CELL DIVISION AND MITOSIS
2nd Semester
Lesson 1 | BIO105p
A.Y. 2023 - 2024
THE CYCLE OF CELL GROWTH AND DIVISION
Single-celled prokaryotic or eukaryotic organisms grow
and divide as long as environmental conditions allow
In multicellular eukaryotes, cell division is under strict
control to develop and maintain different
subpopulations of cells
Cohesin’s hold the sister chromatids together until
they are removed
The equal distribution of chromosomes into each of
two daughter nuclei is called chromosome
segregation
PRODUCTION OF SISTER CHROMATIDS
The life cycle of a cell (cell cycle) is divided into three
parts:
Cell growth and activity, including replication of
DNA
Nuclear division (mitosis)
Division of the cytoplasm (cytokinesis)
Each chromosome consists of one
DNA molecule (a DNA double
helix)
Before DNA
replication
TYPES OF NUCLEAR DIVISION
Mitosis
(growth process)
Meiosis
(process of sexual
reproduction)
divides the replicated
DNA equally and
precisely, generating
daughter cells, which are
exact genetic copies of
the parent cell
produces daughter nuclei
with half the number of
chromosomes of the
parental nucleus
The arrangements of
genes on chromosomes
are different from those in
the parent cell
After DNA
replication
CHROMOSOMES
In eukaryotes, the hereditary information within the
nucleus is distributed among individual, linear DNA
molecules
DNA molecules combine with proteins that stabilize
the DNA molecules, assist in packaging DNA during
cell division, and influence the expression of
individual genes
In a cell, each chromosome is composed of one
DNA molecule and its associated proteins (1:1:1
ratio)
SISTER CHROMATIDS
Before a cell divides in mitosis, duplication of each
chromosome (and its proteins) produces two
identical copies called sister chromatids
Sister chromatids are held together by sister
chromatid cohesion until mitosis separates them,
placing one in each of two daughter nuclei
After cell
division
Each replicated chromosome
consists of a pair of sister
chromatids held tightly together.
Each sister chromatid contains
one DNA molecule, each of which
is an exact copy of the other
In mitosis, the sister chromatids
separate and become daughter
chromosomes, each of which
contains one DNA molecule. The
two daughter cells each receive
one of the daughter molecules
CELL DIVISION AND MITOSIS
2nd Semester
A.Y. 2023 - 2024
Lesson 1 | BIO105p
PLOIDY
The number of chromosome sets in a cell or species
Some microorganisms have only one copy of each
type of chromosome in their nuclei – they are
haploid, or n
Many plant species, have three, four, or even more
complete sets of chromosomes in each cell –They
are polyploid
Most eukaryotes have two copies of each type of
chromosome in their nuclei – they are diploid, or 2n
- The two chromosomes of each pair in a diploid
cell are called homologous chromosomes
Prophase
Chromosomes condense into chromatin
Nucleolus becomes smaller and disappears
The mitotic spindle begins to form between the
two centrosomes as they migrate toward the
opposite ends of the cell, where they will form
spindle poles
THE MITOTIC CELL CYCLE
There are three main events in a cell cycle:
interphase, mitosis (M phase), and cytokinesis
Interphase is divided into three phases:
1. G1 phase, in which the cell grows
2. S phase, in which DNA replicates and
chromosomal proteins are duplicated
3. G2 phase, in which cell growth continues and
the cell prepares for mitosis
THE PHASE OF MITOSIS
Interphase
G1 is also the stage in which many cell types stop
dividing and are shunted into a G0 phase– some
cells in G0 reenter G1, others never resume the
cell cycle
Internal regulatory controls trigger each phase of
the cell cycle and regulate the overall number of
cycles that a cell goes through
Mitosis proceeds in five stages:
Following interphase, mitosis can be divided into
five sequential stages:
1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase
Cytoplasmic division (cytokinesis) coincides with
telophase
Prometaphase
Begins when the nuclear envelope breaks down
Spindle microtubules grow from centrosomes at
