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Ch. 11 How Cells Divide
11.1 Prokaryotes divide for more simply than do eukaryotes
Cell Division in Prokaryotes
- End result for cell division in all organisms is two daughter cells
- Process is the same for all organisms: duplication and segregation of genetic information into
daughter cells, and division of cellular contents
- Division for prokaryotes: binary fission
- Most prokaryotic DNA is a single circular molecule
- DNA is replicated, cell elongates, and pinches into two cells
11.2 The chromosomes of eukaryotes are highly ordered structures
Discovery of Chromosomes
- Walther Fleming noticed threads from nuclei to appear to be dividing lengthwise
- Called this division: mitosis
Chromosome Number
- Most eukaryotes have between 10-50 chromosomes in their body cells
- Humans have 46 chromosomes
- 23 nearly identical pairs (one from Mom and one from Dad)
- Each chromosome contains hundreds or thousands of genes that play
important roles in determining how a person's body develops and
functions
- Monosomy: condition in which a chromosome is missing
- Do not survive embryonic development
- Trisomy: condition in which there is an extra chromosome
- Not always fatal, but embryo doesn't develop properly
The Structures of Eukaryotic Chromosomes
Chromosome Coiling
- Nucleosome - every 200 nucleotides the DNA duplex is coiled around a core of eight
histone proteins
- Further coiling occurs when the string of nucleosomes wraps up into highorder coils called solenoids
- Heterochromatin: domains of chromatin that are not expressed
- Euchromatin: domains of chromatin that are expressed
Chapter 11: How Cells Divide
Composition of Chromatin
- Chromosomes are composed of chromatin
- Chromatin is DNA (40%) and protein (60%)
- DNA is one very long, double stranded fiber that extends unbroken through the entire
length of the chromosome
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Chromosome Karyotypes
- Chromosomes differ widely among species: size, staining properties, location of
centromere, length of arm, and position of constricted regions on arm
- Karyotype: array of chromosomes an individual possesses
- Number of chromosomes in a species
- Haploid (n): one complete set of chromosomes
- Gametes found in gonads
- Diploid (2n): two complete set of chromosomes (one from Mom and Dad)
- Homologous: maternal and paternal chromosomes
- Each chromosome has two homologues
- Humans have diploid dominant life cycle
- Centromere: condensed area found on all eukaryotic chromosomes
- All chromosomes have one prior to replication
- After replication each chromosome has two sister chromatids that share
a common centromere
11.3 Mitosis is a key phase of the cell cycle
The Cell Cycle
- Cell cycle is a division process consisting of 5 phases
Duration of the Cell Cycle
- Length of cell division varies according to organism and cell type
- Average dividing mammalian cell cycle is complete in about 24 hours
- Growth occurs throughout the G1 and G2 phases as well as the S phase
- M phases takes about an hour
- Cells often pause in G1 phase before DNA replication and enter a resting state
- G0 phase - cells can stay in this phase for days to years before resuming cell
division
- At any given time, most of the cells in an animal's body are in G0 phase
Chapter 11: How Cells Divide
The Five Phases
- Interphase (portion of cell cycle between cell divisions) contains 3 of the phases
- G1 - primary growth phase; major portion of cell's life span
- S - synthesis (replication) of chromosomes
- G2 - second growth phase; organelles replicate, chromosomes condense,
microtubles begin to assemble at a spindle
- M - mitosis; phase of cell cycle in which the microtubular apparatus assembles, binds
to the chromosomes, and moves sister chromatids apart
- Continuous process divided into 4 stages (PMAT): Prophase, Metaphase,
Anaphase, Telophase
- C - cytokinesis, phase of the cell cycle when the cytoplasm divides, creating two
daughter cells
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Interphase: Preparing for Mitosis
- G1 - cells undergo the major portion of their growth
- S phase - chromosome replicates to produce two sister chromatids which are attached to each
other by their centromere
- The centromere is a point of constriction on the chromosome, containing a specific
DNA sequence to which is bound a disk of protein called a kinetochore
- G2 - condensation (coiling of chromosomes) begins; in animal cells the centrioles replicate
