Cell Organelles and Biomolecules: Complete Overview (NCERT)

Our body performs countless functions simultaneously—digesting food, transmitting nerve signals, pumping blood, circulating nutrients, synthesizing proteins, and filtering waste—all of which are made possible by the cell, the basic unit of life.

Each cell functions like a miniature factory, equipped with specialized structures known as organelles, each responsible for a specific task. These organelles ensure that life processes continue seamlessly, whether in unicellular organisms (single-celled) or multicellular organisms (complex life forms like humans).

Types of Cells: Prokaryotic vs. Eukaryotic

Cells are broadly categorized into two types based on their nuclear organization and the presence of membrane-bound organelles:

🔹 Prokaryotic Cells (e.g., bacteria)

• Lack of a well-defined nucleus
• Have ribosomes, mesosomes, and in some cases, flagella for movement
• Simpler structure, yet highly efficient

🔹 Eukaryotic Cells (e.g., plant and animal cells)

• Possess a well-organized nucleus
• Contain membrane-bound organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and more
• More advanced and capable of performing complex functions

Advancements in Cell Study

With the development of advanced microscopy, scientists have been able to explore the intricate structure and functioning of cells in greater detail. Understanding how cells work helps us unlock the mysteries of life, from genetics to disease mechanisms.

👉 Want to learn more? Keep reading to discover the amazing parts of a cell, how they work, and why they are so important! 🚀

Plasma Membrane – The Gatekeeper of the Cell

🔹 Definition:
The plasma membrane forms the outer boundary of the cell, controlling what enters and exits while maintaining cell structure.

🔹 Key Features:
✅ Semipermeable – Allows selective movement of molecules.

✅ Fluid Mosaic Model – Proposed by Singer & Nicolson (1972), describes the membrane as a dynamic lipid bilayer with embedded proteins.

✅ Lipid Bilayer – Made of phospholipids with hydrophilic heads (water-loving) and hydrophobic tails (water-repelling)

✅ Proteins in the Membrane:

• Peripheral Proteins – Aid in cell signaling
• Integral Proteins – Embedded in the membrane
• Transmembrane Proteins – Span across the membrane

🔹 Membrane Transport Mechanisms:

1️⃣ Passive Transport (No Energy Required)

• Diffusion & Osmosis – Movement along a concentration gradient
• Facilitated Diffusion – Carrier proteins help transport large molecules (e.g., glucose)

2️⃣ Active Transport (Energy Required – ATP)

• Na⁺-K⁺ Pump – Moves ions against concentration gradient
• Symport & Antiport – Moves molecules together (same/opposite directions)

🔹 Special Structures & Functions:
📌 Mesosomes – Found in prokaryotes, increase membrane surface area
📌 Aquaporins – Water channels for efficient water transport
📌 Ion Channels – Important in nerve and muscle cells

👉 Why It Matters?
The plasma membrane is vital for cell survival, enabling communication, transport, and protection in all living cells! 🚀

Cell Wall – The Protective Barrier

🔹 Definition:

The cell wall is a rigid outer layer found in bacteria, algae, fungi, and plants (but absent in animals). It provides shape, support, and protection to the cell.

🔹 Cell Wall in Different Organisms:

✅ Bacteria – Made of peptidoglycan, prevents osmotic bursting

• Gram-Positive Bacteria: Thick cell wall, single plasma membrane
• Gram-Negative Bacteria: Thin cell wall, dual plasma membrane
  ✅ Fungi – Made of chitin (a polymer of N-acetylglucosamine)
  ✅ Plants – Made of cellulose (a polymer of glucose)

🔹 Key Features & Functions:

📌 Supports & Shapes the Cell – Maintains structural integrity
📌 Prevents Osmotic Lysis – Stops cells from bursting due to water pressure
📌 Cell-Cell Interaction – Middle lamella (calcium pectate) binds plant cells
📌 Plasmodesmata – Tiny channels in plant cells for communication
📌 Defense Mechanism – Protects from infections and mechanical stress

🔹 Special Structures:

🔸 Glycocalyx (Bacteria Only) – Protective sugar-protein coat

• Slime Layer – Loose, flexible covering
• Capsule – Thick, tough barrier against pathogens

👉 Why It Matters?
The cell wall is crucial for structural support, defense, and communication in various organisms! Stay tuned for an in-depth look at its structure and functions in our upcoming post! 🚀

Endoplasmic Reticulum (ER) – The Cell’s Highway

🔹 What is ER?

