Nitrogen metabolism (General)

 Click here for ppt

Click here for study material in Bengali

Enzymes (General)

 Click here for ppt

Click here for study material in Bengali

Carbohydrate Biochemistry (Part II)

To access and download PowerPoint presentation on 'Carbohydrate Biochemistry' click on the link below:

Download slides

 

Disaccharides

Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other.

Example: maltose, lactose, and sucrose

Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid.

N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides.

General formula: Cₙ(H2O)ₙ₋₁

The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s).

When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a non-reducing sugar.

The end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.

The disaccharide maltose contains two D-glucose residues joined by a glycosidic linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other.

Because the disaccharide retains a free anomeric carbon (C-1 of the glucose residue on the right), maltose is a reducing sugar.

Nomenclature of disaccharides (or oligosaccharides)

By convention, the name describes the compound with its nonreducing end to the left.

Give the configuration (α or β) at the anomeric carbon joining the first monosaccharide unit (on the left) to the second.

Name the nonreducing residue; to distinguish five- and six-membered ring structures, insert “furano” or “pyrano” into the name.

Add suffix '-syl' to the name of the first residue. For example, 'glucopyranosyl'.

Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers; for example, (1🠊4) shows that C-1 of the first-named sugar residue is joined to C-4 of the second. 

If the anomeric carbons from both residues are involved in bond formation, a double headed arrow is given.

Name the second residue similarly. Add suffix '-ose' to the name of the second residue in case of reducing sugars and '-oside' in case of non-reducing sugars.

If there is a third residue, describe the second glycosidic bond by the same conventions.

Short name:Glc(α1🠊4)Glc

Sucrose

Sucrose (cane sugar) is made up of α-D-glucose and β-D-fructose linked by a glycosidic bond (α1 ⟺ β2). 

The reducing  groups (anomeric carbon) of both glucose and fructose are involved in glycosidic bond formation. Hence, sucrose is non-reducing sugar and it cannot form osazones.

The systematic name of sucrose is α-D-glucopyranosyl-(1 ⟺ 2)- β-D-fructofuranoside. 

This indicates:

It is composed of two monosaccharides: glucose and fructose

Ring type: Glucose is pyranose and fructose is furanose

Linkage: oxygen on C1 of α-D-glucose  is linked to C2 of β-D-fructose 

Suffix '–oside' and '⟺' indicates that the anomeric carbons of both the monosaccharides participate in glycosidic bond formation

Sucrose is a major carbohydrate produced in photosynthesis. It has the advantage as storage and transport as its functional groups are held together and are protected from oxidative attacks.

Intestinal enzyme, sucrase hydrolyze sucrose to glucose and fructose.

Inversion of sucrose:

Sucrose is dextrorotatory (+66.5°). But when hydrolyzed, it becomes levorotatory (-28.2°). The process of change in optical rotation from dextrorotatory(+) to levorotatory (-) is referred to as inversion. The hydrolyzed mixture of sucrose, containing glucose and fructose, is known as invert sugar.

Sucrose first splits into α-D-glucopyranose (+) and β-D-fructofuranose (+). But β-D-fructofuranose is less stable and gets converted into β-D-fructopyranose (-). The overall effect in the mixture becomes levorotatory (-).

Lactose

Lactose (milk sugar) is composed of β-D-galactose and β-D-glucose held together by   β(1🠊4) glycosidic bond.

The anomeric carbon of C1 of glucose is free. Hence lactose exhibits reducing properties and forms osazones (powder-puff or hedgehog shape).

The systematic name is β-D-galactopyranosyl-(1🠊4)-β-D-glucopyranose.

It is hydrolyzed by intestinal enzyme lactase into glucose and galactose.

Maltose

Maltose (malt sugar) is produced during digestion of starch by enzyme amylase.

Maltose is composed of two α-D-glucose  units held together by α(1🠊4) glycosidic bond. 

A free aldehyde group is present on C1 of the second glucose unit and hence maltose exhibits reducing properties and forms osazones (sunflower shaped).

It can be hydrolyzed by dilute acid or enzyme maltase.

 

In isomaltose, the glucose units are held together by α(1🠊6) glycosidic bond.

