Cell Biology & Cell division for NTA NEET

Cell Biology & Cell Division 

                                                                    By

                                                  T.Arzoo (MBBS Student)

NBMC&H , Darjeeling 

Mock Test 👍

Link:  https://docs.google.com/forms/d/e/1FAIpQLSc4ikWIbaYX2dN9__N_fby0GeoVHTOxPALm53979FprVXoCzg/viewform?usp=sf_link

Someone important points:

Cell Biology 

• Study of form, structure, and composition of cell is called cytology.
• Cell is the structural and functional unit of life. In unicellular organism (amoeba, paramecium, yeast, bacteria) single cell performs all the essential functions of life.
• In multicellular organism, different kinds of tissues perform different function and have division of labour.
• Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus.

• Metthias Schleiden and Theodore Schwann( 1938) proposed the cell theory which was later modified by Rudolf Virchow(1855)-

1. All living organisms are composed of cells and products of cells.

1. All cells arise from pre-existing cells.

Prokaryotic cells	Eukaryotic cells
Membrane-bound nucleus is absent. Cells are smaller in size. Single chromosome is present. Membrane-bound organelles are absent.	Membrane-bound nucleus is present. Cells are larger in size. More than one chromosome is present. Membrane-bound organelles are present.

Shape and size of cells varies greatly according to their position and function. Mycoplasma is the smallest cell and largest isolated cell is the ostrich egg. The shape of cell may be cuboid, columnar, polygonal, thread-like or irregular.

Prokaryotic Cells 

• Prokaryotic cells are represented by Bacteria, Blue-green algae, Mycoplasma and PPLO. They multiply rapidly and vary in size greatly.
• Bacterial cells may be Bacillus (rod-shaped), Coccus (spherical), Vibrio (comma-shaped) and Spirillum (spiral).
• All prokaryotic cells have cell wall surrounding the cell membrane except in Mycoplasma. Genetic material is naked.
• The plasmid DNA, in some bacteria, provides some special features like resistance to antibiotics.
• Cell organelles like Mitochondria, Golgi bodies etc. are absent in prokaryotes. A specialized differentiated cell membrane called Mesosome is the characteristic of prokaryotes.

• In bacterial cell, a chemically complex cell envelope is present, which consist of three layers. The outermost is Glycocalyx, middle one cell wall and inner innermost is the cell membrane.
• Glycocalyax may be as loose sheath in some bacteria called slime layer. In some other bacteria, Glycocalyx may be thick and tough called capsule.
• Plasma membrane is semi-permeable having mesosome in the form of vesicles, tubules, and lamellae. They help in cell wall formation, DNA replication and distribution to daughter cells.
• Motile bacterial cell contains flagella, which is composed of filament, hook and basal body. Pili and fimbriae are the other surface structures that help the bacteria to attach with host and other substances.
• In prokaryotes, ribosome are attached with cell membrane having two sub-units – 50S and 30S to form together 70S prokaryotic ribosomes.

• Ribosomes are site of protein synthesis. Ribosomes attached with mRNA to form a chain are called polyribosomes.

• Reserved materials in prokaryotic cells are present in cytoplasm as cell inclusion bodies, which may contain phosphate, granules, glycogen granules etc.
• Gas vacuoles are found in blue-green algae and purple and green photosynthetic bacteria.

Eukaryotic Cell

• Eukaryotic cells are present in Protista, Plants, Animals, and Fungi. Cytoplasm is divided into compartments due to presence of membrane-bounded organelles.
• The cells contain well-organized nucleus with nuclear membrane. The genetic materials are arranged in chromosomes.
• Plants cells differ in having cell wall, plastids, and large central vacuole as compared to animal cells. Animal cells have centrioles, which are absent in plant cells.

Plant cell

Cell Organelles

Eukaryotes possess many cellular organelles that perform various functions. Some of these organelles, such as the cell wall, are exclusive to plant cells.

• Cell Membrane
• Cell wall
• Mitochondria
• Plastids
• Ribosomes
• Cytoskeletons
• Centrosome
• Centrioles
• Nucleus
• Microbodies
• Cilia and Flagella
• Endomembrane systems which consist of the endoplasmic reticulum and Golgi apparatus. Plant cells have centrally placed vacuoles.

Animal cell

• Cell membrane is composed of lipids that are arranged in bilayer. The lipid component is mainly composed of phosphoglycerides. Later it was found that protein is also present in cell membrane. Ratio of protein and lipids varies in different cells.
• Membrane protein may be integral or peripheral. Integral protein remains buried in membrane but peripheral protein lies on the surface.
• Singer and Nicholson (1972) proposed fluid mosaic model. According to this model, the quasi-fluid nature of lipid enables lateral movement of protein within the bilayer of lipids.

• The main function of plasma membrane is the transport of molecules across it.

