Cell DIVISION/cycle

Learning Objectives
 Infer the significance of cell division.
 Differentiate a DNA molecule, a chromosome, and a
chromatid.
 Characterize the phases of the cell cycle and their control
points.
 Describe the major events associated with stages of mitosis.
 Explain the process of cytokinesis.
 Describe the role of apoptosis in the life cycle of a cell.
 Relate cancer as a result of the malfunction of the cell
during the cell cycle.

Values
Uphold the importance of cell cycle
and reproduction especially in the
perpetuation of generation of
species.
Advocate preservation of life

Cell division plays
important roles in the
lives of organism

Growth and Development
Cell division is associated with
growth and development.
Humans are products of numerous
cellular divisions, as life begins
with only a single cell from the
fusion of parent’s sex cells.

How come bacteria and other minute organisms
reproduce faster and live in almost everywhere
on Earth?
 Reproduction is common process among life forms to
make new organism from one or two parent organism.
 Sexual Reproduction- involves two specialized cells,
called gametes, coming from the parents that will
result to a unique offspring.

Asexual Reproduction
 Is the production of offspring from single parent
without the involvement of gametes.
 The offspring is genetically identical with each other
and to the single parent.
 Most Bacteria and other unicellular organisms,
reproduction involves a simple cell division.
 Binary Fission- the cell pinches apart, splits into two ,
then new cell wall forms between two daughter cells.

Where do cells get the information
they need to function?
 The cell’s genetic material is organized in
tightly-coiled structures called
chromosomes.
 A chromosome is simply a long, continuous
thread of DNA wounded together by DNA-
associated proteins, referred as histones.

DNA is loosely
organized like
scattered
spaghetti on a
plate.

Chromatin
efficiently packages
the DNA into small
volume and fits
inside the nucleus.
Chromatin
undergoes further
condensation,
forming the
chromosome

 The complex set of macromolecules that contain
loose DNA, proteins, and RNA is called chromatin.
 Chromatin is responsible for packaging the DNA
efficiently into smaller volume so that it fits the
nucleus of a cell to protect the DNA structure and
sequence, to prevent DNA damage, to control gene
expression and DNA replication, and to reinforce
the DNA molecule to allow mitosis and meiosis

 Chromatid refers to each
strand of the duplicated
chromosomes.
 Together they are called sister
chromatids, which are held
together by centromere – a
region of condensed
pinched chromosomes.
 Located at the centromere is
a group of proteins called
kinetochore, which is
attached to the spindle fibers
during cell division.

 At the ends of the DNA molecules are structures
referred as telomeres that contain repeated
nucleotides, which contain genetic information that
do not translate into pairs.
 Their role is to prevent the ends of chromosomes from
accidentally attaching to one another and prevent the
loss genes.

 Main Idea: Cell have distinct phases for growth,
reproduction, and normal operations.
 EQ: How do healthy cells decide when to
divide?

THE CELL
CYCLE
Cells follow
definite stages
of growth,
duplication,
and division
known
collectively as
the cell cycle.

INTERPHASE
 Interphase is the growth period in the cell cycle and is
divided into three parts

Gap 1 (G1)
 Gap 1 is the first part of the cell cycle wherein the cell
carries out its normal metabolic functions.
 During Gap 1 phase , an intestinal cell performs its
primary function to absorb nutrients, while a red
blood cells shuttles oxygen to the rest of the body.
 During this phase, cells also increase their size, as
their organelles increase in number.
 Cells have a required size limit.

Synthesis (S)
Synthesis is the second part of cell
cycle, which refers to the time that the
cell makes a copy of the genetic
material in the form of nuclear DNA.
During S phase, the cell spends
considerable amount of time and
energy to make copies of its
chromosomes.

Gap 2 (G2)
 During this stage, cells continue to carry out
their normal functions and also undergo
further growth.
 This stage contain a critical checkpoint
before transitioning to the next stage.
 The cell make sure that everything is in
order, including growing to its correct size
and duplicating DNA without damage.

Mitosis (M)
 Mitosis involves the division of the nucleus and the
genetic material.
 Parts of Mitosis
o Prophase
o Metaphase
o Anaphase
o Telophase
During M stage, the hereditary materials of parent cell is
given to the daughter cells.

