The Cell (Biology): History, Types, Structure, Organelles, Functions

The cell is a structure, containing a mass of liquid surrounded by a semipermeable membrane. cell encloses liquid is called cytoplasm, contains many cell organelles along with nucleus or nuclear material. the main purpose of the cell is to make up all living things.

The cell is functional and Structional unit of life. So cell is considered as the fundamental unit of life. the study of cell structure and function is called cell biology.

The Cell: History, Types, Structure, Organelles, Functions

In cell biology, we understand detailed information of tissues and organisms that cells compose. Some organisms are single-celled (called unicellular Organism) while some are made up of a large number of cells. these organisms are called multicellular organism.

Definition of Cell

A cell can be defined as “Cell is a small compartment that can hold all of the biological equipments which are necessary to keep an organism alive“

1. History of The Cell

Robert Hooke discovered compartments like the structures of the cell wall of cork cambium. These compartments were similar to hollow cavities or empty boxes. Robert Hooke discovered these compartments in 1965 with the help of his microscope.

Robert Hooke coined the term cellula for these compartments. He described his discovery of cellula in his book “Micrographia“. cellula was a later changed into term “cell”.

Another study was done by Grew and Malpighi in the field of cytology. They are also observed small structures in a slice of plants and animals.

Leeuwenhoek was the first person to see free cells. He called them wild animalcules. Leeuwenhoek described his Discovery in “The secret of nature” book.

Leeuwenhoek observed protozoa, sperms, bacteria, RBCs etc under his microscope. That’s why Leeuwenhoek is known as the Father of microbiology.

The Timeline of Cell Biology Discoveries

1595 – The first compound microscope was invented by Jansen
1660– Marcelo Malpighi observed red blood cells in fishtails which were passing through capillaries. he found the connection between arteries and veins in fish
1655 – Robert hook was the first person who saw cells in cork cambium
1668– Francesco Redi saw rotting meat turns into flies. He found that flies can make other flies, but rotting meat can’t. This experiment given an idea about how cells reproduce, cells split to make other cells, which is the third part of the cell theory given by botanist Matthias Schleiden and zoologist Theodor Schwann.
1674 – Leeuwenhoek discovered protozoa. About 9 years later, he saw bacteria. Leeuwenhoek is known as the father of microbiology.
1683 –Discovered Miniature animals- Leeuwenhoek (father of microbiology) made several more discoveries on a microscopic level in his life, He published a letter to the Royal Society in which he explained all the detailed drawings of what he saw during his experiments. Among these, the first protozoa and bacteria were discovered.
1833 – Brown explained the cell nucleus in cells of the orchid plant.
1839 – Cell theory was proposed by botanist Matthias Schleiden and zoologist Theodor Schwann. conclusion of cell theory was, not only plants but animal tissue as well is composed of cells. cell theory ended all the debates that plants and animals were fundamentally different at the level of cellular structure.
1840 – Albrecht von Roelliker discovered that sperm cells (Male Gametes) and egg cells (Female Gametes) are also cells.
1845 -Carl Heinrich Braun reworks the cell theory, and reached to the conclusion that Cell is the basic unit of life.
1855-1856 – N. Pringsheim observed how a sperm cell (male Gamete) penetrated an egg cell (Female Gamete).By this discovery 3rd, part to the cell theory added.
1858 – Rudolf Virchow (physician, pathologist and anthropologist) expounds his famous conclusion: Cells can develop only from existing cells. This discovery clearly explained that cells come from preexisting cells, not from any non-living things.
1857 – Mitochondria was discovered by Kolikar.
1879 – Chromosome behaviour during mitosis was described by Flemming.
1883 – Explained Germ cells are haploid, This was known as chromosome theory of heredity.
1898 – Golgi apparatus (Golgi Body) discovered by the Golgi.
1938 – Differential centrifugation methods used to separate nuclei from the cytoplasm by Behrens.
1939 – First commercial transmission electron microscope by Siemens.
1952 – Gey and coworkers described a continuous human cell line.
1955 – Eagle systematically described the nutritional needs of animal cells in culture.
1957 –The density gradient centrifugation described in caesium chloride solutions for separating nucleic acids by Meselson, Stahl and Vinograd.
1965 – Serum-free medium defined and described by Ham. The first commercial scanning electron microscope by Cambridge Instruments.
1976 – Papers published by Sato and colleagues to show different cell lines require different mixtures of hormones and growth factors in serum-free media.
1981 – Transgenic mice and fruit flies were successfully produced. Mouse embryonic stem cell line described.
1995 – Identification of mutant of GFP with enhanced spectral properties by Tsien.
1998 – Mice were cloned from somatic cells.
1999 –siRNA discovered as part of post-transcriptional gene silencing (PTGS) in plants by Hamilton and Baulcombe.

