Mesosome is a convoluted membranous structure formed in a prokaryotic cell by the invagination of the plasma membrane. Its functions are as follows:

(1) These extensions help in the synthesis of the cell wall and replication of DNA. They also help in the equal distribution of chromosomes into the daughter cells.

(2) It also increases the surface area of the plasma membrane to carry out various enzymatic activities.

(3) It helps in secretion processes as well as in bacterial respiration.

The presence or absence of pigments is the main basis of classification of algae. Chlorophyceae: Chlorophyll a and b are present in them and impart green colour. Chlorophyceae are also called 'blue-green algae'. Phaeophyceae: Chlorophyll a and c and fuxoxanthin are present.

Prophase is the first phase of mitosis, the process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells. During prophase, the complex of DNA and proteins contained in the nucleus, known as chromatin, condenses. During prophase I, the homologous chromosomes condense and become visible as the x shape we know, pair up to form a tetrad, and exchange genetic material by crossing over. During prometaphase I, microtubules attach at the chromosomes' kinetochores and the nuclear envelope breaks down.

Metaphase is a stage of the cell cycle occurring in both mitosis and meiosis cell division processes. During metaphase in mitosis and meiosis, the chromosomes condense and they become visible and distinguishable during alignment at the center of the dividing cell, to form a metaphase plate at the center of the cell. In metaphase, chromosomes are lined up and each sister chromatid is attached to a spindle fiber. In anaphase, sister chromatids (now called chromosomes) are pulled toward opposite poles. In telophase, chromosomes arrive at opposite poles, and nuclear envelope material surrounds each set of chromosomes.

The synaptonemal complex (SC) is a protein structure that forms between homologous chromosomes (two pairs of sister chromatids) during meiosis and is thought to mediate synapsis and recombination during meiosis I in eukaryotes. Prophase 1 of Meiosis is the first stage of meiosis and is defined by five different phases; Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

Prophase immediately follows S and G2 phase of the cycle and is marked by condensation of the genetic material to form compact mitotic chromosomes composed of two chromatids attached at the centromere.

The completion of prophase is characterised by the initiation of the assembly of the mitotic spindle, the microtubules, and the proteinaceous components of cytoplasm that help in the process.

The nuclear envelope starts disintegrating.

meiosis 1 Prophase 1

(1) Leptotene – The chromosomes begin to condense and attain a compact structure during leptotene.

(2) Zygotene – In zygotene, the pairing of homologous chromosomes starts a process known as chromosomal synapsis, accompanied by the formation of a complex structure called synaptonemal complex. A pair of synapsed homologous chromosome forms a complex known as bivalent or tetrad.

(3) Pachytene – At pachytene stage, crossing over of non-sister chromatids of homologous chromosomes occurs at the recombination nodules. The chromosomes remain linked at the sites of crossing over.

(4) Diplotene – Diplotene marks the dissolution of the synaptonemal complex and separation of the homologous chromosomes of the bivalents except at the sites of cross-over. The X-shaped structures formed during separation are known as chiasmata.

(5) Diakinesis – Diakinesis is marked by the termination of chiasmata and assembly of the meiotic spindle to separate the homologous chromosomes. The nucleolus disappears and the nuclear envelope breaks down.

Zygotene

The zygotene stage, otherwise called zygonema, from Greek words signifying "matched threads", happens as the chromosomes approximately line up with one another into homologous chromosome pairs. In some organisms, this is known as the bouquet stage as a result of the manner in which the telomeres bouquet toward one side of the core. At this stage, the synapsis of homologous chromosomes happens, encouraged by gathering of the central component of the synaptonemal complex. Pairing is achieved in a zipper-like style and may begin at the centromere (pro-centric), at the chromosome ends (terminal), or at some other portion. Individuals of a pair are equivalent long and in the position of the centromere. Therefore, pairing is very specific and definite. The paired chromosomes are known as bivalent or quadruplicate chromosomes.

Pachytene

The pachytene stage, otherwise called pachynema, from Greek words signifying "thick threads". At this point, a quadruplicate of the chromosomes has framed known as a bivalent. This is the phase when homologous recombination, including chromosomal hybrid (traverse), happens. At this stage, the Non-sister chromatids of homologous chromosomes may exchange segments over regions of homology. 

