CELL CYCLE

PREPARED BY: MS. CHARIS D. JUANICO,

GENERAL BIOLOGY 1 TEACHER
Learning Objectives

At the end of the lesson, the learner will be able to:

1. illustrate the phases of the cell cycle and their control points;
2. explain the different stages of mitosis/meiosis;
3. compare mitosis and meiosis;
4. explain the significance or applications of mitosis/meiosis;
5. cite some disorders and diseases that result from the failure of the cell
during the cell cycles; and
6. illustrate the two types of cell cycle.
LET’S GET STARTED!

How do we
grow and
develop?
Word Check

1.Cell Cycle
  is a series of events that takes place in a cell as it grows and divides.

2.Meiosis
 is a type of cell division that reduces the number of chromosomes in
the parent cell by half and produces four gamete cells.

3.Mitosis
 is a process where a single cell divides into two identical daughter
cells (cell division). 
 Cell Cycle

 The sequence of events by

which a cell duplicates its
genome, syntheses the
other constituents of the
cell and eventually divides
into two daughter cells is
termed cell cycle.
Cell Cycle
 Eukaryotic cells, such as in plants and animals, have the ability to
grow and reproduce through a complex sequence of events called the
cell cycle.

 Body cells or somatic cells replicate by means of mitosis while sex

cells or gametes reproduce by means of meiosis.

 Like plants, animals can reproduce in two ways either asexual or

sexual.
Cell Cycle
 Reproduction is a cellular process by which an organism produces
others of the same kind. It has something to do with cell division. Cell
division in unicellular organisms produces two new individuals to
increase their numbers while in multicellular organisms, cell division
is not just part of the growth but also it is responsible for the repair
of damaged cells.
Phases of Cell Cycle
A typical eukaryotic cell cycle divides once in approximately
every 24 hours. However, this duration of cell cycle can vary from
organism to organism and also from cell type to cell type. Yeast for
example, can progress through the cell cycle in only about 90
minutes.

The cell cycle is divided into two basic phases:

1. Interphase
2. M Phase (Mitosis Phase)
I. Interphase
 This is called the resting phase, is
the time during which the cell is
preparing for division by
undergoing both cell growth and
DNA replication in an orderly
manner. The interphase is divided
into three further phases: G1 phase
(Gap 1), S phase (Synthesis), and
G2 phase (Gap 2)
A. G1 phase (Gap 1)
It corresponds to the interval between mitosis and initiation of DNA
replication.

During G1 phase the cell is metabolically active and continuously

grows but does not replicate its DNA.

 The cell synthesizes its structural proteins and enzymes (pancreas-

insulin)
B. S phase
(Synthesis)
 S or synthesis phase marks the
period during which DNA synthesis
or replication takes place.
During this time the amount of
DNA per cell doubles. If the initial
amount of DNA is denoted as 2C
then it increases to 4C.
However, there is no increase in
the chromosome number; if the
cell had diploid or 2n number of
chromosomes at G1 , even after S
phase the number of
chromosomes remains the same,
2n.
B. S phase (Synthesis)
 In animal cells, during the S phase, DNA replication begins in the
nucleus, and the centriole duplicates in the cytoplasm.
C. G2 phase (Gap 2)

 During the G2 phase, proteins are synthesized in preparation for

mitosis while cell growth continues.

 Cells prepare to divide, spindle fibers form.

 Some cells in the adult animals do not appear to exhibit division
(e.g., heart cells) and many other cells divide only occasionally, as
needed to replace cells that have been lost because of injury or cell
death. These cells that do not divide further exit G1 phase to enter an
inactive stage called quiescent stage (G0 ) of the cell cycle. Cells in
this stage remain metabolically active but no longer proliferate unless
called on to do so depending on the requirement of the organism.
Control Points of
the Cell Cycle
II. M PHASE (Mitotic Phase)
This is the most dramatic period of the cell cycle, involving a major
reorganization of virtually all components of the cell. Since the number
of chromosomes in the parent and progeny cells is the same, it is also
called as equational division. Though for convenience mitosis has been
divided into four stages of nuclear division (karyokinesis), it is very
essential to understand that cell division is a progressive process and
very clear-cut lines cannot be drawn between various stages.
Karyokinesis involves following four stages: Prophase, Metaphase,
Anaphase, and Telophase.
M Phase (Mitotic Phase)
 Mitosis is the process in which
a new cell nucleus divides into
two new nuclei. Each nuclei has
the same number of
chromosomes (structures in the
nucleus that contain DNA-
Deoxyribonucleic acid) as the
parent cell.

 Mitosis is responsible for the

growth, replacement of worn
out cells and repair.
Structure of a Chromosome
Structure of a Chromosome
1. Prophase
 is the first stage of karyokinesis of mitosis follows the S and G2
phases of interphase.