Cell Theory

French worker H.J. Dutrochet gave the idea of cell theory in 1824. The actual credit of concept of Cell Theory goes to two scientists. One scientist was a botanist and another zoologist. T. Schwann a famous zoologist and M.J. Schleiden if famous botanist where the actual founder of Cell Theory.

Their concept about cell was “All living organisms are composed of cell” T. Schwann & M.J. Schleiden both scientists supported the theory of spontaneous generation. According to them “the new cell arises from the nucleus by budding”

Exceptions to the cell theory

There are exceptions to everything. Likewise, cell theory also has exceptions. Viroids, viruses, Prions do not obey the principles of Cell Theory. Because they are obligate parasite they do not follow cell theory. These organisms are subcellular in nature.

Modifications of Cell Theory

Cell theory modification was done by Rudolf Virchow in 1855. Rudolf Virchow proposed the “law of cell lineage”. According to this principle, cell originates from pre-existing cells (Omnis cellula-e-cellulla). This principle is also known as “cell principle” for “cell doctrine”.

According to this principle

Life exists only in cells.
Membrane-bound cell organelles are unable to live outside the Protoplasm or do not survive alone.
Cells never arise de novo.
The new cells are like the parent in all respect.
Cells show homeostasis nature and remain alive.
The genetic information of any species is stored in deoxyribonucleic acid (DNA) and expressed within the cell.
Cell structure and cell working are controlled by DNA.

The Cell as a Self Contained Unit

Due to the presence of deoxyribonucleic acid (DNA), the autonomy of a cell is believed, otherwise, components of the cell have different shape and functions.

Autonomy in Unicellular Organisms

Unicellular organisms have a totally independent life due to different types of Size, Shape and role of different organelles show division of labour. That’s why all these display homeostases. Unicellular organisms are more active because they have a large surface volume ratio.

Autonomy in Multicellular Organisms

In the case of multicellular organisms, life activities shows by each of the cells independently. Multicellular organisms have an advantage of division of labour over the unicellular organism.

Cellular Totipotancy

Haberlandt suggested the concept of totipotency in the year 1902. Totipotency is the tendency or ability of the cell to divide and Redivide. The cell which has this ability is called totipotent cell and the phenomenon is called totipotency.

Steward et. al. First showed the cellular totipotency in phloem tissues of carrot.

Surface Volume Ratio

Surface volume ratio- the cells which are metabolically active or small in size. They have a higher nucleocytoplasmic ratio for better control and higher surface volume ratio for quicker exchange of minerals between the cell and its outside environment.

Large cells have low surface volume ratio and low nucleoplasmic ratio. When cell size increases surface volume ratio is decreased.

2. Types of Cell

A) Prokaryotic Cell

Prokaryotic cells are the most primitive cells. These are simple in structural organization. Prokaryotic cells have a single membrane system.

Bacteria, Viruses, Blue-green algae, Mycoplasmas, Rickettsias, Spirochetes are prokaryotes which have prokaryotic cells.

The largest and most complex prokaryote organisms are Cyanobacteria or blue-green algae. They have evolved photosynthesis type phenomenon like higher plants. Prokaryotes come under the kingdom Monera and the superkingdom Prokaryota.

