Grade 8

Intermediate

7 min

Lesson 2 of 12

The Structure of DNA

Not Started

Exploring the 'Double Helix' and the chemical building blocks that make up the code of life.

Imagine a library containing the blueprints for every living thing on Earth, from a blue whale to a blade of grass. What if I told you this entire library is written in a code with only four letters, tucked inside a space smaller than a speck of dust?

Learning Objectives

1.Identify the three components of a nucleotide: sugar, phosphate, and nitrogenous base.
2.Apply the base-pairing rules ( $A$ with $T$ , and $C$ with $G$ ) to determine DNA sequences.
3.Describe the double helix shape and how its components are organized.

The LEGO Bricks of Life

DNA stands for Deoxyribonucleic Acid. It is a long molecule made of repeating units called nucleotides. Think of a nucleotide as a single LEGO brick. Each brick is made of three specific parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base. While the phosphate and sugar are always the same, there are four different types of bases that act like a biological alphabet. These nucleotides link together in a long chain to build the instructions for your entire body.

Quick Check

What are the three parts that make up a single DNA nucleotide?

Think of a 'sugar-coated phosphate base'!

Answer

A phosphate group, a deoxyribose sugar, and a nitrogenous base.

The Four-Letter Code

The 'code' of DNA is found in the nitrogenous bases. There are four types: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). These bases follow strict base-pairing rules. Because of their chemical shapes, they fit together like puzzle pieces. $A$ always pairs with $T$ , and $C$ always pairs with $G$ . These pairs are held together by weak hydrogen bonds in the center of the molecule, forming the 'rungs' of the DNA ladder.

If you have one side of a DNA strand, you can easily predict the other side using the base-pairing rules.

1. Look at the first base: $A$ . Its partner is $T$ . 2. Look at the second base: $C$ . Its partner is $G$ . 3. Look at the third base: $G$ . Its partner is $C$ .

So, a strand that reads $A-C-G$ will pair with $T-G-C$ .

Quick Check

If a DNA strand has the sequence $T-T-A-G$ , what is the matching sequence?

Remember:

A

pairs with

T

, and

C

pairs with

G

Answer

$A-A-T-C$

The Twisted Ladder

DNA doesn't just sit flat like a ladder; it twists into a shape called a double helix. Imagine taking a flexible ladder and twisting it into a spiral. The outside 'rails' of the ladder are known as the sugar-phosphate backbone. This backbone is strong and protects the delicate bases inside. The 'rungs' are the nitrogenous base pairs ( $A-T$ and $C-G$ ). This spiral shape is incredibly efficient, allowing six feet of DNA to be packed into the tiny nucleus of a single cell!

Because $A$ always pairs with $T$ , their amounts in a DNA molecule must be equal. This is known as Chargaff's Rule.

1. If a DNA sample is $20\%$ Adenine ( $A$ ), then it must also be $20\%$ Thymine ( $T$ ). 2. Together, $A + T = 40\%$ . 3. The remaining $60\%$ must be split equally between $C$ and $G$ . 4. Therefore, Cytosine ( $C$ ) would be $30\%$ and Guanine ( $G$ ) would be $30\%$ .

A scientist discovers a new organism where $15\%$ of the DNA bases are Guanine ( $G$ ). Calculate the percentage of Thymine ( $T$ ).

1. If $G = 15\%$ , then $C$ must also be $15\%$ . 2. Total $C + G = 30\%$ . 3. Subtract from $100\%$ to find the $A + T$ total: $100\% - 30\% = 70\%$ . 4. Divide by $2$ to find the individual percentage for $T$ : $70\% / 2 = 35\%$ . 5. Answer: $35\%$ of the bases are Thymine.

Key Takeaways

1A nucleotide consists of a phosphate, a deoxyribose sugar, and one of four nitrogenous bases.
2The base-pairing rules are $A-T$ and $C-G$ .
3The DNA molecule is shaped like a double helix, with a sugar-phosphate backbone on the outside and bases on the inside.

Quick Check

Multiple Choice

Which part of the nucleotide makes up the 'rungs' or steps of the DNA ladder?

Multiple Choice

If a DNA molecule contains $10\%$ Cytosine, how much Guanine does it contain?

True/False

The 'double helix' describes a shape that looks like a twisted ladder.

What's Next?

Review Tomorrow

In 24 hours, try to sketch a nucleotide from memory and label its three parts. Then, write down the matching sequence for $G-A-T-T-A-C-A$ .

Practice Activity

Find a partner and play 'Base Match': one person says a base (like 'Cytosine') and the other must immediately say its pair ('Guanine').

Browse

Browse

The Structure of DNA

Learning Objectives

The LEGO Bricks of Life

The Four-Letter Code

The Twisted Ladder

Key Takeaways

Quick Check

What's Next?

The Structure of DNA

Learning Objectives

The LEGO Bricks of Life

The Four-Letter Code

The Twisted Ladder

Key Takeaways

Quick Check

What's Next?