How it heals??

CELL DIVISION Cell need to be replaced as they:

a) Wear out after some time

b) Get damaged (by cuts, UV radiation, environment pollutants)

c) Grow old and die

Most cells do not divide after they have grown and become specialised cells

Eg. – guard cells of the stomata

- nerve cells

- muscle cells

- red blood cells Some cells divide frequently. Eg. –

epithelium cells in intestine, outer layer of skin

Cell division :

Mitosis – by somatic cells, produce new

2 identical daughter cells

Meiosis – by reproductive cells, produce 4 non-identical daughter cells

Phases in cell cycle

Cell cycle – sequence of events taking place in a cell when it is formed until it divides into daughter cells

2 main phases interphase – period between division with 3

subphases, (G1, S, G2)M phase – 2 main subphases, (cytokinesis,

mitosis)

Interphase G1 (Gap 1/growth phase 1)

Cells grow rapidly New organelles are synthesized

S - DNA synthesis Amount of DNA double Chromosomes are replicated

G2 (Gap 2/growth phase 2) Cell prepare for mitosis Protein connected with mitosis are synthesized Organelles replicate, energy increase,

chromosomes condense, mitotic spindle begin to form

M phase

Mitosis Duplicated chromosomes are separated Further divided into prophase,

metaphase, anaphase, telophase

Cytokinesis Division of cytoplasm

– animal cell by cleavage furrow

- Plant cell by cell plate

Early prophase - Chromatids (duplicated chromosome) condense,

visible under microscope- Nucleolus disappear- Paired centrioles move to opposite ends

Late prophase - Nuclear membrane disappear- Spindle forms- Asters form(short microtubules radiate

from centriole)

Metaphase Sister chromatids

line up at spindle equator

Centromere attaches itself to spindle fibre

End of metaphase – centromeres divide

Anaphase Chromosomes pull apart by contraction of

spindle fibres Anaphase completed when chromosomes

arrive at poles

Telophase

Chromosomes cannot be seen clearly Spindle fibres disappear Centrioles replicate Nuclear membrane

reform Nucleolus reappears

in each nucleus

Cytokinesis

Separation of cytoplasm Organelles and other content of cells are

equally divided between two daughter cells In animal cell – cell membrane constricts in

to form cleavage furrow In plant cell – vesicles (carbohydrate)

produced by Golgi body gather at equator of parent cell, fuse to form cell plate, cellulose builds on each side form the cell wall

Similarities (animal and plant cell)

Both have 4 phase – prophase, metaphase, anaphase, telophase

Chromosomes are replicated in S phase Sister chromatids align at equator in metaphase Centromeres divide and sister chromatids

separate in anaphase 2 daughter cells are produce from one parent cell Daughter cells are genetically identical (diploid,

2n) to parent cell

Differences Animal cell Aspect Plant cell

Form during prophase Aster Absence of aster

Centrosome (2 centrioles) located outside nucleus

Centrosome No centriole in plant cell

Form cleavage furrow Cytokinesis Form cell plate

In tissues throughout body except

reproductive cells (sperm and ovum)

Location Mainly at the meristem

cell (root and shoot)

Irregular shape Shape of daughter cells Rectangular shape