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Chapter 10 – Cell Growth and Division 10-1 Cell Growth When a living thing grows, what happens to its cells? Do cells increase in size or are there just more cells? Answer: just more cells Limits to Cell Growth 1. DNA “overload” -remember DNA contains the info that controls a cell’s function -the larger a cell becomes, the more demand the cell places on its DNA -Eukaryotes: DNA stored in nucleus -small cell: info stored in that DNA is able to meet all cell’s needs -larger cell: usually doesn’t make extra copies of DNA -growth w/o limit = “information crisis” -can’t serve increasing needs of the growing cell 2. Exchanging Materials -trouble moving enough nutrients and wastes across cell membrane -remember, the cell membrane regulates what enters and leaves the cell enters: food, water, oxygen leaves: waste products -rate of exchange between food, water, oxygen entering and waste leaving depends on surface area -rate which food and oxygen are used up and waste is produced depends on volume -depends on surface area to volume ratio -cells must divide as they grow! 1 Ratio of Surface Area to Volume *SA= length x width x # sides *V= length x width x height Ratio of the above looks like the following; SA : V ex: 2:1 or 3:1 Volume increases faster than SA, causes SA to V ratio to decrease =serious problems for cell If cell gets too large: -difficult to get oxygen and nutrients in and wastes out ****That’s the reason why cells do not grow much larger even if the organism of which they are a part does.**** If cells can only get so large, then how do cells (and organisms-which are made of cells) grow? Cell Division -process which cells divides into 2 new “daughter” cells -before division occurs the cell replicates (or copies) all of its DNA -solves the problems: -info storage because each daughter cell gets 1 complete set of genetic information 2 -SA:V- each daughter cell has increase SA to V ratio -allows sufficient exchange of materials with the environment STOP 10-2 Cell Division Prokaryotic Cell Division: separate cell contents into 2 parts Eukaryotic Cell Division: 2 Main Stages 1. Mitosis-division of nucleus 2. Cytokinesis- division of cytoplasm Chromosomes -carries the genetic info -made up of DNA and proteins -DNA carries coded genetic information -every organism has specific # of chromosomes humans: 46 fruit flies: 8 carrot cells: 18 Chromosomes are not visible except during cell division because the DNA and protein molecules are spread throughout the nucleus During the cell cycle, a cell grows, prepares for division and divides into 2 daughter cells Interphase is divided into 3 phases: G1, S and G2 *G1: size of the cell increases and it makes new proteins and organelles *S: copying of chromosomes takes place [ 3 *G2: Many of the organelles and molecules needed for cell division are produced M phase, or cell division -includes mitosis and cytokinesis Mitosis is divided into 4 phases: 1. Prophase -Chromosomes condense and become visible -The nuclear envelope breaks down 2. Metaphase -Chromosomes line up across the center of the cell 3. Anaphase -The 2 sets of chromosomes move apart 4. Telophase -Chromosomes move to opposite ends of the cell -Cells lose their distinct shape and 2 nuclear envelopes form Cytokinesis -usually occurs at the same time a telophase -cell membrane pinches the cytoplasm into 2 nearly equal parts -In plant cells, a cell plate forms midway between divided nuclei and a new cell wall begins to form in the cell plate 4 10-3 Regulating the Cell Cycle Controls on Cell Division -most cells will grow until they come into contact with another cell -cell growth and division can be turned off and on -Injury (cut in skin, break in bone) -cells at edges are stimulated to divide rapidly, produces new cells, starts healing process, when healing process nears end,rate of division slows For many years, biologists searched for something that might regulate the cell cycle-something that would tell the cell when it was time to divide, duplicate their chromosomes or enter another phase of the cycle 1980’s- Scientists discovered that cells in mitosis contained a protein that when injected into an nondividing cell would cause a mitotic spindle to form They discovered the amount of this protein rose and fell in time with the cell cycle Cyclins- family of proteins that regulate the timing of the cell cycle in Eukaryotic cells Internal Regulators -respond inside cell -only allow cell cycle to proceed when certain processes have happened Ex: -one regulatory protein makes sure cell doesn’t enter mitosis until all its chromosomes have been replicated -another prevents a cell from entering anaphase until all its chromosomes are attached to the spindle External Regulators -respond to events outside the cell -direct cells to speed up or slow down the cell cycle 5 -ex: growth factors- important during embryonic development and wound healing;-prevent excessive growth and keep the tissues of the body from disrupting each other Cancer- disorder in which some of the body’s own cells lose the ability to control growth -cancer cells do not respond to the signals that regulate the growth of most cells -divide uncontrollably -form masses called tumors -can damage surrounding tissues -cells can break loose from tumors and spread 6