The primary mechanism by which organisms generate new cells is through cell division.

1. Mitosis & Meiosis
Dr. Manju Bhaskar
Assistant Professor
Department of Zoology
D. B. S. College (CSJM University)
Kanpur 208006
Email: drmanjubhaskar19@gmail.com

2. • The primary mechanism by which organisms generate new
cells is through cell division.
• During this process, a single "parent" cell will divide and
produce identical "daughter" cells.
• The parent cell passes on its genetic material to each of its
daughter cells. First, however, the cells must duplicate their
DNA.
• Mitosis is the process by which a cell segregates its
duplicated DNA, ultimately dividing its nucleus into two.

3. • The mechanisms of cell division vary between prokaryotes and
eukaryotes.
• Prokaryotes are single-celled organisms, such as bacteria and
archaea.
• They have a simple internal structure with free-floating DNA.
• They use cell division as a method of asexual reproduction, in
which the genetic makeup of the parent and resulting offspring
are the same.
• One common mechanism of asexual reproduction in
prokaryotes is binary fission.
• During this process, the parent cell duplicates its DNA and
increases the volume of its cell contents.
• Eventually, a fissure emerges in the center of the cell, leading to

4. • The cells of eukaryotes have an organized central
compartment, called the nucleus, and other structures, such
as mitochondria and chloroplasts.
• Most eukaryotic cells divide and produce identical copies of
themselves by increasing their cell volume and duplicating
their DNA through a series of defined phases known as the cell
cycle.
• Since their DNA is contained within the nucleus, they undergo
nuclear division as well.
• "Mitosis is defined as the division of a eukaryotic nucleus,"
said M. Andrew Hoyt, a professor of biology at Johns Hopkins
University, "[though] many people use it to reflect the whole
cell cycle that is used for cell duplication."

5. The Cell
Cycle

6. Stages of the Eukaryotic Cell Cycle
• The eukaryotic cell cycle is a series of well-defined and carefully timed events
events that allow a cell to grow and divide. According to Geoffery Cooper,
author of "The Cell: A Molecular Approach, 2nd Ed." (Sinauer Associates, 2000)
Associates, 2000) most eukaryotic cell cycles have four stages:
• G1 phase (first gap phase): The period prior to the synthesis of DNA. In this
this phase, the cell increases in mass in preparation for cell division. The G1
G1 phase is the first gap phase and grow and carry out various metabolic
activities.
• S phase (synthesis phase): The period during which DNA is synthesized. In
In most cells, there is a narrow window of time during which DNA is
synthesized. The S stands for synthesis.
• G2 phase (second gap phase): The period after DNA synthesis has occurred but
occurred but prior to the start of prophase. The cell synthesizes proteins and
and continues to increase in size. The G2 phase is the second gap phase.

7. • M phase (mitosis): Mitosis involves the segregation of the sister chromatids. A
structure of protein filaments called the mitotic spindle hooks on to the
centromere and begins to contract. This pulls the sister chromatids apart,
slowly moving them to opposite poles of the cell. By the end of mitosis each
each pole of the cell has a complete set of chromosomes. The nuclear
membrane reforms, and the cell divides in half, creating two identical
daughter cells.
• Chromosomes become highly compacted during mitosis, and can be clearly
clearly seen as dense structures under the microscope.
• The resulting daughter cells can re-enter G1 phase only if they are destined to
destined to divide. Not all cells need to divide continuously.
• For example, human nerve cells stop dividing in adults.
• The cells of internal organs like the liver and kidney divide only when needed:
needed: to replace dead or injured cells. Such types of cells enter the G0 phase
phase (quiescent phase).

8. Prophas
e
• In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope breaks down and
spindles form at opposite poles of the cell. Prophase (versus interphase) is the first true step of the mitotic
process. During prophase, a number of important changes occur:
• Chromatin fibers become coiled into chromosomes, with each chromosome having two chromatids joined at
a centromere.
• The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.
• The two pairs of centrioles (formed from the replication of one pair in Interphase) move away from one another
toward opposite ends of the cell due to the lengthening of the microtubules that form between them.
• Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's
equator.
• Kinetochores, which are specialized regions in the centromeres of chromosomes, attach to a type of microtubule
called kinetochore fibers.
• The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers.
• The chromosomes begin to migrate toward the cell center.

9. Metapha
se
• In metaphase, the spindle reaches maturity and the chromosomes align at the
metaphase plate (a plane that is equally distant from the two spindle poles).
During this phase, a number of changes occur:
• The nuclear membrane disappears completely.
• Polar fibers (microtubules that make up the spindle fibers) continue to extend
from the poles to the center of the cell.
• Chromosomes move randomly until they attach (at their kinetochores) to polar
fibers from both sides of their centromeres.
• Chromosomes align at the metaphase plate at right angles to the spindle poles.
• Chromosomes are held at the metaphase plate by the equal forces of the polar
fibers pushing on the centromeres of the chromosomes.