Cellobiose

It is identical to maltose, except that in it the linkage is  β(1🠊4) glycosidic bond.

It is formed during hydrolysis of cellulose.

Trehalose

It is identical to maltose, except that in it the linkage is  (α1⇔α1) glycosidic bond.

It is formed during hydrolysis of cellulose.

It is a non-reducing sugar.

Examples of other oligosaccharides

Raffinose (trisaccharides): Fructose+Galactose+Glucose

Stachyose (Tetrasaccharide):  Galactose+Galactose+Glucose+Fructose

Verbascose (Pentasaccharide): Galactose+Galactose+Galactose+Glucose+Fructose

Polysaccharides

Carbohydrates containing repeating units (more than 10 units) of the monosaccharides or their derivatives linked by glycosidic linkages are called polysaccharides.

They are primarily concerned with 2 important functions:

Structural role

Storage of energy

Polysaccharides can be linear or branched. The occurrence of branched polysaccharides is due to the fact that glycosidic linkages can be formed at any one of the –OH groups of a monosaccharide.

Polysaccharides are of high molecular weight. They are usually tasteless (non-sugars) and form colloids with water.

Polysaccharides are of two types:

Homopolysaccharides (Homoglycans): They, on hydrolysis, yield only one type of monosaccharide. They are named based on the nature of the monosaccharide unit. 

Example: Glucan (polymer of glucose), Fructosan (polymer of fructose)

Heteropolysaccharides (heteroglycans): They, on hydrolysis, yield a mixture of a few types of monosaccharide units or their derivatives. 

Example: Peptidoglycan (polymer of N-acetylglucosamine and N-acetylmuramic acid residues)

Starch

Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals.

Starch is a homopolysaccharide composed of D-glucose units held by glycosidic bonds. 

It is known as glucosan or glucan.

Starch consists of two polysaccharide components:

Water soluble amylose (15-20%)

Water insoluble amylopectin (80-85%)

Chemically amylose is a long unbranched chain with 200-1000 D-glucose units held by (α1🠊4) glycosidic linkage.

Amylopectin is a branched chain with (α1🠊6) glycosidic bonds at the branching points and (α1🠊4) glycosidic bonds everywhere else.

Starches are hydrolyzed by amylases (pancreatic or salivary) to liberate dextrins and finally maltose and glucose units. Amylase acts specifically on the (α1🠊4) glycosidic bonds.

(b) amylopectin

Dextrins

These are the breakdown products of starch by the enzyme amylase or dilute acids.

Starch is hydrolyzed through different dextrins and finally to maltose and glucose. 

The various intermediates (identified by iodine coloration) are soluble starch (blue), amylodextrin (violet), erythrodextrin (red) and achrodextrin (no colour).

Inulin

Inulin is a polymer of fructose.

It occurs in dahlia bulbs, garlic, onion, etc.

It is a low molecular weight (~ 5000) polysaccharide easily soluble in water.

Inulin is not utilized by the body. 

It is used for assessing kidney function through measurement of glomerular filtration rate (CFR).

Cellulose

Cellulose occurs extensively in plants and is totally absent in animals.

Cellulose is composed of β-D-glucose units linked by β(1🠊4) glycosidic bonds.

Cellulose can not be digested by mammals due to lack of the enzyme that cleaves β-glycosidic bonds. Hydrolysis of cellulose yields a disaccharide, cellobiose, which is further broken down to β-D-glucose units.

It is a major constituent of fibers, the non-digestible carbohydrate.

Glycogen

Glycogen is the main storage polysaccharide of animal cells.

Like amylopectin, glycogen is a polymer of (α1🠊4)-linked subunits of glucose, with (α1🠊6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch.

Glycogen is especially abundant in the liver, it is also present in skeletal muscle.

Chitin

Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in β linkage.

The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. 

Peptidoglycan

The rigid component of bacterial cell walls is a  heteropolymer  of  alternating (β1🠊4)-linked N-acetylglucosamine and N-acetylmuramic acid residues.

The enzyme lysozyme kills bacteria by hydrolyzing the   (β1🠊4) glycosidic bond between N-acetylglucosamine and Nacetylmuramic acid.

Carbohydrate biochemistry (Part I)

To access and download PowerPoint presentation on 'Carbohydrate Biochemistry' click on the link below:

Download slides

 Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis.

