Chapter 10 How Cell Divide Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapters Contents Bacteria Cell Division Eukaryotic Chromosomes Overview of the Eukaryotic Cell Cycle Interphase: Preparation for Mitosis M Phase: Chromosome Segregation and the Division of Cytoplasmic Contents Control of the Cell Cycle eukaryotes

Section 10.1 Learning Objectives Describe process of binary fission What is final outcome of this process?

10.1 Bacterial Cell Division Bacteria divide by binary fission Asexual reproduction Reproduction is clonal (all cells identical to parent) Bacterial genome made up of single, circular chromosome tightly packed in the cell at the nucleoid region. Prokaryotes do not have nuclei New chromosomes are partitioned to opposite ends of the cell Occurs when the cell elongates (grows) Septum forms to divide the cell into 2 cells Via protein FtsZ

Bacterial chromosome: Double-stranded DNA Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bacterial cell 1. Prior to cell division, the bacterial DNA molecule replicates. The replication of the double-stranded, Circular DNA mole- cule that constitutes the genome of a bacterium begins at a specific site, called the origin of replica- tion (green area). Origin of replication Bacterial chromosome: Double-stranded DNA 2. The replication enzymes move out in both directions from that site and make copies of each strand in the DNA duplex. The enzymes continue until they meet at another specific site, the terminus of replication (red area). 3. As the DNA is replicated, the cell elongates, and the DNA is partitioned in the cell such that the origins are at the ¼ and ¾ positions in the cell and the termini are oriented toward the middle of the cell.

Septum production via FtsZ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Septum production via FtsZ Septum 4. Septation then begins, in which new membrane and cell wall material begin to grow and form a septum at approximately the midpoint of the cell. A protein molecule called FtsZ (orange dots) facilitates this process. End result 2 identical cells 5. When the septum is complete, the cell pinches in two, and two daughter cells are formed, each containing a bacterial DNA molecule.

Question 1 Prokaryotic cells divide by — Mitosis Cytokinesis Binary fission Replication Conversion Bloom’s level: Knowledge/Understanding

Section 10.2 Learning Objectives Describe the structure of eukaryotic chromosomes. Distinguish between homologues and sister chromatids. Contrast replicated and nonreplicated chromosomes.

DNA Division in Different Organisms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prokaryotes Some Protists Other Protists Yeasts Animals No nucleus, usually have single circular chromosome. After DNA is replicated, it is partitioned in the cell. After cell elongation, FtsZ protein assembles into a ring and facilitates septation and cell division. Nucleus present and nuclear envelope remains intact during cell division. Chromosomes line up. Microtubule fibers pass through tunnels in the nuclear membrane and set up an axis for separation of replicated chromosomes, and cell division. A spindle of micro- tubules forms between two pairs of centrioles at opposite ends of the cell. The spindle passes through one tunnel in the intact nuclear envelope. Kinetochore microtubules form between kinetochores on the chromosomes and the spindle poles and pull the chromo- somes to each pole. Nuclear envelope remains intact; spindle microtubules form inside the nucleus between spindle pole bodies. A single kinetochore microtubule attaches to each chromosome and pulls each to a pole. Spindle microtubules begin to form between centrioles outside of nucleus. Centrioles move to the poles and the nuclear envelope breaks down. Kinetochore microtubules attach kinetochores of chromosomes to spindle poles. Polar microtubules extend toward the center of the cell and overlap. Chromosome Kinetochore microtubule FtsZ protein Spindle pole body Microtubule Kinetochore microtubule Kinetochore microtubule Chromosome Fragments of nuclear envelope Central spindle of microtubules Septum Polar microtubule Nucleus Centrioles Kinetochore Centriole Polar microtubule

Eukaryotic Chromosomes Every species has a different number of chromosomes Humans have 46 chromosomes in 23 nearly identical pairs Additional/missing chromosomes usually fatal with some exceptions (Chapter 13) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 950x © Biophoto Associates/Photo Researchers, Inc.

