Chapter 10: Cell Growth and Division
10.1 Cell Growth, Division, and Reproduction
Limits to Cell Size
- @@There are two main reasons why cells divide instead of continuing to grow@@ * First, the larger a cell becomes, the more demands the cell has on its DNA * Second, a large cell has more trouble moving enough food in and enough waste out * As a cell grows, the size of its cell membrane does not grow as fast as its volume
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Cell Division and Reproduction
- @@Before it becomes too large, a growing cell divides into two new cells that are referred to as “daughter” cells; this process is called cell division@@ * During the process of cell division, a cell makes a copy of its DNA, with each daughter cell getting its own copy; this copying solves the problem of information overload * Cell division also decreases a cell’s volume, which allows for a better exchange of materials in and out of the cell * Cell division can also result in reproduction - the process by which organisms produce offspring (new organisms)
- @@Asexual reproduction@@ @@is a process by which a single parent reproduces by itself, with offspring of asexual reproduction having the same genetic information as their parent@@ * For example, hydras reproduce asexually by budding * As cells divide, the bud grows and eventually separates from the parent * The main advantages of asexual reproduction are that it is quick and that it produces genetically identical offspring * The lack of genetic diversity can be a disadvantage of asexual reproduction, since asexually reproducing organisms may not have needed characteristics if their environment changes rapidly
- @@Sexual reproduction@@ @@is a process by which two cells from different parents fuse, or join together, to produce the first cell of a new organism, with offspring having genetic information from both parents@@ * The main advantage of sexual reproduction is that offspring are genetically different from their parents, letting species “try out” new combinations of genetic information from one generation to the next; if the environment changes rapidly, some members of a species may be able to adjust to those changes * A disadvantage of sexual reproduction is that it is generally slower than asexual reproduction since it takes two parents instead of one to produce offspring
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10.2 The Process of Cell Division
Chromosomes
- Cells package each molecule of DNA into a structure called a chromosome - a threadlike structure that contains the genetic information that is passed from one generation of cells to the next *
- Prokaryotic cells do not have a nucleus; instead, the DNA molecules of prokaryotic cells are found in the cytoplasm. * Usually, the genetic material is contained in a single, circle-shaped chromosome
- The DNA molecules of eukaryotic cells are found in the cell nucleus * Eukaryotic cells have much more DNA than prokaryotes that they package into many chromosomes * The chromosomes in eukaryotic cells form a close relationship with special proteins called histones, which help to keep long molecules of DNA organized in eukaryotic cells * Chromatin is a substance found in eukaryotic chromosomes that consists of DNA tightly coiled around histones * As a cell gets ready to divide, chromatin condenses, and individual chromosomes become visible inside the cell
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The Cell Cycle
- The cell cycle is a series of events a cell goes through as it grows and divides * During the cell cycle, a cell grows, prepares for division by making a copy of its DNA, and then divides to form two daughter cells, each of which then begins the cycle again * Before any cell divides, it must copy, or replicate, its DNA by synthesizing new strands of DNA so that each cell receives a complete set of genetic information
- @@The prokaryotic cell cycle includes cell growth, DNA replication, and cell division@@ * The process of cell division in prokaryotes is a form of asexual reproduction called binary fission - a process that produces two genetically identical organisms
- The Eukaryotic cell cycle has four phases: G1, S, G2, and M; the period of growth between cell divisions is referred to as interphase and includes G1, S, and G2 (eukaryotic cells spend most of their time in interphase) * @@A cell does most of its growing during the G1 phase, increasing in size and producing new proteins and organelles@@ * Though the G in G1 and G2 stands for “gap,” both G phases are periods of growth and activity * @@During the S phase, new DNA is synthesized when chromosomes are replicated@@ * The S stands for “synthesis” * @@In the G2 phase, a cell prepares for cell division by producing many of the needed organelles and molecules@@ * At the end of the G2 phase, a cell is ready to begin the process of cell division * @@The M phase is a series of events that lead to the division of the cell nucleus@@ * The “M” in the M phase stands for mitosis, the first stage during which the nucleus divides * Cytokinesis is the second stage during which the cytoplasm divides * At the end of the M phase, two new daughter cells begin the cycle again