The Prokaryotes have the following characters:

The size of prokaryotic cells is in between 1 to 10 µm.
They found in a variety of forms.
The prokaryotic cell consists of three main components:
(I) Outer covering: a) Cell membrane b) Cell wall c) Slimy capsule (II) Cytoplasm, (III) Nucleoid
In some prokaryotic cells, a small circular double-stranded DNA molecule is present. It is called a plasmid. Plasmids encode proteins required by the organism to resist antibiotic and other toxic material. Plasmids have 1000 to 30,000 base pairs.
A whip-like locomotory structure found in many bacteria. This structure is called a flagellum. flagellum is 150Å thick and 10 to 15µm long.flagellum does not have any surrounding membrane because of this, it grows at the tip. Filament and basal body are two main parts a flagellum.
Pili are short, rod-like non-motile processes or fimbriae. These are present in many bacteria. Pili are formed of a protein called pilin. The thickness of Pili usually less than 10 nm. Pili help in attachment of bacteria to surfaces. They also help bacteria to attach to one another also. Some bacteria have Tubular sex Pili.
All the biochemical mechanisms required to synthesize complex organic materials from simple organic precursors necessary for life are present in Prokaryotic cells.

B) Eukaryotic Cells

The eukaryotic cell is more developed than prokaryotic cells because it has a more developed internal organization.

It is believed that eukaryotic cells are evolved from prokaryotic cells. Prokaryotic cells have Double membrane system in their internal structure.

Primary membranes surround the cell. This primary membrane is called the plasma membrane and the secondary membrane surround the nucleus and other cellular organelles called the nuclear membrane.

Protists, fungi, plants and animals are eukaryotes because they are made up of Eukaryotic cells.

Number- The numbers of cells are correlated with the body size in the case of multicellular organisms. The human blood there are about 30 quadrillions (3 × 1015) corpuscles. About 60 × 1015 cells are present in a 60 kg human. Single-celled stage “Zygote” is the beginning of every multicellular organism.
Zygote developed into an embryo by its mitotic division during development. Embryo converted into whole organism by growth and development.
Shape- A eukaryotic cell may be spherical, cuboidal, oval, disc-like, polygonal, columnar, spindle-like or irregular. Due to this, cells acquire a variety of shapes not only in various organisms but also in different tissues of the same organism. The shape of a cell is correlated with its functions in Eukaryotic organisms.muscles and nerve cells shape is well adapted to their functions.
Many factors are also responsible for the shape of a cell. such as cell functions, cell age, presence or absence of cell wall, the viscosity of cytoplasm etc.
Size- Most of the eukaryotic cells are microscopic. That means we cannot see eukaryotic cells by naked eyes. Size of Eukaryotic cells ranges between 10 to 100µm.
Sporozoites of the malaria parasite are among the smallest cells ( 2µm long). Plasmodium vivax is the name of the malaria parasite. Ostrich egg is biggest eukaryotic cell (175 × 120mm). Nerve cells are the longest eukaryotic cell (few meters long). Human cells generally range from 20 to 30µm.
Components of a Eukaryotic cell are Cell membrane, Cytoplasm, Organelles ( Mitochondria, Centrosomes, Golgi bodies, Plastids, Metaplasm, Cilia, basal bodies and flagella, Microtubule, Basal granules, Ribosomes)

