1 Ass. Lec Ghufran S. Salih Cytology/ Second Stage 2018/ 2019 First semester University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering Lecture Six… Cell Cycle and Cell Division (a) 1. Cell Cycle or Cell Division Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place. All these processes, i.e., cell division, DNA replication, and cell growth, hence, have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control. Cell Lifespan Cell pass through life at four stages - Born - Differentiate - Function - Die or Divide Lifespan Processes - Birth…… Mitosis (except germ cells - Meiosis) - Growth ......Expression of genes and proteins required to grow the cell, its organelles and cytoskeleton - Function……Expression of tissue specific genes and proteins - Division……DNA during cycle, whole cell in Mitosis
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering
Lecture Six… Cell Cycle and Cell Division (a)
1. Cell Cycle or Cell Division
Cell division is a very important process in all living organisms. During
the division of a cell, DNA replication and cell growth also take place.
All these processes, i.e., cell division, DNA replication, and cell growth,
hence, have to take place in a coordinated way to ensure correct division
and formation of progeny cells containing intact genomes. The sequence
of events by which a cell duplicates its genome, synthesizes the other
constituents of the cell and eventually divides into two daughter cells
is termed cell cycle. Although cell growth (in terms of cytoplasmic
increase) is a continuous process, DNA synthesis occurs only during one
specific stage in the cell cycle. The replicated chromosomes (DNA) are
then distributed to daughter nuclei by a complex series of events during
cell division. These events are themselves under genetic control.
Cell Lifespan
Cell pass through life at four stages
- Born
- Differentiate
- Function
- Die or Divide
Lifespan Processes
- Birth…… Mitosis (except germ cells - Meiosis)
- Growth ......Expression of genes and proteins required to grow the
cell, its organelles and cytoskeleton
- Function……Expression of tissue specific genes and proteins
- Division……DNA during cycle, whole cell in Mitosis
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering
- Death…….Apoptosis (programmed cell death) Necrosis (un
programmed cell death)
Cell Lifespan Examples
Intestinal epithelial cells 3–5 days
Red blood cell 120 days
Brain neuron, heart 50 - 100 years
Neutrophil in circulation 6-7 hours; in tissue 4 days
Chromosomes
A. Composition
1. Chromatin
a. DNA and protein
b. Present in non-dividing cells
c. Chromosomes condense during cell division
2. Nucleosomes
a. Part of the chromatin
b. Packaged when different types of histones interact
- Histone (H1) associates with the linker histone, Creates 30 nm diameter
fibers, Fibers form loops held together by scaffolding proteins, These
interact to form the condensed chromosome seen in metaphase
3. DNA
a. Organized into informational units called genes, Chromosomes contain
hundreds to thousands of genes , Humans have less than 30,000 genes
that code for proteins.
b. DNA in chromosomes is packaged in a highly organized way, which
associated with positively charged histone proteins and form beadlike
nucleosomes. Each nucleosome is composed of 146 base pairs wrapped
around 8 histones.
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering
B. Chromosomes of different species differ in number and
information content
1. Humans and several other species of organisms have 46 chromosomes
2. The number of chromosomes does not provide information about the
organism
3. chromosomes carry the genetic information of a cell to the next
generation, and to offspring
Figure 6.1: Structure of chromosome and their compositions
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering
2. Stages of the eukaryotic cell cycle
• The cell cycle describes the status of cells in relationship to growth and division:
1. when cells reach a certain size, growth either stops or the cell must divide
2. most, but not all, eukaryotic cells are capable of dividing
3. cell division is generally a highly regulated process
4. the generation time for eukaryotic cells varies widely, but is usually 8-20 hours
• Cell cycle includes all events from a cell’s formation until it
divides into two major periods (figure 6.2):
a. Interphase
b. Cell division (mitosis: nuclear division and cytokinesis:
cytoplasmic division).
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering
Figure 6.1: Stages of cell cycle
A. interphase
• Metabolic or growth phase
• From cell formation until cell division
• 90% of the cell cycle
• DNA replication
• Preparation for division
• It is divided into subphases:
1. G0: cells that do not divide become arrested in this phase, G0 can last
minutes to years.
2. S: synthetic phase; the DNA is completely replicated (genetic
information duplicated).
3. G1: growth phase with little cell division related activities
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering
4. G2: brief period of growth where enzymes and other proteins necessary
for division are synthesized
B. cell division has two main parts – mitosis and cytokinesis
1. mitosis is the process that distributes a complete copy of the duplicated
genetic information to each daughter cell.
2. cytokinesis is the process of dividing the cytoplasm into two separate
cells. some cells can have mitosis without cytokinesis (most common in
fungi and slime molds)
Mitotic Cell Division
A. Characteristics
1. Daughter cells (2) are identical to mother cell
2. No gain or loss of genetic material
3. Series of continuous events
4. Lasts about two hours
B. Phases of mitosis
1. Cellular characteristics at end of G2
a. Nucleus with nuclear membrane
b. Nucleoli present
c. Two centrosomes present
i. Closely affiliated with nucleus
d. Pair of centrioles present in each centrosome
e. Aster is around each centriole pair
i. Array of microtubules
f. Genetic material is still in chromatin form
2. Characteristics of Prophase
a. Nucleus
- Nucleoli disappear
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering
- Chromatin condenses to form chromosomes
- Each chromosome (duplicated during S phase) forms a pair of sister
chromatids
- Sister chromatids are joined at a centromere by protein tethers
- Centromeres contain a kinetochore where microtubules will bind
- Each sister chromatid has its own kinetochore
b. Cytoplasm
- A system of microtubules, called the mitotic spindle, organizes between
the two poles (opposite ends) of the cell, each pole has a microtubule
organizing center (MTOC), in animals and some other eukaryotes,
centrioles are found in the MTOC.
- Centrosomes migrate toward the poles along the nuclear surface.
- Spindles lengthen.
3. Characteristics of Prometaphase
a. Nuclear membrane disappears and spindles interact with chromosomes
b. Spindles extend from each pole toward the cell’s equator
c. Non-kinetochore microtubules radiate from the centrosomes toward the
metaphase plate, Microtubules overlap with those of the opposite pole
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Ass. Lec Ghufran S. Salih Cytology/ Second Stage
2018/ 2019 First semester
University of Warith Al-Anbiyaa College of Engineering Biomedical Engineering