Lecture Delivered at KMC – IC3 October 2011

Format of LectureIntroduction on Cell CycleConcept of Cell DivisionTypes of Cell DivisionRole of Chromosomes in Cell DivisionMitosis: Technical CharacteristicsMeiosis: Technical CharacteristicsOverall Learning Outcome

Introduction on Cell CycleDistinction between Living and Non-Living

mattersLiving things reproduce and they have a capacity

to procreateProcreation in the living organism has a cellular

basisA great German Physician – Rudolf Virchow put

this distinction very brilliantly – Where a cell exists, there must have been a preexisting Cell, just as the animal arises only from an animal and the plant only from a plant

Continued:Rudolf Virchow (1821 – 1902) explained this

concept of cell cycle by using a Latin axiom “Omnis cellula e cellula” – meaning every cell from a cell.

The continuity of life is based on the reproduction of cells – cell division

Virchow extended his argument on the basis of the existing cell theory proposed by Schleiden and Schwann

Postulates of Cell CycleAll living organisms are composed of cells

and cell productsThere are considerable similarities in the

chemical constitution and metabolic activities of all cells

The total activities of an organism is the sum of the activities and interactions of its independent cell units

Structural Types of CellProkaryotic Cell Eukaryotic Cell----- Prokaryotic Cell – Greek word combination of

the word Pro means first and Karyon means nucleus. The Prokaryotic cells are in the form of bacteria and blue green algae. These are very small and show primitively organised nucleus and cytoplasm

----- Eukaryotic Cell – Greek word combination of the word Eu means true and Karyon means nucleus. The Eukaryotic cells are found in the most of the algae, fungi, plants and animals.

Difference Between Prokaryotic and Eukaryotic CellsProkaryotic Cella)Nuclear membrane

absentb)Nucleolus absent c)Lack of clearly

defined membranesd)Mitotic apparatus

absente)Nucleus diffused or

incipient

Eukaryotic Cella)Nuclear membrane

is presentb)Nucleolus is also

presentc)Chromosomes are

made up of DNAd)Mitotic apparatus

presente)Nucleus is well

organised

Biological Organisation of ProtoplasmProtoplasm is bounded by a cell membrane. It is

further divisible into cytoplasm and nucleoplasmNucleoplasm is enclosed by the nuclear

membraneThe protoplasm is highly organised and it consists

of a number of organelles which are highly ordered structures performing various functions

The Protoplasm also contains a number of depositions called deutoplasmic substances and vacuoles

Concept and Key Roles of Cell DivisionCell Division plays several important roles in

the life of an organismTo explain this – one can take an example of

amoeba – because it is a unicellular organism and when it divides and forms duplicate offsprings - the division of one cell reproduces an entire organism

Cell division is a dynamic process by which cells multiply in number leading to the production of new cells.

Continued:Without Cell division, multicellular organisms

cannot growWhen cell divides, each cell is a complete cell in

itself.Immediately following division, the daughter

cells may be smaller than the parent cell but they grow rapidly and soon reach the size of the original cell.

In real sense, cell division is a process of duplication or multiplication rather than the division in the usual sense.

Continued:Cell division continues to function in renewal

and repairIt replaces cells that die from normal wear

and tear or accidents.Cell division process is an integral part of the

cell cycleHow cell division distributes identical genetic

material to daughter cellsUnderstanding of the cellular mechanics of

cell division is very essential for a scholar

Continued:Knowledge on the molecular control systems

that regulates progress through the cell cycle will also be necessary

What happens when the control system malfunctions will also be useful to understand.

Relevance of Cell Cycle regulation – an active area of research across the globe.

Role of Chromosomes in Cell DivisionA chromosome is a body having special

organisation, individuality and functional qualitiesIt is capable of reproducing itself through

successive cell divisionsThere are two basic types of chromosomes found

in living organisms - a) those found in Prokaryotes – single circular DNA not associated with basic protein (histone); b) true chromosomes characteristics of eukaryotes, this contains DNA, RNA associated with histones and other proteins

ChromosomesThe chromosomes exhibit great variations in

their shape, number and length.The shape of the chromosomes depends on the

location of the centromere – the centromere is the driver of the chromosome because it is responsible for the movement of chromosomes in mitotic spindle.

The longitudinal half of chromosome is known as Chromatid.

The backbone of the chromosomal fibre is made up of DNA molecule

Name (Animals) Chromosome Numbers

Man (Homo Sapiens) 46Dog (Canis Familiaris) 78Cat (Felis Catus) 38Rabbit ( Oryctolagus Cuniculus)

44

Rat (Rattus norvegicus) 42Mouse (Mus Muculus) 40Frog (Rana Pipiens) 26Cockroach (Blatta Germanica)

23

Mosquito (Culex Pipiens)

6

Types of Cell DivisionCell division involves the distribution of

identical genetic material – DNA – to two daughter cells

It is well known that the DNA is passed along from one generation of cells to the next

A dividing cell duplicates its DNA - allocates the two copies to opposite ends of the cell and afterwards split into daughter cells.

Cell DivisionCell division may occur by four methods:a)Amitosis or direct cell divisionb)Endomitosisc)Mitosisd)Meiosis

Amitosis or Direct Cell DivisionAmitosis is the means of asexual

reproductionIt happens in unicellular organisms like

bacteria, yeast etc.In this type the splitting of nucleus is

followed by cytoplasmic constriction

EndomitosisIn this type, chromosomes duplicate but

cytoplasmic division is prevented.This results in the duplication of chromosome

number.