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Mitosis
- Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase
- @@Prophase is the first phase of mitosis@@ * The replicated chromosomes become visible as DNA starts to condense * Each strand of DNA in a doubled chromosome is called a chromatid; sister chromatids are joined at an area called the centromere * The nuclear envelope breaks down, and a spindle begins to form; a pair of structures called centrioles help to organize the spindle fibers
- @@Metaphase is the second phase of mitosis@@ * The chromosomes line up at the center of the cell, and spindle fibers connect the centromere of each chromosome to the spindle
- @@Anaphase is the third phase of mitosis@@ * The centromeres split, and the sister chromatids separate * A complete set of chromosomes move to opposite sides of the cell
- @@Telophase is the fourth and final phase of mitosis@@ * The chromosomes spread out into a tangle of chromatin, a nuclear envelope reforms around each group of chromosomes, and the spindle begins to break apart
- At this point, mitosis is complete, but cell division still has one more step to go
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Cytokinesis
- Cytokinesis completes cell division by splitting one cell into two daughter cells; this process differs between animal and plant cells * *
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10.3 Regulating the Cell Cycle
Controls on Cell Division
- In multicellular organisms, the cell cycle is carefully controlled; in your body, for example, skin cells grow and divide quickly, though most of the cells in your muscles and nerves do not grow and divide
- @@The cell cycle is controlled by regulatory proteins both inside and outside the cell@@ * Proteins that respond to events outside the cell are called external regulators, and they can cause the cell cycle to speed up or slow down * Growth factor is an external regulatory protein that stimulates the growth and division of cells * Proteins that respond to events inside the cell are called internal regulators, and they control the stages of the cell cycle * Cyclins are a group of regulatory proteins that regulate the timing of the cell cycle in eukaryotic cells
- Some cells die by accident because of damage or injury, while other cells, such as skin cells, are “programmed” to die * The process that brings an end to the cell cycle is called apoptosis
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Cancer: Uncontrolled Cell Growth
- Cancer is 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, cells divide uncontrollably@@
- Cancer cells form into a mass of cells called a tumor * Not all tumors are cancerous
- Cancer treatments can include surgery, radiation treatment, and chemotherapy
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10.4 Cell Differentiation
From One Cell to Many
- During the development of an organism, cells differentiate into many types of cells
- An embryo is an organism in its early stage of development * During the development process, the embryo’s cells become different from one another
- Differentiation is a process in which cells become specialized in structure and function * A differentiated cell has become different from the original cell that produced it * Differentiated cells are specialized to perform certain jobs * Ex: In a plant, some cells carry out photosynthesis, some plant cells store sugars, and other plant cells create pathways for moving materials up and down
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Stem Cells and Development
- The fertilized egg and the cells produced by the first few divisions are totipotent - meaning they have the ability to develop into all the tissues of the body
- Early in development, a human embryo forms into a blastocyst - a hollow ball of cells with a group of cells inside called the inner cell mass * The outer cells form tissues that attach the embryo to its mother * The inner cell mass becomes the embryo itself * The cells of the inner cell mass are pluripotent - meaning they are able to develop into all the body’s cell types, but not capable of developing into the tissues surrounding the embryo
- Differentiated, specialized cells develop from unspecialized cells known as stem cells * Embryonic stem cells are the cells that make up the inner cell mass of a blastocyst and can develop into different kinds of body cells * Adult stem cells are multipotent - meaning, they have the ability to produce many, but not all, kinds of differentiated cells * For example, adult stem cells in the bone marrow can develop into several types of blood cells * Stem cells offer the possible benefit of using undifferentiated cells to repair or replace badly damaged cells and tissues; however, human embryonic stem cell research is controversial because it involves issues of life and death
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