Anaphase I

Kinetochore microtubules become short, pulling homologous chromosomes (which comprise of a couple of sister chromatids) to opposite poles. Nonkinetochore microtubules extend, pushing the centrosomes separated apart. The cell stretches for division down the center. Unlike in mitosis, just the cohesin from the chromosome arms is degraded while the cohesin encompassing the centromere stays secured. This enables the sister chromatids to stay together while homologs are segregated.

he mitotic apparatus consists of centrioles with the centre-spheres surrounding them, a cell division spindle with a system of microtubules, and an intermediate substance. Depending on the degree of development of astral rays around the centre-sphere, the mitotic apparatus is classified as astral (characteristic of most animal cells) or anastral (characteristic of plant cells). The mitotic apparatus is formed from macromolecules present in the interphase cell and from material synthesized before division. Its main components are ribonucleoproteins (about 90 percent proteins and 6 percent ribonucleic acid [RNA]). It also contains polysaccharides, lipids, and adenosine triphosphatase.

• Genetic stability- Mitosis helps in the splitting of chromosomes during cell division and generates two new daughter cells. Therefore the chromosomes form from the parent chromosomes by copying the exact DNA.  Therefore, the daughter cells formed as genetically uniform and identical to the parent as well as to each other. Thus mitosis helps in preserving and maintaining the genetic stability of a particular population.
• Growth- Mitosis help in increasing the number of cells in a living organism thereby playing a significant role in the growth of a living organism.

It is a protein complex, visible with the electron microscope, that is the physical basis of the pairing of homologous chromosomes (synapsis) during meiosis. It assembled during zygotene as homologous chromosomes pair up, and it is disassembled during diplotene as homologous chromosomes separate.

<button aria-label="like this post" title="like this post"></button>

S phase – It is the stage during which DNA synthesis occurs. In this phase, the amount of DNA (per cell) doubles, but the chromosome number remains the same.

G2​ phase – In this phase, the cell continues to grow and prepares itself for the division. The proteins and RNA required for mitosis are synthesised during this stage.

Osmolarity defines the solute concentration, i.e. number of moles of solute that contributes to the osmotic pressure of a solution.
Osmolarity can be expressed in milli osmol per litre or mOsmol L^-1  
 

• The arrangement of ovules within the ovary is known as placentation.
• The placentation is of different types namely, marginal, axile, parietal, basal, central and free central.
• In marginal placentation the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge forming two rows. Example- pea

Chromosomes are thread-like structures present in the nucleus, which carries genetic information from one generation to another. They play a vital role in cell division, heredity, variation, mutation, repair and regeneration.

Based on the number of centromeres present

1. Monocentric: having only one centromere
2. Holocentric: having diffused centromere and microtubules are attached along the length of a chromosome
3. Acentric: chromosome may break and fuse together to form a chromosome without a centromere. It cannot attach to the mitotic spindle
4. Dicentric: chromosomal aberration where chromosomes break and fuse together with two centromeres. They are also unstable as two centromeres tend to migrate to opposite poles resulting in fragmentation

The nucleus is a sphere-shaped organelle found in eukaryotic cells. It contains the genetic material of the cell in the form of nucleic acids. It is responsible for controlling all activities of the cell. and contains a nuclear membrane, chromosomes, nucleolus and nucleoplasm.

The Endomembrane system is a membranous component of the eukaryotic cell. The cytoplasm of the cell contains a system of membranous organelles that are suspended in it. The organelles are termed as a system even though they have different structures and functions as they are essential to the working of the cell. All these organelles work in coordination and they include the cell membrane, vacuoles, the nuclear membrane, lysosomes, Golgi complex, vesicles and the endoplasmic reticulum (ER).