The Endoplasmic Reticulum (ER) is a network of membrane-bound tubules and sacs (cisternae) found only in eukaryotic cells. It plays a crucial role in protein synthesis, lipid metabolism, and intracellular transport.

🔹 Types of ER:

✅ Rough Endoplasmic Reticulum (RER) – “Protein Factory”

• Studded with ribosomes on its surface
• Synthesizes proteins for secretion and organelle use
• Transports proteins to the Golgi apparatus for further processing
• Essential for cell communication and secretion

✅ Smooth Endoplasmic Reticulum (SER) – “Lipid Factory”

• No ribosomes; appears smooth
• Synthesizes lipids (phospholipids & cholesterol)
• Plays a role in detoxification (especially in liver cells)
• Stores and regulates calcium ions (Ca²⁺), important for muscle contraction

🔹 Why is ER Important?

📌 Acts as a transport system – Moves proteins and lipids within the cell
📌 Supports cell growth & function – Provides materials for new membranes
📌 Essential for metabolism – Protein & lipid synthesis, detoxification, and calcium storage

Golgi Apparatus – The Cell’s Packaging Hub

What is the Golgi Apparatus?

The Golgi Apparatus (GA) is a membrane-bound organelle responsible for modifying, packaging, and transporting proteins & lipids within the cell. It acts as the shipping and receiving department of the cell.

Structure of Golgi Apparatus

✅ Cisternae – Flattened, stacked pouches (like pancakes)
✅ Cis Face (Receiving Side) – Faces the Endoplasmic Reticulum (ER) and accepts vesicles
✅ Trans Face (Shipping Side) – Sends out processed materials in vesicles to other parts of the cell

Functions of Golgi Apparatus

📦 Modifies Proteins & Lipids – Adds sugars to form Glycoproteins & Glycolipids
🚚 Sorts & Packages Molecules – Sends materials to their correct destinations
📤 Secretes Proteins – Releases materials outside the cell via vesicles
🛠️ Forms Lysosomes – Helps in waste digestion and recycling within the cell

How Does It Work?

1️⃣ Proteins & lipids arrive from the ER in vesicles
2️⃣ Inside the cisternae, they are modified & processed
3️⃣ Packaged molecules exit through the trans face in vesicles
4️⃣ Vesicles transport molecules inside or outside the cell

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Lysosomes – The Cell’s Waste Disposal System

🟢 What are Lysosomes?

Lysosomes are small, membrane-bound vesicles filled with digestive enzymes that help break down waste materials, damaged cell parts, and macromolecules inside the cell. They are known as the “suicide bags” or “recycling centers” of the cell.

🟢 Key Features

🔹 Size – 0.2 – 0.5 microns in diameter
🔹 Membrane – Single-layered, enclosing digestive enzymes
🔹 pH – Works best in acidic conditions
🔹 Found in – Animal cells & some other eukaryotic cells

🟢 Functions of Lysosomes

♻️ Intracellular Digestion – Breaks down food, bacteria, and waste
🛠 Autophagy (Self-Eating) – Recycles damaged organelles & molecules
🛑 Apoptosis (Cell Suicide) – Helps remove damaged cells when necessary
⚠️ Defense Mechanism – Destroys harmful pathogens

🟢 How Lysosomes Work?

1️⃣ Formed from Golgi Apparatus or Endoplasmic Reticulum
2️⃣ Fuses with food vacuoles or damaged cell parts
3️⃣ Releases enzymes to digest & recycle materials
4️⃣ Sends useful components back into the cytoplasm

🟢 Real-Life Impact: Lysosomal Storage Disease

❌ Tay-Sachs Disease – A genetic disorder where lipids build up in brain cells due to a missing digestive enzyme, leading to severe brain damage.

Vacuoles – The Storage Unit of the Cell

What are Vacuoles?

Vacuoles are membrane-bound sacs found inside plant, fungal, and some animal cells. They function primarily as storage compartments for water, nutrients, and waste.

