DNA and RNA Nucleotides: Structure Compared

0:26DNA and RNA: information molecules

DNA and RNA are both information-carrying molecules. In every living cell, DNA holds the genetic information. RNA transfers that genetic information from the DNA to the ribosomes, where it is used. And ribosomes themselves are made of RNA and protein.

So one job each: DNA holds it, RNA carries it. Think of DNA as the master copy that stays safe, and RNA as the working copy that goes out to be used. That picture helps you remember the jobs, but in the exam, use the proper words: holds, and transfers.

1:04Nucleotides as monomers

Both are polymers. A polymer is a long chain made of many repeating units, and the unit here is the nucleotide. Each nucleotide is one link, and the whole chain is only as good as its links. Different nucleotides differ in just one part, and that one part is what carries the information. We will see which part in a moment.

So the question is simple. What is one nucleotide made of?

1:31The three parts of a nucleotide

Every nucleotide has exactly three parts. First, a pentose. That is a sugar with five carbon atoms. Second, a phosphate group. It is the part that later links one nucleotide to the next. And third, a nitrogen-containing organic base. Organic because it is built on carbon, and nitrogen-containing because nitrogen is in its rings. Write both words in the exam.

Pentose, phosphate, base. Notice what is missing. There is no amino acid anywhere.

2:06The DNA nucleotide

Now the DNA nucleotide. The pentose is deoxyribose. The base is one of four. Adenine, cytosine, guanine, or thymine. Shorthand: A, C, G and T.

The sugar and the phosphate are the same in every DNA nucleotide. Only the base changes, so the base is what tells the nucleotides apart.

2:30The RNA nucleotide

The RNA nucleotide is built the same way. But the pentose is ribose, not deoxyribose. And the bases are adenine, cytosine, guanine, and uracil. So RNA has uracil, where DNA has thymine.

The two sugars look almost identical. Ribose has one more oxygen atom, in an OH group, which deoxyribose lacks. A handy memory trick. Deoxyribose has lost an oxygen, and DNA has thymine. Ribose, RNA, uracil.

3:06DNA and RNA side by side

Put them side by side. Both have a phosphate group, and both have a pentose and a base. DNA has deoxyribose and the bases A, C, G, T, and it holds the genetic information. RNA has ribose and the bases A, C, G, U, and it transfers that information to the ribosomes.

4:00Examiner trap and recap

The trap. Never say DNA has uracil, or RNA has thymine. And never call the pentose in DNA ribose.

Recap. Every nucleotide is a pentose, a phosphate group and a base. DNA, deoxyribose and thymine. RNA, ribose and uracil.

Next lesson, we join nucleotides together. A condensation reaction makes a phosphodiester bond, and that is how the double helix is built.