Cell Growth & Division

Cell Growth When cells grow, they increase in volume. What limits cell size?    “1. A cell is a metabolic compartment where a multitude of chemical reactions occur.   2. The number of reactions increase as the volume of metabolic volume within a cell increases. (The larger the volume the larger the number of reactions)    3.All raw materials necessary for metabolism can enter the cell only through its cell membrane.    4.The greater the surface area the larger the amount of raw materials that can enter at only one time.    5.Each unit of volume requires a specific amount of surface area to supply its metabolism with raw materials. The amount of surface area available to each unit of volume varies with the size of a cell.    6. As a cell grows its SA/V decreases.    7. At some point in its growth its SA/V becomes so small that its surface area is too small to supply its raw materials to its volume. At this point the cell cannot get larger.” http://www.ucs.mun.ca/~iemerson/lectures1001/coursenotes/SAtoV.html

The Cell Cycle G1 – cell growth G0 – Cell Diff. S – DNA synthesis decision point G0 – Cell Diff. S – DNA synthesis G2 – Pre division M – Mitosis “All organisms consist of cells that multiply through cell division. An adult human being has approximately 100 000 billion cells, all originating from a single cell, the fertilized egg cell. In adults there is also an enormous number of continuously dividing cells replacing those dying. Before a cell can divide it has to grow in size, duplicate its chromosomes and separate the chromosomes for exact distribution between the two daughter cells. These different processes are coordinated in the cell cycle.” “The cell cycle consists of several phases (see figure). In the first phase (G1) the cell grows and becomes larger. When it has reached a certain size it enters the next phase (S), in which DNA-synthesis takes place. The cell duplicates its hereditary material (DNA-replication) and a copy of each chromosome is formed. During the next phase (G2) the cell checks that DNA-replication is completed and prepares for cell division. The chromosomes are separated (mitosis, M) and the cell divides into two daughter cells. Through this mechanism the daughter cells receive identical chromosome set ups. After division, the cells are back in G1 and the cell cycle is completed. The duration of the cell cycle varies between different cell types. In most mammalian cells it lasts between 10 and 30 hours. Cells in the first cell cycle phase (G1) do not always continue through the cycle. Instead they can exit from the cell cycle and enter a resting stage (G0).” “For all living eukaryotic organisms it is essential that the different phases of the cell cycle are precisely coordinated. The phases must follow in correct order, and one phase must be completed before the next phase can begin. Errors in this coordination may lead to chromosomal alterations. Chromosomes or parts of chromosomes may be lost, rearranged or distributed unequally between the two daughter cells. This type of chromosome alteration is often seen in cancer cells.” “CDK (cyclin dependent kinase) and cyclin together drive the cell from one cell cycle phase to the next. The CDK-molecules can be compared with an engine and the cyclins with a gear box controlling whether the engine will run in the idling state or drive the cell forward in the cell cycle.” http://www.nobelprize.org/nobel_prizes/medicine/laureates/2001/press.html

G0 - Cellular Differentiation Cells Differentiate by Synthesizing specialized proteins Changing organelle and structure ratios Changing cell shape http://www.biologyjunction.com/stemcell_article.htm

G1 - Stem Cells Stem cells are distinguished from other cell types by two important characteristics. First, they are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactivity. Second, under certain physiologic or experimental conditions, they can be forced to differentiate into specific cell types. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart, stem cells only divide under special conditions. Source: http://stemcells.nih.gov/info/basics/pages/basics1.aspx Figure: http://www.stemcellresearchfoundation.org/WhatsNew/Pluripotent.htm

Regeneration

The Cell Cycle In G2, the cell checks to make sure the DNA is correctly replicated. It is also a time when the cell can grow before division. Organelles and structures do not seem to be coordinated with cell division. As long as some of the organelles and structures end up in each daughter cell, that is sufficient.

Cell Division: Mitosis and Cyto-kinesis Early Prophase Late Prophase Metaphase Anaphase Telophase Cell Division: Mitosis and Cyto-kinesis

Mitosis in more detail Key definitions: Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are 1) to package DNA into a smaller volume to fit in the cell, 2) to strengthen the DNA to allow mitosis, 3) to prevent DNA damage, and 4) to control gene expression and DNA replication. A chromatid contains the replicated DNA of each individual chromosome, which are joined by a centromere, for the process of cell division (mitosis ormeiosis). They are normally identical ("homozygous") but may have slight differences in the case of mutations, in which case they are heterozygous. They are called sister chromatids so long as they are joined by the centromeres. The centromere is the part of a chromosome that links sister chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore.[1]Centromeres were first defined as genetic loci that direct the behaviour of chromosomes. Their physical role is to act as the site of assembly of thekinetochore - a highly complex multiprotein structure that is responsible for the actual events of chromosome segregation - e.g. binding microtubulesand signalling to the cell cycle machinery when all chromosomes have adopted correct attachments to the spindle, so that it is safe for cell division to proceed to completion (i.e. for cells to enter anaphase) The centrosome contains a centriole (the mitotubule organizing center of the cell) and a dense mass of special proteins. The centrosome is copied only once per cell cycle so that each daughter cell inherits one centrosome, containing two structures called centrioles.

The Wrap Up