 This leads to the formation of two daughter
cells containing the identical genetic
materials.
 The goal of Mitosis is to distribute an
identical set of genetic instructions- one
copy of each chromosome to the two
daughter cells.
 In M stage the cell’s nuclear membrane
disintegrates, while the DNA condensed,
forming two nuclei.

Cytokinesis
Cytokinesis is basically divides the
cytoplasm of the cell.
Cytokinesis early during telophase and
continues after the nuclei have formed
in the daughter cells.

Rate of Cell Division
 Growth rates vary widely in different organism and
even in the different tissues within an organism.
Common Cell Life Span
Cell Type Approximate Life Span
Skin Cell 14 days
Red Blood Cell 12odays
Liver cell 300-500 days
Internal lining of
intestine
4-5 days
Smooth muscles of
intestine
16 years

 Some parts of the body where cell divides
are rarely believed by scientists to have
entered a stage known as gap zero or G0.
 In this stage, cell are unlikely to divide but
still continue to perform normal function.
 Such cells, like neuron cells and heart
muscle cell that are highly differentiated or
specialized and that body cannot easily
replace, are said to be permanently in G0.

The Mechanism of Cell Division
Mitosis and cytokinesis produce
genetically identical daughter cells.

Mitosis
 Is the division of nucleus into two genetically identical
nuclei containing the same full set of DNA.
 Mitosis occurs in body cells, except the sex cells.
 Mitosis prepares the cells for cytokinesis.
Phases of Mitosis
Prophase
Metaphase
Anaphase
telophase

Prophase: Start of Mitosis
Prophase (pro means “before”).
During prophase, several distinct events
occur.
In prophase, the nucleus disappears.
Chromatin condenses into chromosomes
and the mitotic spindles is formed.
 Spindle fibers are organized microtubules.

Metaphase: Chromosomes at the
Center
 Metaphase (meta means “near”, the spindle fibers
attached to the kinetochore of sister chromatids
facilitates the movement of chromosomes toward the
middle of the cell.
 During metaphase, chromosomes align at the
metaphase plate.

Anaphase: Chromatids to
opposites Poles
 Anaphase ( ana means “up” or “back”), sister
chromatids are tightly paired due to the centromere
and protein cohesion, which is responsible for holding
DNA molecules together in its entire length.
 In anaphase, chromatids separated toward opposite
poles.

Telophase: reformation of Nuclei
 Telophase (telo means “end”), the two complete sets of
identical chromosomes are now positioned at each
pole of the cell, and the reverse of the major events in
prophase happen.
 In telophase, new nuclear envelope forms.
Chromosomes unfold back into chromatin, the nucleoli
reappear, and the cell continues to elongates.

Love of Lab
 Locating the Stages of Mitosis on page 96A.

Cytokinesis
 This stage involves splitting of the
cytoplasm into two cells and completes the
entire cytoplasm divides.
 Cytokinesis typically starts to occur in late
anaphase or telophase.
 It also differs in plants and animal cell
because of some differences in cellular
structure.

 In animals cells, cytokinesis starts with the formation
of cleavage furrow or trench that is pulled inward by
tiny strands of protein actin called microfilaments like
a drawstring
 Slowly membrane begins to pinch roughly in half and
close off, forming a separate cell around each nucleus.
 Daughter cells receive equal portions of parents cell’s
plasma content.

 In plants cell the membrane cannot pinch
off because of the presence of cell wall.
 Instead cell plate form midway between the
two daughter cells’ nuclei.
 This Cell plate is made by the golgi
apparatus, while vesicles supply the new
cells wall material and lipids to form plasma
membrane to the cell plate.
 Cytoplasm is equally divided between both
plants and animal cell.

CYTOKINESIS IN PLANTS AND ANIMALS

Meiosis Mitosis
Type of
Reproduction
Sexual Asexual
Occurs in Humans, animals, plants, fungi. All organisms.
Genetically Different Identical
Crossing Over Yes, mixing of chromosomes can
occur.
No, crossing over cannot occur.
Definition A type of cellular reproduction in
which the number of chromosomes
are reduced by half through the
separation of homologous
chromosomes, producing two
haploid cells.
A process of asexual
reproduction in which the cell
divides in two producing a replica,
with an equal number of
chromosomes in each resulting
diploid cell.

