AP Biology · Lesson 2

Building polymers and nucleic acids

Know Know it

Most large biological molecules are polymers: chains of small monomers joined by covalent bonds. Carbohydrates are built from sugars, proteins from amino acids and nucleic acids from nucleotides. Lipids are not true polymers, but fats are also assembled from smaller parts: glycerol and fatty acids.

Dehydration synthesis joins two monomers. One monomer gives up an -OH and the other an -H, and these leave together as one water molecule. A chain of n monomers is joined by n - 1 bonds, so building it releases n - 1 water molecules. Hydrolysis is the reverse: adding a water molecule breaks the bond between two monomers.

Lipids are mostly nonpolar. A saturated fatty acid has only single bonds and a straight chain; an unsaturated one has at least one double bond that puts a kink in the chain. A phospholipid has a polar phosphate head and two nonpolar tails.

A nucleotide has a five-carbon sugar, a phosphate group and a nitrogenous base. DNA uses the sugar deoxyribose and the bases A, T, G and C. RNA uses ribose and replaces T with U. Nucleotides link sugar to phosphate, forming a sugar-phosphate backbone with a 5' end and a 3' end. DNA has two antiparallel strands held together by hydrogen bonds between paired bases: A with T and G with C.

Trap Which way does the water go?

Building a polymer from monomers gives off water; breaking a polymer into monomers uses water up. When monomers are joined and water is a product, the reaction is dehydration synthesis. When a polymer is split and water is consumed, it is hydrolysis.

Worked example Easy

Which change happens when two glucose molecules are joined to form a disaccharide?

  1. A water molecule is releasedAnswer
  2. A water molecule is used up
  3. A phosphate group is added
  4. A hydrogen bond links the sugars

How to solve it

  1. Joining monomers is a building reaction.
  2. Building reactions here are dehydration synthesis.
  3. Dehydration synthesis releases one water molecule per bond.

Why each choice is right or wrong

  • A. Correct. Joining two monomers is dehydration synthesis, which gives off one water molecule.
  • B. Incorrect. Using up water describes hydrolysis, which breaks a disaccharide apart rather than building it.
  • C. Incorrect. Linking two glucose units adds no phosphate group; the sugars are joined directly.
  • D. Incorrect. The sugars are joined by a covalent bond, not by a weak hydrogen bond.

Explain Explain it

Task verbs: calculate, explain

For a counting question, draw a short chain first. Three beads on a string need two links, so n monomers need n - 1 bonds and release n - 1 water molecules. Then apply the same count to the long chain.

Worked example Hard

One strand of a short DNA molecule reads 5' A-C-G-G-T-A 3'. Which sequence is the partner strand, written from its 5' end to its 3' end?

  1. 5' T-G-C-C-A-T 3'
  2. 5' T-A-C-C-G-T 3'Answer
  3. 5' U-G-C-C-A-U 3'
  4. 5' A-C-G-G-T-A 3'

How to solve it

  1. Pair each base: A with T, C with G, G with C, T with A.
  2. Under 5' A-C-G-G-T-A 3', the partner reads 3' T-G-C-C-A-T 5', because the strands are antiparallel.
  3. Reverse it to write it from 5' to 3': 5' T-A-C-C-G-T 3'.

Why each choice is right or wrong

  • A. Incorrect. T-G-C-C-A-T has the right pairs but is written 3' to 5', so the strand direction is reversed.
  • B. Correct. The complement is 3' T-G-C-C-A-T 5', which reads 5' T-A-C-C-G-T 3' from its own 5' end.
  • C. Incorrect. U-G-C-C-A-U uses uracil, which is found in RNA, not in a DNA strand.
  • D. Incorrect. A-C-G-G-T-A repeats the first strand instead of pairing A with T and G with C.

Explain Your turn

A cell breaks a chain of 8 glucose units into single glucose molecules. Explain what type of reaction this is and how many water molecules it uses, showing how you counted.

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A full answer does this

  • Names the reaction as hydrolysis, which adds water to break bonds between monomers
  • Gives 7 water molecules, because 8 units are held by 7 bonds and each bond takes one water

Model answer

Breaking a polymer into its monomers is hydrolysis: one water molecule is added to split each bond between two glucose units. A chain of 8 glucose units has 8 - 1 = 7 bonds, so the reaction uses 7 water molecules.

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