Difference between Prokaryotic and Eukaryotic cells

	Prokaryotic Cell	Eukaryotic Cell
1.	A Prokaryotic cell is surrounded by a single membrane layer.	In the case of Eukaryotic, there are two surroundings 1. Plasma membrane 2. Nuclear membrane.
2.	In the case of prokaryotic cells, the cell wall is made up of Carbohydrates, lipids, protein and certain amino acids	The eukaryotic cell wall is made up of chitin and cellulose.
3.	Respiratory enzymes are present on the cell membrane in prokaryotic cells	Respiratory enzymes are absent on the cell membrane in Eukaryotic cells
4.	Thylakoids present in the cytoplasm	Thylakoids occur within the chloroplast
5.	SAP Vacuoles are present in prokaryotic cells	SAP vacuoles are commonly present in Eukaryotic cells
6.	70s ribosome are present free in the cytoplasm or attached to mRNA in prokaryotic cells	80s ribosomes are present either free or bound to the endoplasmic reticulum and nuclear Envelope for mRNA
7.	Endocytosis and exocytosis do not occur in prokaryotic cells	Endocytosis and exocytosis takes place in many protists and in animals
8.	The nucleus is absent in prokaryotic cells. nuclear material lies in the cytoplasm and is called nucleoid, contains a single chromosome.	Well developed nucleus found in Eukaryotic cells which surrounds why nuclear membrane nucleus contains two or more chromosomes
9.	Fertilization, gamete formation and meiosis does not occur in prokaryotic cells	Fertilization, gamete formation and meiosis are well developed in Eukaryotic cells
10.	Prokaryotic cells are haploid	Eukaryotic cells are of both types, diploid and haploid
11.	Nuclear envelope or nuclear membrane is absent in prokaryotic cells	Well developed nuclear membrane or nuclear envelope surrounds the nuclear material
12.	The nucleolus is absent in prokaryotic cells	One or more nucleolus is present within the nucleus
13.	Circular DNA is present without associated proteins	Nuclear DNA is associated with proteins,It is linear while extranuclear DNA is present without proteins.
14.	If present Flagella are simple, consist of a single fibril. Flagella are formed of a protein called flagellin.	If present, Flagella are complex in structure, 9+2 pattern of microtubules formed of the protein called tubulin.
15.	Plasmids and pili are present in many prokaryotic cells.	Plasmids and pili are absent in eukaryotic cells.
16.	Most prokaryotes are reproduced by means of asexual reproduction	Most Eukaryotes are reproduced by means of asexual reproduction.

C) Animal Cell

The animal cell can be defined as “an animal cell is a type of cell that is seen specifically in animals (kingdom Animalia). Absence of cell wall is the main characteristics feature of an animal cell. Animal cell organelles enclosed within the cell membrane. As cell wall is absent, animal cells can be in any shape or size. they are surrounded by a plasma membrane.

Types of Animal Cells

There are hundreds of different types of cells found in animals. We can not mention all these types of cells here. We are going to describe some important types of cell which performs important functions of the body.

1. NERVE CELLS

Nerve cells are the message carriers of animal cells. they deliver and receive signals with the help of dendrites and axons. Dendrites and axons are extensions from the cell. Dendrites and axons receive and export signals to and from the cell, respectively.

The communication system of the body of animals completely depends upon Nerve cells. Nerve cells also are known as neurones. There are around 100 billion nerve cells in only brain Nerve cells are the main cells of the nervous system.

The neurones are consists of two major parts

1. Central body
2. Nerve processes

Nucleus and other organelles are present in Central body of the cell. nerve processes are responsible for carrying the message far and wide inside the body.
Axons or Dendrites are the part of nerve processes.axons can be over 1 meter long in length.

2. FAT CELLS

Fat cells are the main component of adipose tissues found in animal. Fat cells are also known as adipocytes. adipocytes work as a storage of energy. The energy stored in the form of triglycerides. Fat cells get shrink after the use of triglycerides.

Fat cells also produce some hormones such as leptin, adiponectin.
leptin controls appetite, and adiponectin makes the body more sensitive to insulin and helps to controls blood sugar levels.

3. MUSCLE CELLS

Muscle cells are long, tubular cells which are important for a huge range of functions. Muscle cells also called myocytes. Muscle cells are responsible for movement, support, and internal functions, such as peristalsis (the movement of food along the gut)

Muscle cells are of three types

Skeletal muscle cells
Cardiac muscle cells
Smooth muscle cells

Skeletal muscle cells are responsible for making general, conscious movements of the body.

Cardiac muscle cells control the contractions of the heart. Cardiac muscle cells generating electrical impulses to control contractions of the heart.

smooth muscle cells control movements of blood vessels, the uterus, and the stomach.

