# 1.5 Proteins · 1.6 Nucleic Acids

F.3 Biology · Topic I Molecules of Life · Lesson 4

What proteins and nucleic acids are made of, how their units join, and how DNA differs from RNA.

## Figure

A nucleotide has three components; in DNA the bases of the two chains pair A with T and G with C.

## Key points

- **Proteins** are organic substances made up of carbon, hydrogen, oxygen and **nitrogen**; some also contain sulphur. Their basic units are **amino acids ／ 氨基酸**.
- Amino acids are joined by **condensation**; the bond between two amino acids is a **peptide bond ／ 肽鍵**. Amino acid → dipeptide → polypeptide.
- **Functions of proteins**: growth and repair of body tissues; **enzymes** (biological catalysts), **hormones** (chemical signals), **antibodies** (body defence), **haemoglobin** (oxygen transport); energy mainly during starvation.
- **Excess amino acids** are not stored: they are **deaminated ／ 脫氨** in the liver to form **urea** and carbohydrates; the carbohydrates are oxidized in respiration to release energy.
- **Nucleic acids** (C, H, O, N and **P**) are **DNA** and **RNA**; they play important roles in heredity and protein synthesis. Their basic units are **nucleotides ／ 核苷酸**.
- **DNA**: two polynucleotide chains coiled into a **double helix ／ 雙螺旋**, held together by hydrogen bonds between the bases. **RNA**: a single polynucleotide chain.
- **ATP** is an energy-rich compound: adenine linked to ribose, which is linked to three phosphate groups.

## Must know

| | Proteins | DNA | RNA |
|----|------|-----|-----|
| **Elements** | C, H, O, N (some also S) | C, H, O, N, P | C, H, O, N, P |
| **Basic unit** | amino acid | nucleotide | nucleotide |
| **Structure** | polypeptide chain(s) | double helix of two polynucleotide chains | single polynucleotide chain |
| **Sugar** | – | deoxyribose | ribose |
| **Bases** | – | A, T, G, C | A, U, G, C |
| **Found mainly in** | all parts of the cell | nucleus | nucleus and cytoplasm |
| **Main roles** | growth and repair; enzymes, hormones, antibodies, haemoglobin | heredity | protein synthesis |

A chain of $n$ units is held together by $n - 1$ bonds, so $n - 1$ water molecules are needed to hydrolyse it completely.

## Common mistakes

- ✗ "Hydrolysing a polypeptide of 15 amino acids needs 15 water molecules." ✓ **14**: one for each peptide bond.
- ✗ "Both DNA and RNA contain ribose." ✓ DNA contains **deoxyribose** and thymine (T); RNA contains **ribose** and uracil (U).
- ✗ "A DNA sample contains 15 % thymine, so it also contains 15 % cytosine." ✓ Thymine pairs with **adenine**: A = T = 15 %, so G + C = 70 % and cytosine = 35 %.
- ✗ "Excess amino acids are stored in the body." ✓ They are deaminated in the liver, forming urea.
- ✗ "Carbohydrates and lipids contain nitrogen too." ✓ Proteins and nucleic acids contain nitrogen; carbohydrates and lipids do not.

## Quick check

1. Name the basic unit of proteins and the bond that joins two of these units.
2. How many water molecules are needed to completely hydrolyse a polypeptide of 20 amino acids?
3. State three differences between DNA and RNA.
4. In the DNA of an organism, 20 % of the bases are guanine. What is the percentage of thymine?
5. What happens to excess amino acids in the body?

## Answers

1. Amino acid; peptide bond.
2. 19 (20 units are held together by 19 peptide bonds).
3. DNA is a double helix, RNA a single chain; DNA has deoxyribose, RNA has ribose; DNA has thymine, RNA has uracil.
4. G = C = 20 %, so A + T = 60 % and thymine = 30 %.
5. They are deaminated in the liver to form urea and carbohydrates; the carbohydrates are oxidized in respiration to release energy.

## Did you know?

Spider silk is a protein. Weight for weight, some spider silk is stronger than steel.