Many, but not all, carbohydrates have the empirical formula (CH₂O)ₙ, [n≥3];  some also contain nitrogen, phosphorus, or sulfur.

Carbohydrate literally means ‘hydrates of carbon’.

Carbohydrates are the most abundant biomolecules on Earth.

Occurrence & Function

Certain carbohydrates (sugar and starch) are a dietary staple and abundant dietary source of energy (4 cal/g) 

Insoluble carbohydrate polymers serve as structural and protective elements:

       in the cell walls of bacteria and plants

       in the connective tissues of animals

       lubricate skeletal joints

       participate in recognition and adhesion between cells

Complex carbohydrate polymers that are covalently attached to proteins or lipids are called glyco-conjugates.

act as signals that determine the intracellular location or metabolic fate of these hybrid molecules

Carbohydrates are precursors of many organic molecules (fats, amino acids, etc.)

They serve as storage form of energy (Ex- Glycogen, Starch)

Classification

The word “saccharide” is derived from the Greek ‘sakcharon’, meaning “sugar”.

Monosaccharides (simple sugars): Consist of a single polyhydroxy aldehyde or ketone unit. Ex- Glucose, Fructose

Glucose

Oligosaccharides: Consist of short chains of monosaccharide units, or residues (2-10), joined by characteristic linkages called glycosidic bonds. Ex- Raffinose

Raffinose

Disaccharides: Consists of  two monosaccharide units joined by glycosidic bond. Ex- Sucrose (Glucose + Fructose)

Sucrose

Polysaccharides: Sugar polymers containing more than 20 or so monosaccharide units, and some have hundreds or thousands of units. Ex- Cellulose, Glycogen

Monosaccharides

Simplest carbohydrates that cannot be hydrolyzed to smaller carbohydrates.

General chemical formula of unmodified monosaccharide is (C.H₂O)ₙ  where n≥3

Consist of a single polyhydroxy aldehyde or ketone unit. 

The most abundant monosaccharide in nature is the six-carbon sugar D-glucose. 

Monosaccharides of more than four carbons tend to have cyclic structures.

Ex- Glyceraldehyde, Glucose, Fructose, etc. 

Classification of monosaccharides

Classified according to 3 different characteristics:

Placement of its carbonyl group

Number of carbon atoms present

Chiral handedness

Classification based on placement of carbonyl group

ALDOSE: Functional group is an aldehyde group (-CHO)

Ex- Glyceraldehyde, Glucose, etc

KETOSE: Functional group is a keto group (>C=O)

Ex- Dihydroxyacetone, Fructose, etc.

Classification based on number of carbon atoms

On the basis of the number of carbon atoms present, monosaccharides can be classified as:

Triose (3 C)

Tetrose (4 C)

Pentose (5 C)

Hexose (6 C)

Heptose (7 C)

Stereoisomers

Stereoisomers: Compounds that have same structural formulae but differ in their spatial configuration. 

A carbon is said to be asymmetric (chiral) when it is attached to four different atoms or groups. 

The number of asymmetric carbon atoms (n) determines the possible number of isomers of a given compound which is equal to 2ⁿ. 

Stereoisomerism is a characteristic feature of all sugars except Dihydroxyacetone. 

Example- 

    Glucose has 4 asymmetric carbon atoms. No. of isomers = 2⁴ = 16

   Glyceraldehyde has 1 asymmetric carbon atom. No. of isomers = 2¹ = 2

   Dihydroxyacetone has no asymmetric carbon atoms. Hence, no isomer is possible.

Classification based on chiral handedness

D and L isomers: Assignment of D or L isomer is made according to the orientation of the asymmetric carbon atom furthest from the carbonyl group.

In a standard Fischer projection if the hydroxyl group is on the right, the molecule is D sugar, and if the hydroxyl group is on the left, the molecule is L sugar. 

D-sugars are biologically more common.

Optical activity of sugars

It is the characteristic feature of compounds with asymmetric carbon atoms. 

When a beam of polarized light is passed through a solution of an optical isomer, it will be rotated to either the right or left. 

The terms dextrorotatory (+) and levorotarory (-) are used to compounds that respectively rotate the plane of polarized light to the right or to the left.

