Cell Cycle and Cell Division Notes

Table of Contents

In this article we will discuss about Cell Cycle and Cell Division Notes:- What is cell cycle, What is cell division, Amitosis, Mitosis and Meiosis

What is Cell Cycle?

Cell cycle is series of events involving cell growth and cell division
Cell growth, DNA replication and cell division constitute a cell cycle
Events happening in a cell cycle are genetically controlled
Duration of cell cycle varies in different organisms and different types of cells. The average duration of the human cell cycle is 24 hrs, whereas a yeast cell completes one cell cycle in 90 minutes
Discovered by Prevost and Dumas (1824)
Cell cycle is series of events that take place in cell, resulting in the duplication of DNA and division of cytoplasm and organelles to produce two daughter cells
Cell cycle is divided broadly into 2 phases

Interphase
M phase

Interphase

Interphase is most active phase
It takes more than 95% time of the cell cycle
Series of metabolic changes occurs during interphase
These changes are not visible under microscope, so termed as resting phase
The Interphase is further divided into G1, S, and G2 phase

I) G1 Phase

Synthesis of enzymes, RNA, amino acids, ATP and nucleotides occur
Raw materials synthesis for S phase
Size of cell is increased

II) S Phase

DNA replication occurs
Synthesis of histone protein takes place
DNA doubles but chromosome number remains same
If animal cell centriole also duplicates

III) G2 Phase

Cell prepares itself for division
Synthesis of proteins and RNA takes place
ATP synthesis occurs

M Phase

Cell division occurs in M phase
This phase has a short duration
M phase is composed of two processes

i) Karyokinesis

Division of nucleus into two daughter nuclei
Four sub stages

Prophase
Metaphase
Anaphase
Telophase

ii) Cytokinesis

Division of cytoplasm resulting in two daughter cells

Cell Cycle Checkpoint

Cell cycle checkpoints are control mechanisms in cell cycle which ensure its proper progression
Three most important checkpoints

G1 Checkpoint
G2 Checkpoint
M checkpoint

I) G1 Checkpoint

This checkpoint checks internal and external conditions are right for division
Check cell size
Check Enzyme, Protein and ATP are synthesized
Check DNA damage or not
If a cell doesn’t get it may leave the cell cycle and enter a resting state called G0 phase

II) G2 Checkpoint

DNA replication completeness
Check DNA completely copied or not
Check Enzyme, Protein and ATP are synthesized
If errors or damage are detected
Cell will pause at G2 checkpoint to allow for repairs
If the damage is irreparable, the cell may undergo apoptosis

III) M Checkpoint

M checkpoint is also known as the spindle checkpoint
Chromosome attachment to spindle at metaphase plate
If chromosome is misplaced, the cell will pauses mitosis, allowing time for spindle to capture the stray chromosome

CELL DIVISION

Production of daughter cell from parent cell is known as cell division
Cell division occurs as part of cell cycle

Types of cell division

Amitosis
Mitosis
Meiosis

Amitosis

Remak discovered amitosis in RBCs of chick embryo
Term was coined by Flemming
Also known as direct cell division
Cell nucleus elongates, constricts in middle and divides directly into two daughter nuclei
This is followed by constriction of cytoplasm to form two daughter cells

Characteristics of Amitosis

Occurs in lower organisms like yeast, fungi, bacteria and in amoeba
Nuclear membrane does not disappear during cell division
Spindle fibers are not formed during cell division
Daughter cell does not have equal number of chromosomes

Mitosis

Mitosis is called as equational division
No change in number of chromosomes in parent cell & daughter cell
Two major processes occur during M Phase: Karyokinesis and Cytoinesis

a) Karyokinesis

Nucleus division is occurs
Chromosomes are also separated
This phase consists of 4 stages