4. BLOOD CELLS

Blood cells are of two types 1. red and 2. white blood cells and 3.platelets. Red blood cells deliver oxygen from the lungs to the rest of the body.it makes up around 99.9% of all blood cells. These cells are the only animal cells that do not have a nucleus.

A vital part of an animal’s immune system is White blood cell. White blood cells help to kill damaging bacteria and other compounds which are harmful to the animal body. platelets help in blood clotting to prevent blood loss after injury

5. SKIN CELLS

Skin is a more complicated and fascinating organ of animals which made up of different layers and several types of cells. Skin is responsible for keeping the body safe and healthy. skin is your largest organ of the animal body which is the main barrier between the animal body and the environment. The skin protects the body from environmental stress.

The skin mostly consists of two types of cells

Keratinocytes
Melanocytes

The term ‘Cyte’ means cell.

Around 90% of all skin cells are made up of Keratinocytes. Keratinocytes produce keratin protein. The keratin helps to make skin an effective layer of protection for the body. hair and nails of animals are made up of keratin protein.

Melanocytes produces a compound called ‘melanin’ melanin makes skin colour. Melanocytes sit underneath keratinocytes in a lower layer of skin cells. athe melanin transported up to the surface layers of cells. The more melanocytes give darker skin colour.

D) Plant Cell

Plant cells are the basic structural and functional unit of organisms which belongs to kingdom Plantae. Plant cells are eukaryotic cells. Plant cells have a true nucleus along with well-developed cell organelles. Plant cells are able to make their food due to the presence of chloroplast. Plants make foods with the help of water, CO2 in the presence of sunlight.

Types of Plant Cells

1. Parenchyma Cells

There are so many types of cells are present in plants. We are going to discuss important types of cells which are involved in various important functions of a plant. Parenchymatous tissues generally present in root stem leaves flowers fruits and seeds.

Parenchymatous cells are the most simple and specialised. These are mainly concerned with the vegetative activities of the plant. Parenchymatous cells are Isodiametric, having a thin wall, soft living cells.

Parenchyma cells are a distinct nucleus having well developed intercellular spaces vacuolated cytoplasm and cellulose cell wall. The shape of parenchyma cells may be spherical, cylindrical, oval, rectangular and star-shaped.

If Parenchymatous cells enclose large air spaces then they are called as aerenchyma. If parenchymatous cells develop chlorophyll they are called chlorenchyma. If Parenchymatous cells are elongated cells with tapering ends they are called as prosenchyma.

2. Collenchyma

Collenchyma occurs in the hypodermis of dicotyledons stems and leaves. Collenchyma is generally absent in monocot and roots. The term “Collenchyma” was coined by Schleiden in 1839.

Collenchyma are compactly arranged cells which are oval spherical or polygonal in outline. Collenchyma cells are extensible, elastic and have the capacity to expand.

collenchyma cells contain Protoplasm and are living without intercellular space. The cell wall of Collenchyma is thickened at the corner and are made up of cellulose, hemicellulose and pectin.

Collenchyma provides mechanical support to petiole, peduncle, roots and fruits. Collenchyma contains chlorophyll so they help in photosynthesis also.

3. Sclerenchyma Cells

In the beginning, the cells are living and have a Protoplasm, but due to the deposit of impermeable secondary wall, they become dead thick and hard. Now, these living cells are known as sclerenchymatous cells for sclerenchyma.

Sclerenchyma Cells consists of thick-walled dead cells. Sclerenchyma Cells vary in shape size and origin. The term “sclerenchyma” was coined by Mettenius in 1805.

Types of sclerenchyma

Sclerenchymatous fibres
Bast fibres
Wood fibres
Surface fibres
Stone cells or Sclereids

4. Xylem Cells

Complex permanent tissue is a group of more than one type of cells having a common origin. Xylem and phloem are an example of complex permanent tissue. Xylem conducts water and mineral nutrients from the roots to the leaves that’s why xylem is a conducting tissue. The term ‘xylem’ coined by Nageli in 1858.