Pairing of
Homologs
Yes No
Function Genetic diversity through sexual
reproduction.
Cellular reproduction
and general growth and
repair of the body.
Number of
Divisions
2 1
Number of
Daughter Cells
produced
4 haploid cells 2 diploid cells
Chromosome
Number
Reduced by half. Remains the same.
Steps (Meiosis 1) Prophase I, Metaphase I,
Anaphase I, Telophase I; (Meiosis 2)
Prophase II, Metaphase II, Anaphase II
and Telophase II.
Prophase, Metaphase,
Anaphase, Telophase.
Karyokinesis Occurs in Interphase I. Occurs in Interphase.
Cytokinesis Occurs in Telophase I and in Telophase
II.
Occurs in Telophase.
Centromeres Split The centromeres do not separate during
anaphase I, but during anaphase II.
The centromeres split
during anaphase.
Creates Sex cells only: female egg cells or male
sperm cells.
Makes everything other
than sex cells.
Discovered by Oscar Hertwig Walther Flemming

Cell cycle must be regulated for
healthy cell growth.
Both internal
and external
factors work
together to
regulate cell
division
Group of proteins
active in
controlling cell
cycle & in
initiating DNA
synthesis
Any various enzymes that
catalyze the transfer of
phosphate group from high
energy phosphate.

External Factors
 External factors from outside the cell that are in the
form of message from nearby cells or from remote
parts of the organism’s body.
 Physical and Chemical External factor help regulate
the cell cycle.
 Physical signs, such as cell-to-cell contact, exist
between cells.
 Mammalian cells experiments grown in in vitro
(Latin,within the glass) laboratory set up grow until a
single layer covers the entire surface of the culture
dish.

 When cell touches another cell, it stops
dividing- a phenomenon called contact
inhibition.
 Another study shown cell that shown cell only
grow if surface is available and stop dividing
when detached from the culture dish – a
phenomenon called anchorage dependent.
 One explanation for this is there are receptors
found in neighboring cell binding each other
and causing the cell’s cytoplasm to form
structures that can block the signal, stopping
the continuous division.

Chemical signals
 Released by the cell such as growth factors
provide instructions for other cells to grow.
 Growth factor are proteins that stimulates
cell division, which have the ability to
activate specific genes to trigger cell growth.

Internal factor
 Come from inside the cell that include several types of
molecules in its cytoplasm.
 The most well-studied internal factors in eukaryotic
cell cycle are kinases and cyclins.
 Kinase is an enzyme that transfers a phosphate group
from one molecule to the target molecules.
 Cyclins are group of proteins active in controlling cell
cycle & in initiating DNA synthesis.

Apoptosis
 Orderly programmed cell death or suicide.
 Apoptosis is needed by the body to ensure
proper cell functioning.
 Apoptosis occurs in body’s webbed fingers
during embryonic period to allow
detachment of fingers and thumbs before
baby is born.

What happens to the
body when there is
uncontrolled cell growth?

CANCER: Growing Out Of Control
 Cancer refers to a group of diseases
characterized by uncontrolled and
abnormal cell division. It occurs when there
is a disruption in the cell cycle.
 Instead of stopping and starting at
appropriate points, cancerous cell divide
continuously until a disorganized solid mass
of cells called tumor is formed.

 Tumors can be categorized as benign or malignant.
 Benign Tumors are cancer cells that remain clustered
together which may be harmless or not and can
probably be cured when removed out of the body.
 Tumors that break away or metastasize are called
malignant tumors. Their ability to break away from
their present location allows them to be transported to
the bloodstream of the lymphatic system to other parts
of the body parts to form more tumors.

Standard cancer treatment option
Local
surgery
radiation
Systematic
chemotherapy
immunotherapy
Involves removal
of the cancerous
body part
Involves exposure
to X-rays to kill
cancer cell and
shrink the tumor
cells
Uses certain
drug to kill
actively
dividing
cells
Uses substances
either from the
body or laboratory
to improve the
body’s defense to
fight cancer

The human Life Cycle and Sexual Reproduction
 Long- Term survival on Earth requires some
form of genetic variation to increase survival
rate of organisms in unstable environment.
 Human life cycle starts with the union of two
sex cells- a sperm cell or spermatozoon
(plural; spermatozoa) from father and an egg
or ovum (plural: ova) from mother through
sexual reproduction.

