how cell division occur in the ell

1. CELL DIVISION

2. CELLS AND CHROMOSOMES
 In both Eukaryotic and Prokaryotic cells, the genetic
material is organized into chromosomes
 Biologists established that all living things are
composed of cells
 Single cell or trillions of cells
 Simplest life forms- viruses are not composed of cells
 Must enter cells in order to function

3. THE CELLULAR ENVIRONMENT
 Living cells are made of many different kinds of molecules
 Water is the most abundant
 Small molecules (salts, sugars, amino acids, and certain
vitamins) readily dissolve in water, and some larger molecules
interact favorably with it
 Hydrophilic and hydrophobic
 Cytoplasm of cell contains both hydrophilic and hydrophobic
substances
 Molecules that make up the cell – Carbohydrates, Lipids, Proteins
(enzymes) etc

4.  Cells are surrounded by membrane
 Specialized structures called Organelles are present inside the cell
 Plant cell wall – cellulose
 Bacterial cell wall - murein
 Walls and membranes separate the contents of the cell
with the outside world, but do not seal it off

5. PROKARYOTIC AND EUKARYOTIC CELLS
 2 kinds of cells – Prokaryotic and Eukaryotic
 Prokaryotic
 Usually less than a thousandth of a millimeter long
 typically lack a complicated system of internal membranes and
membranous organelles
 hereditary material—that is, the DNA—is not isolated in a
special subcellular compartment
 Examples include bacteria (the most abundant life forms on
Earth) and archaea (found in extreme environments such as
salt lakes, hot springs, and deep-sea volcanic vents)
 All other organisms—plants, animals, protists, and fungi—are
eukaryotes.

6.  Eukaryotic cells
 larger than prokaryotic cells, usually at least 10 times bigger
 possess complicated systems of internal membranes
 For example, eukaryotic cells typically contain one or more
mitochondria (singular, mitochondrion-dedicated to the
recruitment of energy from foodstuffs
 Algal and plant cells contain another kind of energy-recruiting
organelle called the chloroplast, which captures solar energy
and converts it into chemical energy
 Both mitochondria and chloroplasts are surrounded by
membranes

7.  In eukaryotic cells, DNA is contained within a large,
membrane-bounded structure called the nucleus
 DNA is organized into discrete structures called
chromosomes
 Individual chromosomes become visible during cell
division when they condense and thicken
 In prokaryotic cells, the DNA is usually not housed
within a well-defined nucleus
 Some of the DNA within a eukaryotic cell is not situated
within the nucleus
 This extranuclear DNA is located in the mitochondria and
chloroplasts.

8.  Both prokaryotic and eukaryotic cells possess numerous
ribosomes (involved in the synthesis of proteins)
 found throughout the cytoplasm
 Although not composed of membranes, in eukaryotic cells they
are often associated with a system of membranes called the
endoplasmic reticulum
 The reticulum may be connected to the Golgi complex
(membranous sacs and vesicles involved in the chemical
modification and transport of substances within cells)
 Other small, membrane-bound organelles may also be
found in eukaryotic cells

9.  In animal cells, lysosomes are produced by the Golgi
complex
 contain different kinds of digestive enzymes that would harm
the cell if they were released into the cytoplasm
 Both plant and animal cells contain peroxisomes
(dedicated to metabolism of substances such as fats
and amino acids)
 The internal membranes and oganelles of eukaryotic
cells create a system of subcellular compartments that
vary in chemical conditions such as pH and salt content
 This variation provides cells with different internal
environments that are adapted to the many processes that
cells carry out.

10. CYTOSKELETON
 The shapes and activities of eukaryotic cells are
influenced by a system of filaments, fibers, and
associated molecules that collectively form the
cytoskeleton
 give form to cells and enable some types of cells to
move through their environment—a phenomenon
referred to as cell motility
 holds organelles in place, and it plays a major role in
moving materials to specific locations within cells—a
phenomenon called trafficking

11. CELL DIVISION
 A cell can divide into two cells, each of which can in turn
divide into two
 Creates a population of cells – clones
 Excluding the errors, all the cells within a clone are
genetically identical
 Cell division is an integral part of the growth of multicellular
organisms, and also the basis of reproduction

12. CELL DIVISION
 Mother cell – a cell that is about to divide
 Daughter cells – the products of division
 When prokaryotic cells divide, the contents of the mother
cell are more or less equally apportioned between the two
daughter cells (fission)
 Under optimal conditions, a prokaryote such as the
intestinal bacterium Escherichia coli divides every 20 to 30
minutes.

