DNA Double Helix and Phosphodiester Bonds

0:19Joining two nucleotides

Two nucleotides join by a condensation reaction. A molecule of water is released, and a bond forms between the phosphate group of one nucleotide and the pentose sugar of the next. That bond is called a phosphodiester bond. Do not confuse it with a peptide bond. Peptide bonds join amino acids. Phosphodiester bonds join nucleotides.

0:43The polynucleotide chain

Keep adding nucleotides and you get a long chain, called a polynucleotide. The pentose sugars and phosphate groups alternate to make the sugar-phosphate backbone. The bases project sideways from the backbone. Notice that every bond along this backbone is a strong, covalent bond. That makes the strand stable.

And because the bases point away from the backbone, they are free to pair up with something else.

1:10Two antiparallel strands

DNA is made of two of these polynucleotide chains, lying side by side. The two strands run in opposite directions, which we call antiparallel. The bases point inwards and meet in the middle, like the rungs of a ladder.

1:24Complementary base pairing

Bases pair up in a very specific way. Adenine pairs with thymine, joined by two hydrogen bonds. Cytosine pairs with guanine, joined by three hydrogen bonds. We call this complementary base pairing.

One thing to keep straight: hydrogen bonds hold the two strands together. They do not join the nucleotides within a strand. Those are phosphodiester bonds.

A with T, C with G. Never A with G.

1:55The double helix

Finally, the whole ladder twists into a double helix. There are lots of hydrogen bonds, so together they hold the molecule stable. But each one is weak, so the strands can be pulled apart when the cell needs to copy the DNA. We will see that next time.

RNA is different. It is a relatively short polynucleotide, and it is a single strand, not a double helix. It still has a sugar-phosphate backbone, joined by phosphodiester bonds, but there is no second strand to pair with. So DNA is the long term store of genetic information, and RNA is the short copy that carries it to the ribosomes.

2:35Why DNA was doubted

A quick bit of history. For years, many scientists doubted that DNA carried the genetic code. DNA seemed too simple. It has only four different bases. Proteins are made from twenty different amino acids, so they looked far more capable of carrying information. The answer is that the code is in the order of the bases, along a very long molecule. Just as twenty-six letters can write every book in a library, four bases in a long enough sequence can store a huge amount of information.

It took experiments over several years to show that DNA, not protein, carries the genetic information.

3:46Examiner trap and recap

The trap is saying that hydrogen bonds join the nucleotides in a strand. Phosphodiester bonds run along each strand. Hydrogen bonds link the two strands.

Recap. Phosphodiester bonds build the backbone, hydrogen bonds pair the bases, and A goes with T, C with G. Next time, how that helix is unwound and copied, in DNA replication.