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CELL DIVISION
1
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Flemming at first studied mitotic division in Salamander
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(iii) Strasburger gave name prophase, metaphase, anaphase, telophase
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(v) Position of chromosome at the time of division was given by Darlington
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(viii) Hormone cytokinin increases rate of cell division
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(iii) Nucleus becomes dumbell shaped, and get divided into two daughter nucleus
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Its results in formation of two
daughter cells
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g
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Blue-green algae, bacteria and Eukaryotic cells
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(ii) Mitosis is a cell division in which parent cell divide to form two daughter cell, in which
number of chromosome amount of DNA number and types of gene are equal to parent cell
(iii) It occurs in somatic cell
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(v) Mitosis results in increase in number of cells in the Mitosis cell division can be divided into
two phases
(1) Interphase (2) Division phase or M phase
(1) Interphase
(i) Interphase is phase between two cell division
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(iii) It is longest phase of cell cycle
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(iv) It is also called resting phase
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(viii) In interphase from the centrosphere of centrioles, asters arises
Interphase can be divided into 3 sub phases
2
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1
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2
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1
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2
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1
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2
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(ii) Mitochondria, Chloroplasts (plants), Lysosomes, Endoplasmic Reticulum Golgi Complex,
Vacuoles and Vesicles are produced
(iii) Structural and functional proteins are formed
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(v) Ribsomes get synthesised in this phase
(vi) Metabolic rate of the cell becomes very high,
(vii) It may be called pre DNA synthesis phase
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(DNA get doubled)
(b) Protein molecules called histones are synthesized that cover each strand of DNA
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(d) In human cell S-phase get completed in 7 hours
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(b) Mitotic spindle begins to form
(c) This phase may be called post DNA synthesis phase
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-phase get completed in 6 hours
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(a) Karyokinesis - Nuclear division
(b) Cytokinesis - Cytoplasmic division
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(iv) Nucleolus start disappearing
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(vi) Around each centriole astral rays are formed in the cytoplasm, spindle fibre also get formed
(B) Metaphase
(i) The chromosome arrange at the equatorial plane
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(iii) Some other fibres of the spindle extend from one pole to the other pole
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(iv) The fibres which occur in between the chromosomes are called interzonal fibres
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each being associated with a
chromatid
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(iii) The daughter chromosomes move towards the opposite pole of the cell
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(iv) The chromosomes attains the shape of J
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(D) Telophase
(i) The daughter chromosomes with their centromereat the poles begin to uncoil and lengthen
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(ii) The nucleolus reappear
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R
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5 Cytokinesis
After nuclear division (Karyokinesis) cytoplasm get divided into two (more or less) equal parts,
this results information of two daughter cells from a single parent cell
(a) In animal Cells
(i) The cell membrane begins to invaginate where spindle equator was present earlier
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(B) In Plant Cells
(i) The spindle fibres do not disappear at the region of equatorial plane
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(iii) As the plate gradually becomes more distinct and develops no into the new cell wall
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Differences in Mitosis of Animal and Plant Cells
S
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Mitosis in Animal cell
Mitosis in Plant cell
(i)
Centriole present
Centriole absent
(ii)
Formation of aster-rays
Absence of aster-rays
(iii)
no cell plate formation
Cell plate formation takes place
(iv)
Forrowing of cytoplasm
Furrow formation absent
during cytokinesis
(v)
Occurs in tissues
Occurs mainly at meristems
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(iii) There is no variation in genetic information
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In many lower animals mitotic cell division is responsible for regeneration and asexual
reproduction
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(ii) In this type of division parent divides to produce four daughter cell
(iii) It is a reduction division in which the number of chromosomes (genetic matter) is reduced to
halfi
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diploid number (2n) 1 becomes haploid (n)
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It
is divided into two main phases
(9) Meiotic-I Meiotic-l is divided into two division
(a) Karyokinesis - Nuclear division
(b) Cytokinesis - Cytoplasmic division Karyokinesis
10
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It is divided into four phases - (a) Prophase-1 (b) Metaphase-l (c)
Anaphase 1 (d) Telophase -1
(A) Prophase-l
It is divided in five sub - stages:
(a) Leptotene (b) zygotene (c) Pachytene (d) diplotene (e) Diakinesis
(a) Leptotene
(i) The volume of the nucleus is increased
(ii) Chromosomes becomes distinct, long and uncoiled
(iii) They occur in diploid number
(iv) They contain a series of beaded structure called chromosome The centriole divides into two
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Each pair consists of a maternal chromosomes (the chromosomes of the mother) and a
paternal chromosomes (the chromosomes of the father)
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(ii) The chromosomes become thicker and shorter
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(iv) The centrioles move to the opposite poles
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As a result, each bivalent now contains four chromatids
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(ii) The homologous chromosomes of each pair begin to separate because of the gradual
disappearance of the force of attraction between them, but the two homologous chromosomes do
not completely separate, but remain attached together at one or more points know as chiasmata
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The broken segment are interchanged
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The interchange of the chromatin materials is described as
crossing over
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(d) Diplotene
(i) In this stage, the homologous chromosomes repel each other
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(ii) By the end of this stage, the chiasmata begin to move along the length of the chromosomes
from the centromere towards the end
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(ii) The bivalents tend to repel each other and migrate to the periphery of the nucleus just inside
the nuclear membrane
(iii) The nucleolus disappears
(iv) The nucleus membrane also begins to disintegrate and disappear
(v) Spindle fibre make their appearance in the cytoplasm
Metaphase-I
The spindle fibres get well developed
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The centromeres lie to wards the poles and the arms
towards the equator
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(C) Anaphase-1
Each chromosome with two chromatids and undivided centromere moves towards the opposite
pole of the cell
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When separated, in each chromosome
the sister chromatids are connected by a centromere
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Shrinking of spindle fibre takes place
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The nuclear membrane and the nucleolus reappear and thus two
daughter nucleiare formed
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Interphase-II
It is period in between meiosis-I and Meiosis-II
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It is generally
present in animal cell
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Like Meiotic -1, Meiotic-Il is
divided into two parts Karyokinesis and cytokinesis
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Each centriole divides into two resulting
in the formation of two centrioles
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They produce asters
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The nuclear membrane and nucleolus disappear
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The two chromatids of each
chromosome are separated by the divisions of centromere and are attached to the spindle fibres
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(d) Telophase II During this stage, the daughter chromsomes again form chromatin thread
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The nucleolus reappears
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13 Cytokinesis
After this there is cleavage of cell membrane in animal cells or cell wall formation in plant cells
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14 Significance of Meiosis
Gametes are produced by meiosis
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In this way by meiotic
cell division number of chromosome is maintained constant in a species
During crossing over exchange of genes takes place in between maternal and paternal (two)
chromosomes exchange of genes leads to variation
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(b) Chromosomes is shortest in metaphase and thickest at anaphase
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(e) Cholchicine inhibits spindle formation at metaphase
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