Skip to main content

Renin Angiotensin Mechanism.

Cell Division: Meiosis

 Cellular level of organization:  Cell Division: Meiosis.

Introduction:

  • Cell is defined as a basic structural and functional unit of the body.

  • Cytology is a branch of science that deals with the study of cells.

  • The animal cell is divided into two major parts as,

Plasma Membrane.

Subcellular organelles.

  1. Nucleus.

  2. Nucleolus

  3. Mitochondria.

  4. Golgi complex.

  5. Ribosomes.

  6. Endoplasmic reticulum.

  7. Lysosomes.

  8. Centrioles.

  • The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. 

  • These events include the duplication of its DNA (DNA replication) and some of its organelles, and subsequently the partitioning of its cytoplasm and other components into two daughter cells in a process called cell division.

  • The events in the cell that include the duplication of its DNA (DNA replication) and some of its organelles, and subsequently the partitioning of its cytoplasm and other components into two daughter cells in a process called cell division.

  • Cell Division: The ability of the cell to produce daughter cells.

  • Normally, cells divide to replace dead or missing cells,

    • intestinal cells divide every 3 days, are broken down by digestion

    • blood cells last 3 months, are replaced by new cell division

    • nerve cells usually don't divide, last for life

    • Embryo: almost constant cell division. Every 30 minutes.

  • Sex cells are unique: in humans, males produce 500,000,000 /day. Females 1/month.

Meiosis.

  • Definition: It's a special type of cell division where the cell genome is reduced to half to produce “Four Haploid Cells”.

  • The cells produced in meiosis are called gametes.

  • As genetic material is reduced to half, it is also called as “Reductional Cell Division”.

  1. Parent Cell (2n)-------> Daughter Cell (1n).

  • Meiosis is divided into following stages,

Meiosis I

Prophase I 

Meiosis II

  1. Prophase I

  1. Leptotene


  1. Prophase II

  1. Metaphase I

2) Zygotene

2) Metaphase II

  1. Anaphase I


3) Pachytene

3) Anaphase II

  1. Telophase I

4) Diplotene

4) Telophase II

  1. Cytokinesis I

5) Diakinesis

5) Cytokinesis II

Meiosis I:

  • It is the phase where actual reduction takes place.

  • It is further divided into,

    • Prophase I

    • Metaphase I

    • Anaphase I

    • Telophase I

    • Cytokinesis I

  1. Prophase I:

  • It is the longest phase and is divided into following phases for convenience of study.

  1.  Leptotene

  2. Zygotene

  1. Pachytene

  2. Diplotene

  3. Diakinesis.

  1. Leptotene:

  • Also called “Leptonema” (Lepto= Thin, Nema= Thread).

  • In this stage condensation of Chromosomes takes place.

  • Chromosomes get converted into thin thread like structures.

  • Each maternal and paternal chromosome starts searching for their “homologous chromosomes”.

  1. Zygotene:

  • Also called “Zygonema” (Zygo= Pair, Nema= Thread).

  • Maternal and Paternal Chromosomes come together to form a pair called “Synapse / Bivalent / Homologous pair”.

  1. Pachytene:

  • Also called “Pachynema” (Pachy= Thick, Nema= Thread). 

  • Most important event takes place i.e. “Crossing Over”.

  • Two nonsister chromatids overlap each other, the point of attachment called  “Chiasmata”.

  • Due to this transfer of genetic material took place between maternal and paternal chromosomes.

  • Pachytene hence is an important step as the basis of evolution takes place here.

  1. Diplotene:

  • Also called “Diplonema” (Diplo= Two, Nema= Thread). 

  • Nuclear membranes and nucleolus start disappearing.

  • Centrioles get connected by spindle fibers.

  • Centrioles start moving to the opposite ends.

  • The overlapped chromosomes start repulsing each other.

  1. Diakinesis:

  • Also called “Terminalization”.

  • Stage is characterized by the ending of cross over.

  • The repulsion between crossed over chromosomes continues, still there is attachment at tip.

  • Nuclear membranes and nucleolus disappear.

  • Centrioles got connected by spindle fibers.

  • Centrioles move to opposite ends.

  • This stage marks the end of Prophase I.

2) Metaphase I:

  • Chromosomes become more prominent.

  • Centrioles are moved towards the opposite ends of the cell and are connected with spindle fibers.

  • The crossed over chromosomes get arranged at the equatorial plane.

  • Crossed chromosomes are still attached at the tip with each other.

  • Metaphase is the best stage to study the chromosomes.

3) Anaphase I:

  • Spindle fibers start contracting.

  • Results in dys-junctioning of chromosomes.

  • The separated chromosomes start moving to the opposite ends of the cell.