Prophase
Metaphase
Anaphase
Telophase

I) Prophase

First stage of mitosis
Chromatin material starts to condensation and take the shape of chromosomes
In each chromosome 2 chromatids are attached at centromere
Nuclear membrane disintegrates
Each centriole separate and migrates towards opposite pole of cell
Around centrole astral ray are formed
In addition longer spindle fibres are formed
Spindle fibres along with asters known as amphiaster or mitotic apparatus

II) Metaphase

Condensation of chromosomes is completed & observed clearly under microscope
Chromosomes lie at the equatorial plane
Spindle fibres extending from one pole to other are called continuous spindle fibres
Fibres extending from a pole to centromere of chromosome are called discontinuous spindle fibres
Metaphase is best stage in mitosis for analyzing chromosomes and to study their morphology

III) Anaphase

Chromatids of each chromosome are separated and form two chromosomes called daughter chromosomes
Daughter chromosomes move to opposite poles
Fibres which develop between the separating centromeres are called interzonal fibres
Migration of daughter chromosomes due to contraction of spindle fibres and stretching of interzonal fibres
The chromosomes appear as V, L, J and I shaped depending upon the position of centromere
At end of anaphase, two groups of chromosomes are formed, one at each pole
Number of chromosomes at each pole is equal to number of chromosome in the parent nucleus

IV) Telophase

Final stage of karyokinesis
Telophase is a reversal of prophase
Telophase chromosome became decondensed and uncoiled and form chromatin
Nucleolus, nuclear envelope and nucleoplasm is reformed
Spindle fibres break down and absorbed in cytoplasm

b) Cytokinesis

Division of cytoplasm into two daughter cells is called cytokinesis
Microtubules and microfilaments arrange on equator to form midbody
Periphery of equator contractile ring formed that is made up of actin and myosin
Due to actin and myosin contract, furrow forms from outside to inside in cell
Furrow deepens continuously and ultimately cell divides into two daughter cells

Significance of Mitosis

Mitosis ensure equal distribution of chromosomes and cytoplasm between daughter cells
Chromosomal number is maintained constant
Mitosis helps in growth and development of organisms
Helps in repair of tissues
Helps in asexual reproduction of organisms

Mitosis Plants vs. Animals

Animal Cell

Centrioles are present
Aster formation occurs
No cell plate is formed
Furrowing of cytoplasm occurs
Occurs in tissues throughout the body

Plant Cell

Centrioles are absent
No aster formation
Cell plate is formed
No furrowing of cytoplasm at cytokinesis
Occurs mainly in meristems

Types of Mitosis

Anastral Mitosis

During mitosis these cells do not produce centrioles
Spindle fibres forms but no asters are observed
Commonly found in plants

Amphiastral Mitosis

During mitosis these cells produce two centrioles
Spindle fibres is associated with two asters
Commonly found in animals cell

Intranuclear Mitosis

Mitotic divisions take place inside the nucleus
Nuclear envelope remains intact
Spindle fibres forms inside the nucleus
Found in some protozoa and fungi

Endomitosis

Chromosomes replicate without division of the cell nucleus
Resulting in polyploidy nucleus
Occurs in salivary glands of Drosophila and other flies

Free Nuclear Division

Karyokinesis is not followed by cytokinesis
It arises a multinucleated condition
Found in some fungi

Dinomitosis

Dinomitosis is type of mitotic division
Occurs in the class Dinophyceae
Nuclear membrane intact
No spindle fibres are formed
Chromosomes remain attached to nuclear membrane

Meiosis

Discovered in plant cells by Strasburger
The term was coined by Farmer and Moore
Meiosis also known as reductional division
Sexual reproduction in organisms takes place through the fusion of male & female gametes
They contain only half the number of chromosomes
Meiosis leads to formation of haploid cells
Meiosis is composed of two rounds of cell division: Meiosis I and Meiosis II
Each round of division contains a period of karyokinesis and cytokinesis

Meiosis I

Meiosis 1 separates the pair of homologous chromosomes
Reduces the diploid cell to haploid
The different stages of meiosis 1

