DNA & RNA

Slide 3

Slide 4

Next, we will take a more in-depth look at what DNA, deoxyribonucleic acid, and RNA, ribonucleic acid, are composed of.  In slide 3 the DNA molecule is seen as a double stranded helix or a “ladder” shaped molecule.  The rungs of the ladder are actually hydrogen bonds (H) & the sides of the ladder are phosphodiester bonds made by joining nucleotides together. 

In slide 4, a nucleotide is seen to be composed of 3 chemical groups: a nitrogenous base or nitrogen containing base, a sugar molecule & a phosphate (PO₄) group.  There are 2 types of nitrogenous bases found in DNA or RNA known as purines or pyrimidines; a nucleotide contains either one.  There are 5 purine or pyrimidine bases found in DNA or RNA; these are adenine, thymine, guanine, cytosine, or uracil.  These will be discussed more later. 

This nitrogen containing base is linked to a 5 carbon sugar – either deoxyribose, which has a hydroxyl group (OH) at carbon 3’, as labeled in slide 4, or ribose, which has OH groups at carbons 2’ & 3’.  The phosphate group is attached to the sugar at carbon 5’.  Notice that each nucleotide has a direction – the top part is the 5’-phosphate end & the bottom is the 3’-OH end.

Each nucleotide can be linked together endlessly to form a nucleic acid.  The phosphate group of the lower nucleotide links to the –OH group at the 3’ carbon on the upper nucleotide to form a phosphodiester bond.  The 5’ end will always will have a free or unbound phosphate group & the 3’ end will always have an unbound 3’-OH group.  Direction is an important concept to remember!

Slide 5

In slide 5 you can observe how nucleotides are linked together to form a

nucleic acid.  Notice that each side of the DNA helix or ladder is in the opposite direction of each other.  In slide 5, the left side of the DNA molecule has the 5’ end at the top and the right side has the 5’ end at the bottom.  This is called an anti-parallel structure.  Each side is always anti-parallel.  Also notice that the ladder structure is further stabilized by hydrogen bonds that join the nitrogenous bases together, which as are the “rungs of the ladder.”

The H bonds are very systematic in how they form.  Of the 4 bases in DNA - adenine, guanine, cytosine & thymine – Adenine always bonds with Thymine & Guanine always bonds with Cytosine. 

These are called base pairs or “Watson-Crick base pairing” after the two scientists who proposed the structure of DNA in the early 1950’s.  This is also called complementary base pairing – bases only bond with their complement. 

In RNA, 3 of the 4 bases are the same as in DNA – adenine, guanine, cytosine -- the only difference is uracil is present in place of thymine.  Thus, base pairs in RNA are Adenine & Uracil and Guanine & Cytosine.   Complementary base pairing is a very important concept to keep in mind. 

Slide 6

On the right side of slide 6, is a segment of DNA, where H bonds form between A-T & C-G and nucleotides are linked together in an anti-parallel fashion.  Therefore, DNA is a complementary & anti-parallel structure.  On the left side of the slide, RNA tends to be single-stranded, but it can loop on itself and when it does, it is also in a complementary & anti-parallel fashion.  Also notice the RNA has a direction as well…from the 5’ to 3’ end.