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

  • Chromosomes make it possible to separate DNA precisely during cell division

• 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

  • In animal cells, the cell membrane pinches in
  • In plant cells, a cell plate forms

  

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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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