Chapter 10:

Cell Growth and Division

10.1Cell Growth, Division, and Reproduction

• S C . 9 1 2 . L . 1 6 . 1 7 C o m p a r e a n d c o n t r a s t m i t o s i s a n d m e i o s i s a n d r e l a t e t o t h e p r o c e s s o f s e x u a l a n d a s e x u a l r e p r o d u c t i o n a n d t h e i r c o n s e q u e n c e s f o r g e n e t i c v a r i a t i o n .

Limits To Cell Size • Cells divide rather than

continuing to grow because the larger a cell becomes, the more demands the cell places on its DNA. In addition, a larger cell is less efficient in moving nutrients and waste materials across the cell membrane.

Information “Overload” • Cells store DNA which is

necessary for cell growth. While cells increase in size, DNA does not.

• Cells need to duplicate and not grow because if they didn’t, the cell would grow to large and experience an information crisis where the DNA would not be able to serve the needs of the growing cell.

Exchanging Materials • The second reason why cells

must divide and not grow is correlated to the exchange of cellular items.

• The rate of food, oxygen, water, and waste product passage in and out of the cell is dependent on the surface area and volume of the cell. If the cell grew larger, cell functions in sufficient amounts would be more difficult to undergo.

Division Of The Cell

• Before becoming too large, the cell divides to form two new daughter cells in the process called cell division. Cell division reduces all of the problems that would be caused by the overgrowth of cells.

• DNA overload does not occur because each daughter cell gets its own copy of the genetic material. The problem of exchanging materials does not occur because the surface area and volume of each cell are proportional.

Cell Division and Reproduction • Reproduction is an

essential step in an organism. Cells can undergo either sexual or asexual reproduction.

Asexual Reproduction • Asexual reproduction is the

production of genetically identical offspring from a single parent.

• Asexual reproduction, like cell division, is the only form of reproduction for most single-celled organisms. It is simple and efficient, and increases population size very quickly.

Sexual Reproduction • The other form of reproduction

that organisms undergo is sexual reproduction. Most plants and animals reproduce sexually.

• Sexual reproduction is a form off reproduction where the fusion of each parent’s gametes form a new individual. Offspring produced by sexual reproduction inherit some of their genetic information from each parent.

Sexual Reproduction Vs.Asexual Reproduction

Sexual Reproduction• Sexual reproduction can pass

on acquired favorable traits and adaptations for specific instances, and creates genetic diversity. However; finding a mate and the development of offspring requires more time than asexual reproduction.

Asexual Reproduction• Asexual reproduction

produces organisms more quickly with a higher survival rate that are genetically identical to the parent. However; asexual reproduction results in a lack of genetic diversity.

Both asexual and sexual reproduction have positives and negatives, but either form of reproduction is more favorable to a specific species with certain circumstances.

10.2The Process of Cell Division

• S C . 9 1 2 . L . 1 6 . 1 4 D e s c r i b e t h e c e l l c y c l e , i n c l u d i n g t h e p r o c e s s o f m i t o s i s . E x p l a i n t h e r o l e o f m i t o s i s i n t h e f o r m a t i o n o f n e w c e l l s a n d i t s i m p o r t a n c e i n m a i n t a i n i n g c h r o m o s o m e n u m b e r d u r i n g a s e x u a l r e p r o d u c t i o n .

Chromosomes • Chromosomes are packages of DNA containing carefully bundled genetic information.

• In order for genetic information to be correctly distributed in cellular division, cells must first make a complete copy of their genetic information prior to division.

Prokaryotic Chromosomes • The DNA molecules in

prokaryotes are found in the cytoplasm along with most of the other contents of the cell.

• Most prokaryotes contain a single, circular DNA chromosome that contains all of the cell’s genetic information.

Eukaryotic Chromosomes• Chromosomes make it possible to separate DNA

precisely during cell division.• Chromosomes are made up of histones coiled with DNA

that form nucleosomes that condense during cell division.

The Cell Cycle

• During the cell cycle, a cell grows, prepares for division, and divides to form two new daughter cells.

• Each daughter cell then moves into a new cell cycle of activity, growth, and division.