13. CELL DIVISION
 Eukaryotic cell division is more complex than that of
prokaryotic cells
 Many chromosomes must be duplicated, and the
duplicates must be distributed equally and exactly to the
daughter cells
 For the cellular entities the distribution process is not
equal and exact
 Mitochondria and chloroplasts are randomly apportioned to the
daughter cells
 The ER and the Golgi complex are fragmented at the time of
division and later are re-formed in the daughter cells

14.  Each time a eukaryotic cell divides, it goes through a
series of phases that collectively form the cell cycle
 G1 → S → G2 → M
 S - chromosomes are duplicated (requires DNA synthesis)
 M (Mitosis) phase - the mother cell actually divides
 has two components:
 (1) mitosis- distributes the duplicated chromosomes equally and
exactly to the daughter cells, and
 (2) cytokinesis - physically separates the two daughter cells from
each other
 The G1 and G2 phases are “gaps” between the S and M
phases

16.  The length of the cell cycle varies among different types of
cells
 In embryos, where growth is rapid, the cycle may be as short as
30 minutes
 In slow-growing adult tissues, it may last several months
 Some cells, such as those in nerve and muscle tissues, cease to
divide once they have acquired their specialized functions
 The progression of eukaryotic cells through their cycle is
tightly controlled by different types of proteins
 When the activities of these proteins are disrupted, cells divide
in an unregulated fashion
 deregulation of cell division may lead to cancer

17. MITOSIS
 When Eukaryotic cells divide, they distribute their genetic
material equally and exactly to their offspring
 Each chr in a mother is duplicated prior to division (in S
phase)
 Chrs are extended and thin - chromatin
 During mitosis, the chromosomes shorten and thicken
(condense)
 5 stages – Interphase, Prophase, Metaphase, Anaphase,
Telophase

19. MITOSIS
 Interphase
 Individual chrs cannot be seen
 Chrs duplicate to produce sister chromatids
 Distribution of duplicated chr to the daughter cells is
organized and executed by microtubules (tubulins)
 Microtubules later assemble to form spindle
 MTOCs (Microtubule organizing centers) found near the
nucleus helps in spindle formation
 MTOCs are differentiated into small organelles called
centrosomes ( absent in plant cells)
 Each centrosome contains a barrel-shaped centromere,
aligned at right angles to each other

20. MITOSIS
 Pericentriolar material , a diffuse matrix surrounds the
centrioles
 As the cells enter mitosis, microtubules develop around
each of the daughter centrosomes to form a sunburst
pattern called aster
 The initiation of spindle formation and condensation of
duplicated chromosomes marks the start of prophase

21. MITOSIS
 PROPHASE
 Formation of spindle is accompanied by fragmentation of many
intracellular organelles
 Nucleolus disappears, mitochondria and chloroplast remains
intact
 Nuclear envelope breaks up and disappears along with the ER
 Some microtubules attach to the kinetochores (protein structures
associated with centromeres)
 Attachment of spindle microtubules to the kinetochores indicate
the start of metaphase

22. MITOSIS
 METAPHASE
 Duplicated chrs move to positions midway to the spindle poles
 Movement is controlled by changes in the length of spindle
microtubules and by the action of force-generating motor
proteins that work near the kinetochores
 Additional microtubules (those not attached to the kinetochores)
stabilize the spindle apparatus
 The duplicated chrs come to lie in a single plane in the middle of
the cell- METAPHASE PLATE
 Each sister chromatid is connected to a different pole