I) Karyokinesis I

Prophase I
Metaphase I
Anaphase I
Telophase I

II) Cytokinesis I

Prophase 1

Longest duration in Meiosis-I
It is divided into five sub-stages

Leptotene

Size of cell and nucleus increases
Chromosomes appear long, uncoiled thread like in structure bearing many bead like structures
Nuclear membrane and nucleolus remain as it is

Zygotene

Pairing of homologous chromosomes is called Synapsis
Paired homologous chromosomes are referred as bivalents or tetrad
Homologous chromosomes are held together by Synaptonemal complex

Pachytene

Longest phase because crossing over takes place
Chiasmata formation occurs
Small fragment of chromosome exchange between two non-sister chromatids

Diplotene

Bivalents begin to separate due to dissolution of synaptonemal complex
Non sister chromatids remain attached at the sites of crossovers.
Nuclear membrane and nucleolus begins to disappear

Diakinesis

Movement of the chiasmata towards the end of chromosome is called terminalization
End of diakinesis the nuclear membrane and nuleolus get completely disappeared
Chromosomes are free in the cytoplasm
Spindle fibres begin to form

Metaphase I

Bivalents align at the equatorial plate
Spindle fibers are attached with the centromeres of the homologous chromosomes

Anaphase 1

Spindle fibres shorten
Homologous chromosomes of each bivalent segregate and move to opposite poles
Segregation of homologous chromosomes during anaphase 1 is called disjunction
Division of centromere is absent

Telophase I

Separated chromosomes form two haploid nuclei
Nuclear membrane and nucleolus reappears
The chromosomes get uncoiled into chromatin thread
The spindle fibres disappear totally

Cytokinesis I

In animals, cytokinesis occurs by the constriction of the cell membrane
Also known as furrowing method
In plants, it occurs through the formation of the cell plate
Resulting in the creation of two daughter cells

Interphase II or Interkinesis

Two daughter cells formed pass through a short stage called interphase-II
Resting phase between meiosis-I and meiosis-II
Synthesis of RNA and protein may take place
Centrosome undergo replication
No DNA synthesis occurs

Meiosis II

Daughter cell divides mitotically and results in four haploid cells
This division is also known as the homotypic division
This division does not include the exchange of the genetic material and the reduction of the chromosome number
The different stages of meiosis II

I) Karyokinesis II

Prophase II
Metaphase II
Anaphase II
Telophase II

I) Cytoinesis II

Prophase II

Centrioles move towards the opposite poles
Nuclear membrane and nucleolus disappear
Spindle fibre starts to form
Chromosome again become compact

Metaphase II

Chromosomes get arranged on the equator of the cell
Spindle fibres organize between poles and attaches to centromere of chromosome

Anaphase II

Centromere of each chromosome divides
Sister chromatids separates to form two daughter chromosome
Sister chromatids are pulled away towards the opposite poles
Daughter chromosome moves towards the opposite poles due to the shortening of Spindle fibres and the stretching of interzonal fibres

Telophase II

Meiosis ends with telephase II
Daughter chromosome enclosed by nuclear envelope
Chromosome elongates to form thin networks of chromatin
Nucleolus reappears due to the synthesis of ribosomal RNA

Cytokinesis II

The result of cytokinesis is four haploid daughter cells
Cytokinesis takes place by cell plate formation in plant cell
- Successive methods: cytokinesis followed by each nuclear division resulting in 4 haploid cells. Eg. Monocot plants
- Simultaneous methods: cytokinesis occurs only after meiosis-II to form 4 haploid cells. Eg. Dicot plants
In animal cells, cytokinesis occurs by furrow formation

Significance of Meiosis

Meiosis helps to maintain a constant number of chromosomes by reducing the chromosome number in the gametes
Essential for sexual reproduction in higher animals and plants
Meiosis helps in the formation haploid gametes and spores for sexual reproduction
Number of chromosome remain fixed in a species from generation to generation
Crossing over occurring brings genetic variations in offspring which helps in evolution of organisms
Failure disjunction in Meiosis leads mutation to the formation of polypoid forms