Active Transport	Passive Transport
The transport involves an expenditure of energy by the cells. It occurs against the concentration gradient. It is a rapid process.	The cells do not spend energy in passive transport. This transport is always along the concentration gradient. It is comparatively slow process.

• The movement of water from higher concentration to lower concentration by diffusion is called osmosis.
• Cell wall is present in plant cells and fungi. Algae have cell wall made up of cellulose, galactans and minerals like calcium carbonate. In other plants, it consists of cellulose, hemicellulose, pectin, and proteins.
• Primary cell wall of young plant is capable of growth, which diminish in mature cells. Secondary cell wall is formed on inner side of the cells.
• Plasmodesmata connects the cytoplasm of neighboring cells.
• Endomembrane system of cell includes endoplasmic reticulum, golgi complex, lysosomes and vacuoles.

• Endoplasmic Reticulum are the tubular structure scattered in the cytoplasm.

1. Rough endoplasmic reticulum bears ribosomes on its surface. RER is involved in protein synthesis and secretion.
2. Smooth endoplasmic reticulum does not bear ribosomes on its surface. SER is involved in lipid synthesis and steroidal hormones.

• Golgi apparatus was first observed by Camillo Golgi in 1898 near nucleus. They consist of many flat, disc-shaped sacs or cisternae stacked parallel to each other.
• Golgi apparatus performs the function of packaging of materials and its transportation. A number of protein synthesized by ribosomes are modified in cisternae of Golgi apparatus. Golgi apparatus is the site for synthesis of Glycoproteins and glycolipids.

• Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus. They are rich in hydrolytic enzymes- lipase, protease, carbohydrases active at acidic PH. These enzymes are capable of digesting carbohydrates, proteins, lipids, and nucleic acids.
• Vacuoles are membrane-bound space found in cytoplasm containing water, sap and excretory product. They are bound by single membrane. They form contractile vacuole and food vacuole in many organisms.
• Mitochondria is double membrane-bound structure with the outer membrane and inner membrane dividing its lumen in two compartments. The inner membrane forms a number of infoldings called cristae towards the matrix.

• Two membranes have their own specific enzyme.
• Mitochondria are sites for aerobic respiration. They produce cellular energy in form of ATP so, they are called power house of the cells. The matrix of mitochondria also contain circular DNA molecules, a few RNA molecules, ribosomes and components of protein synthesis.
• Plastids are found in plant cells and in Euglenoids.

• Chloroplast contains chlorophyll that traps solar energy for photosynthesis. Chromoplast provides yellow, orange and red colours to different parts of plants.
• Leucoplasts are colourless plastids that store food, amyloplasts (carbohydrates), elaioplasts (oils) and aleuroplasts (proteins).
• Chloroplasts are double membrane structures. The space limited by inner membrane is called stroma. Thylakoids are present in stroma as stacks like the piles of coins called grana.

• Stroma contains enzymes for synthesis of protein and carbohydrates. Double strand circular DNA and ribosomes are also present in stroma.
• Eukaryotic cells have 80S ribosomes. They have granuler structure with two subunits.
• Centrosome is an organelles containing two cylindrical structures called centrioles. Each centrioles is made up of 9 fibrils of tubulin protein. Central part of centriole is called hub and peripheral fibrils are called spokes.
• Nucleus has highly extended, elaborate nucleoprotein fibers called chromatin, nuclear matrix, and nucleoli. The outer membrane is continuous with endoplasmic reticulum and bears ribosomes.
• The chromatin materials change into chromosome during active cell division. It consists of DNA and histone proteins.
• Every chromosome has a primary constriction or the centromere, on the sides of which disc-shaped kinetochores are present.

• On the basis of position of centromere, chromosomes are of following types-

Some chromosomes have non-staining secondary constriction at certain location. This gives a small fragment called satellite.

——————————————————————————————————————————

Cell Division :

There are two types of cell division: mitosis and meiosis. Most of the time when people refer to “cell division,” they mean mitosis, the process of making new body cells. Meiosis is the type of cell division that creates egg and sperm cells. ... Mitosis and meiosis, the two types of cell division.

Cell Cycle 

Cell Cycle and Cell Division

Table of Contents

• Define Cell Cycle
• Interphase
• Mitosis
• Meiosis
• Meiosis I
• Meiosis II

Define Cell Cycle

Cell Cycle is defined as the series of changes that occur in a cell which leads to division of cells into two daughter cells. It takes about 1 hour to complete the cycle.

Cell Cycle is divided into two phases - Interphase and Mitosis

Interphase

It is a phase in which a cell prepare itself for cell division. It is further divided into following phases:

G1 phase is also known as First Gap Phase. During this phase, biosynthetic activities occur at a very fast rate. Cell synthesizes more proteins, increase the number of mitochondria and ribosomes.

S phase is the phase where DNA is replicated. At the end of DNA replication, each chromosome is with two sister chromatids. Thus, the amount of DNA gets doubled during this phase, but the ploidy remains same.