opposite spindle poles toward the center of the
cell
A kinetochore forms on each sister chromatid at
the centromere (the point where chromatids are
joined in sister chromatid adhesion)
Kinetochore microtubules bind to the
kinetochores
Non kinetochore microtubules overlap those
from the opposite spindle pole
Metaphase
Spindle microtubules move chromosomes into
alignment at the spindle midpoint (metaphase
plate or equatorial plane)
Condensation gives each chromosome a
characteristic shape, determined by length and
centromere location
CELL DIVISION AND MITOSIS
Lesson 1 | BIO105p
Anaphase
The spindle separates sister chromatids and
pulls them toward opposite spindle poles
Movement continues until the separated
chromatids (daughter chromosomes) have
reached the two poles
At this point, chromosome segregation is
complete
Telophase and Cytokinesis
Telophase:
The spindle disassembles and chromosomes at
each spindle pole decondense, returning to the
extended state typical of interphase
The nucleolus reappears, RNA transcription
resumes
A new nuclear envelope forms around the
chromosomes at each pole producing the two
daughter nuclei
At this point, nuclear division is complete –the
cell has two nuclei
Cytokinesis:
Cytokinesis produces two daughter cells, each
with one of the two daughter nuclei
The plane of cytoplasmic division is determined
by the layer of microtubules that persists at the
former spindle midpoint
CYTOKINESIS
Cytokinesis by Furrowing
In animals, protists, and many fungi, a furrow girdles
the cell and deepens until it cuts the cytoplasm into two
parts
Cytokinesis by Cell Plate Formation
CHECKPOINTS
The cell cycle has three key checkpoints to
prevent critical phases from beginning until the
previous phases are completed correctly:
1. G1/S checkpoint
2. G2/M checkpoint
3. Mitotic spindle checkpoint
Checkpoints are signals to stop – inactivation of a
checkpoint allows the cell cycle to proceed
G1/S CHECKPOINT
The G1/S checkpoint is the main point in the cell
cycle at which a cell decides whether to divide or
not
The cell cycle arrests at the G1/S checkpoint if DNA
is damaged by radiation or chemicals – if the DNA
damage is repaired, the cycle starts again
Cell cycle arrest also occurs at this checkpoint if the
cell is nutritionally deficient or growth factors are
absent
CELL DIVISION AND MITOSIS
Lesson 1 | BIO105p
G2/S CHECKPOINT
The G2/M checkpoint commits a cell to mitosis
Cells arrest at the G2/M checkpoint if DNA was not
replicated accurately in S or if the DNA has been
damaged by radiation or chemicals
Accurate DNA replication is essential for producing
genetically identical daughter cells
MITOTIC SPINDLE CHECKPOINT
Occurs before metaphase in the M phase
- This checkpoint assesses whether
chromosomes are attached properly to the
mitotic spindle so that they align correctly at
the metaphase plate
Once the cell begins anaphase, it is irreversibly
committed to completing M
CYCLINS AND CYCLIN-DEPENDENT KINASES
Direct regulation of the cell cycle itself involves an
internal control system consisting of proteins called
cyclins and enzymes called cyclin-dependent
kinases (Cdks)
A Cdk is a protein kinase, which phosphorylates
and regulates the activity of target proteins
- Cdk enzymes are active only when bound to
cyclin
Concentrations of cyclins change as the cell cycle
progresses
CANCER FROM ABNORMAL CELL CYCLES
Cancer occurs when cells lose normal controls over
division – cancer cells divide continuously and
uncontrollably, producing a rapidly growing mass
called a tumor
Cancer cells typically lose adhesions to other cells
and spread throughout the body (metastasis)
producing new tumors in other body regions
Metastasis is promoted by changes that block
contact inhibition and alter cell-surface molecules
that link cells together or to the extracellular matrix
Cancer cells typically have a number of mutated
genes that promote uncontrolled cell division or
metastasis
The mutated form of the genes, called oncogenes
(cancer genes) encode altered versions of these
products
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