It may be noted that the D and L configurations of sugars are primarily based on the structure, optical activities may be different.

Racemic mixture: If dextrorotatory and levorotatory isomers are present in equal concentration, it is known as racemic mixture or DL mixture. Racemic mixture does not exhibit any optical activity, since the dextro- and levorotatory activities cancel each other.

Epimers

If two monosaccharides differ from each other in their configuration around a single specific carbon (other than anomeric carbon), they are referred to as epimers to each other.

D-glucose and D-mannose differ only in the stereochemistry at C-2, are epimers.

D-glucose and D-galactose which differ at C-4, are epimers.

Inter-conversions of epimers (eg.- glucose to galactose and vice versa) is known as epimerization and is catalyzed by a group of enzymes called epimerases.

Common Monosaccharides Have Cyclic Structures

In aqueous solution, aldotetroses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl group along the chain. 

The formation of these ring structures is the result of a general reaction between alcohols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals. 

These structures contain an additional asymmetric carbon atom and thus can exist in two stereoisomeric forms.

Hemiacetal formation

Hemiketal formation

D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asymmetric and producing two stereoisomers, designated  as α and β.

These six-membered ring compounds are called pyranoses because they resemble the six membered ring compound pyran.

The systematic names for the two ring forms of D-glucose are α-D-glucopyranose  and β-D-glucopyranose.

Only aldoses having five or more carbon atoms can form pyranose rings.

Aldohexoses and ketohexoses also exist in cyclic forms having five membered rings, which, because they resemble the five membered ring compound furan, are called furanoses.

The six-membered aldopyranose ring is much more stable than the aldofuranose ring and predominates in aldohexose solutions.

Anomers

Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atom are called anomers.

The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon.

In case of α-anomer, the –OH group held by anomeric carbon is on the opposite side of the –CH2OH group of the sugar ring. The opposite is true for β-anomers.

The α- and β-anomers of D-glucose interconvert in aqueous solution by a process called mutarotation.

Thus, a solution of α-D-glucose and a solution of β-D-glucose eventually form identical equilibrium mixtures having identical optical properties. This mixture consists of about one-third α-D-glucose (36%), two-thirds β-D-glucose (63%), and very small amounts of the linear and five-membered ring (glucofuranose) forms (1%).

             

 D-glucose      ⃡       Equilibrium mixture        ⃡         β-D-glucose 

   +112.2°                             +52.7°                                  +18.7°

Ketohexoses also occur in α and β anomeric forms.

In these compounds the hydroxyl group at C-5 (or C-6) reacts with the keto group at C-2, forming a furanose (or pyranose) ring containing a hemiketal linkage.

D-Fructose readily forms the furanose ring, the more common anomer of this sugar in combined forms or in derivatives is D-fructofuranose.

The specific optical rotation of fructose is -92° at equilibrium. 

Monosaccharides Are Reducing Agents

Monosaccharides can be oxidized by relatively mild oxidizing agents such as ferric (Fe3+) or cupric (Cu2+).

The carbonyl carbon is oxidized to a carboxyl group.

Sugars capable of reducing ferric or cupric ion are called reducing sugars. They have free aldehyde or ketone group present in their structure.

Ex- Glucose

Sugars not capable of reducing ferric or cupric ion are called non-reducing sugars. They do not have free aldehyde or ketone group present in their structure.

Ex- Sucrose

This property is the basis of Fehling’s reaction, a qualitative test for the presence of reducing sugar.

Monosaccharide derivatives

There are a number of sugar derivatives in which a hydroxyl group in the parent compound is replaced with another substituent, or a carbon atom is oxidized to a carboxyl group.

• In amino sugars, an –NH2 group replaces one of the -OH groups in the parent hexose. 
• Substitution of –H for –OH produces a deoxy sugar.
• The acidic sugars contain a carboxyl group, which confers a negative charge at neutral pH.

Sugar acids: Oxidation of aldehyde or primary alcohol groups in the monosaccharide results in sugar acids.

The acidic sugars contain a carboxyl group, which confers a negative charge at neutral pH.

Examples:

Gluconic acid is produced from glucose by oxidation of aldehyde group.