 In the S and G2 phases the new DNA molecules formed are not
distinct but intertwined.

 Prophase is marked by the initiation of condensation of

chromosomal material. The chromosomal material becomes
untangled during the process of chromatin condensation
Key Features of Prophase:

1. Chromosomal material condenses to form

compact mitotic chromosomes.
Chromosomes are seen to be composed of
two chromatids attached together at the
centromere.

2. Centrosome which had undergone

duplication during interphase, begins to
move towards opposite poles of the cell.
Each centrosome radiates out microtubules
called asters. The two asters together with
spindle fibers forms mitotic apparatus.
2. Metaphase
 The complete disintegration of the nuclear envelope marks the start
of the second phase of mitosis, hence the chromosomes are spread
through the cytoplasm of the cell.

 condensation of chromosomes is completed and they can be

observed clearly under the microscope. This then, is the stage at
which morphology of chromosomes is most easily studied.
 metaphase chromosome is made up of two sister chromatids, which
are held together by the centromere.

 Small disc-shaped structures at the surface of the centromeres are

called kinetochores. These structures serve as the sites of attachment
of spindle fibres (formed by the spindle fibres) to the chromosomes
that are moved into position at the centre of the cell.
 the metaphase is characterized by all the chromosomes coming to
lie at the equator with one chromatid of each chromosome
connected by its kinetochore to spindle fibers from one pole and its
sister chromatid connected by its kinetochore to spindle fibers from
the opposite pole

 The plane of alignment of the chromosomes at metaphase is

referred to as the metaphase plate.
Key Features of Metaphase:

 Spindle fibers attach to

kinetochores of
chromosomes.

 Chromosomes are moved

to spindle equator and get
aligned along metaphase
plate through spindle fibers
to both poles.
3. Anaphase
 At the onset of anaphase, each chromosome arranged at the
metaphase plate is split simultaneously and the two daughter
chromatids, now referred to as daughter chromosomes of the future
daughter nuclei, begin their migration towards the two opposite
poles.

 As each chromosome moves away from the equatorial plate, the

centromere of each chromosome remains directed towards the pole
and hence at the leading edge, with the arms of the chromosome
trailing behind.
Key Features of
Anaphase:

 Centromeres split and

chromatids separate.

 Chromatids move to
opposite poles.
4. Telophase
 the beginning of the final stage of karyokinesis

 the chromosomes/chromatids that have reached their respective

poles decondense and lose their individuality.

 The individual chromosomes can no longer be seen and each set of

chromatin material tends to collect at each of the two poles.
Key Features of Telophase:

 Chromosomes cluster at opposite

spindle poles and their identity is lost as
discrete elements.

 Nuclear envelope develops around the

chromosome clusters at each pole
forming two daughter nuclei.

 Nucleolus, Golgi complex and ER

reform.
Cytokinesis
 Mitosis accomplishes not only
the segregation of duplicated
chromosomes into daughter
nuclei (karyokinesis), but the cell
itself is divided into two daughter
cells by the separation of
cytoplasm called cytokinesis at
the end of which cell division gets
completed.
 In an animal cell, this is achieved by the appearance of a furrow in
the plasma membrane. The furrow gradually deepens and ultimately
joins in the center dividing the cell cytoplasm into two.

 In plant cells, wall formation starts in the center of the cell and
grows outward to meet the existing lateral walls. The formation of the
new cell wall begins with the formation of a simple precursor, called
the cell-plate that represents the middle lamella between the walls of
two adjacent cells.
 At the time of cytoplasmic division, organelles like mitochondria and
plastids get distributed between the two daughter cells. In some
organisms karyokinesis is not followed by cytokinesis as a result of
which multinucleate condition arises leading to the formation of
syncytium (e.g., liquid endosperm in coconut).
Significance of
Mitosis
 Mitosis or the equational division is usually restricted to the diploid
cells only (2n).

 Mitosis usually results in the production of diploid daughter cells with

identical genetic complement.

 . The growth of multicellular organisms is due to mitosis. Cell growth

results in disturbing the ratio between the nucleus and the cytoplasm.
It therefore becomes essential for the cell to divide to restore the
nucleo-cytoplasmic ratio.
 A very significant contribution of mitosis is cell repair. The cells of
the upper layer of the epidermis, cells of the lining of the gut, and
blood cells are being constantly replaced.

 Mitotic divisions in the meristematic tissues – the apical and the

lateral cambium, result in a continuous growth of plants throughout
their life.
Meiosis
 The production of offspring by sexual reproduction includes the
fusion of two gametes, each with a complete haploid set of
chromosomes.

 This specialized kind of cell division that reduces the chromosome

number by half results in the production of haploid daughter cells.
This kind of division is called meiosis.
 Meiosis ensures the production of haploid phase in the life cycle of
sexually reproducing organisms whereas fertilization restores the
diploid phase.

