DNA: Adenine, Guanine, Cytosine, Thymine & Complementary Base Pairing

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  • 0:06 Nucleic Acids Review
  • 0:46 The Function of DNA
  • 2:15 Pyrimidines and Purines
  • 3:13 Complementary Base Pairing
  • 4:05 DNA Strands are Antiparallel
  • 6:28 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
Learn the language of nucleotides as we look at the nitrogenous bases adenine, guanine, cytosine and thymine. Armed with this knowledge, you'll also see why DNA strands must run in opposite directions.

Nucleic Acids Review

Previously on DNA & RNA:

Miss Crimson: The testimony of my expert witness will not only clear my client of all wrongdoing, but will also reveal the identity of the true killer of our poor departed Mr. Bones.

Professor Pear: Nucleic acids are the molecules that cells use to store, transfer and express genetic information. DNA stands for deoxyribonucleic acid. It's the molecule that stores genetic information in an organism. That makes the nucleotide the most basic subunit of DNA, or, more generally, of any nucleic acid.

The four nitrogenous bases in DNA
Four Bases

The Function of DNA

Miss Crimson: So, Professor, you told us that a DNA nucleotide consists of a phosphate group, a sugar and a nitrogenous base. Can you tell us how nucleotide structure pertains to the case at hand?

Professor Pear: Oh, yes. You see, you need to understand the chemistry behind DNA to fully appreciate the importance and function of the molecule. The phosphate group and sugar are the same in every nucleotide, but there are four different nitrogenous bases: guanine, adenine, thymine and cytosine. They are often abbreviated by the first letter of each nitrogenous base: G, A, T and C.

They essentially function as a four-letter alphabet. Or, if I may make an analogy to the case at hand, the information in DNA is like a recipe in one of our poor victim's cookbooks. 'Reading' the DNA code ultimately tells a cell how to make proteins that it can use to perform various functions necessary for life. For instance, reading a specific sequence of DNA tells one cell how to make hemoglobin protein to carry oxygen molecules throughout the body. On the other hand, another cell might read a different recipe, which tells it how to make insulin protein to control blood sugar levels. Oh, and 'reading', or transcribing, DNA is really an intriguing process.

Pyrimidines and Purines

Miss Crimson: Yes, Professor, I'm sure DNA transcription is very interesting, but let's stick to the basic characteristics of DNA that pertain to the trial at hand. You were telling us about the nitrogenous bases.

Cytosine bonds with guanine and adenine bonds with thymine
Complementary Base Pairing

Professor Pear: You're quite right. The bases can be categorized into two different groups. The single-ring nitrogenous bases, thymine and cytosine, are called pyrimidines, and the double-ring bases, adenine and guanine, are called purines. (Miss Crimson has a puzzled look.) I guess you might wonder how I can remember that, but it's really quite simple. 'All Gods are pure.' Adenine and guanine are purines. And, by process of elimination, that means cytosine and thymine have to be pyrimidines. See?

Miss Crimson: Yes, yes. That's a very nice mnemonic aid. Adenine and guanine are purines, but we're getting off track. You were telling us why the chemical structure of nucleotides is important.

Complementary Base Pairing

Professor Pear: Oh, yes. The chemistry of the nitrogenous bases is really the key to the function of DNA. It allows something called complementary base pairing. You see, cytosine can form three hydrogen bonds with guanine, and adenine can form two hydrogen bonds with thymine. Or, more simply, C bonds with G and A bonds with T. It's called complementary base pairing because each base can only bond with a specific base partner. The structures complement each other, in a way, like a lock and a key. C will only bond with G and A will only bond with T in DNA. Because of complementary base pairing, the hydrogen-bonded nitrogenous bases are often referred to as base pairs.

DNA Strands are Antiparallel

The sugar and phosphate ends of a DNA strand are referred to by their carbon numbers
Sugar and Phosphate Ends

Remember how I said that DNA polynucleotides look like half of a ladder? Well, hydrogen bonding completes the ladder. Since the nitrogenous bases can hydrogen-bond, one polynucleotide can bond with another polynucleotide, making the nitrogenous bases the rungs of the ladder. Each polynucleotide participating in this ladder is often referred to as a strand. Because the bases can only fit together in a specific orientation, a parallel orientation between the strands won't work. The strands must be antiparallel, or upside-down, relative to one another.

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