Glucuronic acid is formed from glucose by oxidation of primary alcohol group (C6).

Amino sugars: When one or more hydroxyl groups of the monosaccharide are replaced by amino groups, the products formed are called amino sugars.

They are present as constituents of heteropolysaccharides.

Examples:

D-glucosamine

D-galactosamine

They are sometimes acetylated.

Example:

N-acetyl-D-glucosamine

Deoxysugars: They contain one oxygen less than that of their parent molecule. 

The groups –CHOH and –CH2OH become –CH2 and –CH3 due to absence of one oxygen atom.

Examples: 

D-2-Deoxyribose

L-Rhamnose

L-Fucose

Sugar alcohols: Sugar alcohols (polyols) are produced by reduction of aldoses or ketoses.

Examples: 

Sorbitol from glucose

Mannitol from mannose

Alditols: The monosaccharides on reduction yield polyhydroxy alcohols known as alditols.

Examples:

Ribitol (constituent of flavin coenzymes)

Glycerol (Component of lipid)

Xylitol (Sweetener used in sugarless gums and candies)

Cell Cycle Regulation (General)

 Click here for PPT

Cell Division (General)

 

Cell Division

Cell division is a fundamental process by which a parent cell divides into two or more daughter cells. This process is essential for growth, repair, tissue regeneration, and reproduction in organisms. Before division, the cell duplicates all its components, including its DNA (genetic material). 

This fundamental biological process ensures genetic continuity across generations.

Types of Cell Division

1. Amitosis (Direct Cell Division)

• A simpler, less precise form of division, typically seen in certain lower organisms or specific cell types.

• The nucleus elongates and divides directly without forming spindle apparatus or chromosome condensation. The cytoplasm divides afterwards.

• Example: bacteria, yeast, and some protozoa

2. Mitosis (Equational Division)

• A process where a single cell divides into two identical daughter cells (daughter cells have the same number of chromosomes as the parent cell, e.g., 2n → 2n).

• It is crucial for growth, repair, and asexual reproduction.

• Example: Division in somatic (vegetative) cells

3. Meiosis (Reductional Division)

• A process where a single cell divides twice to produce four genetically distinct daughter cells, each with half the number of chromosomes as the parent cell e.g., 2n → n.

• It is essential for sexual reproduction and generating genetic variation.

• Example: Division in germ cells (reproductive cells) to form gametes (sperm and egg cells).

Mitosis

Mitosis produces two genetically identical daughter cells, each containing the same number of chromosomes as the parent cell. This process is essential for growth, tissue repair, and asexual reproduction.

Mitosis is preceded by Interphase (G1, S, G2 phases), where the cell grows and the DNA is replicated. The division itself, the M phase, includes Karyokinesis (nuclear division) and Cytokinesis (cytoplasmic division).

Phases of Mitosis

Prophase

This is the first and longest stage of mitosis, where the cell prepares for the separation of chromosomes. The events of this phase are as follows: 

Events of Prophase	Significance
Chromatin Condensation	The loose, thread-like chromatin coils and condenses to form discrete, compact chromosomes. Each chromosome is duplicated, consisting of two identical sister chromatids joined at the centromere.
Nucleolus Disappears	The nucleolus, where ribosomes are synthesized, disintegrates.
Centrosome Migration	In animal cells, the two centrosomes (which were duplicated in Interphase) begin to move away from each other toward opposite poles of the cell.
Mitotic Spindle Formation	Microtubules begin to polymerize from the centrosomes, forming the mitotic spindle, which will later separate the chromosomes.
Nuclear Envelope Breakdown	The nuclear membrane completely fragments towards late prophase. Allows the spindle microtubules to access and interact with the chromosomes.

Metaphase

This phase is defined by the alignment of the chromosomes at the center of the cell. The events of this phase are as follows: 

Events of Metaphase	Significance
Chromosome condensation	Chromosomes attain the maximum condensed stage.
Spindle Attachment	The spindle microtubules attach to the kinetochores (protein structures) located at the centromere of each sister chromatid. The attachment to kinetochores is essential for the precise movement and separation of chromatids.
Chromosome Alignment (Metaphase Plate)	Chromosomes are pulled by the spindle fibers to align along the cell's equatorial plane, forming the Metaphase Plate. This central alignment ensures that sister chromatids will separate correctly and equally into the two daughter cells.

