Chapter 10 Cell Growth and Division Cell Growth - In most cases, living things grow by producing more cells. - the cells of an adult animal are no larger than those of a young animal—there are just more of them There are two main reasons why cells divide rather than continuing to grow indefinitely. 1. The larger a cell becomes, the more demands the cell places on its DNA. 2. The cell has more trouble moving enough nutrients and wastes across the cell membrane.
Division of the Cell The process by which a cell divides into two new daughter cells is called cell division Before cell division occurs, the cell replicates, or copies, all of its DNA. Each daughter cell then gets a complete copy of that information. In prokaryotes, cell division is a simple matter. In eukaryotes, cell division is more complex and occurs in two main stages. 1. The first stage, division of the cell nucleus, is called mitosis 2. The second stage, division of the cytoplasm, is called cytokinesis 1
Bacteria Simply Split - A bacterium is a single, prokaryotic cell. - It has a cell wall but lacks a nucleus and membrane-bound organelles. - A bacterium's single DNA molecule is not coiled around proteins to form chromosomes. - Instead, its DNA is a circular chromosome - For these tiny organisms, cell division takes place in two stages: 1. the DNA is copied 2. then the cell splits. -
To make a copy of itself, the DNA molecule begins to "unzip" lengthwise, exposing its two strands As this happens, each strand is made into a complete DNA molecule. the cell now has two identical copies of DNA then the bacterium splits into two equal halves through a process called binary fission. Binary fission is a form of asexual reproduction that produces identical offspring
Chromosomes - DNA (deoxyribonucleic acid) is a long, thin molecule that contains the information needed to direct a cell's activities and to determines a cell's characteristics - This vast amount of information is encoded in DNA is organized into genes. - A gene is a segment of DNA that transmits information from parent to offspring - A single molecule of DNA has thousands of genes - when a cell prepares to divide, the DNA molecule coils and twists into a dense structure called a chromosome. - A chromosome is a rod-shaped structure that forms when a single DNA molecule and its associated proteins coil tightly before cell division - If DNA did not coil and form a chromosome, DNA strand would be about 5 cm (approximately 2 in.) long and not fit inside a cell. - chromosomes are approximately 40 percent DNA and 60 percent protein. - Chromosomes are visible through a microscope only after they have condensed prior to cell division - each chromosome forms a copy of itself 2
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The two copies of each chromosome are called chromatids Chromatids ensure each new cell will have the same genetic information as the old cell The two chromatids are attached by a protein disk at a point called a centromere
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Human cells have 23 different chromosomes human cells contain two copies of each chromosome, for a total of 46 chromosomes. The two copies of each chromosome are called homologous chromosomes Homologous chromosomes are similar in shape and size and have similar genetic information Each person receives one homologous chromosome from your mother and the other from your father. When a cell contains two homologous chromosomes, it is termed diploid Biologists use the symbol 2n to represent the diploid number of chromosomes in a cell. For humans, 2n = 46
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- Not all cells are diploid - gametes - egg cells and sperm cells-are haploid. - A haploid cell contains only one of each chromosome. - The fusion of two haploid gametes forms a diploid zygote - A zygote (zy goht) is a fertilized egg cell - Being haploid ensures that when an egg and a sperm fuse, the resulting zygote will contain the characteristic diploid number of chromosomes for that organism - Biologist use the symbol n to represent the haploid number of chromosomes - For humans, n = 23.
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The Cell Cycle Eukaryotic Cells Undergo Nuclear Division - Eukaryotic cells are more complex than prokaryotic cells - eukaryotic cells must take into account the nucleus with the chromosomes inside and the many other Internal organelles, all of which must be strategically maneuvered before the cell can properly divide. The cell cycle is the 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 daughter cells, each of which then begins the cycle again.
