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CELL GROWTH AND DIVISION. Chapter 10, section 1. Limits to Cell Growth. There are two main reasons why cells divide rather than continuing to grow indefinitely. The larger a cell becomes, the more demands are placed on its DNA
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CELL GROWTH AND DIVISION Chapter 10, section 1
Limits to Cell Growth • There are two main reasons why cells divide rather than continuing to grow indefinitely. • The larger a cell becomes, the more demands are placed on its DNA • The cell has more trouble moving material into and out of the cell.
DNA “Overload” • When a cell is small, the information stored in DNA is able to meet all of the cell’s needs. • If a cell increases in size without limits, an information crisis would occur.
Compare a cell to a growing town: • In a small town, this main street is fine for a small population.
But imagine that the population grows, • The tiny street of the small town can no longer handle all of the people.
Exchanging Materials • In a cell, the rate at which oxygen and food enter a cell, and waste is removed from the cell depends on the total surface to volume ratio of a cell.
Ratio of Surface Area to Volume • Image a cell that is shaped like a cube. The cell has a length of 1 cm. • Surface area would be equal to length x width x number of sides, • Or, 1cm x 1cm x 6 = 6 cm squared.
The volume of the cell would be equal to: • Length x width x height • Or, 1 cm x 1 cm x 1 cm = 1 cm cubed. • The surface area by the volume ratio would be 6:1
If the cell size was doubled or tripled, the volume increases more rapidly than the surface area. • This makes it much more difficult to get materials into and out of the cell.
Division of a Cell • Before a cell becomes to large, a growing cell divides to form two “daughter” cells.
Before cell division occurs, the cell replicates, or copies, all of its DNA. • Each daughter cell is identical to the original.
CELL DIVISION Chapter 10, section 2
In eukaryotes, the first stage of cell division is called mitosis. • The second stage of cell division is called cytokinesis. • Reproduction by mitosis and cytokinesis is classified as asexual, because each new cell is identical to the parent.
Chromosomes • Chromosomes carry genetic information from one generation to the next. • Chromosomes are made up of DNA. • Humans have 46 chromosomes.
A chromotid is one of two “sister” parts of a duplicated chromosome. • Centomeres hold the chromatids together
The Cell Cycle • The cell cycle is a series of events that cells go through as they grow and divide. • During the cell cycle, a cell grows, prepares for division, and divides to form two identical daughter cells.
Events of the Cell Cycle • The cell cycle consists of four phases.
Interphase – is divided into three phases: • G1- During this stage, cells increase in size and make new proteins and organelles.
S – in this stage, chromosomes are replicated. • G2 – this is usually the shortest stage, many of the organelles and molecules required for cell division are produced.
Mitosis or the M phase • Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase.
Prophase – chromosomes become visible. The centrioles, separate and take up positions on opposite sides of the nucleus.
Metaphase – chromosomes line up across the center of the cell.
Telophase – chromosomes begin to disperse, the nuclear envelope reforms, and spindle fibers break apart.
Cytokinesis Cytokinesis is still part of the M phase. During cytokinesis, the cell membrane separates to form two identical new cells.