Chromosomes Composition Chromosomes are composed of chromatin – complex of DNA and protein Typical human chromosome 140 million nucleotides long In the nondividing nucleus Heterochromatin – not expressed Euchromatin – expressed

Heterochromatin vs. Euchromatin http://www.histology.leeds.ac.uk/cell/nucleus.php

Chromosome Structure Nucleosome The complex of DNA and histone proteins is termed a nucleosome. DNA duplex coiled around 8 histone proteins every 200 nucleotides Histones are positively charged and strongly attracted to negatively charged phosphate groups of DNA

The double stranded DNA is coiled around a core of eight histones proteins, the complex is termed a nucleosome. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. DNA Double Helix (duplex) Nucleosome Histone core DNA

Nucleosomes wrapped into higher order coils called solenoids Leads to a fiber 30 nm in diameter This 30-nm fiber is the usual state of nondividing (interphase) chromatin During mitosis, chromatin in solenoid arranged around scaffold of protein to achieve maximum compaction

Levels of Eukaryotic Chromosomal Organization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mitotic Chromosome Rosettes of Chromatin Loops Chromatin Loop Solenoid Scaffold protein Scaffold protein Chromatin loop

Levels of Eukaryotic Chromosomal Organization Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. DNA Double Helix (duplex) Nucleosome Histone core DNA

Chromosome Karyotypes Particular array of chromosomes in an individual organism is called karyotype. Humans are diploid (2n) 2 complete sets of chromosomes 46 total chromosomes Haploid (n) – 1 set of chromosomes Pair of chromosomes are homologous Each one is a homologue

© CNRI/Photo Researchers, Inc. A Human Karyotype Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 500x © CNRI/Photo Researchers, Inc.

Chromosome Replication Prior to replication, each chromosome composed of a single DNA molecule After replication, each chromosome composed of 2 identical DNA molecules Held together by cohesin proteins Visible as 2 strands held together as chromosome becomes more condensed One chromosome composed of 2 sister chromatids

Homologous chromosomes Homologous chromosomes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Homologous chromosomes Homologous chromosomes Kinetochore Replication Cohesin proteins Centromere Kinetochores Sister chromatids Sister chromatids

Question 3 The unexpressed form of DNA is _____; the expressed form of DNA is ______. Heterochromatin; homochromatin Euchromatin; mesochromatin Mesochromatin; homochromatin heterochromatin; euchromatin None of the above Bloom’s level: Knowledge/Understanding

Question 10 Homologous chromosomes and sister chromatids are the same thing. This is true This is false Bloom’s level: Knowledge/Understanding

Question 9 Where would a researcher find histones? In chromosomes with DNA coiled around them In the spindle apparatus At the formation of the cell plate Surrounding a nuclear pore Bound to a ribosome Bloom’s level: Knowledge/Understanding

Learning Objectives 10.3 Cell Cycle 10.4 Interphase Describe the eukaryotic cell cycle.  10.4 Interphase Describe the events that take place during interphase. Illustrate the connection between sister chromatids after S phase.

Eukaryotic Cell Cycle Interphase G1 (gap phase 1) S (synthesis) Primary growth phase, longest phase S (synthesis) Replication of DNA G2 (gap phase 2) Organelles replicate, microtubules organize M (mitosis) Subdivided into 5 phases C (cytokinesis) Separation of 2 new cells Interphase

Duration of Cell Cycle Time it takes to complete a cell cycle varies greatly Mature cells take longer than those embryonic tissue to grow Growth occurs during G1, G2, and S phases Most variation in length of G1 Resting phase G0 – cells spend more or less time here

Duration of Cell Cycle Most variation in the length of the cell cycle between organisms or cell types occurs in G1 Cells often pause in G1before DNA replication and enter a resting state called G0 Resting phase G0 – cells spend more or less time here before resuming cell division. Most of cells in animal’s body are in G0 phase Muscle and nerve cells remain there permanently Liver cells can resume G1 phase in response to factors released during injury

The Cell Cycle M Phase Metaphase Prometaphase Anaphase Prophase Telophase G2 G1 S Interphase G2 Mitosis M Phase Cytokinesis S G1

Interphase: Preparation for Mitosis G1, S, and G2 phases G1 – cells undergo major portion of growth S – replicate DNA produce two sister chromatids attached at the centromere G2 – chromosomes coil more tightly using motor proteins; centrioles replicate; tubulin synthesis Centromere – point of constriction Kinetochore – attachment site for microtubules Each sister chromatid has a centromere Chromatids stay attached at centromere by cohesin