The Prokaryotic Cell Cycle • Cell division in prokaryotes is

a form of asexual reproduction known as binary fission.

• Once the chromosome has replicated, the cell membrane begins to indent and pinch inwards until the cytoplasm and chromosomes divide to form two new daughter cells.

The Eukaryotic Cell Cycle

• The eukaryotic cell cycle consists of four phases: G1, S, G2 (Interphase), and M.

• G1 Phase: Cell Growth:

• During G1 phase, the cells do most of their growing. They increase in size and synthesize new proteins and organelles.

• S Phase: DNA Replication:• During S phase, new DNA is

synthesized when the chromosomes are replicated. The cell at the end of the S phase contains twice as much DNA as it did at the beginning.

The Eukaryotic Cell Cycle

• G2 Phase: Preparing for Cell Division:• G2 phase Is the shortest phase

of interphase. During G2 phase, many of the organelles and molecules required for cell division are produced.

• M Phase: Cell Division:• During M phase, two

processes are undergone. First is the process of cell division known as mitosis. Second is the division of the cytoplasm, known as cytokinesis.

Mitosis • Mitosis is the part of eukaryotic cell division during which the cell nucleus divides.

• Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase.

Prophase

During prophase, the longest stage of mitosis, chromosomes begin to condense and are visible through a light microscope. The cell grows spindle fibers. The spindle fibers extend from the centrioles, which begin to move to opposite sides of the cell. The nuclear envelope begins to disintegrate and the nucleolus divides.

Metaphase

During metaphase, the spindle fibers attach to the centromeres of the chromosomes. The chromosomes are moved to the center of the cell and line up in the center as they are held there by the spindle fibers.

Anaphase

During anaphase, the sister chromatids divide at the centromere to form separate chromosomes. Spindle fibers in the center of the cell lengthen and move the chromosomes to opposite ends of the cell. Anaphase ends when the chromosomes are completely separated into two groups.

Telophase

Telophase is the final step of mitosis. During telophase, the mitotic spindle fibers disassemble. The nuclear envelope begins to reform around the chromosomes and the nucleolus becomes visible. The chromosomes unwind into a tangle of chromatin.

Cytokinesis• Cytoplasm completes

the process of cell division- it splits one cell into two.

• The process of cytokinesis differs in animal and plant cells.

Cytokinesis in Animal Cells

In animal cells, the fibers of the cell membrane contract and completely split the cell into two equal parts.

Cytokinesis in Plant Cells

In plant cells, a cell plate forms and grows until it eventually divides the nuclei and cell in two.

10.3Regulating the Cell Cycle

• S C . 9 1 2 . L . 1 6 . 8 E x p l a i n t h e r e l a t i o n s h i p b e t w e e n m u t a t i o n , c e l l c y c l e , a n d u n c o n t r o l l e d c e l l g r o w t h p o t e n t i a l l y r e s u l t i n g i n c a n c e r.

Controls on Cell Division

• Cell growth and division can be turned on and off. Most cells divide until they come in contact with each other, where once they touch they stop dividing and growing.

• Inside the body when an injury occurs, the cells at the edge of the injury are stimulated to divide to start the healing process. The rate of cell division slow and everything goes back to normal when the injury is healed.

The Discovery of Cyclins • Cyclin is one of a family of

proteins that regulates the cell cycle in eukaryotic cells.

• Biologists discovered that the protein cyclin tells the cells when it is time to divide, duplicate their chromosomes, and enter another phase of the cell cycle.

Regulatory Proteins • Through further research,

scientists have learned that cyclin is just one of dozens of proteins that regulate the cell cycle.

• The cell cycle is controlled by regulatory proteins both inside and outside the cell.

Internal Regulators

• Internal regulatory proteins respond to events occurring inside a cell. They allow the cell cycle to proceed only when certain events have occurred in the cell itself.

• Some prevent mitosis from occurring until chromosomes from replicating.

External Regulators • External regulatory proteins

respond to events outside the cell. They produce direct cells to speed up or slow down the cell cycle.

• One group of external regulatory proteins are growth factors. Growth factors stimulate the growth and division of cells and are important in the healing of wounds and embryonic development.

