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Cell and cell reproduction

Created Date @December 24, 2024
Contents

Discovery of Cells
What is a Cell?
Key Characteristics of Cells
Cell Size and Scale
Types of Cells
1. Prokaryotic Cells
2. Eukaryotic Cells
Comparison of Prokaryotic and Eukaryotic Cells
Cell organelles
Major Cell Organelles and Their Functions
Cell Division
Cell cycle
Interphase
Mitotic phase
Meiosis
Meiosis I
Meiosis II

Discovery of Cells
Cell was first discovered and named by Robert Hooke in 1665. He saw the dead cell walls of a cork as it appeared
under a microscope.

Robert Hooke first observed cells in 1665 while

examining a thin slice of cork under a microscope. He
named these structures "cells" because they reminded
him of small rooms (cells) that monks lived in. The
term cell is derived from a latin word ‘cella’ meaning
small room.
Using a primitive microscope he built himself, Hooke
saw a honeycomb-like structure of tiny boxes in cork
tissue. What he actually observed were the cell walls
of dead plant cells, as cork is composed of dead plant
tissue.

🔬 Historical Note: Hooke documented his

observations in "Micrographia," which included
detailed drawings of his microscopic
observations. This work laid the foundation for
future cell biology studies.

Hooke’s cells when he views cork under 🔬

What is a Cell?
A cell is the basic structural and functional unit of all living organisms. It is the smallest unit of life that can
replicate independently. Both living and non-living beings are made up of molecules made from chemical

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 elements such as carbon, hydrogen, oxygen and nitrogen. The organisation of these molecules into cells is one
feature that distinguishes living things from all other matter.

Key Characteristics of Cells

Cell membrane - A protective barrier that regulates what enters and leaves the cell

Cytoplasm - A gel-like substance where cellular components are suspended

Genetic material - DNA/RNA that contains instructions for cell function

Organelles - Specialized structures that perform specific functions

Types of Cells
1. Prokaryotic Cells
Simple cells without a nucleus, found in bacteria and archaea.

2. Eukaryotic Cells
Complex cells with a membrane-bound nucleus, found in
plants, animals, fungi, and protists.

Cell Size and Scale 💡 Fun Fact: The human body contains approximately
Most cells are microscopic, ranging from 1-100 37.2 trillion cells!
micrometers in diameter, though some
specialized cells can be larger.

Comparison of Prokaryotic and Eukaryotic Cells

Feature Prokaryotic Cells Eukaryotic Cells

Cell Size Typically 0.1-5 µm Typically 10-100 µm

Nucleus No true nucleus Membrane-bound nucleus

DNA Structure Single circular chromosome Multiple linear chromosomes

Membrane-bound Organelles Absent Present

Cell Division Binary fission Mitosis/Meiosis

Ribosomes 70S type 80S type

Examples Bacteria, Archaea Plants, Animals, Fungi

🧬 Note: The structural differences between prokaryotic and eukaryotic cells reflect their evolutionary history
and functional requirements. Eukaryotic cells are generally more complex and specialized.

Feature Plant Cell Animal Cell

Cell Wall Present (cellulose) Absent

Chloroplasts Present Absent

Vacuole Large, central vacuole Small vacuoles if present

Shape Fixed, rectangular Irregular, round

Storage Starch Glycogen

Lysosomes Rare or absent Present

Centrioles Absent Present

🔍 Note: These differences reflect the distinct functions and survival needs of plants versus animals. For
example, plant cells have cell walls for structural support and chloroplasts for photosynthesis, while animal
cells need flexibility for movement.

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 Cell organelles
Cell organelles are specialized structures within cells that perform specific functions, much like organs in the
human body. Each organelle has a unique role in maintaining cellular health and function.

Major Cell Organelles and Their Functions

Cella membrane or plasma membrane: It is a biological membrane that separates the interior of the cells from
outside environment. It is selectively permeable to ions and organic molecules and controls the movement of
substances in and out of the cell. It consists of phospholipid bilayers, proteins and carbohydrates. It is
involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signalling.

Cytoplasm: the fluid inside the membrane is called as cytoplasm. It is composed of water, salts and organic
molecules. Plat, fungi and bacterial cells have an extra structural layer outside the cell membrane called cell
wall.

