biological mother and one from our biological father. Our chromosomes contain the instructions for the development and functioning of all our cells.

Chromosomes are composed of long strands of DNA tightly wrapped around proteins.  Each cell in our body contains approximately 1.8 meters (6 feet) of DNA in total, although each strand is less than one millionth of a centimeter thick. [2]

Our DNA contains around 25,000 genes.[3]   Each person has the same set of genes.   A gene is a piece of DNA that contains the instructions to make a specific protein.    Proteins are important because they tell each of our cells what to do and when to do it.  Some proteins give our body shape and structure while others help cells to do things like digest our food or carry oxygen in our blood.

What Makes us Unique?

DNA Variations

Our DNA has two strands. Each of these strands has a sequence made up of a series of four letters,  A,T,C,G called bases.  The two strands are joined together like a ladder  through their bases.  A always pairs with T, and C pairs with G.

Variations in our DNA sequence are what make each of us unique.   We can have different types of  variations in our DNA sequence.  For example, we can have a rearranged sequence, pieces missing in our sequence, called deletions, or pieces added called insertions.

The most common variation occurs when one genetic “letter” (A, T, C, or G) is replaced by any of the other three letters.  This is called a single nucleotide polymorphism or a SNP, pronounced SNIP.   In this picture the G in the first sequence (GGC ) has been replaced by the A in the second sequence (CAC).   Since the two strands of DNA are complementary you only have to show the sequence of one strand.

Gene Variations

The sequence of DNA that is located in a gene is called the gene sequence.   SNPs often occur in a gene sequence. When  this happens it can affect the protein produced by the gene.  This can affect how we look or how our body functions.    We typically inherit one gene from our mother and one from our father.     In this picture, this person has inherited an T SNP variation from their mother and an A variation for their father.   Other people could inherit AA, AT or TT.    

How do our Variations make us Unique?

Different variations have different effects.  Even SNP variations can have a significant effect on how we look and how our body functions. Some variations don’t have any effect.   Some variations can influence things like our hair color or the size of our feet.  Other variations influence our weight.  Other variations can influence  how likely we will be to get a certain disease like diabetes.   In many cases it is our genes in combination with our environment that have the greatest effect.


[1] Bianconi E et al.  An estimation of the number of cells in the human body. Ann Hum Biol. 2013 Nov-Dec;40(6):463-71.

[3] National Human Genome Research Institute.  Chromosome Abnormalities.

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