The cell membrane is also known as the plasma membrane. It is the outermost covering of animal cells. It is a semi-permeable membrane composed of lipids and proteins. The main functions of the cell membrane include:

1. Protecting the integrity of the interior cell.

2. Providing support and maintaining the shape of the cell.

3. Helps in regulating cell growth through the balance of endocytosis and exocytosis.

4. The cell membrane also plays an important role in cell signalling and communication.

5. It acts as a selectively permeable membrane by allowing the entry of only selected substances into the cell.

Prokaryotic cell envelope possess chemically complex cell envelope. The cell envelope comprises of three layers: the outermost glycocalyx, the cell wall and the plasma membrane. All the three layers together act as a single protective unit.
Bacteria is classified into two groups based on the response to the gram staining method −
Gram positive:- Cells that take up the gram stain and are stained dark blue or purple.
Gram negative:- Cells that do not take up the gram stain and are stained pink or light red.
If glycocalyx is present as a loose sheath it is called slime layer. At times glycocalyx is present as thick and tough layer then it is called capsule.
The cell wall determines the shape of the cell and also provides structural support to the cell. The plasma membrane is semi-permeable in nature.
A special membranous structure called mesosome is formed by the extensions of plasma membrane into the cell.
Bacteria may be motile or non-motile based on the presence of thin filamentous extensions called flagella. Flagella helps in motility. Bacterial flagella are composed of three parts −
1.Filament
2.Hook
3.Basal Body
Pili are elongated structures made up of pilin proteins. Fimbriae are small bristle like fibres which mostly help in attachment of the bacteria to different surfaces.

Prokaryotic cells have different characteristic features. The characteristics of the prokaryotic cells are mentioned below.

1. They lack a nuclear membrane.

2. Mitochondria, Golgi bodies, chloroplast, and lysosomes are absent.

3. The genetic material is present on a single chromosome.

4. The histone proteins, the important constituents of eukaryotic chromosomes, are lacking in them.

5. The cell wall is made up of carbohydrates and amino acids.

6. The plasma membrane acts as the mitochondrial membrane carrying respiratory enzymes.

7. They divide asexually by binary fission. The sexual mode of reproduction involves conjugation.

CELL THEORY

• Schleiden and Schwann together formulated the cell theory.
• Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells.
• Cell theory states that
• All living organisms are composed of cells and products of cells.
• All cells arise from pre-existing cells.

Each alveolus is made up of highly-permeable and thin layers of squamous epithelial cells. Similarly, the blood capillaries have layers of squamous epithelial cells. Oxygen-rich air enters the body through the nose and reaches the alveoli. The deoxygenated (carbon dioxide-rich) blood from the body is brought to the heart by the veins. The heart pumps it to the lungs for oxygenation. The exchange of O₂ and CO₂ takes place between the blood capillaries surrounding the alveoli and the gases present in the alveoli.

Thus, the alveoli are the sites for gaseous exchange. The exchange of gases takes place by simple diffusion because of pressure or concentration differences. The barrier between the alveoli and the capillaries is thin and the diffusion of gases takes place from higher partial pressure to lower partial pressure. The venous blood that reaches the alveoli has lower partial pressure of O₂ and higher partial pressure of CO₂ as compared to alveolar air. Hence, oxygen diffuses into blood. Simultaneously, carbon dioxide diffuses out of blood and into the alveoli.

Each alveolus is made up of highly-permeable and thin layers of squamous epithelial cells. Similarly, the blood capillaries have layers of squamous epithelial cells. Oxygen-rich air enters the body through the nose and reaches the alveoli. The deoxygenated (carbon dioxide-rich) blood from the body is brought to the heart by the veins. The heart pumps it to the lungs for oxygenation. The exchange of O2 and CO2 takes place between the blood capillaries surrounding the alveoli and the gases present in the alveoli.

Thus, the alveoli are the sites for gaseous exchange. The exchange of gases takes place by simple diffusion because of pressure or concentration differences. The barrier between the alveoli and the capillaries is thin and the diffusion of gases takes place from higher partial pressure to lower partial pressure. The venous blood that reaches the alveoli has lower partial pressure of O2 and higher partial pressure of CO2 as compared to alveolar air. Hence, oxygen diffuses into blood. Simultaneously, carbon dioxide diffuses out of blood and into the alveoli.