Mitosis
Prophase: Formation of the Mitotic Apparatus
- Chromosomes become visible
- Centriole pair begins to move apart and microtubules form between them
- Microtubules connect the kinetochores on each pair of the sister chromatids to
the two poles of the spindle
- Nuclear envelope breaks down
Metaphase
- Chromosomes align in the center of the cell
- Positioned by the microtubules attached to the kinetochores of the centromeres,
all of the chromosomes line up on the metaphase plate
Cytokinesis
- During mitosis, organelles were reassorted to areas that will separate into new cells
- Cell divides
Cytokinesis in Animal Cells
- Achieved by means of a constricting belt of actin filaments
- Filaments slide past one another and diameter of belt decreases pinching the
cell and creating a cleavage furrow... eventually it separates cell
Chapter 11: How Cells Divide
Anaphase and Telophase: Separation of the Chromatids and Reformation of the Nuclei
Anaphase
- Shortest stage of mitosis
- Centromeres divide
- Centromere splits in two, freeing the two sister chromatids from each other
- Centromeres of all the chromosomes divide simultaneously
- Sister chromatids are pulled rapidly toward the poles to which their kinetochores are
attached
Telophase
- Spindle apparatus disassembles as the microtubules are broken down
- Nuclear envelope forms around each set of sister chromatids (which can now be called
chromosomes because each set has its own centromere)
- Chromosomes uncoil into the more extended form that permits gene expression
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Cytokinesis in Plant Cells
- Cells assembles membrane components in their interior, at right angles to the spindle
aparatus... this is called a cell plate
- Cell plate continues to grow outward until it reaches the interior surface of the
plasma membrane and fuses with it... which eventually separates the cell
11.4 The cell cycle is carefully controlled
General Strategies of Cell Cycle Control
- Two irreversible points in the cell cycle
- The replication of genetic material
- The separation of the sister chromatids
- Cell cycle can be put on hold at specific points called checkpoints
- The process checks for accuracy and can be halted if there are errors
Architecture of the Control System
- Cell uses three main checkpoints to assess the internal state of the cell and integrate
external signals
- G1/S, G2/M, and late metaphase
- Passage through the checkpoints is controlled by kinase enzymes made of
enzymatic subunit partnered with a protein called cyclin
- Enzymes are called cyclin-dependent kinases or Cdk's
- These drive the cell cycle
G1/S Checkpoint
- Primary point at which the cell decides whether or not to divide
- This is the checkpoint at which external signals can influence events of the cycle
- Growth factors, internal signals such as nutritional state, and intact genome
- The restriction point (R point) - the point at which the cell decides to divide
Spindle Checkpoint
- Ensures that all of the chromosomes are attached to the spindle in preparation for
anaphase
Molecular Mechanisms of Cell Cycle Control
Controlling the Cell Cycle in Multicellular Eukaryotes
- Growth factors: proteins that stimulate cell division
- When neighboring cells have used up what little growth factor is present, not
enough is left to trigger cell division in any one cell
Chapter 11: How Cells Divide
G2/M Checkpoint
- Passage through this checkpoint represents the comitment to mitosis
- Assesses the success of DNA replication and can stall the cycle if DNA had not been
accurately replicated
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Chapter 11: How Cells Divide
Cancer and the Control of Cell Proliferation
- Cancer: the unrestrained, uncontrolled growth of cells
- Disease of cell division - a failure of cell division control
- p53 - gene plays a role in the G1 checkpoint of cell division
- p53, the product of p53, monitors the integrity of DNA and checks for damage
- If the DNA is damaged it halts the cell division and stimulates the activity of
special enzymes to repair the damage. Once repairs have been made, p53
allows the cell division to continue
- By halting division in damaged cells, the development of many mutated cells is
prevents and p53 is considered a tumor-suppressor gene
- When p53 is nonfunctional these cancer cells are able to repeatedly undergo cell
division without being halted at the G1 checkpoint
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