Lesson Notes By Weeks and Term v4 - SHS 3
CELL STRUCTURE AND FUNCTIONS
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CELL STRUCTURE AND FUNCTIONS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Biology
Class: SHS 3
Term: 1st Term
Week: 15
Grade code: 3.2.2.LI.2
Strand code: 2
Sub-strand code: 1
Content standard code: 3.2.1.CS.3
Indicator code: 3.2.2.LI.2
Theme: LIFE IN THE FUNDAMENTAL UNIT
Subtheme: CELL STRUCTURE AND FUNCTIONS
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This lesson explores the fascinating concept of human variation—the differences we see among people every day. In Ghana, we see this diversity in our families and communities, from the shade of our skin and the texture of our hair to our ability to taste certain substances. While this lesson falls under the broad topic of life's fundamental units (as variation originates at the genetic level within our cells), our focus today is not on cell organelles but on the practical, real-world *applications* of these variations.
A. What is Variation?
Variation refers to the differences that exist between individuals of the same species. No two individuals (except identical twins) are exactly alike. These differences are a result of both genetic factors (heredity) and environmental factors.
Types of Variation: Continuous Variation: Definition: These are variations that show a complete range of measurements from one extreme to the other, with many intermediate values. There are no clear-cut categories. Characteristics: It is controlled by many genes (polygenic inheritance) and is often influenced by the environment. Graphical Representation: When plotted, it gives a normal distribution curve (a bell-shaped curve). Ghanaian Examples: Height: People in your class are not just "short" or "tall." There is a wide range of heights. Skin Colour: There is a beautiful spectrum of skin tones in Ghana, from very light to very dark, with countless shades in between. Weight: An individual's weight can vary over a continuous range. Discontinuous Variation: Definition: These are variations that fall into distinct, clear-cut categories with no intermediate forms. Characteristics: It is controlled by one or a few genes and is not usually affected by the environment. Ghanaian Examples: ABO Blood Groups: You are either blood group A, B, AB, or O. There is no "in-between." Ability to Taste Phenylthiocarbamide (PTC): You are either a "taster" (it tastes bitter) or a "non-taster" (it's tasteless). Sickle-Cell Trait: A person's genotype is either AA (normal), AS (carrier/trait), or SS (has the disease). These are distinct categories. Fingerprints: While the patterns are complex, they fall into distinct types like arches, loops, and whorls. B. Causes of Genetic Variation
The primary source of the differences we see is our genes. The main mechanisms are: Meiosis: The process of forming gametes (sperm and egg cells). Crossing Over: During Prophase I, homologous chromosomes exchange segments of DNA. This shuffles alleles, creating new combinations on a single chromosome. Independent Assortment: During Metaphase I, pairs of homologous chromosomes line up randomly at the cell's equator. This means the combination of maternal and paternal chromosomes that goes into each gamete is completely random. Sexual Reproduction: The random fusion of a male gamete (sperm) with a female gamete (egg) during fertilisation brings together genetic material from two different parents, creating a unique individual. Mutation: A sudden, random change in the structure of a gene or chromosome. This is the ultimate source of all new alleles and, therefore, new traits in a population. While many mutations are harmful, some can be neutral or even beneficial. C. Applying Knowledge of Variation to Improve Human Life