complete cell division

1. Cell
Division
Mitosis &
Meiosis
From the Virtual Cell Biology Classroom on ScienceProfOnline.com
Image: Cell Division, Wikipedia

2. CELL DIVISION
• It’s the process by which a cell divides to
form two new cells
• • Three types of cell division or cell
reproduction in an organism
• • Prokaryotes (bacteria)
- Binary fission
- • Divides forming two new identical cells

3. • Eukaryotes
-Mitosis
• Cell or organism growth • Replacement or
repair of damaged cells
-Meiosis • formation of sex cells, or gametes

4. WHY DO CELLS DIVIDE?
• Cells divide for growth, development,
repair of worn-out tissues and
reproduction
• To facilitate the exchange of materials
• To control DNA overloading

5. PROKARYOTIC CELL
DIVISION
• 1. Binary Fission
• Three (3) major steps;
• DNA Replication DNA is copied resulting into two
identical chromosomes
• Chromosome Segregation Chromosomes
separate and move towards ends (poles) of cell
• Cytokinesis (Separation) Cytoplasm divides
forming two (2) cells
• Each new daughter cell is Genetically Identical to
parent cell

7. EUKARYOTIC CELL DIVISION
• Cell division that results in two daughter cells each
having the same number and kind of chromosomes
as the parent cell
• 1. MITOSIS
• Two (2) main steps:
1. Mitosis
• Fours steps;
[Prophase>Metaphase>Anaphase>Telophase]
2. Cytokinesis
• Cytoplasm divides forming two new daughter cells
• Each daughter cell is Genetically Identical to parent
cell

9. • Cell division that results in four daughter cells
2. MEIOSIS
• Two (2) major steps:
1. Mitosis Fours steps;
[Prophase>Metaphase>Anaphase>Telophase]
2. Cytokinesis
• Cytoplasm divides forming two new daughter
cells
• Each daughter cell is NOT Genetically Identical to
parent cell

10. THE CELL CYCLE
• The sequence of events from the time a cell first
arises as a result of cell division until the time when
that cell itself divides.
• Arise – Divide
• This consist of periods of;
• Growth and Development
• DNA Replication
• Preparation For Division
• Cell Division
• Cell after division begins a new cycle

11. The Cell Cycle
• Consist of two(2) main periods;
I. Interphase
II. Mitotic Phase

12. Interphase
• Interphase: period of growth and DNA replication
between cell divisions
• Three (3) phases:
• G1 Phase ‒ Cell increases in size
• S Phase ‒ Replication of DNA ‒ Two sister strands
of DNA called chromatids are produced
• G2 Phase ‒ Organelles double ‒ New cytoplasm
forms ‒ All other structures needed for mitosis form

13. CELL CYCLE

14. Mitotic Phase
• Mitotic phase is the stage when a cell divides
• Mitosis – the division of a single nucleus into two
genetically identical daughter nuclei • This division
involves two(2) processes;
‒ Division of the nucleus
‒ Separation of the cytoplasm and the new nuclei
into daughter cells

15. Mitotic Phase
• Divided into two (2) mitotic phases
• 1st Mitotic Phase contain four stages (P-
MAT)
‒ Prophase, metaphase, anaphase and
telophase
• 2nd Mitotic Phase is cytokinesis

16. Mitosis
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
•P-MAT

17. Interphase
• The period when the cell is in a non-dividing state
• A cell spends most of its time in this phase
• During this time, it grows, replicates its
chromosomes and prepares for cell division.
• The cell then leaves interphase, undergoes
mitosis, and completes its division.

18. Early Prophase
• Chromatids condense becoming chromosomes
• Nucleolus disappears
• Centrioles separate and start moving to opposite
ends of the cell
• Spindle begins to form

19. Late Prophase
• The nuclear membrane fragments and the
microtubules invade the nuclear area
• Centrioles have moved to the opposite poles
• The spindle is completely formed

20. Metaphase
• The chromosomes are aligned at the metaphase
plate
• Centrioles move at polar ends and projects
spindle fibers to connect each chromosome

21. Anaphase
• The paired chromosomes (sister chromatids)
separate
• Separated chromatids move to opposite pole
• Partial division of cytoplasm begins