 Both sperm cell and the egg cell are
produced in a type of cell division called
meiosis.
 The sex cells produced in meiosis are
haploid cells because they contain only half
the number of chromosomes.
 The haploid cell is represented with the
symbol n. As a result of the union
spermatozoon (n) and a haploid ovum (n),
a diploid (2n) zygote is formed.

 The process of cell division by mitosis is a
continuing process for the rest of an
organism as a way to form new tissues and
organs, and to repair damaged and old
tissues. As a result of mitosis, each somatic
cell , also called body cell, of the human
body contains diploid number of
chromosomes, except sex cell- 23
chromosomes

 In human life cycle, the process of meiosis is
important to reduce the number of
chromosomes from diploid (2n= 46) to
haploid (n=23)
 All cell in our body have specialized
functions. But all these cells can be divided
into two major group: somatic cells (Greek
word soma, meaning body) and germ cells.
 Somatic cells or body cells compose you body
tissues and organs.

 Each diploid cell contain 23 pairs of homologous
chromosomes.
 Homologous chromosomes refer to a set of
chromosomes having the same length and appearance
that was inherited from the parent.
 Scientist assigned a number to each pair of homologous
chromosome according to their size from largest to
smallest.
 Viewing the arranged chromosomes can be done in the
process called karyotyping with the use of a diagram
known as a karyogram.

 Karyotype of human showing the pairs of homologous
chromosomes arranged into autosomes and sex
chromosomes.
 Chromosomes pairs 1 to 22 are designated as
autosomes, which are chromosomes associated with
genes not directly related to the sex of an organism.
 The twenty-third pair is a special one called sex
chromosomes, which are directly linked to the
development of sexual characteristics of an organism.
Humans have two different sex chromosomes, known as
the X and Y chromosomes.

 Humans and mammals use the XY system to
determine the gender. An organism having two X
chromosome is female, while an organism having X
and Y is male.
 The X and Y chromosome, although paired with 23
set, are not homologous. The X chromosome is larger
containing more genes and other unrelated ones to
sexual characteristics, The Y chromosome is the
smallest chromosome

 Meiosis -Is important for sexual reproduction,
is the form of cell division that involves the
formation of sex cells.
 Meiosis divides the number of the gametes,
the process is also called reduction division.
 Meiosis involves two divisions of the nucleus
known as meiosis I and meiosis II

1. Prophase I
 Is characterized by he breaking down of
nuclear envelope. Spindle fiber begin to
assemble.
 Duplicated chromosomes condense, while
the homologous chromosomes pair and
line-up by gene precisely in its length.
 Aside from pairing along their length, they
cross over at a point called chiasma.

2. Metaphase I
 The pairs of homologous chromosomes are
randomly moved by the spindle to the
equator of the cell.
 This result in the lining up of 23
chromosomes alongside each other in the
middle of the cell.

3. Anaphase I
 Homologous chromosomes separate from
each other. Like in mitosis, chromosomes of
each pair are pulled to the opposite sides of
the cell by the action of spindle fibers. Sister
chromatids remain together and don not
separate at their centromeres throughout
meiosis.

4. Telophase 1
The individual chromosomes that
have pulled in opposite directions
now gather at each pole. Both poles
contain one chromosomes from
each pair of the homologous
chromosomes.

Meiosis II
 The goal of meiosis II is to divide sister chromatids,
resulting in sex cell with only half the chromosome.

5. Prophase II
New spindle forms around the
chromosome. The nuclear envelope
breaks down with chromosomes pulled
at opposite sides of the cell by the
spindle fibers.

6. Metaphase II
Chromosomes line up along the
equator through the spindle fibers. At
this stage, each chromosome has sister
chromatids still attached to the
centromere.

7. Anaphase II
Centromeres divide and sister
chromatids are individually pulled
apart, then move to opposite poles of
the cell.

8. Telophase II
 Nuclear envelopes forms around each set of
chromosomes at opposite ends of the cell.
The spindle fibers breaks down and the cell
undergoes cytokinesis.
 The result of meiosis is four haploid cells
with a recombination of the chromosomes
both from the parents.

Gametogenesis
 Meiosis is the process of gamete formation. The
formation of sperm cells is called spermatogenesis while
the formation of egg cells is called oogenesis.