Key Features

✔️ Membrane – Covered by a special membrane called Tonoplast
✔️ Size – Large in plants (occupies up to 90% of the cell), smaller in fungi & animals
✔️ Contents – Stores water, nutrients, waste, pigments, and enzymes

Types of Vacuoles & Their Functions

🟢 Plant Vacuoles

• Central Vacuole – Stores water & maintains cell shape (turgor pressure)
• Lytic Vacuole – Helps in digestion & waste removal
• Protein Storage Vacuole (PSV) – Stores proteins & nutrients
• Storage Vacuole – Holds pigments, toxins, or metabolites

🟠 Fungal Vacuoles

• More complex than plant vacuoles
• Helps in pH regulation, osmoregulation, and digestion

🔵 Animal & Protist Vacuoles

• Food Vacuoles – Engulf & digest food (found in protists)
• Contractile Vacuoles – Helps in excretion & water balance (e.g., Amoeba)

Why are Vacuoles Important?

✔️ Stores essential nutrients & waste
✔️ Provides structural support in plants
✔️ Protects against biotic stress (toxins, pathogens)
✔️ Maintains water balance in cells

Mitochondria: The Powerhouse of the Cell

What are Mitochondria?

Mitochondria are double-membrane-bound organelles found in almost all eukaryotic cells. They generate ATP (the energy currency of the cell) through cellular respiration.

Key Features

✔️ Shape & Size – Typically sausage or rod-shaped, 3.0 – 10.0 μm long.

✔️ Number per Cell – Varies depending on energy demand (some cells have thousands)

✔️ Independent Organelle – Has its own DNA, ribosomes (70S), and RNA, allowing it to synthesize some of its proteins.

Structure of Mitochondria

🟠 Outer Membrane – Smooth, acts as a protective barrier.

🔵 Inner Membrane – Has folds called Cristae, increasing surface area for energy production.

🟢 Intermembrane Space – Region between inner & outer membranes.

🟡 Mitochondrial Matrix – Contains enzymes, DNA, and ribosomes, involved in TCA Cycle (Krebs Cycle) & other metabolic processes.

Functions of Mitochondria

⚡ ATP Production – Generates energy via cellular respiration.

🧬 DNA Replication & Protein Synthesis – Can produce some of its own proteins.

💡 Metabolism – Plays a crucial role in TCA cycle, fatty acid oxidation, and amino acid metabolism.

🧪 Apoptosis (Cell Death Regulation) – Releases cytochrome c, which triggers programmed cell death.

Why is Mitochondria Important?

✔️ Essential for energy production (without ATP, cells can’t function)
✔️ Has its own genetic material (supports Endosymbiotic Theory)
✔️ Plays a role in aging & metabolic diseases

Plastids: The Colored Organelles of Plant Cells

What are Plastids?

Plastids are double-membrane-bound organelles found in plant cells and some protists. They play a major role in photosynthesis, storage, and pigmentation. Plastids are large and visible under a microscope due to their pigment content.

Types of Plastids

1️⃣ Chromoplasts 🟠🔴
✔️ Contain colored pigments (yellow, red, pink, violet)
✔️ Found in fruits, flowers, and aging leaves
✔️ Pigments include:

• Carotenoids (orange, red)
• Xanthophylls (yellow)
  ✔️ Function: Enhances pollination and seed dispersal by attracting pollinators & animals

2️⃣ Leucoplasts ⚪
✔️ Colorless plastids, mainly for storage
✔️ Types of Leucoplasts:

• Amyloplasts – Store starch (e.g., potatoes)
• Aleuroplasts – Store proteins
• Elaioplasts/Lipoplasts – Store oils & fats

3️⃣ Chloroplasts 🌿☀️
✔️ Green plastids, responsible for photosynthesis
✔️ Contain chlorophyll a, chlorophyll b, carotenoids, and xanthophylls
✔️ Found mainly in mesophyll cells of leaves
✔️ Size & Shape:

• Generally lens-shaped, oval, spherical, discoid, or ribbon-like
• 2–4 μm wide & 5–10 μm long
  ✔️ Largest organelles in a plant cell

Structure of Chloroplasts

🟢 Outer & Inner Membrane – Protects and encloses internal components.

🟢 Thylakoids – Flattened membranous sacs that contain pigments.

🟢 Grana (Granum – singular) – Stacks of thylakoids (look like coin stacks)

🟢 Stroma – Fluid-filled space containing DNA, ribosomes (70S), and enzymes.

🟢 Stroma Lamellae – Tubular structures connecting grana.

🟢 Lumen – Space inside thylakoids, crucial for light reactions of photosynthesis.

Functions of Plastids

✔️ Photosynthesis (Chloroplasts) – Convert solar energy into chemical energy.

✔️ Storage of Starch, Proteins & Oils (Leucoplasts).

