chapter 10 - cell growth and division. this liver cell has almost completed the process of cell...
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Chapter 10 -
Cell Growth and Division
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• This liver cell has almost completed the process of cell division. During cell division, a cell splits into two roughly equal daughter cells (magnification: 11,500×).
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10-1 Cell Growth, Division, and Reproduction
Limits to Cell Size
What are some of the difficulties a cell faces as it increases in size?
•The larger a cell becomes, the more demands the cell places on its DNA and the more trouble the cell has moving enough nutrients and wastes across the cell membrane.
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Limits to Cell Size
• DNA Overload– As a cell grows in size, its DNA does not– “information crisis”
• Exchanging Materials– getting food into and wastes out of the
cell
• Ratio of Surface Area to Volume– volume increases more rapidly than
surface area
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Division of the Cell
• Before a cell becomes too large, it divides into two new “daughter cells”
• This process is called “cell division”
• Cell division solves all 3 problems
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Solutions:
• DNA Overload– Before cell division occurs, the cell
replicates (copies) all of its DNA so that each daughter cell will get a copy of genetic information
• Exchanging Materials– Reduces cell volume
• Ratio of Surface Area to Volume– Increases Surface Area to Volume Ratio
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Cell Division and Reproduction
How do asexual and sexual
reproduction compare?
• Reproduction (the formation of new individuals) is one of the most important characteristics of living things.
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• Asexual Reproduction– Offspring are produced from a single
parent cell – Simple, efficient, effective– Enables populations to increase in number
very quickly– The two daughter cells are genetically
identical to the parent cell (in most cases)– Exs. – Bacteria
– Single-celled organisms
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• Sexual Reproduction– Offspring are produced by inheriting some
of their genetic information from each parent cell
– Involves the fusion of genetic information from two separate parent cells
– Allows for genetic diversity in populations – The daughter cells are genetically different
from the parent cell – Exs. – Most animals and plants
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Comparing Methods of ReproductionAsexual
• Faster Reproduction– when conditions are right
• Lack of Genetic Diversity
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Sexual
• Slower Reproduction
• More Genetic Diversity– Able to survive changes in
environmental conditions
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Comparing Asexual & Sexual Reproduction
Asexual
Reproduction
Sexual
Reproduction
Offspring
Parent Cells
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Prokaryotic Chromosomes
• Prokaryotes do not have a nucleus
• Their DNA is found in the cytoplasm
• Most Prokaryotes contain a single, circular DNA chromosome
10-2 The Process of Cell Division
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Eukaryotic Chromosomes
• DNA is contained in the nucleus
• Chromosomes are made up of DNA and proteins.
• Chromosomes are not visible except during division.
• Before division, each chromosome is replicated (copied).
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• Chromosomes become visible at the beginning of cell division.
• Each chromosome consists of two identical “sister” chromatids.
• Each pair of chromatids is attached at an area called the centromere. Centromeres are usually located near the middle of the chromatids, some lie near the ends.
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A Human Chromosome
• This is a human chromosome shown as it appears through an electron microscope. Each chromosome has two sister chromatids attached at the centromere.
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The Prokaryotic Cell Cycle
• Takes place very rapidly under ideal conditions
• DNA is replicated when bacteria reach a certain size
• Cell Division begins when replication is complete
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• The 2 DNA molecules attach to different regions of the cell membrane
• The cell is pinched inward, dividing the cytoplasm and chromosomes between the two new cells
• This results in a form of asexual reproduction called binary fission
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The Eukaryotic Cell Cycle
• The cell cycle consists of 4 Phases:
G1, S, G2, and M.
• The length of the cell cycle and the length of each phase depends on the type of cell.
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• Interphase – – a period of growth between
cell divisions– Divided into 3 parts:
• G1 - Cell Growth–Cells do most of their growing in this phase
• S – DNA Replication (Synthesis)–DNA is replicated (copied)– The cell contains 2 copies of its DNA
• G2 – Preparing for Cell Division–Usually the shortest phase–When completed, cell division begins
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• M Phase – Cell Division– Produces 2 identical daughter cells– Two Stages:• Mitosis- Division of the Nucleus• Cytokinesis – Division of the Cytoplasm
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Events of the Cell Cycle
• During the cell cycle, the cell grows, replicates its DNA, and divides into two daughter cells.
