🧱 Centrosome and 🔁 Centrioles — The Spindle Organisers

The centrosome is a specialized organelle found mainly in animal cells, located near the nucleus. It plays a crucial role in cell division and in the formation of cilia and flagella.

🔍 Structure of Centrosome

  • A centrosome contains two centrioles.
  • The two centrioles are arranged perpendicular (at right angles) to each other.
  • They are surrounded by an amorphous protein-rich material called the pericentriolar material (PCM).

🔑 Memory Line:

Centrosome = two perpendicular centrioles + PCM.

🧩 Structure of a Centriole

Each centriole is a cylindrical structure with a characteristic cartwheel arrangement.

🧬 Microtubule Organisation

  • Composed of 9 peripheral triplets of microtubules.
  • Each triplet is made of tubulin protein.
  • No central microtubules are present.

👉 This is called the 9 + 0 arrangement.

🧠 Central Hub & Spokes

  • The central proteinaceous region is called the hub.
  • The hub is connected to peripheral triplets by radial spokes.

🔑 Key Diagram Line:

Centriole = 9 peripheral triplets + no central pair (9+0).

⚙️ Functions of Centrosome & Centrioles

  • Formation of spindle fibres during cell division (mitosis and meiosis).
  • Formation of basal bodies, which give rise to cilia and flagella.

🔑 Exam Point:

Centrosome is essential for spindle formation in animal cells.

🧠 Nucleus — Control Centre of the Cell

The nucleus is a large, prominent, membrane-bound organelle that controls all cellular activities and stores the genetic material.

  • First described by Robert Brown (1831).
  • The deeply staining nuclear material was later called chromatin by Flemming.

🔑 Memory Line:

Nucleus stores DNA and controls the cell.

🧱 Structure of Interphase Nucleus

(The nucleus when the cell is not dividing)

1️⃣ Nuclear Envelope

  • Made of two membranes (inner & outer).
  • Space between them = perinuclear space (10–50 nm).

🚪 Nuclear Pores

  • Formed where inner and outer membranes fuse.
  • Act as selective gateways for transport.

🔁 Transport Through Pores:

  • RNA (mRNA, tRNA, rRNA subunits)
  • Proteins (enzymes, histones)

🔑 Key Point:

Nuclear pores regulate two-way transport between nucleus and cytoplasm.

2️⃣ Nucleoplasm (Nuclear Matrix)

  • Semi-fluid substance inside nucleus.
  • Contains chromatin and nucleolus.

3️⃣ Nucleolus 🟣

  • Dense, spherical, non-membrane-bound body.
  • Site of ribosomal RNA (rRNA) synthesis.
  • Assembly of ribosomal subunits occurs here.

🔑 High-Yield Line:

More active protein synthesis → larger nucleolus.

🧬 Chromatin and Chromosomes

Chromatin

  • Present in interphase nucleus.
  • Appears as long, thin, thread-like material.

Composition of Chromatin

  • DNA
  • Histone proteins (basic)
  • Non-histone proteins
  • RNA

Chromosomes

  • During cell division, chromatin condenses to form chromosomes.

Key Structural Features

  • Centromere: Primary constriction holding sister chromatids together.
  • Kinetochore: Protein disc on centromere for spindle attachment.

🔑 Memory Line:

Chromatin (interphase) → Chromosomes (division).

📊 Classification of Chromosomes (Based on Centromere Position)

Type Centromere Position Shape
Metacentric Middle V-shaped
Sub-metacentric Slightly off-centre L-shaped
Acrocentric Near one end J-shaped
Telocentric At terminal end I-shaped

🔑 Exam Tip:

This classification is frequently asked in NEET.

⭐ Special Chromosomal Structures

Satellite
  • Some chromosomes show a secondary constriction.
  • A small detached part beyond it is called a satellite.
🧪 Microbodies
  • Small, membrane-bound vesicles found in plant and animal cells.
  • Contain various oxidative enzymes.

🔑 Key Note:

Microbodies are enzyme-rich but do not contain DNA.

🧠 One-Page Memory Capsule

  • Centrosome → two perpendicular centrioles
  • Centriole → 9+0 triplet arrangement
  • Nucleus → control centre, DNA storage
  • Nuclear pore → regulated transport
  • Nucleolus → rRNA synthesis
  • Chromatin → DNA + histones
  • Chromosome types → meta, sub-meta, acro, telo

💡 Questions & Answers

Q1. What is a centrosome? State its functions.

A1. A centrosome is an organelle found near the nucleus in animal cells. It contains two centrioles and helps in spindle formation during cell division.

🔑 Key Points:

  • Two centrioles
  • Forms spindle fibres
  • Helps in cell division

Q2. Describe the structure of a centriole.

A2. A centriole is a cylindrical structure made of nine peripheral triplets of microtubules arranged in a cartwheel pattern, with no central microtubules.

🔑 Key Points:

  • 9 peripheral triplets
  • 9+0 arrangement
  • Made of tubulin

Q3. Why is the nucleus called the control centre of the cell?

A3. The nucleus controls the cell because it contains DNA, which regulates all cellular activities like growth, metabolism, and protein synthesis.

🔑 Key Points:

  • Contains DNA
  • Regulates gene expression

Q4. What are nuclear pores and what is their function?

A4. Nuclear pores are openings in the nuclear envelope that regulate the movement of RNA and proteins between the nucleus and cytoplasm.

🔑 Key Points:

  • Two-way transport
  • Selective gateways

Q5. What is the nucleolus and why is it important?

A5. The nucleolus is a dense structure inside the nucleus where rRNA is synthesized and ribosomal subunits are assembled.

🔑 Key Points:

  • rRNA synthesis
  • Ribosome assembly

Q6. What is a centromere? How does it help in chromosome classification?

A6. A centromere is the primary constriction on a chromosome. Its position is used to classify chromosomes into metacentric, sub-metacentric, acrocentric, and telocentric types.

🔑 Key Points:

  • Primary constriction
  • Basis of chromosome classification