 In males, the process by which sperm cells are produced
is called spermatogenesis.
 Spermatogenesis in males yields four mature sperm
cells.

 In females, the process of gamete formation
is called oogenesis
 Only one of the resulting cells gets nearly all
of the cytoplasm. This cell receives almost all
the organelles, cytoplasm, and nutrients
because it is this cell that will ultimately give
rise to an egg cell. The other cell is very small
called a polar body.
 The larger cell undergoes meiosis II, and the
division of the egg cell during meiosis is
again unequal.

 The large cell develops into a gamete called
an ovum or egg. The polar body may divide
again, but its cell will not survive and the
polar body will die in a process called
apoptosis.
 Oogenesis yields a single mature egg and
nonfunctional polar bodies

Importance of Meiosis
Three mechanisms that contribute to
the genetic variation
1. Independent assortment
2. Crossing –over
3. Random fertilization

Independent Assortment
The random distribution of
homologous chromosomes during
meiosis is called independent
assortment.

Crossing-Over and Random Fertilization
 Another factor that contributes to genetic
variation is crossing –over. This occurs during
prophase I of meiosis, where chromosomes
line up in the process called synapsis, while
sections of their DNA are exchanged.
 DNA exchange during crossover adds more
recombination probabilities to the
independent assortment of chromosomes that
occur later in meiosis.

 Chromosomal errors are associated with
several factors such as error in mitosis,
meiosis, environment, and age of the mother.
 Mistakes during the process of meiosis can
create an abnormal number of chromosomes.
 If meiosis does not occur properly, the sperm
or egg cell may end up with more or less than
required number of chromosomes.

The process where chromosomes or
chromatids fail to separate during meiosis
is called nondisjunction. Nondisjunction
can occur either in meiosis I (happens
when homologous pairs fail to separate) or
meiosis II (involves normal separation of
chromosomes in meiosis I, but sister
chromatids fail to separate during
anaphase I.

Turner Syndrome (45,XO)
 Turner syndrome is a chromosomal condition seen in
female individuals. Affected females have small and
underdeveloped ovaries, uterus, and oviducts.
 In early childhood, individuals with Turner Syndrome loses
the ability of normal ovarian function. They do not
experience puberty at a normal age.
 Affected individual have the following characteristics:
short, broad chest, webbed neck, and puffiness or swelling
of hands and feet.
 They also have normal intelligence and can live fairly
normal lives.

Down Syndrome (Trisomy 21)
 The most common disorder of trisomy is
Down Syndrome, wherein the 21th
chromosome has three instead of two
chromosomes. It occurs in one out of 800
newborns.
 Most cases of Down Syndrome is not due to
inheritance but on random mistakes during
formation of reproductive cells of one of the
parents.

 Affected offspring has the following physical
attributes: short stature, round head, flat face,
slanting eyes, stubby fingers, wide gap between
the first and second toes, short neck, short
arms,and short legs.
 They have muscle tone and loose joints that
improve with age. They are born with a number
of birth defects in the heart, intestine or
breathing. Delayed development and behavioral
problem are also observed.

Klinefelter Syndrome (47, XXY)
 Klinefelter Syndrome is a chromosomal abnormality
affecting the physical and cognitive development of males.
Those affected have underdeveloped prostate gland and
testes, there is shortage of testosterone production.
 This lead to delayed or incomplete puberty, lack of facial
and body hair, breast enlargement, and unusual small
penis.
 Affected males have big hands and feet, and unusually long
arms and legs. They may have delayed speech and language
development, and learning disabilities.
 Some variants of Klinefelter have more than extra
chromosome in each cell such as in 48, XXXY or 49, XXXXY
that are associated with severe forms of abnormality.

Trisomy X (47, XXX)
 The abnormal presence of an X chromosome is also known as
Trisomy X syndrome, which affects one in 1000 females. Affected
individuals usually have mild symptoms or none at all. If
symptoms are more pronounced, these may include
development delays and language- based learning disabilities.
 In addition to errors in meiosis, the age of the mother can also
add up to the equation. The older the woman is during
pregnancy, the older is the age of her eggs because she was born
with all the eggs she will ever have.
 A woman of 45 years during pregnancy has a 45-year-old egg as
well.
 Errors in meiosis have a higher risk due to the aging process.

Cell DIVISION/cycle

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Cell DIVISION/cycle