Apoptosis• Just as new cells are produced

everyday, cells die on a regular basis. Cells either die from accidental damage or apoptosis.

• Apoptosis is a process of programmed cell death.

• Apoptosis is important in development by shaping body structures through the removal of cells and sometimes disease can occur if apoptosis doesn’t happen.

Cancer: Uncontrolled Cell Growth

• Cancer is a disorder in which body cells loses its ability to control growth.

• Cancer cells do not respond to the signals that regulate the growth of most cells. As a result, the cells divide uncontrollably.

• Cancer cells from cancerous tumors which are malignant as they invade and destroy surrounding healthy tissue. As the body is disrupted, life-threatening illness results.

What Causes Cancer? • Cancers are caused by

defects in the genes that regulate cell growth and division.

• There are several sources of such defects including: smoking or chewing tobacco, radiation exposure, defective genes, and viral infection.

Treatment For Cancer

• Great advances have been made in chemotherapy and have even made it possible to cure certain types of cancer; however it produces serious, harmful side-effects.

• Tumorous cancers can often be removed especially with skin diseases like melanoma.

10.4Cell Differentiation

• H E . 9 1 2 . C . 1 . 8 A n a l y z e s t r a t e g i e s f o r p r e v e n t i o n , d e t e c t i o n , a n d t r e a t m e n t o f c o m m u n i c a b l e a n d c h r o n i c d i s e a s e s .

From Once Cell to Many

• 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.

Defining Differentiation

• The process by which cells become specialized is known as differentiation.

• Our bodies contain highly differentiated cells that carry out the jobs we need to perform to stay alive.

• During the development of an organism, cells differentiate into many types of cells.

Mapping Differentiation

• The process of differentiation determines a cell’s ultimate identity.

• In some organisms, a cell’s role is rigidly determined at a specific point in the course of development.

• Biologists have mapped the outcome of each and every cell division from fertilized egg to adult in many organisms.

Differentiation in Mammals

• In mammals, cell differentiation is controlled by a number of interacting factors in the embryo.

• Adult cells generally reach a point at which their differentiation is complete: when they can no longer become other types of cells.

Stem Cells and Development

• Scientists wonder how so many specialized cells can come from one single cell. That cell would have to be totipotent, meaning that it is literally able to do everything and develop into any type of cell.

• Only the fertilized egg and the cells produced by the first few cell divisions are truly totipotent.

Human Development • A human embryo forms

into a blastocyst in which its cells have begun to specialize.

• The outer cells form tissue to attach the embryo to the mother. The inner cells become the actual embryo.

• The inner cells are pluripotent, meaning they can develop into most but not all cell types.

Stem Cells • The unspecialized cells from which differentiated cells develop are known as stem cells.

• Because of their potential to develop into other cell types, stem cells are the subject of intense research around the world.

Embryonic Stem Cells

Embryonic stem cells are pluripotent cells found in the embryo and have the capacity to produce just about any cell type in the human body.Researchers have been working closely with embryonic stem cells to differentiate them for different medical uses.

Adult Stem Cells • Adult stem cells are groups of

cells that differentiate to renew and replace cells in the adult body.

• Adult stem cells are multipotent, meaning that they can develop into many types of differentiated cells.

• Evidence from such cells suggest that the body contains pools of stem cells from which new skin and blood cells can be produced.

Frontiers in Stem Cell Research

• Scientists would like to learn exactly which signals tell a cell to become specialized, and how others remain multipotent.

• Scientists wish to solve the mystery around how stem cells retain the capacity to differentiate.

Potential Benefits

• Stem cells offer the potential benefit of using undifferentiated cells to repair or replace badly damaged cells and tissues.

• The hope for stem cell research is that they can be used to repair cellular damage and maybe one day be used to regrow even whole, malfunctioning body parts.

Ethical Issues

• Although there are benefits to stem cell research, many ethical questions are raised.

• Embryological stem cells are harvested from embryos that are inevitably going to be destroyed. Since some people feel that embryos are entitled to rights, government funded research has been highly viewed as unethical and unjust.