Xylem composition- composed of four types of cells

Xylem Tracheids
Xylem vessels
Xylem parenchyma
Xylem fibres

Types of xylem on the basis of origin

Primary xylem
Secondary xylem

5. Phloem Cells

Phloem is also a complex permanent tissue which helps in the transport of organic food material from leaves to stem and roots in a downward direction. I Phloem is also known as Leptome.

The term ‘Phloem’ was given by Nageli.

Types of phloem on the basis of the position

External Phloem – It is present outside the xylem.
Intraxylary phloem – It originates from procambium, occurs on the inner side of primary xylem.
Interxylary phloem – It originates from cambium, occurs in groups within the secondary xylem.

Types of phloem on the basis of origin

Primary phloem – Formed by procambium during primary growth.
Secondary phloem- Formed by vascular cambium during secondary growth.

Difference between Animal Cell and Plant Cell

SN	Plant Cell	Animal Cell
1.	Cell wall present in plant cells	The cell wall is absent in animal cells
2.	In plant cells, nucleus usually lies near the Periphery due to vacuole	Nucleus present near the centre in animal cells.
3.	In the case of plant cells, the centrosome is usually Absent from higher plant cells except for lower motile cells.	Centrosome is present that helps in the formation of spindle fibres during cell division.
4.	Plastid are present except Fungi	Plastids are totally absent in animal cells
5.	In plant cells, mitochondria are generally is spherical or oval in shape.	Mitochondria are generally tubular in shape in the case of animal cells.
6.	Single large central vacuole is present in plant cells	Many vacuoles occurs which are smaller in size
7.	Plant cells are capable of forming all the essential amino acids coenzymes and vitamins	Animal cells cannot form all the amino acids enzymes and vitamins which are important for metabolism, growth and development.
8.	There is no contractile vacuole in plant cells	Contractile vacuole may occur to pump excess water in animal cells
9.	Spindle formed during cell division is Anastral spindle.	Spindle formed during cell division, are Amphistral.
10.	Lysosomes present in less number in plant cells	Number of lysosomes are present in animal cells.

3. Cell Structures

A) Plant Cell Structure

B) Animal Cell Structure

SN	Organelles	Plant Cell	Animal Cell
1.	Cell wall	Present	*Absent*
2.	Cell Membrane	Present	Present
3.	Cytoplasm	Present	Present
4.	Chloroplast	Present	*Absent*
5.	Endoplasmic reticulum	Present	Present
6.	Golgi complex	Present	Present
7.	microtubules	Present	*Absent*
8.	mitochondria	Present	Present
9.	Nucleus	Present	Present
10.	Peroxisomes	Present	Present
11.	Plasmodesmata	Present	*Absent*
12.	Ribosomes	Present	Present
13.	Lysosomes	*Absent*	Present
14.	Centrosome	*Absent*	Present
15.	Villi	*Absent*	Present
16.	Flagella	*Absent*	Present
17.	Glyoxysomes	May be present	Absent.
18.	Desmosomes/ Tight junction	*Absent*	Present

5. Cell Divisions

Cells are reproduced with the help of cell divisions. Cell reproduction is the process by which a mature cell divides and forms to nearly equal daughter cells. These daughter cells resemble the parental cell in a number of characters.

Cell cycle- according to Howard and Pelc cell cycle is the sequence of events which occur during cell growth and cell division. Events are collectively known as the cell cycle.
The cell cycle completes in to measure its steps

Interphase
Dividing phase (M phase)

Interphase- Interphase is the period between one cell division to the beginning of the next cell division. Interphase is the resting phase of the cell in which cell not divides.

M phase – M phase is dividing phase of the cell. It in two major steps

Karyokinesis
Cytokinesis

Types of cell divisions

A) Amitosis

This is direct cell division was discovered by Remake in 1855 in RBC of the chick embryo. There is no differentiation of chromosomes and spindle fibres in this type of cell division. The nuclear envelope does not degenerate.