22. Telophase
• Chromosomes are at the poles
• Chromosomes uncoil-turn chromatin
• Nuclear envelops reforms
• Spindle fiber disappear

23. Cytokinesis
• Occurs at the end of mitosis
• Animal cells: a cleavage furrow separates the
daughter cells
• Plant cell: a cell plate separates the daughter cells
• Daughter cells are genetically identical

24. CONTROL OF THE CELL
CYCLE
• Regulatory proteins called cyclins control the cell
cycle at checkpoints:
• G1 Checkpoint—decides whether or not cell will
divide
• S Checkpoint—determines if DNA has been
properly replicated
• Mitotic Spindle Checkpoint—ensures
chromosomes are aligned at mitotic plate

25. MEIOSIS
• Meiotic cell division occurs in germ cells of all
living organisms.
• During meiosis, the genetic material of a diploid
germ cell undergoes two nuclear divisions and
resulting in four haploid daughter cells.
• Each daughter cells has one half of the number
of chromosomes as the parent cell.
• There are two successive nuclear divisions in
meiosis as compared to the one division found
in mitosis.

26. • The two stages of meiosis are
1. Meiosis I
2. 2.Meiosis II
• Meiosis I also called as Reductional Division
• Meiosis II also called as Equational Division
• Before a dividing cell enters meiosis, it
undergoes a period of growth called Interphase.

28. INTERPHASE
• The interphase just prior to the entry of cell in to
meiosis is known as pre meiotic interphase.
• During the S phase of pre meiotic interphase,
chromosome replication takes place.

29. MEIOSIS I
• Meiosis I separate homologous chromosomes
and produce two cells with haploid chromosome
number (N) for that reason it is known as
Reductional Division.
• Meiosis I consist of four stages,
• Prophase I
• Metaphase I
• Anaphase I and
• Telophase I.

30. PROPHASE I
• Prophase I is the longest in duration compared to
Prophase in mitosis.
• It takes about 85 - 95 percent of the total time for
meiosis and also much more complex.
• The Prophase I divided into 5 stages (Le Za Pa Di
Dia).
• Leptotene
• Zygotene
• Pachytene
• Diplotene and
• Diakinesis

31. i. LEPTOTENE
• The first stage of Prophase I is called Leptotene or
leptonema.
• All the chromosomes begin
to condense, so, they become
visible as fine thread.
• There is a marked increase
in the nuclear volume.
• A process of 'homology search' which is essential
to the initial pairing of
• homologs, begins
• during this phase.

32. ZYGOTENE
• The zygotene stage is also known as zygonema.
• This stage begins with the initiation of pairing between
homologous chromosomes, and it ends with complete
pairing.
• The process of pairing (at end to end) between homologous
chromosomes is known as Synapsis (Homologous dyads).

33. • The synaptonemal complex (a protein structure
that forms between homologous chromosomes
during meiosis and is thought to mediate
chromosome pairing, synapsis, and
recombination) is formed during this zygotene
stage.
• At the completion of zygotene, the paired
homologs take the form of bivalents.
• The number of bivalents in each species is equal
to the haploid number.

34. PACHYTENE
• The pachytene stage is also known as pachynema.
• The process of synapsis is complete.
• The two homologous of each bivalent appear to be
attached to each other at one or more points, these
attachments are known as chiasmata.

35. • Crossing over is a precise breakage,
swapping, and reunion between two non-
sister chromatids.
• Crossovers make new gene combinations,
and which are an important source of
genetic variations in populations.

36. DIPLOTENE
• The diplotene stage is also known as diplonema.
• DNA recombination is complete.
• The chromatids continue to shorten and thicken
and the four sister chromatids in a group are called
tetrad.
• The synaptonemal complex begins to break down.

37. • The paired chromatids begin to pull apart,
causing the strands to separate
longitudinally.
• The chiasmata tend to become terminals
as the meiotic prophase continues.

38. DIAKINESIS
• The chromosomes become shorter and thicker
due to condensation.
• Nucleolus and nuclear envelope disappear
towards the end of diakinesis.
• The spindle apparatus becomes organized.
• The centrioles migrate away from one another.

39. METAPHASE I
• All the bivalents migrate within a cell migrate to
metaphase plate.
• One homolog is pulled above the metaphase
plate, the other below.
• The centromeres of homologous chromosomes of
each bivalent stretch out on either side.

