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Gene Technology and Molecular
Techniques (CLAS 475)
Chapter 1
The Cell Cycle, Mitosis and Meiosis
Dr. Ali Al-Minawi
Overview: The Key Roles of Cell Division
• The ability of organisms to
reproduce best
distinguishes living things
from nonliving matter
• The continuity of life is
based on the reproduction
of cells, or cell division
which is an integral part of
the cell cycle, the life of a
cell from formation to its
own division
•In unicellular organisms: division of one cell reproduces the entire organism (example:
Binary Fission in Bacteria)
•In Multi-cellular organisms:
- Most Eukaryotic cell division occurs
through Mitosis {Mitosis (the division
of the nucleus) and Cytokinesis (the
division of the cytoplasm)} resulting in
daughter cells with identical genetic
information, DNA
-
A special type of cell division produces
non-identical daughter cells (gametes:
sperm and egg cells) that have only one
set of chromosomes, half as many as
the parent cell produced by a variation
of cell division called meiosis
-
That’s because Multi-cellular
organisms depend on cell division for:
a) Reproduction, b) Development
from a fertilized cell, c) Growth, d)
Tissue renewal and Repair
Cellular Organization of the Genetic Material
•
All the DNA in a cell constitutes the cell’s
genome
•
A genome can consist of a single DNA
molecule (common in prokaryotic cells)
or a number of DNA molecules (common
in eukaryotic cells)
•
DNA molecules in a cell are packaged
into chromosomes
•
The number of chromosomes in somatic
cells varies widely among species: 18 in
cabbage plants, 56 in elephants, 90 in
hedgehogs, and 148 in one species of alga.
•
Eukaryotic chromosomes consist of
chromatin, a complex of DNA and
protein that condenses during cell division
Chromosomes in eukaryotes and prokaryotes are different
PROKARYOTES
EUKARYOTES
single chromosome plus plasmids
many chromosomes
circular chromosome
linear chromosomes
made only of DNA
made of chromatin, a nucleoprotein
(DNA coiled around histone proteins)
found in cytoplasm
found in a nucleus
copies its chromosome and divides
immediately afterwards
copies chromosomes, then the cell
grows, then goes through mitosis to
organise chromosomes in two equal
groups
Important: Chromosomes in eukaryotes
• Found in the nucleus
• Condensed and visible during cell division
• At the beginning of mitosis they can be seen to consist of two threads
(sister chromatids) joined by a centromere
• The sister chromatids are identical copies
• During mitosis the sister chromatids separate and are placed into two
nuclei
Numbers of chromosomes
•
Constant for each cell in the body (except sex cells which only
have half sets).
•
Constant throughout the life of an individual (you don’t lose or
gain chromosomes)
•
Constant for all members of a species
Identifying chromosomes
Chromosomes can be identified by:
• Their size
• Their shape (the position of the centromere)
NB Chromosomes are flexible
• Banding patterns produced by specific stains
(Giemsa)
Chromosomes are analysed by organising them
into a KARYOTYPE
Development and chromosomes
•
•
Differences in chromosomes are associated with difference in the way we
grow.
The karyotypes of males and females are not the same:
– Females have two large X chromosomes
– Males have a large X and a small Y chromosome
– The X and the Y chromosomes are called sex chromosomes
– The sex chromosomes are placed at the end of the karyotype
•Unusual growth can be associated
with chromosome abnormalities
e.g. People who develop Down’s
syndrome have trisomy 21
Trysomy-21 
Down’s syndrome
Chromosomes and cell division
•
Multicellular organisms copy their chromosomes
before cell division.
•
They must grow to a mature size.
•
The nucleus divides, distributing the chromosomes
into two equal groups (mitosis).
•
The cytoplasm then divides (cytokinesis) each part
taking a nucleus.
Interphase
Distribution of Chromosomes During
Eukaryotic Cell Division
•
In preparation for cell division, DNA is replicated and the chromosomes
condense
–
i.e. Chromatin fiber becomes densely coiled and folded (shorter and thick).
•
The centromere is the narrow “waist” of the duplicated chromosome, where the
two chromatids are most closely attached and the part of a chromatid on either
side of the centromere is referred to as an arm of the chromatid.
