Lesson Overview Lesson Overview Cell Growth, Division, and Reproduction Lesson Overview 10.1 Cell Growth, Division, and Reproduction

Lesson OverviewLesson Overview Cell Growth, Division, and ReproductionCell Growth, Division, and Reproduction

Lesson OverviewLesson Overview10.1 Cell Growth, Division, 10.1 Cell Growth, Division,

and Reproductionand Reproduction


Information “Overload”Living cells store critical information in DNA.

As a cell grows, that information is used to build the molecules needed for cell growth.

As size increases, the demands on that information grow as well. If a cell were to grow without limit, an “information crisis” would occur.


Information “Overload” Compare a cell to a growing town. The town library has a limited number of books. As the town grows, these limited number of books are in greater demand, which limits access.

A growing cell makes greater demands on its genetic “library.” If the cell gets too big, the DNA would not be able to serve the needs of the growing cell.


Exchanging MaterialsFood, oxygen, and water enter a cell through the cell membrane. Waste products leave in the same way.

The rate at which this exchange takes place depends on the surface area of a cell.

The rate at which food and oxygen are used up and waste products are produced depends on the cell’s volume.

The ratio of surface area to volume is key to understanding why cells must divide as they grow.


Ratio of Surface Area to VolumeImagine a cell shaped like a cube. As the length of the sides of a cube increases, its volume increases faster than its surface area, decreasing the ratio of surface area to volume.

If a cell gets too large, the surface area of the cell is not large enough to get enough oxygen and nutrients in and waste out.


Traffic ProblemsTo use the town analogy again, as the town grows, more and more traffic clogs the main street. It becomes difficult to get information across town and goods in and out.

Similarly, a cell that continues to grow would experience “traffic” problems. If the cell got too large, it would be more difficult to get oxygen and nutrients in and waste out.


Division of the CellBefore a cell grows too large, it divides into two new “daughter” cells in a process called cell division.

Before cell division, the cell copies all of its DNA.

It then divides into two “daughter” cells. Each daughter cell receives a complete set of DNA.

Cell division reduces cell volume. It also results in an increased ratio of surface area to volume, for each daughter cell.


In multicellular organisms, cell division leads to growth. It also enables an organism to repair and maintain its body.

In single-celled organisms, cell division is a form of reproduction.

Asexual Reproduction


Asexual ReproductionAsexual reproduction is reproduction that involves a single parent producing an offspring. The offspring produced are, in most cases, genetically identical to the single cell that produced them.

Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring.

Both prokaryotic and eukaryotic single-celled organisms and many multicellular organisms can reproduce asexually.


Examples of Asexual Reproduction

Bacteria reproduce by binary fission.

Starfish can reproduce by fragmentation.

Hydras reproduce by budding.


Sexual ReproductionIn sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow.

The offspring produced inherit some genetic information from both parents.

Most animals and plants, and many single-celled organisms, reproduce sexually.


Comparing Sexual and Asexual Reproduction


Lesson OverviewLesson Overview10.2 The Process 10.2 The Process

of Cell Divisionof Cell Division


The genetic information that is passed on from one generation of cells to the next is carried by chromosomes.

Every cell must copy its genetic information before cell division begins.

Each daughter cell gets its own copy of that genetic information.

Cells of every organism have a specific number of chromosomes.

Chromosomes


Prokaryotic ChromosomesProkaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm.

Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information.


The Prokaryotic Cell CycleThe prokaryotic cell cycle is a regular pattern of growth, DNA replication, and cell division.

Most prokaryotic cells begin to replicate, or copy, their DNA once they have grown to a certain size.

When DNA replication is complete, the cells divide through a process known as binary fission.


Binary fission is a form of asexual reproduction during which two genetically identical cells are produced.

For example, bacteria reproduce by binary fission.

The Prokaryotic Cell Cycle


The Eukaryotic Cell CycleThe eukaryotic cell cycle consists of four phases: G1, S, G2, and M.

Interphase is the time between cell divisions. It is a period of growth that consists of the G1, S, and G2 phases. The M phase is the period of cell division.


G1 Phase: Cell Growth

In the G1 phase, cells increase in size and synthesize new proteins and organelles.


S Phase: DNA ReplicationIn the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.


G2 Phase: Preparing for Cell Division

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


M Phase: Cell DivisionIn eukaryotes, cell division occurs in two stages: mitosis and cytokinesis.

Mitosis is the division of the cell nucleus.

Cytokinesis is the division of the cytoplasm.


Important Cell Structures Involved in Mitosis

Chromatid – each strand of a duplicated chromosome

Centromere – the area where each pair of chromatids is joined

Centrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindle

Spindle – a fanlike microtubule structure that helps separate the chromatids


ProphaseDuring prophase, the first phase of mitosis, the duplicated chromosome condenses and becomes visible.

The centrioles move to opposite sides of nucleus and help organize the spindle.

The spindle forms and DNA strands attach at a point called their centromere.

The nucleolus disappears and nuclear envelope breaks down.


MetaphaseDuring metaphase, the second phase of mitosis, the centromeres of the duplicated chromosomes line up across the center of the cell.

The spindle fibers connect the centromere of each chromosome to the two poles of the spindle.


AnaphaseDuring anaphase, the third phase of mitosis, the centromeres are pulled apart and the chromatids separate to become individual chromosomes.

The chromosomes separate into two groups near the poles of the spindle.


TelophaseDuring telophase, the fourth and final phase of mitosis, the chromosomes spread out into a tangle of chromatin.

A nuclear envelope re-forms around each cluster of chromosomes.

The spindle breaks apart, and a nucleolus becomes visible in each daughter nucleus.


Cytokinesis is the division of the cytoplasm.

