AP Biology · Unit 1 · Topic 1.6
A nucleotide has three parts and a strand has two different ends. Build one, then find out what holds two strands together.
Learning objective 1.6.A Describe the structure and function of DNA and RNA.
Drag a phosphate, a sugar and a base together until they snap into one nucleotide. Add more and they join into a strand, and the two ends of that strand turn out not to be the same.
A nucleotide is a five-carbon sugar, a phosphate and a nitrogenous base. That is true of every nucleotide in every nucleic acid. What differs between one nucleotide and the next is only which base is attached.
Nucleotides join through their sugars and phosphates, building a sugar–phosphate backbone. Because each link runs from the phosphate of one nucleotide to the sugar of the next, the chain has a direction.
The 5′ end . The 3′ end . Assemble a strand on the bench and the two ends are labeled for you — they are never the same, and an exam question asking you to describe the ends of a nucleotide chain wants exactly this.
The backbone is identical all the way along, so it carries no information. Everything is in the sequence of bases hanging off it. Change the order and you change the message, which is what makes a nucleic acid an information molecule rather than just a polymer.
Press bases to write a template strand. The tool pairs it, counts the hydrogen bonds holding the two strands together, and then lets you add a chemical that breaks those bonds.
An A–T pair is held by two hydrogen bonds and a G–C pair by three. Those bonds run between the bases in the middle of the double helix, holding one strand against the other.
Add a chemical that disrupts hydrogen bonds and those pairings break, so the two strands separate. Each strand stays whole, because the sugar–phosphate backbone is held together by covalent bonds, which the chemical does not touch. That distinction is the whole answer to the exam question.
Because G–C pairs carry three hydrogen bonds and A–T only two, a sequence rich in G and C is held together more tightly. Write a run of Gs and then a run of As on the bench and compare the hydrogen bond count for the same number of pairs.
DNA is typically double stranded and RNA is typically single stranded. DNA uses thymine where RNA uses uracil, and DNA's sugar is deoxyribose where RNA's is ribose. Switch the strand type on the build tab to see the parts change.
Answer these in your own words before you open the answers.
1. A five-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base — adenine, thymine, guanine or cytosine in DNA, with uracil replacing thymine in RNA.
2. The two ends are different. One end carries a phosphate group attached to the 5′ carbon of the sugar. The other carries a free hydroxyl group on the 3′ carbon. This is why a strand has a direction and is written 5′ to 3′.
3. The hydrogen bonds between the paired bases would break, so the two strands would separate from one another. Each individual strand would stay intact, because its sugar–phosphate backbone is held together by covalent bonds, which are not affected.
4. The G–C rich sample. Each G–C pair is held by three hydrogen bonds while each A–T pair has only two, so for the same number of base pairs the G–C rich sample has more hydrogen bonds to break and needs more energy.