•
Each duplicated chromosome has two sister chromatids, which separate during
cell division and move into two new nuclei, one forming at each end of the cell.
–
•
The two chromatids, each containing an identical DNA molecule, are
initially attached all along their lengths by adhesive protein complexes
called cohesins; this attachment is known as sister-chromatid cohesion.
Once the sister chromatids separate, they are considered individual chromosomes.
Thus, each new nucleus receives a collection of chromosomes identical to that of
the parent cell
The nuclelus must first divide before the cell can !
•
•
•
Therefor Chromosomes must duplicate
Chromosomes are copied (# doubles)
Chromosomes appear as threadlike coils
(chromatin) at the start, but each chromosome
and its copy(sister chromosome) change to
sister chromatids at end of this phase
The mitotic phase alternates with interphase in the cell cycle
Phases of the Cell Cycle
•
The cell cycle consists
of:
a) Interphase : in which
cell growth and copying of
chromosomes in
preparation for cell division
≈ 90% of cell cycle).
b) Mitotic (M) phase :
that includes mitosis and
cytokinesis, (is usually the
shortest part of the cell
cycle ≈ 10% ).
a) Interphase
•During Interphase a cell grows (G1), continues to grow as it copies its
chromosomes (duplicated) (S), grows more as it completes preparations for cell
division (G2), and divides (M).
•Thus Interphase can be divided into three sub phases: a) G1 phase (“first gap”),
b) S phase (“synthesis”), and c) G2 phase (“second gap”)
•During all three sub-phases, the cell grows by producing proteins and
cytoplasmic organelles such as mitochondria and endoplasmic reticulum.
•After Mitosis the daughter cells may then repeat the cycle.
b) Mitosis
•Mitosis is conventionally divided into five phases: a) Prophase, b) Prometaphase,
c) Metaphase, d) Anaphase, and e) Telophase
•Overlapping with the latter stages of mitosis, cytokinesis is well underway by late
telophase and completes the mitotic phase.
The Mitotic Division of
an Animal Cell
Mitosis produces identical offspring (2N---2N)
Kinetochore (centromere)
The Mitotic Spindle: A Closer Look
•
The mitotic spindle is an apparatus of microtubules that controls chromosome
movement during mitosis
•
During prophase, assembly of spindle microtubules begins in the centrosome, the
microtubule organizing center
•
The centrosome replicates, forming two centrosomes that migrate to opposite ends
of the cell, as spindle microtubules grow out from them
•
An aster (a radial array of short microtubules) extends from each centrosome
•
The spindle includes the centrosomes, the spindle microtubules, and the asters
Cytokinesis: A Closer Look
•
In animal cells, cytokinesis
occurs by a process known as
cleavage, forming a cleavage
furrow
For Your Own Knoledge:
Binary Fission:
•
Prokaryotes (bacteria and
archaea) reproduce by a type
of cell division called binary
fission
•
In binary fission, the
chromosome replicates
(beginning at the origin of
replication), and the two
daughter chromosomes
actively move apart
The eukaryotic cell cycle is regulated by a molecular
control system
•
•
•
The frequency of cell division
varies with the type of cell
These cell cycle differences result
from regulation at the molecular
level.
Moreover, the cell cycle appears
to be driven by specific chemical
signals present in the cytoplasm
(Internal Signals) or by External
signals:
–
•
For You Own Knowledge
EXPERIMENT
Experiment 1
S
Experiment 2
G1
M
G1
M
M
RESULTS
Growth factors
– Fibronectin,etc
– Carcinogens
Some evidence for this hypothesis
comes from experiments in which
cultured mammalian cells at
different phases of the cell cycle
were fused to form a single cell
with two nuclei
S
S
When a cell in the
S phase was fused
with a cell in G1,
the G1 nucleus
Immediately entered
the S phase—DNA was
synthesized.
When a cell in the M phase
was fused with a cell in G1,
the G1 nucleus immediately
began mitosis—a spindle
formed and chromatin
condensed,even though the
chromosome had not been
duplicated.