✔️ Pigmentation (Chromoplasts) – Attracts pollinators & aids in fruit/flower coloration.

Why are Plastids Important?

✔️ Essential for plant survival & metabolism
✔️ Self-replicating (have their own DNA & ribosomes)
✔️ Support Endosymbiotic Theory (similar to mitochondria)

👉 Did you know? Just like mitochondria, chloroplasts have their own DNA & 70S ribosomes, allowing them to produce some of their proteins independently!

Ribosomes: The Protein Factories of the Cell

🔹 What are Ribosomes?

✔️ Ribosomes are membraneless organelles found in both prokaryotic & eukaryotic cells
✔️ They function as protein-synthesizing factories
✔️ Discovered by George Palade in 1955 using an electron microscope

🔹 Structure of Ribosomes

✔️ Composed of ribosomal RNA (rRNA) & proteins
✔️ Made up of two subunits (small & large)
✔️ The subunits remain separate when not actively synthesizing proteins

🔹 Types of Ribosomes (Based on Sedimentation Rate – Svedberg Unit)

📌 Why do mitochondria & chloroplasts have 70S ribosomes?

This supports the Endosymbiotic Theory, which suggests that these organelles originated from ancient prokaryotic cells that were engulfed by a larger cell.

🔹 Location of Ribosomes in the Cell

✔️ Free Ribosomes 🏗️ – Suspended in the cytoplasm, synthesize proteins used inside the cell.

✔️ Bound Ribosomes 📦 – Attached to the endoplasmic reticulum (Rough ER), synthesize proteins for secretion or membrane insertion.

🔹 Function of Ribosomes

✔️ Primary site of protein synthesis.

✔️ Help in polypeptide chain formation by translating messenger RNA (mRNA).

✔️ Ribozymes – Some rRNAs have catalytic activity, aiding in peptide bond formation.

🔹 Fun Fact:

✔️ A rapidly growing mammalian cell contains ~10 million ribosomes! 🧬

Microbodies: Small but Powerful Organelles

🔹 What are Microbodies?

✔️ Small, single-membrane-bound cell organelles
✔️ Found only in eukaryotic cells
✔️ Usually located near the endoplasmic reticulum (ER)
✔️ Involved in metabolism & detoxification

🔹 Types of Microbodies

Microbodies are classified into two types based on their function:
1️⃣ Peroxisomes
2️⃣ Glyoxysomes

Peroxisomes 🧪

✔️ Function: Involved in energy metabolism & detoxification
✔️ Formation: Derived from the ER and replicate by fission
✔️ Morphology: Similar to lysosomes but assembled like mitochondria & chloroplasts
✔️ Key Feature: Lacks its own DNA (unlike mitochondria & chloroplasts)

Peroxisome Functions in Different Cells

🔸 In Animal Cells:

✅ Oxidation reactions (breakdown of fatty acids & amino acids)
✅ Lipid biosynthesis
✅ Alcohol detoxification (liver cells)
✅ Neutralizes hydrogen peroxide (H₂O₂) with catalase

🔸 In Plant Cells:

✅ Fatty acid → Carbohydrate conversion (in seeds)
✅ Photorespiration (in leaves)

Glyoxysomes 🌱

✔️ Specialized peroxisomes found in plants & fungi
✔️ Located in fat storage tissues of germinating seeds
✔️ Increases in number & activity when oil-rich seeds germinate

Functions of Glyoxysomes

✅ Fatty acid oxidation → Converts stored lipids into energy
✅ Glyoxylate cycle → Helps in carbohydrate synthesis
✅ Gluconeogenesis → Produces glucose for the growing seedling

📌 Why are Glyoxysomes Important?
👉 Young seedlings cannot perform photosynthesis immediately, so they depend on glyoxysomes to convert stored fats into glucose until they mature.

👉 Coordination with Mitochondria & Plastids

• Glyoxysomes oxidize fatty acids
• Mitochondria & plastids help convert them into glucose

Key Differences: Peroxisomes vs. Glyoxysomes

Cytoskeleton: The Structural Framework of Cells

🔹 What is the Cytoskeleton?

