Back To CourseBiology 101: Intro to Biology
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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.
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.
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.
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.
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.
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.
Miss Crimson: What do you mean antiparallel?
Professor Pear: Well, remember that the backbone is made of phosphate groups and sugars. Therefore, each strand will always have a phosphate at one end and a sugar at the other end. Rather than having to refer to the phosphate or sugar end, scientists simply refer to the ends of the DNA by the closest carbon in the sugar ring. Since the carbons in the sugar are numbered one to five, the sugar end of the strand is called the 3' end and the phosphate end of the strand is called the 5' end. Remember that complementary base pairing works like a lock and key, so there's only one orientation in which hydrogen bonding will work. If you try to orient the two strands parallel to each other, the sugar ends of the polynucleotides are both at one end and the phosphate groups are at the other end. However, the nitrogenous bases can't hydrogen-bond in this orientation. The key can't fit into the lock.
For hydrogen bonding to work, the two DNA strands must run in opposite directions. The 3' end of one strand can hydrogen-bond with the 5' end of the other strand. If we represent the strands as arrows with the arrowhead at the 3' end of the stand, we can see that the strands in a DNA molecule are organized antiparallel relative to each other.
Miss Crimson: Okay. Let me stop you again, Professor, so I can summarize your testimony for the jury.
There are four nitrogenous bases found in DNA that are called guanine, adenine, thymine and cytosine. They are abbreviated by the first letter in their name, or G, A, T and C. The bases can be divided into two categories: Thymine and cytosine are called pyrimidines, and adenine and guanine are called purines. Each nucleotide base can hydrogen-bond with a specific partner base in a process known as complementary base pairing: Cytosine forms three hydrogen bonds with guanine, and adenine forms two hydrogen bonds with thymine. These hydrogen-bonded nitrogenous bases are often referred to as base pairs.
Because of the alternating nature of the phosphate groups and sugars in the backbone of nucleic acids, a nucleic acid strand has directionality. The end of a nucleic acid where the phosphate group is located is called the 5' end. The end of the nucleic acid where the sugar is located is called the 3' end. Finally, DNA strands are antiparallel, meaning that the strands in a DNA molecule are parallel, but are oriented in opposite directions. Essentially, the 5' end of one strand pairs with the 3' end of the other strand.
To be continued . . .
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Back To CourseBiology 101: Intro to Biology
21 chapters | 137 lessons