G2 phase is the phase where the cell prepare itself for mitosis.

Mitosis

Also, known as Equational Division because, the number of chromosomes will remain same in the daughter cell as present in parent cell. It is a phase where nuclear division occurs. This is known as Karyokinesis. It is divided into following phases:

Prophase

During this phase chromatin condenses to form chromosomes.There are two identical copies of each chromosome which are attached to centromere. During the end of the prophase, nucleolus dissolves. Nuclear membrane also disintegrates at the end. The centrosome moves to the opposite poles. Spindle fibers starts to appear.

Fig. 2. Prophase of mitosis

Metaphase

During this phase, chromosomes aligned themselves on the equatorial plate. The analysis of the metaphase chromosome is used in cytogenetics and cancer studies.

Fig. 3. Metaphase of mitosis

Anaphase

It is the shortest stage of the cell cycle. During this phase, the replicated chromosomes move apart and daughter chromatids moves to the opposite poles. Chromosomes are maximally condensed during late anaphase. The chromosomes appear Y- shaped as they move to the opposite poles.

Fig. 4. Anaphase of mitosis

Telophase

This is where two daughter nuclei are formed. Nucleolus and nuclear membrane re-appear. The late telophase is marked by cytokinesis. This is last phase of mitosis.
 

Fig. 5. Telophase of mitosis

Cytokinesis

The division of cytoplasm, organelles, and cell membrane to form two cells is known as Cytokinesis. It divides the cell into two daughter cells which are identical to the parent cell.
 

Fig. 6. Cytokinesis

Cell Cycle Exit

Some cells divide at a fast rate whereas some divides slowly and some even do not divide once they are formed. Those cells which do not divide once formed, enter into a phase known as GO phase. For Example, neuronal cells once formed will not divide.

Meiosis

Also, known as Reductional Division because it reduces the number of chromosomes to half the parent cell. It is of two types meiosis I and meiosis II. Gametes such as sperm or egg under meiosis.

Meiosis I

It is divided into following phases:

Prophase I

It is the longest phase of meiosis I. During this phase, homologous chromosomes pairs and DNA segments are exchanged (known as recombination). It is further divided into following-

Leptotene

• It is the first stage of meiosis.
• Each individual chromosome exists with two sister chromatids.
• Elements of synaptonemal complex assemble.
• Condensation and coiling of chromosomes occur.

Zygotene

• Chromosomes align as homologous pair of chromosomes.
• Synapsis of homologous chromosomes occur.
• The paired chromosomes are known as Bivalent or Tetrad.

Fig. 7. Different stages of prophase I of meiosis I

Pachytene

• During this stage, crossing over and homologous recombination occurs.
• Chiamata is formed where homologous chromosomes exchange there segments.

Diplotene

• Homologous chromosomes start to separate.
• Synaptonemal complex disassemble.
• Chromosomes remain bound at the chiasmata.

Diakinesis

• Chromosomes condenses further, so that four parts of the tetrads are visible.
• The nucleoli disappear, nuclear membrane disintegrates.
• Mitotic spindle starts to form.

Metaphase I

Homologous chromosomes are aligned on the metaphasic plate. The replicated chromosomes are joined together via protein known as Cohesin.

Fig. 8. Metaphase I

Anaphase I

Kinetochore microtubules shorten, pulling homologous chromosomes to opposite poles. The cohesin (protein complex) from the chromosome arms is degraded while the cohesin surrounding the centromere remains safe. This allows the sister chromatids to attach together while homologs are segregated.

Fig. 9. Metaphase I

Telophase I

Each daughter cell now has half the number of chromosomes as compared to parent cell. The microtubules that forms the spindle starts to disappear. The chromosomes again forms chromatin. Sister chromatids remain attached during telophase I.

Fig. 10. Telophase I

Meiosis II

Meiosis II is the second meiotic division. It is similar to mitosis, but the genetic results are different. It results in the formation of four haploid cells from the two haploid cells produced in meiosis I.

The four main steps of Meiosis II are as follows:  Prophase II, Metaphase II, Anaphase II, and Telophase II.

In prophase II nucleoli and nuclear membrane disappears. Shortening and thickening of the chromatids also occur during this phase. Centrosomes move to the poles and spindle fibers are formed for the second meiotic division.

Fig. 11. Different stages of meiosis II

In metaphase II, the centromeres are present with two kinetochores that are attached to spindle fibers from the centrosomes at opposite poles. The metaphasic plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.

Anaphase II is accompanied by sister chromatid segregation. The remain of the cohesin is degraded to allow sister chromatid segregation.

Telophase II, which is like telophase I, de-condensation of chromosomes occurs. Nuclear envelopes reorganize and cleavage or cell plate formation starts to produce four daughter cells, each with a haploid set of chromosomes.

—————————————————————————————————————————————————-