Amitosis is a very primitive type of division which is found in prokaryotes protozoans East foetal membrane of mammals cartilage tissues of mammals

B) Mitosis

Mitosis was first observed by Strassburger in 1875 while studying plant cell. W Flemming first observed mitosis cell division in an animal cell in 1879. The term mitosis was coined by Flemming 1882.

This is also called an indirect cell division. Mitosis is responsible for the growth and development of the body of an organism so it is also called somatic cell division. In this division, chromosomes number is kept constant in daughter cell equal to parents cell. That’s why it is also called equational division.

Mitosis takes place in somatic cells of animals and plants. In plants, mitosis occurs in growth area or meristematic cells such as root Apex and shoot Apex. Mitosis is completed into major steps

I) Karyokinesis- This is the division of the nucleus. Karyokinesis completes in 4 steps-

Prophase
Metaphase
Anaphase
Telophase

II) Cytokinesis- this is the division of cytoplasm. Cytokinesis is done by two methods

Cell furrow method- in animal cell
Cell plate method– in plant cell

C) Meiosis

Meiosis is a slow process of cell division as compared to Mitosis. Mitosis occurs in reproductive cells (Male and Female gametes) in which nucleus divides twice but chromosomes replicate only once to form four haploid cells.

After meiosis division each haploid cell having the half number of chromosomes present in the parents it is also known as reduction division because it is reducing the number of chromosomes as compared to the parent cell.

Process of meiosis is completed in 2 steps – meiosis I and meiosis II

Meiosis I

Chromosome number is reduced by half. Meiosis I result in the formation of two haploid cells from one diploid cell or parent cell. Meiosis I completed into two steps

Karyokinesis I – karyokinesis I is involvement of division of cell nucleus. Karyokinesis is divided into four phases

Prophase I- Laptop, zygotene, pachytene, diplotene & diakinesis
Metaphase I- chromosomes came on the equator
Anaphase I- separation of homologous chromosomes
Telophase I- two daughter nuclei are formed

Cytokinesis I – Cytokinesis sometimes may not be present. If present, it occurs why cell furrow formation in animal cell and cell plate formation in plant cell

Meiosis II

This is the homotypical division. There is no change in the number of chromosomes after meiosis I. This is same as mitotic division. Completed in two steps karyokinesis II and cytokinesis II.

Difference between Mitosis and Meiosis

Mitosis takes place in all types of cells whereas news is usually occurring in reproductive cells
Mitosis may continue throughout the life of plant and animals whereas music takes place at the time of reproduction only.
In the case of Mitosis nucleus undergoes a single division whereas in the case of meiosis nucleus undergoes two divisions first is reductional division and second is equational.
The number of chromosomes not reduced after completion of division whereas the number of chromosomes reduced (half) after completion of meiosis division.

Conclusion

Robert Hook first time described the texture of a piece of cork as “cell” in 1665. Similar structures were also observed in many living organisms by many scientists.

Schwann T. stated that all living organisms are composed of cells after examining a variety of plant and animal tissues in 1839.

Basically two types of cells “Prokaryotic” and “Eukaryotic” are there. Prokaryotic cells are the primitive cells as compare to eukaryotic. Prokaryotic cells include bacteria, blue-green algae, viruses and photosynthetic cells cyanobacteria etc.

Prokaryotic cell size varies from 1 to 10 um. they consist of mainly three components: 1) the outer covering ( cell membrane, cell wall and a slimy capsule).

2) Cytoplasm 3) Nucleoid lacks a nuclear membrane. the plasmid (Additional small circular DNA) may also be present.

In some prokaryotic cells, Flagella and pili like structure are also present. Eukaryotic cells are more developed and are surrounded by double membranes.

Shape, size and number vary in multicellular organisms. It is also composed of three main components.

The cell membrane is a thin elastic living covering.

The cytoplasm of a cell is a semi-fluid, homogenous, translucent consisting of many cell organelles, inclusions, cilia, flagella, basal bodies and microtubules.

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