Growth: Increase in bulk of body.3 ways of Growth1. Multiplicative:- increase in cell no.2. Auxetic:- increase in cell size.3. Accretionary :- increase accumulation of intercellular substances.
1. Multiplicative growth: cell division causes increase in cell no.3 categoriesA) Cat I : do not divide in post-natal period. e.g.- nerve cellsB) Cat II : continuous loss is replaced by stem cells. e.g.- epidermis of skin -intestinal epithelial cell -RBCsC) Cat III : divides only in altered conditions. e.g.- Liver in resection
2. Auxetic growth : increase in cytoplasmic volume. e.g.- oocytes & some Neurons Nucleus is not sufficient to fulfill the nutritional need. So surrounded by small cells to provide nutrition.3. Accretionary growth : increase accumulation of intercellular substances e.g.- connective tissue:- Bones & Cartilage.
CONTROL :-Local EndocrinalLOCAL:- During early embryonic lifeENDOCRINAL:-Affects rate Generally- Somatotropic hormone. Affects rate Locally – Progesterone.-by affecting General metabolic rate-Thyroid hormoneOR affecting protein synthesis-some Corticosteroid & Somatotropic hormone*local control is imp in wound healing in Adults.*Chalones are imp. Which inhibits cell division normally.
CELL CYCLE Definition : is the period of time between the birth of the cell & it’s own division to produce two daughter cells. Rate: - Short life span( fast turnover ) - according to demand as in healing of wounded skin Faulty coupling of replacement – NeoplasmPhases of cell cycleA. Interphase :- G1, S ,G2B. Mitotic phase :- M
G1-Phase :-Responds to growth factors to initiates cycle -once made, this is irreversible -molecular machinery for cell cycle generated G0 Phase/Quiescent phase- cells retain capacity for proliferation but no longer dividing. - GH can stimulate it to enter cell cycle.S-Phase :- DNA Replication occursG2-Phase :- -cell prepares for division -period ends with start of breakdown of nuclear membrane & onset of chromosomal condensation.Timing :-S phase :6-8hrs G1 phase : great variationG2 phase : 2-4 hrs 2hrs in rapidly dividing toM phase : 1-2 hrs >100hrs in some cells
Regulation of transitions between cell cycle phases- Cyclins at there maximum abundance at G1-S,G2-M transitions.- High level cyclins protein activates CDKs- Activation of various cyclin-CDK complex regulates G1- S,G2-M transitions.*There are check points in the cell cycle at which progress will be arrested.**P53 – Tumor suppressor gene
Mechanism of Cell DivisionDivision -Direct/amitotic- random nuclear material distribution -Indirect- a) Mitosis- somatic cells b) Meiosis- Germ cells Two events - Karyokinesis - Cytokinesis
MITOSISDefinition:- “Mitosis is the process that results in the distribution of identical copies of the parent cell genome to the two daughter somatic cells.”Four phases:- -Prophase -Metaphase -Anaphase -Telophase
Prophase Condensation of chromatin Centriole separation & movement at opposite pole Microtubule formation, Aster formation Nucleoli disappears Nuclear membrane disintegration into vesicles to release chromosomes(**this event marks end of prophase)Prometaphase Microtubules extends into central region Attaches chromosomes & move it towards Metaphase/Equatorial plate or plane
Metaphase :- Chromosomes are in equatorial plane/aster formation Arranged in ring viewed from poles Cytoplasmic movements during late metaphase Centromere is doubled structureAnaphase :- Centromere separates Chromosomes moves apart towards poles Infolding of cell equator begins & it deepens during telophase as cleavage furrow.
Telophase :- Chromosome decondense Nuclear membrane forms Nucleoli appears Cytoplasmic division continues Spindle remnants disintegrates cleavage furrow- due to band of actin & myosinINHIBITORS OF CELL CYCLERadiation exposure -chromosomal damage-inhibition of cell cycle-ulceration of skin & mucous membraneChemical agents - Colchicine & It’s derivatives(**imp. In Karyotyping & cancer therapy)
MEIOSISDefinition:- “The process in which two cell divisions occurring one after the other, following only one round of DNA replication; shows chromosomal pairing and chromosomal seperation in the first division and chromatid seperation in second; leading to the formation of four haploid daughter cells from a single 2n parent cell.”
Stages of meiosis Meiosis-Io Prophase- I - Leptotene - Zygotene - Pachytene - Diplotene - Diakinesis / Prometaphaseo Metaphase – Io Anaphase – Io Telophase- I Meiosis- IIo Metaphase – IIo Anaphase- II similar to mitosiso Telophase- II
Prophase- ILeptotene:- Long thin thread to beaded chromomeres End is attached to nuclear membraneZygotene Homologus chromosomes are arranged in pairs lengthwise / Synapsis One paternal / one maternal
Pachytene: Chromosome splits longitudinally except at centromere Tetrad formationDiplotene: Form chiasma Break by endonuclease & joined by ligases Cross over Imp. For continual reshuffling of the genes / structural basis of hereditary diversity