✔️ A fibrous protein network that maintains cell shape & organization
✔️ Provides mechanical support to the cell
✔️ Plays a crucial role in cell division, movement, and intracellular transport

🔹 Major Components of the Cytoskeleton

The cytoskeleton consists of three main types of filaments, differing in protein composition & size:

1. Microtubules 🌀

✔️ Made of tubulin protein (α & β subunits form a dimer)
✔️ Hollow, rod-like structures composed of 10–15 protofilaments
✔️ Dynamic (can grow & shrink as needed)
✔️ Main Functions:
✅ Provides structural support to the cell
✅ Acts as tracks for intracellular transport of vesicles & organelles
✅ Forms spindle fibers during cell division
✅ Responsible for cilia & flagella movement

2. Actin Filaments 🏋️‍♂️

✔️ Composed of actin protein
✔️ Thin filaments found near the plasma membrane
✔️ Main Functions:
✅ Muscle contraction (present in skeletal muscle)
✅ Cytokinesis (helps in cell division)
✅ Provides mechanical strength to the plasma membrane
✅ Plays a role in cell movement

3. Intermediate Filaments 🏗️

✔️ Strong, rope-like filaments
✔️ Composed of various proteins (e.g., keratin, vimentin, lamin)
✔️ Main Functions:
✅ Provides mechanical strength to the cell
✅ Helps maintain nuclear & cellular integrity
✅ Involved in cell-to-cell connections

Summary: Cytoskeleton at a Glance!

• Microtubules → Intracellular transport, spindle fibers, cilia/flagella movement
• Actin Filaments → Muscle contraction, cytokinesis, cell shape
• Intermediate Filaments → Structural support, mechanical strength

Cilia and Flagella: Cell Movement Structures

What are Cilia & Flagella?

✔️ Microscopic, hair-like projections found on the cell surface
✔️ Function: Provide cell motility & movement of substances around the cell
✔️ Structure: Made up of microtubules with a 9+2 arrangement

Differences Between Cilia & Flagella

Structure of Cilia & Flagella

✔️ Both arise from a basal body (like a centriole)
✔️ Bounded by a plasma membrane (continuous with the cell membrane)
✔️ Axoneme structure:
✅ “9+2” arrangement of microtubules
✅ 9 outer doublets + 2 central singlets enclosed in a sheath
✔️ Microtubules are composed of tubulin protein

Functions of Cilia & Flagella

🔹 Cilia:
✅ Helps in movement of mucus & particles in the respiratory tract
✅ Assists in egg transport in the female reproductive tract
✅ Aids movement in unicellular organisms (e.g., Paramecium)

🔹 Flagella:
✅ Propels sperm cells in mammals
✅ Enables motility in unicellular organisms like Chlamydomonas & bacteria
✅ In bacteria, flagella help in chemotaxis (movement toward nutrients)

📌 Key Takeaways:
✔️ Cilia → Short, numerous, moves fluid/substances
✔️ Flagella → Long, few in number, moves the whole cell
✔️ Both have a “9+2” arrangement of microtubules

Centrosome and Centrioles: The Microtubule Organizing Centers

What is a Centrosome?

✔️ A specialized region near the nucleus in animal cells
✔️ Consists of two centrioles placed perpendicular to each other
✔️ Surrounded by an amorphous pericentriolar material

Structure of Centrioles

✔️ Cylindrical structures made up of microtubules
✔️ Each centriole is composed of 9 triplets of microtubules arranged in a ring-like pattern (9×3 arrangement)
✔️ No central microtubules, unlike cilia/flagella

📌 Key Difference from Cilia & Flagella:
Centrioles have a 9×3 arrangement, while cilia/flagella have a 9+2 arrangement

Function of Centrosome & Centrioles

🔹 Cell Division:
✅ Duplicates in the S-phase of the cell cycle
✅ Moves to opposite poles during mitosis (M-phase)
✅ Organizes the mitotic spindle, which helps in chromosome separation

🔹 Microtubule Organization:
✅ Serves as the microtubule-organizing center (MTOC)
✅ Helps in cytoskeleton arrangement

🔹 Basal Body Formation:
✅ Centrioles act as basal bodies for cilia & flagella formation

📌 Key Takeaways:
✔️ Centrosome = Two centrioles + Pericentriolar material
✔️ Centrioles have a 9×3 microtubule arrangement
✔️ Essential for spindle formation & cell division

Nucleus: The Command Center of the Cell

What is the Nucleus?