40. • The centrioles are at opposite poles of the
cell.
• Spindle fibers from one pole of the cell
attach to one chromosome and spindle
fibers from the opposite pole attach to the
homologous chromosome.

41. ANAPHASE I
• Chromosomes move to the opposite poles.
• The microtubules and the kinetochore fibers
interact, which causes the movement.
• A difference between mitosis and meiosis is that
sister chromatids remain joined after metaphase
in meiosis I, whereas in mitosis they separate.

42. • During Anaphase I original chromosomes
separate, so reduction in the number of
chromosomes from 2N to N number, yet
the sister chromatids remain together

43. TELOPHASE I
• The homologous chromosome completes its
migration to the two poles because of the
shortening of spindles.
• The nuclear envelope is organized around two
groups of chromosomes.
• The nucleolus also
• reappears.

44. Cytokinesis
• Cytokinesis involves the formation of a cleavage furrow,
resulting in the pocketing of the cell into two cells.
• At the end of Telophase, I and Cytokinesis, two daughter
cells are produced, each with one-half of the number of
chromosomes (haploid set of replicated chromosomes)
of the original parent cell.

45. INTERKINESIS
• Interkinesis (Interphase II) is similar to
interphase
• The cell now rests for a bit before beginning the
second meiotic division. During this period,
called interkinesis, the nuclear membrane in
each of the two cells reforms around the
chromosomes. In some cells, the spindle also
disintegrates, and the chromosomes relax.

46. MEIOSIS II
• Meiosis II is the second part MEIOSIS II of the
meiotic process.
• Meiosis II consists
• Prophase II Each dyad is composed of a pair of
sister chromatids attached by a common
centromere.
• Metaphase II Centromeres are positioned at the
equatorial plane.
• Anaphase II Centromeres divide, and the sister
chromatids of each dyad are pulled to opposite
poles

47. Telophase II
• One member of each pair of homologous
chromosomes is present in each pole.
• Each chromosome is referred to as a monad (a
combination of maternal and paternal genetic
information).
• Nuclei reform around chromosomes at the poles.
• Following cytokinesis and finally four haploid
gametes result from a single meiotic event.

48. Functions of Meiosis
1. Production of haploid (n) gametes: so, that
fertilization restores the normal somatic (2n)
chromosome number.
2. Production of tremendous amounts of genetic
variation.
3. Segregation of the two alleles of each gene.
This takes place due to the pairing between the
two homologs of each chromosome and their
separation at the first anaphase.

49. 4. Recombination between linked genes due to
crossing over during pachytene stage.
5. Meiosis facilitates segregation and independent
assortment of chromosomes and genes.
6. In sexually reproducing species, meiosis is
essential for the continuity of generation. Because
meiosis results in the formation of male and female
gametes and union of such gametes lead to the
development of zygotes and thereby new
individual.

50. Mitosis Meosis
1 Occurs in somatic cells Occurs in reproductive cells
2 One cell produces two daughter cells One cell produces four
daughter cells
3 It is an equational division separating sister
chromatids.
It is a reduction division.
The first stage is a reduction
division which separates
homologous chromosomes
at first anaphase. Sister
chromatids separate in an
equational division at II
anaphase.
4 Only one division per cycle i.e. one cytoplasmic
division (cytokinesis) per equational division
Two divisions per cycle i.e.
two cytoplasmic divisions,
one reduction division and
equation division.
5 Chromosomes fail to synapse. No chiasmata
formation.
Chromosomes synapse and
form chiasmata.
6 Genetic exchange between homologous
chromosomes does not occur
Genetic exchange through
chiasmata occurs between
homologous chromosomes.

51. Mitosis Meosis
7 Genetic contents of daughter cells are
identical
Genetic contents of
daughter cells are
different.
8 Chromosome number of daughter cells is
the same as that of mother cell.
Chromosome number of
daughter cells is half of
that of mother cells.
9 Daughter cells are capable of undergoing
additional mitotic divisions.
Daughter cells are not
capable of undergoing
another meiotic division
although they may
undergo mitotic division.
10 Start at the zygote stage and continues
through the life of the organism.
Occurs only after
puberty, in higher
organisms, but occurs in
the zygote of algae and
fungi.