Anaphase

This is the shortest phase, characterized by the separation of sister chromatids. The events of this phase are as follows: 

Events of Anaphase	Significance
Sister Chromatid Separation	The proteins (cohesins) holding the sister chromatids together at the centromere break down, allowing the centromeres to split. This is the crucial step for ensuring that each future daughter nucleus receives a complete and identical set of genetic information.
Movement to Poles	The newly separated sister chromatids (now considered individual daughter chromosomes) are rapidly pulled by the shortening kinetochore microtubules toward opposite poles of the cell. This action separates the two full genomes destined for the daughter cells.
Cell Elongation	Non-kinetochore microtubules lengthen, which pushes the poles apart, causing the entire cell to elongate. Prepares the cell for the physical splitting of the cytoplasm.

Telophase

This is essentially the reversal of prophase, where two new nuclei are formed. The events of this phase are as follows: 

Events of Telophase	Significance
Nuclear Envelope Re-formation	A new nuclear envelope forms around each complete set of chromosomes at the two poles of the cell. This marks the end of karyokinesis (nuclear division) and creates two distinct nuclei.
Chromosomes Decondense	The chromosomes unwind and uncoil, returning to their long, dispersed chromatin form. The DNA returns to its functional state for gene expression in the newly formed daughter cells.
Spindle Disassembly	The mitotic spindle fibers depolymerize and break down. The components are recycled for use in the cytoskeleton of the new cells.
Nucleoli Reappear	The nucleoli re-form within the new nuclei.

Cytokinesis (Not a phase of Mitosis, but part of the M-phase)

Event	Description
Cytoplasm Division	The physical division of the cytoplasm and its contents.
Animal Cells	A cleavage furrow (a groove in the cell surface) forms and deepens, pinching the parent cell into two daughter cells.
Plant Cells	A cell plate (which will become the new cell wall) forms in the middle of the cell, dividing the parent cell into two.

Significance of Mitosis

• Growth: Increases the number of cells in a multicellular organism, leading to growth.

• Repair and Regeneration: Replaces damaged, dead, or worn-out cells (e.g., healing a wound).

• Asexual Reproduction: The basis for reproduction in many unicellular and simple multicellular organisms (e.g., fission in bacteria, vegetative propagation in plants).
• Maintains Chromosome Number: Ensures the two daughter cells receive the exact same number and type of chromosomes as the parent cell (equational division).

Phases of mitosis

Meiosis

Meiosis produces four haploid gametes (sperm or egg cells) with half the chromosome number, ensuring genetic diversity through crossing over and independent assortment. This process is essential for sexual reproduction.

Meiosis involves two sequential cycles of nuclear and cell division: Meiosis I and Meiosis II. It is preceded by a single interphase where DNA is replicated.

Phases of Meiosis

Meiosis I: Reductional Division

Meiosis I is the first division where homologous chromosomes separate, reducing the chromosome number from diploid (2n) to haploid (n).