 We come across meiosis during gametogenesis in plants and

animals. This leads to the formation of haploid gametes.
Key Features of Meiosis:
 Meiosis 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.
 Four haploid cells are formed at the end of meiosis II. Meiotic events
can be grouped under the following phases:
1. Meiosis I
 Before entering meiosis I, a cell must first go through
interphase. As in mitosis, the cell grows during G1 ​phase, copies
all of its chromosomes during S phase, and prepares for division
during G2 phase.
Stages of Meiosis I
Prophase I
 Prophase of the first meiotic division is typically longer and more
complex when compared to prophase of mitosis. It has been further
subdivided into the following five phases based on chromosomal
behavior: Leptotene, Zygotene, Pachytene, Diplotene and
Diakinesis.
A. Leptotene
 the stage the chromosomes become gradually visible under the light
microscope. The compaction of chromosomes continues throughout
leptotene.
B. Zygotene

 the chromosomes start pairing

together and this process of
association is called synapsis. Such
paired chromosomes are called
homologous chromosomes (a pair of
chromosomes with the same gene
sequence, loci, chromosomal length,
and centromere location).
 The complex formed by a pair of synapsed homologous
chromosomes is called a bivalent or a tetrad. However, these are
more clearly visible at the next stage.
C. Pachytene
 this stage, the four chromatids of each bivalent chromosomes
becomes distinct and clearly appears as tetrads.

 is characterized by the appearance of recombination nodules, the

sites at which crossing over occurs between non-sister chromatids of
the homologous chromosomes. Crossing over is the exchange of
genetic material between two homologous chromosomes.
Crossing Over of Traits
D. Diplotene
 a late stage of prophase during meiosis, in which the
chromatid pairs of the tetrads begin to separate and chiasmata
can be seen.
E. Diakinesis

 is marked by terminalization of chiasmata.

 the chromosomes are fully condensed and the meiotic spindle is

assembled to prepare the homologous chromosomes for separation.

 by the end of diakinesis, the nucleolus disappears and the nuclear

envelope also breaks down. Diakinesis represents transition to
metaphase.
Metaphase I
 The bivalent chromosomes
align on the equatorial plate

 The microtubules from the

opposite poles of the spindle
attach to the kinetochore of
homologous chromosomes.
Anaphase I
  the homologues are pulled
apart and move apart to
opposite ends of the cell. The
sister chromatids of each
chromosome, however,
remain attached to one
another and don't come
apart.
Telophase I
 the chromosomes arrive at opposite poles of the cell. In some
organisms, the nuclear membrane re-forms and the chromosomes
decondense, although in others, this step is skipped—since cells will
soon go through another round of division, Meiosis II.
Cytokinesis
 Cytokinesis usually occurs at the same time as telophase I,
forming two haploid daughter cells.
Performance Task:
3D Animal/ Plant Cell

Deadline: October 15, 2021

Rubric
Criteria 10 pts 7 pts 5 pts 3 pts 1 pt

Cell
Construction
Cell model Cell model Cell model Cell model Cell model was
accurately displayed displayed displayed only poorly
Distinguish displayed all almost all cell several cell few cell constructed,
es the cell organelles, organelles, is organelles, is organelles, is majority of the
characteris is neat, and neat, and neat, and neat and cell organelles
tics of an colorful. colorful. colorful. colorful. are not included.
animal cell.
 Accurate
labeling of
Accurate labeling Less than half or
almost all cell
of all cell Accurate not all cell
organelles, 85 -
organelles, 100 % labeling of Accurate labeling organelles are
90 % of all
of all other several cell of few cell labeled and
Cell other cell
structures are organelles, 80 organelles, 75 - multiple spelling
Labeling structures are
identified, -85 % of all other 80 % of other cell errors, less than 75
identified,
writing is legible, structures are structures are % of other cell
writing is
and no spelling identified, identified, eligible structures are
Knows: legible, and few
errors. It is clear writing is legible writing, and a few identified, eligible
or no spelling
which label goes and few or no spelling errors. writing, and
errors. It is clear
with each spelling errors. multiple spelling
which label
structure. errors.
goes with each
structure.
 Information is
clearly 85 - 90 % of
Visual 75 - 80% of
provided. the assigned Less than 75 %
Appeal and 80 - 85 % of the assigned
100% of the structures of assigned
Originality the assigned structures are
assigned are structures are
structures are constructed
structures are constructed constructed
constructed accurately and
constructed accurately accurately and
The model accurately colored
accurately and colored colored neatly.
is colorful, and colored neatly.
and colored neatly.
accurate neatly.  
neatly. Design Picture is  
and neat
is original and organized.
organized.