Nucleus: Large often rounded organelle. All plants and animal cells have nucleus which contain a copy of the
DNA of the organism. It is the control center of the cell that contains genetic material (DNA) and directs
cellular activities. The chemical codes on the DNA are codes to produce every protein. The Eukaryotic genetic
material is divided into different linear molecules called chromosomes insides the nucleus. Eukaryotic cells
contain one nucleus which contains genome(complete set of genetic or hereditary information of an
organism).

📌 Gene Sequencing: It is the process of determining the precise order of nucleotides within a DNA
molecule. Modern sequencing methods include:

Next Generation Sequencing (NGS): Allows rapid sequencing of large amounts of DNA

Sanger Sequencing: Traditional method used for smaller DNA fragments

Nanopore Sequencing: Real-time analysis of DNA sequences by passing DNA through tiny pores

Mitochondria: Double membrane bounded

organelle present in all the eukaryotic cells. They
process macro molecule or food to obtain energy
through aerobic respiration. They have their own
DNA and ribosomes in their matrix. The cellular
energy(Adenosine Triphosphate or ATP) is made
here. It converts the chemical bond energy in food
into ATPs. Aka powerhouse of the cell.

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 Endoplasmic Reticulum (ER): It is a system of membranous tubules and sacs. The primary function is to act
as an internal transport system allowing molecules to move from one part of the cell to another.

Rough ER: Studded with Ribosomes involved in protein synthesis. It is an extension of the nuclear
envelope which allows the mRNA to be transported to Ribosomes where they are translated to proteins

Smooth ER: It has no ribosomes. It is nvolved in lipid and steroid synthesis, regulation of the calcium level
in muscles and detoxification in liver cells

📌 Protein synthesis:
graph TD

B[DNA]
Protein synthesis is the process by which cells build
proteins. It occurs in two main stages: C[RNA]
D[Ribosomes]
1. Transcription: DNA is used as a template to
E[tRNA]
create messenger RNA (mRNA) in the nucleus
F[Amino Acids]
2. Translation: mRNA travels to ribosomes where it
is decoded to create specific proteins B[DNA as genetic blueprint]
B --> C[RNA for carrying genetic
The process requires:
C --> D[Ribosomes as protein-bui
DNA as the genetic blueprint D --> E[tRNA to transport amino
RNA for carrying genetic information E --> F[Amino acids as protein b

Ribosomes as protein-building factories

tRNA to transport amino acids

Amino acids as protein building blocks

This process is essential for cell growth, repair, and

the production of enzymes and other vital proteins.

Golgi Apparatus: Processes(maturation and modification) and packages proteins and lipids into vesicles for
distribution in a small bag-like structure. These structures are made up of biological membranes and are
called as transport vesicles. Aka post office of the cell.

Lysosomes: Small spherical organelle which contain digestive enzymes that break down cellular waste and
foreign materials. Aka suicide bags of the cells and are common in animal and fungi cells but are rare in plant
cells.

Ribosomes: Sites of protein synthesis

Vacuoles: Storage compartments for water, nutrients, and waste products

Chloroplasts: Found in plant cells, responsible for photosynthesis

Plastids: These are organelles found primarily in plant cells. The main types include:

Chloroplasts: Green plastids that carry out photosynthesis. Chloroplasts are structured with: A double-
membrane envelope, Thylakoids stacked into grana, Stroma containing carbon-fixing enzymes, Light-
capturing chlorophyll in thylakoids, DNA and ribosomes for protein synthesis within the chloroplast

Chromoplasts: Contain pigments that give fruits and flowers their colors

Leucoplasts: Colorless plastids that store nutrients like starch, proteins, and lipids

Amyloplasts: Specialized leucoplasts that store starch, particularly in roots and tubers

Peroxisomes:Peroxisomes are small, membrane-bound organelles that contain enzymes (including many
oxidizers and at least 50 enzymes) involved in various metabolic processes. Their main functions include:

Breakdown of lipids, fatty acids, ethanol, and amino acids through oxidation, which produces H₂O₂

Detoxification of harmful substances, especially in liver cells

Production and breakdown of hydrogen peroxide

Synthesis of specialized lipids needed for nerve cell insulation

Organization and Interaction

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 These organelles don't work in isolation but form an interconnected network within the cell. They communicate
and interact with each other through various cellular pathways and mechanisms to maintain proper cell function.