Kinetochores Sister chromatids Cohesin proteins Centromere region of Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sister chromatids Cohesin proteins Centromere region of chromosome Kinetochore Kinetochore microtubules Metaphase chromosome

Interphase G2 Centrioles (replicated; animal cells only) Chromatin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Centrioles (replicated; animal cells only) Chromatin (replicated) Interphase G2 Aster Nuclear membrane Nucleolus Nucleus • DNA has been replicated • Centrioles replicate (animal cells) • Cell prepares for division

Question 4 What happens during the S phase of the cell cycle? Growth and maturation Replication of nuclear DNA Production of extra organelles Separation of chromosomes into sister chromatids Bloom’s level: Knowledge/Understanding

Question 16 Mature neurons can spend the entire animal’s lifetime in — G0 G1 G2 S M Bloom’s level: Application

Section 10.5 Learning Objectives Describe the phases of mitosis. PMAT Explain the importance of metaphase. Compare cytokinesis in plants and animals.

M phase: Chromosome Segregation and the Division of Cytoplasmic Contents Mitosis is divided into 5 phases: 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase

Prophase Individual condensed chromosomes first become visible with the light microscope Condensation continues throughout prophase Spindle apparatus assembles 2 centrioles move to opposite poles forming spindle apparatus (no centrioles in plants) Asters – radial array of microtubules in animals (not plants) Nuclear envelope breaks down

Prophase • Chromosomes condense and become visible • Chromosomes appear as two sister chromatids held together at the centromere • Cytoskeleton is disassembled: spindle begins to form • Golgi and ER are dispersed • Nuclear envelope breaks down Mitotic spindle beginning to form Condensed chromosomes Prophase

Prometaphase Transition occurs after disassembly of nuclear envelope Microtubule attachment 2nd group grows from poles and attaches to kinetochores Each sister chromatid connected to opposite poles Chromosomes begin to move to center of cell – congression Assembly and disassembly of microtubules Motor proteins at kinetochores

Prometaphase • Chromosomes attach to microtubules at the kinetochores Centromere and kinetochore Mitotic spindle • Chromosomes attach to microtubules at the kinetochores • Each chromosome is oriented such that the kinetochores of sister chromatids are attached to microtubules from opposite poles. • Chromosomes move to equator of the cell Prometaphase

© Andrew S. Bajer, University of Oregon Metaphase Alignment of chromosomes along metaphase plate Not an actual structure Future axis of cell division 57µm Polar microtubule Centrioles Kinetochore microtubule Aster Metaphase plate Sister chromatids © Andrew S. Bajer, University of Oregon

Chromosomes aligned on metaphase plate Kinetochore microtubule • All chromosomes are aligned at equator of the cell, called the metaphase plate • Chromosomes are attached to opposite poles and are under tension Metaphase Polar microtubule

Anaphase Begins when centromeres split Key event is removal of cohesin proteins from all chromosomes Sister chromatids pulled to opposite poles

Polar microtubule Chromosomes • Proteins holding centromeres of sister chromatids are degraded, freeing individual chromosomes • Chromosomes are pulled to opposite poles • Spindle poles move apart Kinetochore microtubule Anaphase

Telophase Spindle apparatus disassembles Nuclear envelope forms around each set of sister chromatids Now called chromosomes Chromosomes begin to uncoil Nucleolus reappears in each new nucleus

Telophase • Chromosomes are clustered at opposite poles and decondense Kinetochore microtubule Nucleus reforming • Chromosomes are clustered at opposite poles and decondense • Nuclear envelopes re-form around chromosomes • Golgi complex and ER re-form Polar microtubule Telophase

Cytokinesis Cleavage of the cell into equal halves Animal cells – constriction of actin filaments produces a cleavage furrow Plant cells – cell plate forms between the nuclei

Cytokinesis in Animal Cell b. 25 µm a: © David M. Phillips/Visuals Unlimited; b: © Guenter Albrecht-Buehler, Northwestern University, Chicago

Cytokinesis in Plant Cell 19,000× Vesicles containing membrane components fusing to form cell plate Cell wall (top): © E.H. Newcomb & W.P. Wergin/Biological Photo Service

Outcome of Mitosis After PMAT and cytokinesis Used for Two identical daughter cells Number and types of chromosomes remains the same 2n mitosis  2n n  mitosis  n Used for Asexual reproduction Growth repair

