How can adenine distinguish thymine from cytosine

What are the units used for the ideal gas law? How does Charle's law relate to breathing? What is the ideal gas law constant? How do you calculate the ideal gas law constant? How do you find density in the ideal gas law? Does ideal gas law apply to liquids? Impact of this question views around the world. In DNA nucleotide subunits, there are four nitrogenous bases:. Each of these bases can be divided into two categories: purine bases and pyrimidine bases.

Adenine and guanine are examples of purine bases. This means their structure is a nitrogen-containing six atom ring joined with a nitrogen-containing five atom ring that share two atoms to combine the two rings. Thymine and cytosine are examples of pyrimidine bases. These bases are made up of a single nitrogen-containing six atom ring. Chargaff's rule, also known as the complementary base pairing rule, states that DNA base pairs are always adenine with thymine A-T and cytosine with guanine C-G.

A purine always pairs with a pyrimidine and vice versa. However, A doesn't pair with C, despite that being a purine and a pyrimidine.

This rule is named after the scientist Erwin Chargaff who discovered that there are essentially equal concentrations of adenine and thymine as well as guanine and cytosine within almost all DNA molecules. These ratios can vary between organisms, but the actual concentrations of A are always essentially equal to T and same with G and C.

For example, in humans, there's approximately:. It has to do both with the hydrogen bonding that joins the complementary DNA strands along with the available space between the two strands. Two purines and two pyrimidines together would simply take up too much space to be able to fit in the space between the two strands. This is why A cannot bond with G and C cannot bond with T.

But why can't you swap which purine bonds with which pyrimidine? A base pair is two chemical bases bonded to one another forming a "rung of the DNA ladder.

Each strand has a backbone made of alternating sugar deoxyribose and phosphate groups. Attached to each sugar is one of four bases--adenine A , cytosine C , guanine G , or thymine T.

The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine. Base pair describes the relationship between the building blocks on the strands of DNA.