10.1 Cell Growth, Division, and Reproduction Lesson Overview 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. In addition, a larger cell is less efficient in moving nutrients and waste materials across its cell membrane.

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 Materials Food, 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 Volume Imagine 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 Problems To 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 Cell Before 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.

Cell Division and Reproduction How do asexual and sexual reproduction compare? The production of genetically identical offspring from a single parent is known as asexual reproduction. Offspring produced by sexual reproduction inherit some of their genetic information from each parent.

Sexual Reproduction In 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

10.2 The Process of Cell Division Lesson Overview 10.2 The Process of Cell Division

Chromosomes 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.

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

Eukaryotic Chromosomes In eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin.

The Cell Cycle What are the main events of the cell cycle? During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells.

The Prokaryotic Cell Cycle Binary fission is a form of asexual reproduction during which two genetically identical cells are produced. For example, bacteria reproduce by binary fission.

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

G1 Phase: Cell Growth In the G1 phase, cells increase in size and synthesize new proteins and organelles.

S Phase: DNA Replication In 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 Division In 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

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

Prophase During 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.

Prophase During 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.

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

Metaphase During 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.

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

Anaphase During 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.

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

Telophase During 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.

Telophase During 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 How do daughter cells split apart after mitosis? Cytokinesis completes the process of cell division – it splits one cell into two.

Cytokinesis in Animal Cells The 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 Cells In 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.

10.3 Regulating the Cell Cycle Lesson Overview 10.3 Regulating the 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 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.

Apoptosis Apoptosis 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 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 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.

Lesson Overview 10.4 Cell Differentiation

From One Cell to Many How 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 Differentiation The 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.

Stem Cells and Development What are stem cells? The unspecialized cells from which differentiated cells develop are known as stem cells.

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.

Frontiers in Stem Cell Research What 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 Benefits Stem 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 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.