Biomolecules and the Chemistry of Life
Topics in Depth
The Theme of Nucleic Acids in Biomolecules and the Chemistry of Life
Nucleic acids—our buddies DNA, or deoxyribonucleic acid, and RNA, ribonucleic acid—consist of long chains of nucleotides, which are monomers. (Sensing a pattern yet? Good. Biology is all about patterns.)
Every nucleotide has three parts:
- A sugar
- A phosphate group
- A nitrogen-containing base
In DNA, the sugar is deoxyribose ("ose" means "sugar"), and in RNA, the sugar is ribose. Hence the "D" and "R" in DNA and RNA. Both these sugars have five carbons. In fact, the only difference between them is that ribose has an extra hydroxyl group (OH), and deoxyribose just has a hydrogen in the same spot. "Deoxy" means "without oxygen."
Do you remember that in amino acids, the mysterious R group varied and gave each amino acid its unique identity? In a nucleic acid, the nitrogen-containing base plays the same role as the R group. Deoxyribose can bind to four different Rs, or kinds of nitrogen-containing bases. Here they are in all their glory:
- Adenine, or A
- Guanine, or G
- Cytosine, or C
- Thymine, or T
Ribose binds to four kinds of bases as well. Three are the same as deoxyribose: adenine, guanine, and cytosine. But, instead of binding to thymine, ribose binds with the base uracil. RNA just had to be different.
Therefore, the four bases of RNA are:
- Adenine, or A
- Guanine, or G
- Cytosine, or C
- Uracil, or U
How do you remember the switch between thymine and uracil? Uracil has an "R" and so does RNA. Easy peasy.
Nucleotides bind together by—say it with us—dehydration synthesis. Shocking! A hydrogen from the phosphate (–H2PO4R) group of one nucleotide combines with a hydroxyl group (–OH) from the sugar of another nucleotide. Water is produced, and the two nucleotides are officially stuck to each other. When lots of these bonds form, you wind up with a long chain of nucleotides; the "backbone" of this chain therefore consists of alternating sugars and phosphates.
RNA is single-stranded, meaning it has one linear chain, and the nitrogen-containing bases aren’t bound to anything else. In contrast, DNA is double-stranded; each base binds to another base, which, of course, is part of its own nucleotide. A helpful tip for remembering this is to realize that double-stranded and DNA both start with the letter D. The result is a double-stranded molecule. It looks like a ladder when it is all stretched out, but coils into a spiral, or helix, under normal conditions. This is the reason that DNA is called a double helix.
With all this base-binding going on, you might wonder how bases decide which other base they should bind to? It turns out there is a pretty simple rule:
Adenine (A) and thymine (T) always pair together (A–T), and cytosine (C) and guanine (G) always pair together (C–G).
In other words, A and T are complementary bases, as are C and G.
Here are the basic components of a generic nucleotide.
Here are DNA's nucleotides in action:
The sequence of nucleotides in DNA provides information that is later used to make proteins. Proteins, as you already know, have many different functions and are critical to building organisms. DNA is like a really, really long instruction book. D is for directions. On the other hand, RNA translates the DNA message to a format that can be read by ribosomes, or the cellular organelles that assemble proteins. RNA also plays a role in recruiting the correct amino acids to the protein assembly sites. When organisms reproduce, their DNA is copied and passed on to their offspring, ensuring that every living organism has a master copy of the instruction book.
To sum up:
|Nucleoid Acid||Oxygen?||T or U?||Strand Type||Role|
|DNA||Deoxy (No OH)||Thymine||Double-Stranded||Director|
|RNA||Ribose (OH)||URacil||Single-Stranded||Reader, Recruiter|
You will learn a lot more about DNA and RNA a little later. For now, just recognize that they contain a lot of important information, and, in the case of DNA, pass that info on to the next generation.
Some nucleotides are not part of DNA or RNA but still play important roles in a cell. Nucleotides are busy little fellas. For instance, cyclic adenosine monophosphate (cAMP) is an intracellular signal: it communicates information from one part of the cell to another. Adenosine triphosphate (ATP) is a common and critical energy transfer molecule. The bonds that hold three phosphate groups to adenosine store energy. They form when energy is released and transfer that energy to other places in a cell. Other nucleotides are coenzymes, which are molecules that help enzymes work properly. Don't try to stuff all of these guys in your head right now; we will explain more about them later.
Scientists are able to tell the sequence of a genome by identifying or "reading" the code found in the nucleic acids. Read more here.
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