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Mitosis• Mitosis – the part of cell division during
which the nucleus divides.
• Biologists divide the events of mitosis into four phases:– prophase– metaphase– anaphase– telophase
• Mitosis may last anywhere from a few minutes to several days.
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Prophase• Longest phase
• Chromosomes become visible
• Centrioles move to opposite sides of the nucleus
• Spindles form
• Nuclear membrane
breaks down
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Metaphase
• Lasts only a few minutes
• Chromosomes line up in the center of the cell
• Microtubules connect the centromere to the spindles
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Anaphase• Centromeres joining sister chromatids
separate to become individual chromosomes
• Chromosomes move apart
• Ends when chromosomes are at the poles of the spindle.
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Telophase• Chromosomes begin
to fade into a tangle of dense material
• Nuclear envelope reforms
• Spindle breaks apart
• Nucleolus becomes visible
• Last phase of mitosis
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Cytokinesis • Last phase of the M phase
• Division of the cytoplasm occurs
• Cell plate is formed in plants
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10-3 Regulating the Cell Cycle
• Controls on Cell Division– Cells grown in the lab will continue to
divide until they come into contact with other cells. Then they stop growing.
– If you remove cells, the cells will divide again until they touch other cells.
– This shows that controls on cell growth and division can be turned on and off.
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Cell Growth
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The Discovery of Cyclins
• For many years, biologists searched for a signal that would regulate the cell cycle – something that would tell cells when it was time to divide, replicate their chromosomes, or enter another phase of the cell cycle.
• In the 1980’s, a protein was discovered that when injected, would cause a nondividing cell to form a mitotic spindle.
• They named this protein Cyclin.
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• Cyclins are proteins that regulate the timing of the cell cycle.
• Scientists have discovered a family of cyclins that regulate the timing of the cell cycle in eukaryotic cells.
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Cell Cycle Regulation
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Regulatory Proteins
• The cell cycle is controlled by regulatory proteins both inside and outside the cell.
• Internal Regulators – proteins that monitor and respond to events inside the cell.– Examples:• Making sure the a cell does not enter mitosis
until its chromosomes have replicated• Preventing a cell from entering anaphase
until the spindle fibers have attached to the chromosomes
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• External Regulators – proteins that respond to events outside
the cell. – Can direct the cell to speed up or
slow down their cell cycles.
• Examples:– Growth Factors• Stimulate the growth and division of
cells
• Important during embryonic development and wound healing
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Cell Growth and Healing
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Apoptosis
• A process of programmed cell death
• Once triggered, a cell undergoes a series of controlled steps leading to its self-destruction:– The cell and its chromatin shrink– Then parts of the cell’s membranes
break off– Neighboring cells then quickly clean up
the cell’s remains
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• Apoptosis plays a key role in development by shaping the structure of tissues and organs in plants and animals.
• Example – the embryonic development of a mouse’s foot– The space between the toes is caused by
cell death through Apoptosis
• When Apoptosis does not occur as it should, a number of diseases can result– Examples: Cell loss in AIDS and Parkinson’s
disease from too much Apoptosis
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Apoptosis
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Cancer: Uncontrolled Cell Growth
• Cancer is a disorder in which body cells lose the ability to control cell growth and division
• Cancer cells do not respond to the signals that regulate the growth of most cells.
• As a result, most cancer cells divide uncontrollably.
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• Cancer cells form a mass of cells called a tumor
• Not all tumors are cancerous
• Some tumors are benign, or noncancerous
• A benign tumor does not spread to surrounding healthy tissue or to other parts of the body.
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• Cancerous tumors are malignant
• Malignant tumors invade and destroy surrounding healthy tissue
• As cancer cells spread:– They absorb the nutrients needed by other
cells– Block nerve connections– Prevent organs from functioning properly
• This disrupts the delicate balances of the body, and life-threatening illness results
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Lung Cancer
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What Causes Cancer?
• Cancer is caused by defects in the genes that regulate cell growth and division
• Sources of defects:– Smoking or chewing tobacco– Radiation exposure– Defective genes– Viral Infection
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• All cancers have one thing in common The control over the cell cycle has broken down.
• Many cells have a defect in the gene p53. This gene normally stops the cell cycle until all of the chromosomes have properly replicated.
• Cells lose the information they need to be able to respond to the signals that normally control cell growth.