Phase	Sub-phases	Key Events	Significance
Prophase I		The longest and most complex phase and consists of 5 sub-phases:	Crossing over is crucial for genetic recombination (variation). The paired chromosomes ensure proper separation.
	Leptotene	Chromosome Condensation Begins, Axial Elements Form	The chromatin threads start to coil and condense, making the individual chromosomes (each with two sister chromatids) visible as long, thin strands. Protein scaffolds assemble along the length of each homologous chromosome.
	Zygotene	Synapsis Begins, Synaptonemal Complex Forms, Bivalents/Tetrads Form	Homologous chromosomes begin to align precisely, coming together side-by-side in a process called synapsis. A ladder-like protein structure, the synaptonemal complex, forms between the paired homologous chromosomes, holding them tightly together. The paired structure consisting of two homologous chromosomes (four sister chromatids total) is now fully formed and referred to as a bivalent or tetrad. Precise alignment of homologous chromosomes is established, which is essential for accurate crossing over.
	Pachytene	Synapsis Complete, Crossing Over Occurs	The paired homologous chromosomes are fully synapsed. Genetic exchange (recombination) takes place between the non-sister chromatids of the homologous chromosomes. This happens at specific points called recombination nodules. Genetic Variation is introduced by shuffling alleles between maternal and paternal chromosomes, creating recombinant chromosomes.
	Diplotene	Synaptonemal Complex Dissolves, Homologues Begin to Separate, Chiasmata Become Visible	The protein complex holding the homologues together disintegrates. The homologous chromosomes start to repel and move apart, but they remain attached at the sites where crossing over occurred. The points of attachment where genetic exchange occurred are now physically visible as X-shaped structures called chiasmata (plural; chiasma, singular). In many female mammals, oocytes enter a prolonged resting stage (Dictyotene) during Diplotene, which can last for years or decades.
	Diakinesis	Chromosomes Fully Condensed, Terminalization of Chiasmata, Nuclear Disintegration, Spindle Assembly begins	The chromosomes reach their maximum condensation. The chiasmata move towards the ends (terminalize) of the homologous chromosomes, further separating them while still keeping the pairs linked. The nucleolus disappears, and the nuclear envelope breaks down. The meiotic spindle begins to assemble, marking the end of Prophase I and the transition to Metaphase I.
Metaphase I		Homologous pairs (bivalents) align randomly at the cell's equatorial plate (metaphase plate). Spindle fibers from one pole attach to one full homologous chromosome (both sister chromatids).	Independent Assortment (random orientation) occurs here, which is a second major source of genetic variation.
Anaphase I		Homologous chromosomes separate and are pulled to opposite poles. Sister chromatids remain attached at their centromeres.	This is the point of reduction: each pole receives a haploid set of chromosomes, though each chromosome is still duplicated.
Telophase I & Cytokinesis I		Chromosomes gather at the poles. The nuclear envelope may reform. Cytokinesis divides the cell into two haploid daughter cells (n), each with duplicated chromosomes.	The chromosome number has been reduced by half.22 A brief interphase (Interkinesis) may follow, but no DNA replication occurs.

Sub-phases of Mitotic Prophase I

Phases of Meiosis

Meiosis II: Equational Division

Meiosis II resembles mitosis but begins with a haploid cell (n) and involves the separation of sister chromatids. It is called equational because the chromosome number remains haploid (n).

Phase	Key Events
Prophase II	The nuclear envelope (if reformed) breaks down, and the spindle apparatus re-forms in both haploid daughter cells. Prepares the cell for the second round of division.
Metaphase II	Individual chromosomes (composed of two sister chromatids) align along the equatorial plate of each of the two cells. Kinetochore microtubules attach to the centromere of each sister chromatid. The chromosomes are now set up for the final separation of genetic material.
Anaphase II	The centromeres split, and the sister chromatids separate, moving to opposite poles. Each chromatid is now considered an individual chromosome. Sister chromatid separation is achieved, similar to mitosis.
Telophase II & Cytokinesis II	Chromosomes arrive at the poles and decondense. New nuclear envelopes form around each of the four sets of chromosomes. Cytokinesis fully separates the cells, resulting in four unique haploid (n) daughter cells (gametes). Produces the final, genetically diverse gametes ready for fertilization.

Significance of Meiosis

• Formation of Gametes: Essential for the production of haploid gametes (sex cells) for sexual reproduction.

• Maintenance of Chromosome Number: Ensures that the species-specific diploid chromosome number (2n) is restored after fertilization when two haploid gametes fuse.

• Genetic Variation: Crossing over (Prophase I) and Independent Assortment (Metaphase I) reshuffle genetic material, producing unique combinations and contributing to the diversity necessary for evolution.

Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Type of Cell	Somatic (vegetative) cells	Germ (reproductive) cells
Number of Divisions	One	Two (Meiosis I and Meiosis II)
Daughter Cells	Two	Four
Chromosome Number	Remains the same as parent (Diploid, 2n)	Reduced to half the parent number (Haploid, n)
Genetic Identity	Genetically identical to the parent cell	Genetically different from the parent cell and each other
Purpose	Growth, repair, tissue regeneration, asexual reproduction	Sexual reproduction (gamete formation) and genetic variation
Pairing of Homologous Chromosomes	No	Yes, in Prophase I (synapsis)
Crossing Over	No	Yes, in Prophase I