GENETICS_Chromosomes_L4

🧬 BIOLOGY FORM 3 – GENETICS
Lesson 4: Chromosomes
🎯 Lesson Objectives

By the end of this lesson, you should be able to:

Define chromosomes and describe their structure.

State the functions of chromosomes.

Explain the types and number of chromosomes in humans.

Understand the relationship between chromosomes, genes, and DNA.

🧠 1. Definition of Chromosomes

A chromosome is a thread-like structure found in the nucleus of a cell that carries genetic information in the form of DNA.

Chromosomes are visible during cell division (mitosis and meiosis).

Each chromosome consists of DNA tightly coiled around proteins called histones.

🔬 2. Structure of a Chromosome

Each chromosome is made up of:

Part Description
Chromatid One of the two identical strands formed when a chromosome replicates. Two chromatids are joined at a centromere.
Centromere The point where two sister chromatids are joined. It plays a role in movement during cell division.
Short arm (p arm) The shorter section above the centromere.
Long arm (q arm) The longer section below the centromere.
Telomere The end region of a chromosome that protects it from deterioration.

🧬 Note: When the chromosome replicates, it becomes a double-stranded structure (two chromatids).

🧬 3. Composition of Chromosomes

Chromosomes are made of:

Deoxyribonucleic acid (DNA): Carries the genetic code.

Proteins (histones): Support and package the DNA.

Genes: Sections of DNA that control specific traits.

🧩 4. Relationship Between DNA, Genes, and Chromosomes
Level Description
DNA The molecule that carries genetic instructions.
Gene A segment of DNA that codes for a specific trait.
Chromosome A structure that organizes and contains many genes.

📘 Example:
A human has 46 chromosomes containing thousands of genes, each made of DNA.

👨‍👩‍👧 5. Types of Chromosomes
Type Description Example
Autosomes Non-sex chromosomes that determine body traits. Humans have 44 autosomes (22 pairs).
Sex Chromosomes (Gonosomes) Determine the sex of an individual. Males: XY, Females: XX
🧮 6. Number of Chromosomes

Humans: 46 chromosomes (23 pairs).

Domestic cat: 38 chromosomes.

Pea plant: 14 chromosomes.

Fruit fly (Drosophila): 8 chromosomes.

🧠 Each parent contributes 23 chromosomes through gametes (sperm and ovum), giving the offspring 46 in total.

⚙️ 7. Functions of Chromosomes

Carry genetic information from one generation to another.

Control cell activities through the genes they contain.

Ensure accurate distribution of genetic material during cell division.

Determine inherited characteristics (e.g., eye color, height, etc.).

Influence sex determination (via sex chromosomes).

🌿 8. Abnormalities in Chromosome Number (Chromosomal Disorders)

Changes in chromosome number can lead to genetic disorders, e.g.:

Down syndrome: Extra chromosome 21 (Trisomy 21).

Turner’s syndrome: Missing one X chromosome (XO) in females.

Klinefelter’s syndrome: Extra X chromosome in males (XXY).

📊 9. Summary Table
Concept Key Point
Chromosome Thread-like structure carrying genetic material.
Number in humans 46 (23 pairs).
Types Autosomes (22 pairs), Sex chromosomes (1 pair).
Made of DNA, proteins, and genes.
Function Transmission and expression of genetic information.
📘 10. Review Questions

Define a chromosome.

State the main components of a chromosome.

Differentiate between autosomes and sex chromosomes.

How many chromosomes are found in human gametes?

Explain the relationship between chromosomes, genes, and DNA.

🧪 11. Practical Activity

Title: Observing Chromosomes under a Microscope

Prepare slides of onion root tip cells.

Stain with acetocarmine to make chromosomes visible during cell division.

Observe under high power of the microscope.

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