10. Anaph
ase
• In anaphase, the paired chromosomes (sister chromatids) separate and begin moving to
opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and
elongate the cell. At the end of anaphase, each pole contains a complete compilation of
chromosomes. During anaphase, the following key changes occur:
• The paired centromeres in each distinct chromosome begin to move apart.
• Once the paired sister chromatids separate from one another, each is considered a "full"
chromosome. They are referred to as daughter chromosomes.
• Through the spindle apparatus, the daughter chromosomes move to the poles at opposite ends
of the cell.
• The daughter chromosomes migrate centromere first and the kinetochore fibers become
shorter as the chromosomes near a pole.
• In preparation for telophase, the two cell poles also move further apart during the course of

11. Telopha
se
• In telophase, the chromosomes are cordoned off into distinct new nuclei
in the emerging daughter cells. The following changes occur:
• The polar fibers continue to lengthen.
• Nuclei begin to form at opposite poles.
• The nuclear envelopes of these nuclei form from remnant pieces of the
parent cell's nuclear envelope and from pieces of the endomembrane
system.
• Nucleoli also reappear.
• Chromatin fibers of chromosomes uncoil.
• After these changes, telophase/mitosis is largely complete. The genetic

12. Cytokin
esis
•Cytokinesis is the division of the cell's cytoplasm.
•It begins prior to the end of mitosis in anaphase
and completes shortly after telophase/mitosis.
•At the end of cytokinesis, two genetically
identical daughter cells are produced.
•These are diploid cells, with each cell containing a
full complement of chromosomes.

13. Mitosi
s

14. Meiosi
s
• A type of cell division that results in four daughter cells each with half the
number of chromosomes of the parent cell, as in the production of gametes and
plant spores.
• It involves two sequential cycles of nuclear and cell division called meiosis I
and meiosis II but only a single cycle of DNA replication.
• Meiosis I is initiated after the parental chromosomes have replicated to
produce identical sister chromatids at the S phase.
• Meiosis involves pairing of homologous chromosomes and recombination
between non-sister chromatids of homologous chromosomes. l Four haploid
cells are formed at the end of meiosis II. Meiotic events can be grouped under
the following phases
• Meiosis I Meiosis II
• Prophase I Prophase II
• Metaphase I Metaphase II
• Anaphase I Anaphase II

15. Prophas
e I
• Leptotene- This phase is the start of prophase-I. It is marked by the
condensation of the chromosomes.
• 2. Zygotene- In this phase the homologous chromosomes start pairing
up, called the synapsis. The synaptonemal complex starts building up.
This complex is required to hold the homologous chromosomes at a
place close to each other. Bivalent chromosomes are visible at this stage.
• 3. Pachytene- In this stage, this non-sister chromatids of homologous
chromosomes exchange their parts, the process is called the crossing
over. The attachment point of the crossing-over of the non-sister
chromatids is called chiasma.
• 4. Diplotene- The crossing-over process is completed by this stage. The
homologous chromosomes remain attched at the point of chiasma.
• 5. Diakinesis- The homologous chromosomes start to separate and
synaptonemal complex disappears. The nuclear membrane also
disappears.

17. Meiosis
Meiosis I

18. Metapha
se I
• In metaphase I of meiosis I, the homologous pairs of chromosomes line
up on the metaphase plate, near the center of the cell.
• Known as reductional division.
• While the chromosomes line up on the metaphase plate with their
homologous pair, there is no order upon which side the maternal or
paternal chromosomes line up. This process is the molecular reason
behind the law of segregation.
Anaphase I
• Much like anaphase of mitosis, the chromosomes are now pulled
towards the centrioles at each side of the cell.
• The centrosomes holding the sister chromatids together do not dissolve
in anaphase I of meiosis, meaning that only homologous chromosomes
are separated, not sister chromatids.

19. Telophas
e I
• In telophase I, the chromosomes are pulled completely apart and new
nuclear envelopes form.
• The plasm membrane is separated by cytokinesis and two new cells are
effectively formed.

20. Meiosis
II
• Two new cells, each haploid in their DNA, but with 2 copies, are the result
of meiosis I. Again, although there are 2 alleles for each gene, they are on
sister chromatid copies of each other. These are therefore considered
haploid cells. These cells take a short rest before entering the second
division of meiosis, meiosis II.
Prophase II
• This phase resembles prophase I.
• The nuclear envelopes disappear and centrioles are formed.
• Microtubules extend across the cell to connect to the kinetochores of
individual chromatids, connected by centromeres.
• The chromosomes begin to get pulled toward the metaphase plate.

21. Metaphase II
• Now resembling mitosis, the chromosomes line up with their centromeres on
the metaphase plate.
• One sister chromatid is on each side of the metaphase plate.
• At this stage, the centromeres are still attached by the protein cohesin.
Anaphase II
• The sister chromatids separate.
• They are now called sister chromosomes and are pulled toward the centrioles.
• This separation marks the final division of the DNA.
• Unlike the first division, this division is known as an equational division,
because each cell ends up with the same quantity of chromosomes as when the
division started, but with no copies.

22. Telophase II
• As in the previous telophase I, the cell is now divided into two and the
chromosomes are on opposite ends of the cell.
• Cytokinesis or plasma division occurs, and new nuclear envelopes are formed
around the chromosomes.
Result
• At the end of meiosis II, there are 4 cells, each haploid, and each with only 1 copy of
the genome.
• These cells can now be developed into gametes, eggs in females and sperm in males.

23. Meiosis II

24. Referen
ce
• Bailey, Regina. "The Stages of Mitosis and Cell Division." ThoughtCo,
Aug. 27, 2020, thoughtco.com/stages-of-mitosis-373534.
• http://www.biologycorner.com/bio4/notes/mitosis.php
• https://biologydictionary.net/meiosis/
• https://www.istockphoto.com/photos/cell-division

25. Thank You