Limits to Cell Growth Why do cells divide rather than continuing to grow indefinitely?  The larger a cell becomes, the more demands the cell places on its DNA. The DNA of the cell would not be able to serve the increasing needs of the cell.  The cell has more trouble moving enough nutrients and wastes across the cell membrane.

Division of the Cell Cell division is the process by which a cell divides into two daughter cells.  Before cell division, the cell replicates all of its DNA. Each daughter cell gets a complete copy of the genetic information.  Each daughter cell has an increased ratio of surface area to volume allowing for efficient exchange of materials across the cell membrane.  Why do cells divide? – Asexual reproduction - the cells produced are genetically identical to the parent cell. – The source of new cells when multicellular organisms grow and develop. – Wound repair

M phase G 2 phase S phase G 1 phase The Cell Cycle

Four phases of the cell cycle: Interphase – The cell grows and replicates its DNA and centrioles. G1 phase – cell growth and synthesis of new proteins and organelles. S phase – DNA replication. G2 phase – preparation for mitosis; organelles and proteins needed for cell division are produced. Cell division – M phase; mitosis and cytokinesis take place.

Mitosis – the first stage of cell division in which division of the nucleus occurs. Cytokinesis – the second stage of cell division in which the division of the cytoplasm occurs.

Chromosomes The cells of every organism have a specific number of chromosomes. Chromosomes are not visible except during cell division when they are condensed into compact structures. Before cell division, each chromosome is replicated. After replication, each chromosome consists of two identical “sister” chromatids. – Each pair of chromatids are attached at the centromere. – When the cell divides, the “sister” chromatids separate from each other.

Mitosis has four phases:  Prophase – chromatin condenses into chromosomes; centrioles separate; spindle begins to form; and nuclear envelope breaks down.  Metaphase – chromosomes line up across the center of the cell. Each chromosome is connected to a spindle fiber at its centromere.  Anaphase – sister chromatids separate into individual chromosomes and are moved apart.  Telophase – chromosomes gather at opposite ends of the cell and lose their distinct shapes. Two new nuclear envelopes will form.

Centrioles Chromatin Interphase Nuclear envelope Cytokinesis Nuclear envelope reforming Telophase Anaphase Individual chromosomes Metaphase Centriole Spindle Centriole Chromosomes (paired chromatids) Prophase Centromere Spindle forming

Control of Cell Division Cells in a petri dish will continue to grow until they come into contact with other cells. Controls on cell growth and division can be turned on and off.

A sample of cytoplasm is removed from a cell in mitosis. The sample is injected into a second cell in G 2 of interphase. As a result, the second cell enters mitosis. Effect of Cyclins on Cell Division The timing of the cell cycle is regulated by cyclins.

Cell Cycle Regulators Internal Regulators – proteins that respond to events inside the cell. Allow the cell cycle to proceed only when certain processes have happened inside the cell (i.e. all the chromosomes have been regulated). External Regulators – proteins that respond to events outside the cell. Direct cells to speed up or slow down the cell cycle. For example, growth factors stimulate the growth and division of cells. Growth regulators important during embryonic development and wound healing.

Uncontrolled Cell Growth Why is cell growth regulated so carefully?  Consequences of uncontrolled cell growth can be severe.  Cancer cells do not respond to the signals that regulate the growth of most cells. As a result, they divide uncontrollably.  Many cancer cells have a defect in a gene called p53, which normally halts the cell cycle until all the chromosomes have been properly replicated.