Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Describe the structure of a DNA molecule (double helix).
• Explain complementary base pairing rules (A-T, C-G).
• Explain the process of DNA replication.
• Understand how DNA carries genetic information.
• Define the term 'genetic code'.

Lesson summary

DNA, deoxyribonucleic acid, is the molecule that holds the genetic instructions for all living organisms and many viruses. It's essentially the blueprint for life! Understanding DNA is crucial because it impacts numerous aspects of our lives, from understanding inherited diseases like sickle cell anaemia (prevalent in some South African communities) to developing new disease treatments and understanding human evolution, including the origins of the diverse populations found in South Africa. Forensic science, particularly in solving crimes, heavily relies on DNA analysis.

Lesson notes

2.1 The Structure of DNA DNA is a polymer, specifically a nucleic acid. This means it's made up of repeating units called nucleotides.

Each nucleotide has three parts: Deoxyribose Sugar: A five-carbon sugar.

This is what gives DNA its name: deoxyribo*nucleic acid.

Phosphate Group: A molecule containing phosphorus and oxygen. The phosphate groups form the backbone of the DNA strand.

Nitrogenous Base: A molecule containing nitrogen. There are four different nitrogenous bases in DNA: Adenine (A) Guanine (G) Cytosine (C) Thymine (T) The deoxyribose sugar and phosphate group are the same for every nucleotide, but the nitrogenous base varies. The sequence of these bases is what carries the genetic information.

The Double Helix: DNA doesn't exist as a single strand. Instead, two strands of DNA twist around each other to form a double helix, resembling a twisted ladder. The deoxyribose sugar and phosphate group form the sides of the ladder (the sugar-phosphate backbone), and the nitrogenous bases form the rungs.

Base Pairing Rules: The two strands of DNA are held together by hydrogen bonds between the nitrogenous bases.

However, there are specific rules for which bases can pair together.

This is called complementary base pairing: Adenine (A) always pairs with Thymine (T) (A-T) Guanine (G) always pairs with Cytosine (C) (G-C) This complementary base pairing is crucial for DNA replication. For example, if one strand has the sequence ATGC, the other strand will have the sequence TACG.

Worked example

Imagine a short sequence of DNA on one strand is: `5'-C-A-T-G-G-C-T-A-3'`. (The 5' and 3' denote the directionality of the strand, which is important, but we will discuss it later.)

What is the sequence of the complementary strand?

Solution:

Using the base pairing rules:

C pairs with G

A pairs with T

T pairs with A

G pairs with C