23. MITOSIS
 ANAPHASE
 Sister chromatids of duplicated chrs are separated during this
stage
 Accomplished by shortening of microtubules and by degrading
the materials that holds the sisters together
 Sister chromatids are pulled to the opposite poles
 The poles themselves begin to move apart
 This double movement cleanly separates the two sets of chrs
into distinct spaces within the dividing cell
 Chrs decondense to chromatin, organelles lost at the onset of
mitosis reform, nuclear membrane reappears

24. MITOSIS
 TELOPHASE
 Decondensation of chrs and restoration of internal organelles are a
characteristic of telophase
 When mitosis is complete, the 2 daughter cells are separated by
the formation of membranes between them
 In plants, a wall is also laid down between the daughter cells
 The physical separation of the daughter cells – CYTOKINESIS
 The daughter cells are genetically identical and has a complete set
of chrs as that of the mother
 Mistakes may occur occasionally

25. MEIOSIS
 Reductional Division
 Reduces the chr no by half
 Reduces the diploid no of chrs to the haploid state i.e. reduces
the chrs in a cell by half
 The resulting haploid cells either directly become gametes or
divide to produce cells that later become gametes
 Meiosis plays a key role in reproduction among Eukaryotes

27. MEIOSIS
 Meiosis involves 2 cell divisions
 Chr duplication, associated with DNA synthesis occurs
prior to these divisions
 Chr duplication Meiosis I Meiosis II

28. MEIOSIS - I
 Divided into :
 Prophase-I
 Leptonema
 Zygonema
 Pachynema
 Diplonema
 Diakinesis
 Metaphase-I
 Anaphase –I
 Telophase - I

29. MEIOSIS - I
 Leptonema/ Leptotene
 Greek – thin threads
 Individual chrs can be barely seen with a light microscope
 With an electron microscope, each of the chrs appear to consist
of 2 sister chromatids
 As chromosome condensation continues, the cell progresses
into zygonema

30. MEIOSIS - I
 Zygonema / Zygotene
 Greek – paired threads
 Homologous chrs come together intimately, pairing occurs
(SYNAPSIS)
 Synaptonemal complex – proteinacious structure forms between
the pairing chrs
 Pairing may be facilitated by a tendency for homologous chrs to
remain in the same region of the nucleus during interphase
 Condensation progresses resulting in thickened chrs

31. MEIOSIS - I
 Pachynema / Pachytene
 Greek – thick threads
 Paired chrs can be easily seen with a light microscope
 Each pair consists of duplicated chrs, which themselves
contains sister chromatids
 The pair is referred to as a bivalent of chrs
 If we count the strands, it is called a TETRAD of chromatids
 The paired chrs may exchange material (CROSSING OVER)

32. MEIOSIS - I
 Diplonema / Diplotene
 Greek – two threads
 The paired chrs separate slightly but still remain in close contact
at sites of crossing over
 Contact points are called ‘chiasmata’ (singular chiasma)
 This stage may last a very long time

33. MEIOSIS - I
 Near the end of Prophase-I, the chrs condense further, the
nuclear membrane fragments, and a spindle apparatus forms
 Microtubules attach to kinetochores of the chrs
 The chrs move to a central plane of the cell that is perpendicular
to the axis of the spindle apparatus
 Last stage of Prophase-I and start of Metaphase-I

34. MEIOSIS - I
 Metaphase – I
 The paired chrs orient towards the opposite poles
 This ensures that when the cell divides, one member of each
pair will go to each pole
 The chiasmata that holds the bivalents together slip away from
the centromeres towards the ends of the chrs –
TERMINALIZATION
 Reflects the growing repulsion between the members of each
chr pair

35. MEIOSIS - I
 Anaphase – I
 The paired chrs separate from each other definitively
 This separation – ‘chromosome disjunction’; mediated by spindle
apparatus acting on each of the bivalents
 As the separating chrs gather at opposite poles, the first meiotic
division comes to an end

36. MEIOSIS - I
 Telophase –I
 The spindle apparatus is disassembled
 The daughter cells are separated by membranes
 The chrs decondense, nucleus is formed around the chrs in
each daughter cell
 In some, daughter nuclei do not form, the daughter cells proceed
to 2nd meiotic division

37. MEIOSIS - II
 Also called Equational Division
 Same as mitosis