Lesson Notes By Weeks and Term v4 - SHS 2
CELL STRUCTURE AND FUNCTIONS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
CELL STRUCTURE AND FUNCTIONS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Biology
Class: SHS 2
Term: 1st Term
Week: 14
Grade code: 2.2.2.LI.1
Strand code: 2
Sub-strand code: 2
Content standard code: 2.1.2.CS.1
Indicator code: 2.2.2.LI.1
Theme: LIFE IN THE FUNDAMENTAL UNIT
Subtheme: CELL STRUCTURE AND FUNCTIONS
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
This lesson introduces the "blueprint of life" – nucleic acids, specifically DNA and RNA. Every living thing, from the smallest *kontomire* plant to the largest elephant, and including ourselves, relies on these molecules. In Ghana, understanding DNA helps us tackle important issues like diagnosing hereditary diseases such as sickle cell anaemia, improving our crops like maize and cassava through scientific research, and even identifying family relationships. Today, we will uncover the beautiful structure of DNA, as discovered by Watson and Crick, and understand why this structure is perfectly designed for its job of carrying our genetic inheritance.
2.1 What are Nucleic Acids? Nucleic acids are large biological molecules (macromolecules) that are essential for all known forms of life. They carry the genetic information that is passed from one generation to the next and are responsible for directing the synthesis of proteins.
There are two main types of nucleic acids: Deoxyribonucleic Acid (DNA) Ribonucleic Acid (RNA) 2.2 The Building Block: The Nucleotide Imagine a wall made of bricks. The entire wall is the nucleic acid, and each individual brick is a nucleotide. A nucleotide is the monomer, or basic repeating unit, of a nucleic acid. Every nucleotide has three distinct parts: A Pentose (5-Carbon) Sugar: In DNA, this sugar is called deoxyribose. In RNA, this sugar is called ribose. The only difference is that ribose has one more oxygen atom than deoxyribose. A Phosphate Group: This is what makes the molecule an "acid". It is rich in energy and helps link the nucleotides together. A Nitrogenous Base: This is the most important part for carrying information. There are five different bases, which fall into two chemical groups: Purines (Double-ring structure): Adenine (A) and Guanine (G). Pyrimidines (Single-ring structure): Cytosine (C), Thymine (T), and Uracil (U). Important Rule: DNA uses Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). RNA uses Adenine (A), Guanine (G), Cytosine (C), and Uracil (U). Uracil replaces Thymine in RNA.
Diagram of a Nucleotide: ``` (Phosphate Group) ----- (Pentose Sugar) ----- (Nitrogenous Base) / \ | O=P-O | | \ | O O-CH2 | \ / \ / C---C | | C---C \ / O ``` *(Teacher's note: Draw this on the board, clearly labelling the three parts. Emphasise the connection points.)* 2.3 The Watson-Crick Model of DNA In 1953, James Watson and Francis Crick proposed a model for the structure of DNA that revolutionised biology. It is so important that we still use it today.
Key Features of the Watson-Crick Model: The Double Helix: DNA is not a single strand but a double helix. Imagine a ladder that has been twisted into a spiral. The two long sides of the ladder are made of alternating sugar and phosphate molecules. This is called the sugar-phosphate backbone. The rungs of the ladder are made of pairs of nitrogenous bases. Complementary Base Pairing: The bases do not pair randomly. There is a strict rule: Adenine (A) always pairs with Thymine (T). They are held together by two hydrogen bonds (A=T). Guanine (G) always pairs with Cytosine (C). They are held together by three hydrogen bonds (G≡C). This rule is called complementary base pairing. Because of this, if you know the sequence of bases on one strand, you automatically know the sequence on the other strand. *Analogy:* Think of it like a puzzle where the 'A' piece only fits with the 'T' piece, and the 'G' piece only fits with the 'C' piece. Antiparallel Strands: The two strands of the DNA helix run in opposite directions. This is described as being antiparallel. We label the ends of each strand based on the position of the carbon atoms in the deoxyribose sugar. One strand runs from the 5' (five-prime) end to the 3' (three-prime) end. The opposite strand runs from the 3' end to the 5' end. *Analogy:* Imagine the N1 highway from Accra to Kumasi. One side of the road goes north, and the other side goes south. They are parallel but run in opposite directions.