⚡ Important: Each organelle works in coordination with others to maintain cellular homeostasis. The
dysfunction of any organelle can lead to cellular problems and diseases.

Cell Division
Cell division is a very important process in all living organisms. During the division of cells the cell growth takes
place along with replication of DNA and other organelles all these processes have to take place in a coordinated
way to ensure correct division and formation of cells.

Cell cycle
A highly coordinated and regulated sequence of events during which a cell undergoes systematic changes to
replicate its genetic material (DNA), duplicate its essential organelles and cellular components, and ultimately
complete the process of division to produce two daughter cells. This carefully orchestrated process ensures
accurate distribution of cellular contents and maintains the integrity of genetic information.

Interphase
Interphase is the longest phase of the cell cycle, during which the cell grows and prepares for division. Resting or
the non-mitotic phase of the cell cycle. It consists of three main stages:

G1 Phase (First Gap): The cell grows in size and produces more proteins and organelles. The cell also checks
if conditions are suitable for DNA replication.

S Phase (Synthesis): DNA replication occurs during this phase. The cell makes an exact copy of all its genetic
material.

G2 Phase (Second Gap): The cell continues to grow and produce proteins. It checks if DNA replication was
successful and prepares for cell division.

During interphase, although the cell appears to be "resting" when viewed under a microscope, it is actually
very active at the molecular level, carrying out essential functions and preparations for division.

Mitotic phase
There are two kinds of cell division—mitosis and meiosis. Mitosis is a process by which new cells are generated. It
is responsible for producing somatic cells. While meiosis is the process by which gametes are generated for
reproduction. It is responsible for producing germ cells or sex cells. The mitotic phase is the active division phase
of the cell cycle where one cell divides into two identical daughter cells. It consists of several stages:

Prophase: Chromatin condenses into chromosomes, nuclear membrane breaks down, nucleolus also
disappears, and mitotic spindle fibers begin to form at the opposite poles of the cell.

Metaphase: Chromosomes align at the cell's equator (metaphase plate), attached to spindle fibers.

Anaphase: Sister chromatids separate and move to opposite poles of the cell. At the end of anaphase a
complete set(46) of daughter chromosomes is found at each pole.

Telophase: Nuclear membranes reform around the separated chromosomes, spindles disappear and
chromosomes begin to de-condense.

Following mitosis, cytokinesis occurs - a cleavage furrow is formed by a contractile ring and eventually leads to
the physical division of the cell's cytoplasm to form two separate diploid daughter cells.

Each daughter cell receives an identical set of chromosomes, ensuring genetic continuity. The entire process
is highly regulated to prevent errors that could lead to cellular abnormalities.

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 Meiosis
Meiosis is a specialized type of cell division which involves not one but two cell division that produces gametes
(sex or germ cells) with half the normal number of chromosomes. This process is crucial for sexual reproduction
and genetic diversity. Unlike mitosis, which produces two identical diploid cells, meiosis produces four unique
haploid cells.

Reduction Division: Reduces chromosome number from diploid (2n) to haploid (n)

Genetic Recombination: Creates genetic diversity through crossing over of chromosomes

Two Divisions: Consists of Meiosis I and Meiosis II

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 Homologous Pairs (Homologues):

Pairs of chromosomes with the same gene sequence, structure, and length

One chromosome comes from the mother, the other from the father

They carry genes for the same traits but may have different versions (alleles)

In humans, there are 23 pairs of homologous chromosomes (22 autosomes + 1 sex chromosome pair)

During meiosis, homologous pairs:

Line up side by side in a process called synapsis

Exchange genetic material through crossing over

Separate during Anaphase I, ensuring genetic diversity

Meiosis I Meiosis II
The first division is unique and includes: The second division is similar to mitosis:

Prophase I: Homologous chromosomes pair up and Prophase II: Chromosomes condense in

exchange genetic material (chiasmata ot crossing over) each cell

Metaphase I: Paired chromosomes align at the equator Metaphase II: Chromosomes align at the
equator
Anaphase I: Homologous chromosomes separate to
opposite poles Anaphase II: Sister chromatids separate

Telophase I: Two cells form, each with half the original Telophase II: Four haploid cells are formed
number of chromosomes

The process of meiosis is essential for maintaining

the correct chromosome number across
generations and introducing genetic variation in
offspring through crossing over and independent
assortment.

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