The process of cytokinesis is different in animal and plant cells.

Cytokinesis


Cytokinesis in Animal CellsThe cell membrane is drawn in until the cytoplasm is pinched into two equal parts.

Each part contains its own nucleus and organelles.


Cytokinesis in Plant CellsIn plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall.

Instead, a cell plate forms between the divided nuclei that develops into cell membranes.

A cell wall then forms in between the two new membranes.


Lesson OverviewLesson Overview10.3 Regulating 10.3 Regulating

the Cell Cyclethe Cell Cycle


Controls on Cell Division How is the cell cycle regulated?

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


The controls on cell growth and division can be turned on and off.

For example, when an injury such as a broken bone occurs, cells are stimulated to divide rapidly and start the healing process. The rate of cell division slows when the healing process nears completion.


The Discovery of CyclinsCyclins are a family of proteins that regulate the timing of the cell cycle in eukaryotic cells.

This graph shows how cyclin levels change throughout the cell cycle in fertilized clam eggs.


Regulatory ProteinsInternal regulators are proteins that respond to events inside a cell. They allow the cell cycle to proceed only once certain processes have happened inside the cell.

External regulators are proteins that respond to events outside the cell. They direct cells to speed up or slow down the cell cycle.

Growth factors are external regulators that stimulate the growth and division of cells. They are important during embryonic development and wound healing.


ApoptosisApoptosis is a process of programmed cell death.

Apoptosis plays a role in development by shaping the structure of tissues and organs in plants and animals. For example, the foot of a mouse is shaped the way it is partly because the toes undergo apoptosis during tissue development.


Cancer: Uncontrolled Cell Growth How do cancer cells differ from other cells?

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


Cancer is a disorder in which body cells lose the ability to control cell growth.

Cancer cells divide uncontrollably to form a mass of cells called a tumor.


A benign tumor is noncancerous. It does not spread to surrounding healthy tissue.

A malignant tumor is cancerous. It invades and destroys surrounding healthy tissue and can spread to other parts of the body.

The spread of cancer cells is called metastasis.

Cancer cells absorb nutrients needed by other cells, block nerve connections, and prevent organs from functioning.


What Causes Cancer?Cancers are caused by defects in genes that regulate cell growth and division.

Some sources of gene defects are smoking tobacco, radiation exposure, defective genes, and viral infection.

A damaged or defective p53 gene is common in cancer cells. It causes cells to lose the information needed to respond to growth signals.


Treatments for CancerSome localized tumors can be removed by surgery.

Many tumors can be treated with targeted radiation.

Chemotherapy is the use of compounds that kill or slow the growth of cancer cells.


Lesson OverviewLesson Overview10.4 Cell Differentiation10.4 Cell Differentiation


THINK ABOUT ITThe human body contains hundreds of different cell types, and every one of them develops from the single cell that starts the process. How do the cells get to be so different from each other?


From One Cell to ManyHow do cells become specialized for different functions?

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


All organisms start life as just one cell.

Most multicellular organisms pass through an early stage of development called an embryo, which gradually develops into an adult organism.


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

For example, a plant has specialized cells in its roots, stems, and leaves.


Defining DifferentiationThe process by which cells become specialized is known as differentiation.

During development, cells differentiate into many different types and become specialized to perform certain tasks.

Differentiated cells carry out the jobs that multicellular organisms need to stay alive.


Mapping DifferentiationIn some organisms, a cell’s role is determined at a specific point in development.

In the worm C. elegans, daughter cells from each cell division follow a specific path toward a role as a particular kind of cell.


Differentiation in MammalsCell differentiation in mammals is controlled by a number of interacting factors in the embryo.

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


Stem Cells and DevelopmentWhat are stem cells?

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


One of the most important questions in biology is how all of the specialized, differentiated cell types in the body are formed from just a single cell.

Biologists say that such a cell is totipotent, literally able to do everything, to form all the tissues of the body.

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


Human DevelopmentAfter about four days of development, a human embryo forms into a blastocyst, a hollow ball of cells with a cluster of cells inside known as the inner cell mass.

The cells of the inner cell mass are said to be pluripotent, which means that they are capable of developing into many, but not all, of the body's cell types.


Stem CellsStem cells are unspecialized cells from which differentiated cells develop.

There are two types of stem cells: embryonic and adult stem cells.


Embryonic Stem CellsEmbryonic stem cells are found in the inner cells mass of the early embryo.

Embryonic stem cells are pluripotent.

Researchers have grown stem cells isolated from human embryos in culture. Their experiments confirmed that embryonic stem cells have the capacity to produce most cell types in the human body.


Adult Stem CellsAdult organisms contain some types of stem cells.

Adult stem cells are multipotent. They can produce many types of differentiated cells.

Adult stem cells of a given organ or tissue typically produce only the types of cells that are unique to that tissue.


Frontiers in Stem Cell ResearchWhat are some possible benefits and issues associated with stem cell research?

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

Human embryonic stem cell research is controversial because the arguments for it and against it both involve ethical issues of life and death.


Potential BenefitsStem cell research may lead to new ways to repair the cellular damage that results from heart attack, stroke, and spinal cord injuries.

One example is the approach to reversing heart attack damage illustrated below.


Ethical IssuesMost techniques for harvesting, or gathering, embryonic stem cells cause destruction of the embryo.

Government funding of embryonic stem cell research is an important political issue.

Groups seeking to protect embryos oppose such research as unethical.

Other groups support this research as essential to saving human lives and so view it as unethical to restrict the research.

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Lesson Overview Lesson Overview Cell Growth, Division, and Reproduction Lesson Overview 10.1 Cell Growth, Division, and Reproduction