  • The chromosome number gets reduced to half.

4) Telophase I:

  • It is just opposite to Prophase I.

  • Spindle fibers disappear.

  • Nuclear membrane and nucleolus reappears.

  • Formation of Two Haploid Nuclei takes place at opposite ends of cells.

  • The chromosomes undergo recoiling to form Chromatin threads.

5) Cytokinesis I:

  • The furrow formation takes place in the cell membrane.

  • The furrow starts growing and contracting outside to inside and results in formation of two daughter cells.

  • The formed daughter cells after brief rest undergoes Meiosis II.

Between Meiosis I and Meiosis II there is a short resting period of the cells called “Interkinesis”.

Meiosis II:

  • It is just like Mitosis.

  • Each haploid cell formed during meiosis I undergoes Meiosis II.

  1. Prophase II:

  • Nuclear membranes and nucleolus disappear.

  • Centrioles start moving to opposite ends of the cell and are connected by Spindle fibers.

  • Chromatin material undergoes condensation to form chromosomes.

  1. Metaphase II:

  • Chromosomes become more prominent.

  • Centrioles are moved towards the opposite ends of the cell and are connected with spindle fibers.

  • The chromosomes get arranged at the equatorial plane.

  • Metaphase is the best stage to study the chromosomes.

  1. Anaphase II:

  • Spindle fibers start contracting.

  • Results in dys-junctioning of chromosomes.

  • The separated chromosomes start moving to the opposite ends of the cell.

  1. Telophase II:

  • It is just opposite to Prophase II.

  • Spindle fibers disappear.

  • Nuclear membrane and nucleolus reappears.

  • Formation of Two Nuclei takes place at opposite ends of cells.

  • The chromosomes undergo recoiling to form Chromatin threads.

  1. Cytokinesis II:

  • The furrow formation takes place in the cell membrane.

  • The furrow starts growing and contracting outside to inside and results in formation of two gametes.

Commonly Asked Questions.

  1. Write a short note on “Meiosis”.

  2. Why is meiosis called “Reductional Cell Division”?

 

Labels:

Popular posts from this blog

Rate of Drying Curve.

  Definition Drying is defined as the removal of liquid from a product usually with application of heat. Rate of Drying Curve. Drying process can be divided into three periods Initial Adjustment Period. Constant drying rate period. First falling drying rate period. Second falling rate period. Initial Adjustment Period (A-B): Also called the “ Heating up” period . In this period the substance gets heat and increases in temperature. Drying has not yet started. Constant drying rate period (B-C): During this period the temperature of the solid and the rate of drying remain constant. The moisture evaporating from the surface is replaced by water diffusing from the interior of the solid at a rate equal t o the rate of evaporation.  The moisture content at the end of constant rate (point C) is referred to as the critical moisture content (CMC).  At CMC, dry spots start appearing and drying rate starts falling . First falling drying rate period (C-D): This period is also called ...
Read more

Heat Exchangers and Heat Interchangers.

  In pharmaceutical industries many types of equipments are used for transfer of heat, they can be classified as follows, Heat Exchangers. Heat Interchangers. Heat Exchangers: These devices are used for transferring heat from a fluid (Hot Gas or Steam) to another fluid (Liquid) through a metal wall. Heat Interchangers: These devices are used for transferring heat from a One liquid to another liquid or one gas to another gas through a metal wall. HEAT EXCHANGERS; The equipment used for heat transferring are known as heat exchangers. Some of the processes that involves heat transfer in pharmaceutical industries are: Preparation of starch paste (in steam jacketed kettle). Crystallization. Evaporation. Distillation.  Classification of heat exchangers On the basis of transfer of heat, heat exchangers are classified as: Direct transfer type:  The hot and cold fluids are separated by a metal wall through which the heat is transferred from hot fluid to cold fluid. E.g. shell and ...
Read more

Flash Distillation.

  Principle: When a hot mixture is allowed to enter from a high-pressure zone into a low pressure zone, the entire liquid mixture is suddenly vaporized. This process is known as flash vaporization .  During this process, the chamber is cooled.  The less volatile fraction is condensed and the more volatile component remains in the vapor phase .  This process requires time, hence liquid and vapor are kept in intimate contact until equilibrium is achieved. Flash distillation is also called equilibrium distillation because separation of two liquids takes place when liquid and vapor phases are at equilibrium. Equipment used for Flash Distillation: Construction: It consists of a pump, which is connected to a feed reservoir.  Pumps help in pumping the feed into the heating chamber.  The heating chamber is heat supplied by steam.  The other end of the pipe is directly introduced into the vapor-liquid separator through a reducing valve.  The vapor outlet...
Read more