✔️ A membrane-bound organelle found in eukaryotic cells
✔️ Acts as the control center for cellular activities
✔️ Contains genetic material (DNA) and regulates gene expression

The Nuclear Envelope

📌 Structure:

✔️ Double-membrane structure that encloses the nucleus
✔️ Outer membrane is continuous with the endoplasmic reticulum (ER)
✔️ Inner membrane is lined with a protein network (lamins) for structural support

📌 Function:

✅ Regulates entry & exit of molecules between the nucleus and cytoplasm
✅ Prevents direct access to genetic material, ensuring controlled gene expression

The Nuclear Pore Complex (NPC)

📌 Structure:
✔️ Composed of nucleoporins (pore-forming proteins)
✔️ Eight structural subunits arranged in a ring around a central channel
✔️ Approximate diameter: 120 nm

📌 Function:
✅ Selective transport of proteins, RNAs, and small molecules
✅ Regulates nuclear-cytoplasmic exchange
✅ Maintains the nuclear environment

Nucleoplasm (Nuclear Sap)

📌 Structure:
✔️ A clear, gel-like fluid inside the nucleus
✔️ Contains nuclear matrix (protein fibrils)
✔️ Suspends nucleolus and chromatin

📌 Function:
✅ Maintains nuclear shape & structure
✅ Provides an environment for DNA replication & transcription

Key Takeaways

✔️ Nucleus = Control center of the cell
✔️ Nuclear envelope = Double membrane for genetic protection
✔️ Nuclear pore complex = Regulates molecular traffic
✔️ Nucleoplasm = Fluid medium for nuclear processes

Nucleolus: The Ribosome Factory

What is the Nucleolus?

✔️ A non-membranous nuclear body located inside the nucleus.

✔️ Acts as the site for rRNA synthesis & ribosome assembly.

✔️ Forms around nucleolar organizing regions (NORs) on chromosomes.

Structure & Features

✔️ Dense, spherical structure found inside the nucleus.

✔️ Lacks a membrane but is compartmentalized for different nuclear processes.

✔️ Composed of proteins, rRNA, and chromosomal DNA regions.

Functions of the Nucleolus

✅ rRNA synthesis – Transcribes and processes ribosomal RNA.

✅ Ribosome assembly – Combines rRNA with ribosomal proteins to form ribosomal subunits.

✅ Gene regulation – Helps in transcriptional regulation & gene silencing.

✅ Cell stress response – Plays a role in DNA repair and cellular stress response.

Nucleolar Organizing Regions (NORs)

✔️ A chromosomal region containing multiple rRNA genes
✔️ Provides instructions for rRNA synthesis
✔️ Essential for nucleolus formation & ribosome production

Key Takeaways

✔️ Nucleolus = Site for rRNA production and ribosome assembly
✔️ No membrane but plays a crucial role in nuclear organization
✔️ Vital for protein synthesis and cellular function

Chromosome: The Genetic Blueprint of Life

What is a Chromosome?

✔️ A thread-like structure formed by the coiling of DNA around proteins
✔️ Found in the nucleus of eukaryotic cells and cytoplasm of prokaryotes
✔️ Contains all the genetic material (DNA) of an organism

Types of Chromosomes

1️⃣ Autosomes (Body Chromosomes): Carry genes for general traits (e.g., height, eye color)
2️⃣ Allosomes (Sex Chromosomes): Determine the sex of an organism (XX in females, XY in males in humans)

✔️ Humans have 23 pairs (46 chromosomes)
✔️ 22 pairs of autosomes + 1 pair of sex chromosomes

Chromosome Numbers in Different Organisms

🔹 Fun Fact: Humans were initially believed to have 48 chromosomes, but in 1956, Joe Hin Tjio corrected it to 46!

Chromosomes in Prokaryotes vs. Eukaryotes

✔️ Prokaryotes (Bacteria, Blue-Green Algae)

• Contain a single circular chromosome in the cytoplasm (called nucleoid)
• Some bacteria like Vibrio cholerae have more than one chromosome

✔️ Eukaryotes (Plants, Animals, Fungi, Protists)

• Chromosomes are present inside the nucleus
• Exist as chromatin (loosely packed DNA) during interphase
• Condense into visible chromosomes during cell division

Chromatin & DNA Packaging

✔️ Chromatin: A complex of DNA + histone proteins
✔️ Nucleosome: The basic unit of chromatin, where DNA wraps around histone proteins
✔️ Compact DNA Packaging: If unwound, all DNA in a human cell stretches to 6 feet!

Importance of Chromosomes

✅ Store genetic information & determine traits
✅ Ensure accurate DNA replication & cell division
✅ Prevent genetic disorders (errors can lead to diseases like Down syndrome)

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