Cell Cycle: The life of a eukaryotic cell is traditionally diagrammed as: G1
S
G2
M
C
1. The G1 (G stands for gap) phase is the growth phase of a cell. During this phase, a cell grows rapidly and carries out its routine functions. For most organisms, this phase occupies the major portion of the cell's life between cell divisions. 2. The S phase (S = synthesis) is when the DNA is copied. At the end of this phase, an individual chromosome consists of two chromatids attached at the centromere. 3. In the G2 phase, preparations are made for nuclear division. Mitochondria and other organelles replicate. Microtubules are reassembled; they will be used to form, the spindle apparatus that moves the chromosomes. 4. The M phase is the phase in which mitosis occurs. Mitosis is the process by which the nucleus of a cell is divided into two nuclei, each with the same number and kinds of chromosomes. 5. The C phase is when the cytoplasm divides during a process called cytokinesis.
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What Happens During Mitosis - A eukaryotic cell spends most of its life in the G1, S, and G2 phases, which are collectively called interphase. - During interphase, a cell does a great deal of growing. - Chromosomes are loosely wound, and genes are being used to make the enzymes that direct the activities of the cell. Forming Spindle Fibers - As interphase ends and mitosis begins, the chromosomes begin to condense. - Enzymes in the cell begin to break down the nuclear envelope. - In the center of an animal cell, a pair of cylindrical structures called centrioles start to separate, each moving toward opposite poles of the cell. - As the centrioles move apart, a network of protein cables, called the spindle, forms - The spindle will help move chromosomes apart. - Each cable is called a spindle fiber and is made of microtubules, long hollow tubes of protein. - Recall that plant cells do not have centrioles. However, plant cells form a spindle that is almost identical to that of an animal cell
Interphase
Prophase
Attaching Spindle Fibers to Centromeres - As the chromosomes continue to condense, a second group of microtubules extends out from a region of the centromere of each chromosome called the kinetochore - The kinetochore is a disk of protein that serves as a platform for assembling the microtubules. - The two sets of microtubules extend out toward opposite poles of the cell. - Each set of microtubules continues to grow longer until it makes contact with the pole of the spindle. - When the process is complete, one chromatid is attached by a set of microtubules to one pole, and the other chromatid is attached to the other pole.
Metaphase 7
Separating Chromatids - Once the microtubules are attached to the centromeres, the centromeres split, freeing the chromatids from each other . - Mitosis is now simply a matter of reeling in the microtubules and dragging the chromatids, each now considered a chromosome, to the poles, - At the poles, the ends of the spindle fibers are dismantled bit by bit. As the fibers become shorter and shorter, the attached chromosomes move closer and closer to the poles. - When they finally arrive, each pole has one complete set of chromosomes.
Dividing the Cell - In the final step of cell division, a new nuclear envelope forms around each pole forming two nuclei, and the chromosomes within uncoil. - Cytokinesis then takes place; the cytoplasm of the cell divides in half - Each half includes one of the two new nuclei and an assortment of organelles, which replicated earlier in the cell cycle.
Stages of Mitosis Biologists traditionally divide the process of mitosis into four general stages: 1. prophase 2. metaphase 3. anaphase 4. telophase
Prophase - chromosomes begin condensing and become visible - The nuclear envelope begins to break down - the network of spindle fibers becomes visible.
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Metaphase -
the chromosomes move to the center of the cell and line up each chromosome is held in place by the microtubules attached to the kinetochore
Anaphase -
the two chromatids physically separate when the centromere divides The chromatids, each which now may be called a chromosome, move toward opposite poles of the cell as the fibers attached to them shorten.
Telophase -
The chromosomes, now at opposite ends of the cell, uncoil A new nuclear envelope forms The spindle fibers break down and disappear, mitosis is complete.
Cytokinesis -
As mitosis ends, cytokinesis begins During cytokinesis the cytoplasm of the cell is cleaved in half, and the cell membrane grows to enclose both cells.
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Uncontrolled Cell Growth Cancer, a 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. As a result, they divide uncontrollably and form masses of cells called tumors Cancer cells may break loose from tumors and spread throughout the body, disrupting normal activities and causing serious medical problems or even death.
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