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Treatments for Cancer
• When a cancerous tumor is located, it can often be removed by surgery– Example – Melanomas (skin cancer)
• Cancer cells grow rapidly so they must copy their DNA quickly.
• This makes them vulnerable to damage from radiation
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• Chemical compounds that would kill cancer cells (or at least slow their growth) are used in chemotherapy.
• Great advances in chemotherapy has made it possible to cure some forms of cancer.
• However, because chemotherapy compounds target rapidly dividing cells, they also interfere with cell division in normal, healthy cells.
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• Chemotherapy produces some serious side effects in some patients
• Researchers are searching to find highly specific ways in which cancer cells can be targeted for destruction while leaving healthy cells unaffected
• Cancer is a serious disease. It is a disease of the cell cycle and conquering it will require a deeper understanding of the processes that control cell division
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10-4 Cell Differentiation
From One Cell to Many
How do cells become specialized for different functions?
• Multicellular organisms start life as just one cell
• Living things pass through a developmental stage called an embryo from which the adult organism is gradually produced
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• During the development process, an organism’s cells become more and more differentiated and specialized for particular functions
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Differentiation• Differentiation is the process by which
cells become specialized
• During the development of an organism, cells differentiate into many types of cells
• A differentiated cell has become different from the embryonic cell that produced it
• The cell is specialized to perform certain tasks– Exs. – contraction, photosynthesis,
protection
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Mapping Differentiation
• The process of differentiation determines a cell’s ultimate identity
• In some organisms, a cell’s role is determined at a specific point in development
• Each time an organism develops, the process is the same
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Differentiation in Mammals
• In mammals and other organisms, cell differentiation is a flexible process that is controlled by a number of interacting factors in an embryo
• Adult cells generally reach a point at which their differentiation is complete (they can no longer become other types of cells)
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• How are all of the specialized, differentiated types of cells in the body formed from just a single cell?
• Such a cell is called “totipotent”
• It is literally able to do everything and to develop into any type of cell in the body
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Stem Cells
What are Stem Cells?
• Stem cells are unspecialized cells from which differentiated cells develop
• They are at the base of a branching “stem” of development from which different cell types form
• They have the potential to develop into other cell types
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Human Development
• After about 4 days of development, a human embryo forms into a blastocyst, a hollow ball of cells with a cluster of cells inside called the inner cell mass
• Even at this early stage, the cells of the blastocyst begin to specialize.
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• The cells of the inner cell mass are pluripotent
• Pluripotent cells can develop into most (but not all) of the body’s cell types
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Embryonic Stem Cells
• These are pluripotent cells found in the early embryo
• These cells can be grown in culture and coaxed to differentiate into nerve cells, muscle cells, and even into sperm and egg cells
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• Typically, stem cells of a given organ or tissue produce only the types of cells that are unique to that tissue
• Examples:– Adult stem cells in bone marrow can
develop into several different types of blood cells
– Stem cells in the brain can produce neurons (nerve cells)
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Adult Stem Cells
• These are groups of cells that differentiate to renew and replace cells in the adult body
• They have limited potential and are called multipotent, meaning that they can develop into many types of differentiated cells
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Potential Benefits
• Stem cells offer the potential benefit of using undifferentiated cells to repair or replace badly damaged cells and tissues
• Stem cells may have an important impact on human health
• Stem cells may be able to repair the cellular damage of some conditions such as:– Heart muscle cells following a heart attack– Brain cell damage caused by a stroke– Paralysis from spinal cord injuries
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Ethical Issues
• Human embryonic stem cell research is controversial because the arguments for it and against it both involve ethical issues of life and death
• Most techniques for harvesting embryonic stem cells cause the destruction of the embryo
• Unlike embryonic stem cells, adult stem cells have raised few ethical questions as they can be obtained from the body of a willing donor
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• In the future, it may be possible to address these concerns with a technological solution
• Recent experiments suggest that it may be possible to extract a small number of embryonic stem cells without damaging the embryo itself
• Other experiments have shown that it is possible to reprogram adult cells by switching “on” a few genes, causing them to function like embryonic stem cells
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• In this way, there would be no need to involve embryos at all
• It could also make it possible to tailor specific therapies to fit the needs of each individual patient
• If successful, methods like these might allow research to go forward while avoiding any destruction of embryonic life