Cell Cycle and Cell Division Notes | Class 11 & NEET Free Notes

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

1. Interphase
2. 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

1. Prophase
2. Metaphase
3. Anaphase
4. 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

1. G1 Checkpoint
2. G2 Checkpoint
3. 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

1. Amitosis
2. Mitosis
3. 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

1. a) Karyokinesis

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

1. Prophase
2. Metaphase
3. Anaphase
4. 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