10.3 Regulating the Cell Cycle

Lesson Overview Regulating the Cell Cycle

10.3 Regulating

the Cell Cycle

Lesson Overview Regulating the Cell Cycle

Controls on Cell Division

The cell cycle is controlled by regulatory proteins.

• The controls can be turned on and off.

Ex: After an injury, cells are stimulated to divide. The rate of division slows when the healing process nears

completion.

Lesson Overview Regulating the Cell Cycle

The Discovery of Cyclins

Cyclins 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.

Lesson Overview Regulating the Cell Cycle

Regulatory Proteins

Internal regulators: proteins that respond to events inside a cell.

• Allow the cell cycle to proceed only once certain events have happened inside the cell.

External regulators: proteins that respond to events outside the cell.

• Direct cells to speed up or slow down the cell cycle.

• Ex: Cells stop dividing when they run out of room.

Growth factors: external regulators that stimulate the growth and division of cells.

• Important during embryonic development and healing.

Lesson Overview Regulating the Cell Cycle

Cells That Stop Dividing

• Once mature, some cells enter a resting phase

(G ₀ ).

• These cells no longer go through cell division.

• May be temporary or permanent.

• Ex: neurons, blood cells

Lesson Overview Regulating the Cell Cycle

Apoptosis

Apoptosis: programmed cell death.

• Plays a role in development by shaping the structure of tissues and organs.

Ex: Human feet and hands are shaped the way they are partly because they

undergo apoptosis during tissue

development.

Lesson Overview Regulating the Cell Cycle

Numbers of Chromosomes

Diploid (2n): two copies of each chromosome

• Human body cells are diploid

Haploid (n): one copy of each chromosome

• Sex cells (gametes): sperm, egg

Mitosis produces 2n daughter

cells from 2n parent cell.

Lesson Overview Regulating the Cell Cycle

Cancer: Uncontrolled Cell Growth

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.

Lesson Overview Regulating the Cell Cycle

A benign tumor is noncancerous.

• Does not spread to surrounding healthy tissue.

A malignant tumor is cancerous.

• Invades and destroys surrounding healthy tissue.

• 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.

Lesson Overview Regulating the Cell Cycle

Lesson Overview Regulating the Cell Cycle

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, mistakes during replication, and viral infection.

A damaged or defective p53 gene is common in cancer cells.

• Causes cells to lose the information needed to respond to

growth signals.

Lesson Overview Regulating the Cell Cycle

Treatments for Cancer

Some 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.

• Inhibits mitosis in

rapidly-dividing cells.

10.4 Cell Differentiation

From One Cell to Many

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.

Ex: A plant has specialized cells in its roots, stems,

and leaves.

Defining Differentiation

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

Differentiated cells carry out the jobs that

multicellular organisms need to stay alive.

Mapping Differentiation

In 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 Mammals

Cell 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.

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.

Stem Cells and Development

Human Development

After 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 Cells

Stem cells are

unspecialized cells from which differentiated cells develop.

There are two types of

stem cells: embryonic

and adult stem cells.

Embryonic Stem Cells

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

Embryonic stem cells are pluripotent.

Researchers have grown stem cells isolated from human embryos in culture.

• Experiments confirmed that embryonic stem cells have

the capacity to produce most cell types in the human

body.

Adult Stem Cells

Adult 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.

Potential Benefits

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

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

Frontiers in Stem Cell Research

Ethical Issues

Most 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.