Chapter 10 -

Cell Growth and Division

• 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×).

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.

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

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

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

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.

• 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

• 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

Comparing Methods of ReproductionAsexual

• Faster Reproduction– when conditions are right

• Lack of Genetic Diversity

Sexual

• Slower Reproduction

• More Genetic Diversity– Able to survive changes in

environmental conditions

Comparing Asexual & Sexual Reproduction

Asexual

Reproduction

Sexual

Reproduction

Offspring

Parent Cells

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

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).

• 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.

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.

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

• 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

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.

• 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

• M Phase – Cell Division– Produces 2 identical daughter cells– Two Stages:• Mitosis- Division of the Nucleus• Cytokinesis – Division of the Cytoplasm

Events of the Cell Cycle

• During the cell cycle, the cell grows, replicates its DNA, and divides into two daughter cells.

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.

Prophase• Longest phase

• Chromosomes become visible

• Centrioles move to opposite sides of the nucleus

• Spindles form

• Nuclear membrane

breaks down

Metaphase

• Lasts only a few minutes

• Chromosomes line up in the center of the cell

• Microtubules connect the centromere to the spindles

Anaphase• Centromeres joining sister chromatids

separate to become individual chromosomes

• Chromosomes move apart

• Ends when chromosomes are at the poles of the spindle.

Telophase• Chromosomes begin

to fade into a tangle of dense material

• Nuclear envelope reforms

• Spindle breaks apart

• Nucleolus becomes visible

• Last phase of mitosis

Cytokinesis • Last phase of the M phase

• Division of the cytoplasm occurs

• Cell plate is formed in plants

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.

Cell Growth

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.

• 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.

Cell Cycle Regulation

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

• 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

Cell Growth and Healing

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

• 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

Apoptosis

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.

• 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.

• 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

Lung Cancer

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

• 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.

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

• 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.

• 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

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

• During the development process, an organism’s cells become more and more differentiated and specialized for particular functions

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

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

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)

• 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

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

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.

• The cells of the inner cell mass are pluripotent

• Pluripotent cells can develop into most (but not all) of the body’s cell types

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

• 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)

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

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

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

• 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

• 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