Download Cell division and mitosis

MITOSIS AND CELL CYCLE
Learning objectives:
At the end of the lecture, students should be able to:
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Identify steps of cell division:Mitosis( prophase, prometaphase, metaphase,anaphase,telophase)
Explain the significance of mitosis.
Define cell cycle.
Explain various stages of cell cycle.
Recall the events of somatic cell division.
Discuss the significance of S phase of cell cycle.
Integrate the knowledge of cell cycle to the basis of development of cancer.
Lecture outline
MITOSIS
• Division of somatic cell into two identical daughter cells.
• Mitosis is conventionally divided into five phases:
– Prophase
– Prometaphase
– Metaphase
– Anaphase
– Telophase
• Cytokinesis is well underway by late telophase
Functions of cell division:
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The continuity of life is based upon the reproduction of cells, or cell division
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Necessary to overcome the limitations to Cell Growth like:
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DNA “Overload” – there is a limit to the number of proteins a DNA molecule can code for at
one time. As the cell gets bigger, it needs more proteins, but, it may have to wait if there’s
already too many proteins being made
Exchange of Materials – there is a limit to the amount of materials a cell can move across it’s
cell membrane. As the cell gets bigger, it needs more food/water to come in AND more waste to
move out. Eventually it either starve or poison itself with wastes.
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Cell division is integral part of cell cycle
Functions of Cell Division
Life cycle of Cell
The life cycle of a cell is the period from cell reproduction to the next cell reproduction.
When mammalian cells are not inhibited and are reproducing as rapidly as they can, this life cycle may
be as little as 10 to 30 hours.
It is terminated by a series of distinct physical events called mitosis that cause division of the cell into
two new daughter cells.
Phases of the Cell Cycle
The cell cycle consists of
Mitotic (M) phase (mitosis and cytokinesis)
Interphase (cell growth and copying of chromosomes in preparation for cell division)
Cell Division ; M Phase
Mitosis – nuclear division
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis – division of cytoplasm and organelles
Results in 2 identical “daughter” cells
Cell Division; Interphase
(about 90% of cell cycle)
can be divided into subphases:
G1 phase (“first gap”)
S phase (“synthesis”)
G2 phase (“second gap”)
Chromosome Structure
Chromosome – condensed form of DNA that is visible during cell division
Sister Chromatids – the 2 copies of DNA that make up the chromosome and are separated into the 2
daughter cells during cell division
Centromere – where the 2 sister chromatids attach
The degree of coiling can vary in different regions of the chromatin:
Heterochromatin refers to highly coiled regions where genes aren’t expressed.
Euchromatin refers to loosely coiled regions where genes can be expressed.
Karyotype
An ordered, visual representation of the chromosomes in a cell
Chromosomes are photographed when they are highly condensed, then photos of the individual
chromosomes are arranged in order of decreasing size:
In humans each somatic cell has 46 chromosomes, made up of two sets, one set of chromosomes comes
from each parent
Prophase
Chromatin condenses, this causes the chromosomes to begin to become visible
Centrosomes separate, moving to opposite ends of the nucleus
The centrosomes start to form a framework used to separate two sister chromatids called the mitotic
spindle that is made of microtubules
Nucleolus disappears
Pro-metaphase
Nuclear envelope fragments
Chromosomes become more condensed
A kinetochore is formed at the centromere, the point where the sister chromatids are attached
Microtubules attach at the kinetochores
Metaphase
Shortest phase of mitosis
Centrioles are at poles
Chromosomes line up in the middle along equator
Spindle fibers attach to chromosomes on the centromere
Anaphase
Each centromere splits making two chromatids free
Each chromatid moves toward a pole
Cell begins to elongate , caused by microtubules not associated with the kinetochore
Telophase
Formation of nuclear membrane and nucleolus
Short and thick chromosomes begin to elongate to form long and thin chromatin
Formation of the cleavage furrow - a shallow groove in the cell near the old metaphase plate
Cytokinesis
division of the cytoplasm
In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow
Summary; Mitotic Division of an Animal Cell
Regulation of cell cycle
External signals: growth factors fibronectin,etc
Internal signals carcinogens
Apoptosis
Immune functions
SIGNIFICANCE OF S PHASE OF CELL CYCLE
• Synthesis Phase:
• It is the period during which synthesis of DNA takes place.
• DNA synthesis results in preparation of an exact replica of
the genetic material.
• Replication of centrioles also take place during this phase.
• The S phase takes approx 8 hours to complete.
Loss of Cell Cycle Control ; Cancer
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Cancer cells do not respond normally to the body’s control mechanisms
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Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
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If abnormal cells remain at the original site, the lump is called a benign tumor
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Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to
other parts of the body, where they may form secondary tumors
Normal brain cell
Cancer brain cell
Reference:
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Cell Division- Mitosis: Basic Histology by Junqueira
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Cell Cycle: Basic Histology by Junqueira