The Cell Cycle Control System
•
The sequential events of the cell cycle are directed by a distinct cell cycle control
system, which is similar to a clock, regulated by both internal and external
controls
•
The clock has specific checkpoints where the cell cycle stops until a go-ahead
signal is received
•
If the cell does not receive the go-ahead signal, it will exit the cycle, switching
into a nondividing state called the G0 phase
For You Own Knowledge
Loss of Cell Cycle Controls in Cancer Cells
•
Cancer cells do not respond normally to the body’s control mechanisms
•
Cancer cells exhibit neither density-dependent inhibition nor anchorage
dependence
•
Cancer cells. Cancer cells usually continue
to divide well beyond a single layer,
forming aclump of overlapping cells, They
do not exhibit anchorage dependence or
density-dependent inhibition.
•
Cancer cells may not need growth factors
to grow and divide:
–
They may make their own growth
factor
–
They may convey a growth factor’s
signal without the presence of the
growth factor
–
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
25 µm
They may have an abnormal cell
cycle control system
25 µm
(a) Normal mammalian cells (b) Cancer cells
•
A normal cell is converted to a cancerous cell by a process called transformation
•
Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
•
If abnormal cells remain at the original site, the lump is called a benign tumor
•
Malignant tumors invade surrounding tissues and can metastasize, exporting
cancer cells to other parts of the body, where they may form secondary tumors
Meiosis reduces the number of chromosome sets from diploid
to haploid
•
•
•
Like mitosis, meiosis is preceded by the
replication of chromosomes (i.e. Interphase)
Meiosis takes place in two sets of cell
divisions, called meiosis I and meiosis II
resulting in four haploid daughter cells (each
having half as many chromosomes as the
parent cell), rather than the two daughter cells
in mitosis
Meiosis I is preceded by interphase, in which
chromosomes are replicated to form sister
chromatids
•Interphase
•Homologous pair of chromosomes
•in diploid parent cell
•Chromosomes
•replicate
•Homologous pair of replicated chromosomes
•Diploid cell with
•Sister
•replicated
•chromatids
•chromosomes
•Meiosis I
•1 •Homologous
•chromosomes
•separate
•
The sister chromatids are genetically identical •Meiosis II
and joined at the centromere
•2 •Sister chromatids
•separate
•
The single centrosome replicates, forming two
centrosomes
•Haploid cells with unreplicated chromosomes
•Metaphase I
•Prophase I
•Telophase I and
•Anaphase I
•Prophase II
•Metaphase II
•Anaphase II
•Cytokinesis
•Centrosome
•chromatids
•Centromere
•Chiasmata
•Spindle
•Cytokinesis
•Sister chromatids
•(with centriole pair)
•Sister
•Telophase II and
•remain attached
•(with kinetochore)
•Metaphase
•plate
•Sister chromatids
•Homologous
•chromosomes
•Homologous
•Cleavage
•chromosomes
•furrow
•separate
•Haploid daughter cells
•forming
•separate
•Fragments
•Microtubule
•of nuclear
•attached to
•envelope
•kinetochore
Division in meiosis I occurs in four
phases:
Division in meiosis II also occurs in
four phases:
•Prophase I
•Metaphase I
•Anaphase I
•Telophase I and cytokinesis
•Prophase II
•Metaphase II
•Anaphase II
•Telophase II and cytokinesis
•Note: Meiosis II is very similar to
mitosis
IMPORTANT: A Comparison of Mitosis and Meiosis
•
Mitosis conserves the number of chromosome sets, producing cells that are genetically
identical to the parent cell. While Meiosis reduces the number of chromosomes sets from two
(diploid) to one (haploid), producing cells that differ genetically from each other and from the
parent cell
•
The mechanism for separating sister chromatids is virtually identical in meiosis II and
mitosis
•
Three events are unique to meiosis, and all three occur in meiosis l:
–
Synapsis and crossing over in prophase I: Homologous chromosomes physically
connect and exchange genetic information.
–
Homologs on the metaphase plate: At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead of individual replicated chromosomes
–
Separation of homologs: At anaphase I, it is homologous chromosomes, instead of
sister chromatids, that separate.
The End