PRESICE AND SHORT NOTES TO BE ABLE TO CRACK NEET EXAM COMING FROM AN MBBS STUDENT. CAN BE USEFUL IN BOARD EXAMS AS WELL. INFORMATION FROM AAKASH COACHING CENTRES AND NCERT TEXTBOOKS HAVE BEEN ADDED.
➔ Replication of genetic material followed by division of the cell in Two
➔ Prevost and Dumas First Study cell division during the Cleavage of zygote of frog
➔ Rudolf Virchow found new cells developed from pre - existing cells, Cell lineage theory, doctrine of
genetic continuity - (omnis cellula-e cellula)
➔ Mature cell divides into two daughter cells
➔ Cell growth continuous process
➔ DNA synthesis specific stage - S phase
Phases of cell cycle : Period required to complete one cell cycle is called generation time - 24 hours in
humans, 90 minutes in yeast. - Divided into two basic phases :
➔ Interphase (longest phase in cell cycle - 95% of the whole time)
◆ Between end of one cell division to beginning of next cell division - growing phase / resting
phase (no division) - Metabolically active phase, preparing a cell for the next division - All
mature cells occur in interphase
◆ Nerve cells of mammals have the longest interface, and do not divide after formation at
birth - stomach cells replaced every 3 days due to high acidic environment
◆ Three stages :
● G1 phase : longest in interphase major checkpoint - G1 to S phase or Go phase
deciding.
○ Synthesis of rRNA and mRNA, ribosomes and protein
○ metabolic rate is high - organelle duplication
○ cell size increase
○ no change in DNA amount
● S phase :
○ most active phase in cell cycle.
○ DNA synthesis period
○ DNA becomes double, 2C to 4C in the 2n diploid cell
○ No increase in chromosomes - diploid stays diploid but has duplicate
genes
○ Synthesis of histone proteins and non histone chromosomal proteins and
new chromatin fibres formed.
○ duplication of centrioles in cytoplasm. - absent in higher plants - needed in
M phase.
● G2 phase : minor checkpoint.- G2 to M transition - checks DNA damage.
○ Tubulin (mitotic spindle protein) synthesis begins
○ Chromosome condensation factor appears
○ Repair damaged DNA
○ ATP, RNA, NHC are synthesised
● Go Phase (inactive phase)
○ Quiescent stage
○ Cells remain metabolically active but do not proliferate unless needed
○ Cells exit G1 Phase to enter this inactive phase
○ Eg : NERVE CELLS
➔ M phase or mitosis phase : Dividing stage / mitotic stage - most dramatic Phase of cell
division : Karyokinesis and cytokinesis - Equatorial division, somatic cell division, chromosome
number is constant
◆ Karyokinesis 4 phases : nuclear division :
● Prophase :
○ Longest phase, chromatin fibres thicken and shorten, form chromosome,
overlap like ball of wool, chromosome divides into two chromatids attached to
centromere, nuclear membrane and nucleolus disintegrates, spindle
, formation begins, centrioles move opposite poles, cell components
disappear.
● Metaphase :
○ Chromosomes size (thickest and shortest) can be measured, move towards
equatorial plane of spindles, arranged with arms towards pole, centromere
towards equator (CONGRESSION), spindle fibres attached to
kinetochores, chromosomes during metaphase have 2 chromatids,
metaphase best stage for studying chromosome morphology.
● Anaphase,
○ Shortest stage, chromatids move towards opposite poles due to anaphasic
movement, shape is best studied at anaphase .
● Telophase
○ Chromosomes reach poles by spindle fibres, form two groups, begin to uncoil,
form chromatin net, nuclear membrane nucleolus and other components re-
appear, spindle fibres disappear, centrioles organised into centrosomes,
reverse of prophase.
◆ Cytokinesis : division of cytoplasm :
● Stars at mid - anaphase and completed at telophase
● Syncytium : only karyokinesis, no cytokinesis - leads to multiple nuclei - eg : liq
endosperm in coconut
● two methods :
○ Cell furrow method - centripetal manner - mostly in animals
◆ Construction of furrow deepens centripetally and divides mother into
two daughter cells - help of microfilaments
○ Cell plate method :
◆ Mostly in plant cells
◆ From Middle lamella, centrifugal plate formation, materials are on
both sides of the cell plate, forming two daughter cells.
➔ Significance of Mitosis :
◆ Constant chromosome number, genetic stability, all similar genetic constituency provides
new cells for healing, eg : cells of upper layer of epidermis and blood cells, helps at sexual
reproduction by fragmentation, budding, stem cutting.. somatic variations maintained,
important role in speciation.
➔ Meiosis :
◆ In mature diploid reproductive cells, the nucleus divides twice but chromosome
replicates once, four haploid cells formed, half chromosomes of parent, reductional
division, occurs only once.
◆ In diploid germ cells of sex organs, can be studied in young flower buds, in Pollen mother
cells
◆ Meiosis 1 is initiated after parent chromosomes have replicated to produce sister chromatids
during S phase, involving recombination of chromosomes, four haploid cells formed.
◆ Two Steps :
● meiosis 1 : (4 chromatids and 2 centromeres)
○ Pro Phase 1 - Longest phase, 5 subphases : leptotene, zygote, pachytene,
diplotene, diakinesis :
◆ Leptotene : Long chromosome is called chromomeres, shows
bouquet arrangement - condensation, coiling
◆ Zygotene : Chromosomes short thicken, paired homologous
chromosomes are seen, Pairing of chromosomes occur -> synapsis,
formation of synaptonemal complex (ladder like nucleoproteins
complex) - AKA bivalent
◆ Pachytene : Longest stage, recombination or crossing over occurs,
characterised by appearance of recombination, 4 chromatids and 2
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