Question 13 If a researcher looked at a cell and noticed a straight line of sister chromatids, which phase would they be looking at? Prophase Metaphase Anaphase Telophase Interphase Bloom’s level: Application

Question 5 Cytokinesis is the division of the nucleus, and mitosis is the division of the cytoplasm. This is true This is false Bloom’s level: Knowledge/Understanding

Section 10.6 Learning Objectives Distinguish the role of checkpoints in the control of the cell cycle Characterize the role of the anaphase-promoting complex/cyclosome (APC/C) in mitosis. Describe cancer in terms of cell cycle control. Proto-oncogenes & tumor suppressor genes

Control of the Cell Cycle Current view integrates 2 concepts Cell cycle has two irreversible points Replication of genetic material Separation of the sister chromatids Cell cycle can be put on hold at specific points called checkpoints Process is checked for accuracy and can be halted if there are errors Allows cell to respond to internal and external signals

3 Checkpoints G1/S checkpoint G2/M checkpoint Cell “decides” whether or not to divide Primary point for external signal influence G2/M checkpoint Cell makes a commitment to mitosis Assesses success of DNA replication Can stall the cycle if DNA has not been accurately replicated. Late metaphase (spindle) checkpoint Cell ensures that all chromosomes are attached to the spindle

3 Checkpoints G2/M checkpoint Spindle checkpoint M G2 G1/S checkpoint (Start or restriction point) G1 S

Cyclin-dependent kinases (Cdks) Enzyme kinases that phosphorylate proteins Primary mechanism of cell cycle control Cdks partner with different cyclins at different points in the cell cycle For many years, a common view was that cyclins drove the cell cycle – that is, the periodic synthesis and destruction of cyclins acted as a clock Now clear that Cdk itself is also controlled by phosphorylation

Checkpoints of the Yeast Cell Cycle G2/M Checkpoint Spindle Checkpoint Cdc2/Mitotic Cyclin APC • Replication completed • Chromosomes attached at metaphase plate • DNA integrity Active Cdc2 Inactive APC M Active APC Inactive Cdc2 G2 G1/S Checkpoint Cdc2/G1 Cyclin • Growth factors • Nutritional state of cell G1 S • Size of cell Active Cdc2 Inactive Cdc2

Cyclin-dependent kinase (Cdk) Cyclin P Cdk – cyclin complex Also called mitosis-promoting factor (MPF) Activity of Cdk is also controlled by the pattern of phosphorylation Phosphorylation at one site (red) inactivates Cdk Phosphorylation at another site (green) activates Cdk

The Action of MPF Once thought that MPF was controlled solely by the level of the M phase-specific cyclins Although M phase cyclin is necessary for MPF function, activity is controlled by inhibitory phosphorylation of the kinase component, Cdc2 Damage to DNA acts through a complex pathway to tip the balance toward the inhibitory phosphorylation of MPF

Anaphase-promoting complex (APC) Also called cyclosome (APC/C) At the spindle checkpoint, presence of all chromosomes at the metaphase plate and the tension on the microtubules between opposite poles are both important Function of the APC/C is to trigger anaphase itself Marks securin for destruction; no inhibition of separase; separase destroys cohesin

Checkpoints of the Mammalian Cell Cycle G2/M Checkpoint Spindle Checkpoint Cdk1/Cyclin B APC • Replication completed • Chromosomes attached at metaphase plate • DNA integrity Active Cdk1 Inactive APC M Active APC Inactive Cdk1 G2 G1/S Checkpoint Cdk2/Cyclin E • Growth factors • Nutritional state of cell G1 S • Size of cell Active Cdk2 Inactive Cdk2

Cancer Unrestrained, uncontrolled growth of cells Failure of cell cycle control Two kinds of genes can disturb the cell cycle when they are mutated Tumor-suppressor genes Proto-oncogenes

Tumor-suppressor genes p53 plays a key role in G1 checkpoint p53 protein monitors integrity of DNA Prevent the development of mutated cells containing mutations p53 is absent or damaged in many cancerous cells In many cancers, p53 is not working (has been mutated)

Normal p53 protein destroys cells that have irreparable damage to their DNA Abnormal p53 protein fails to stop cell division, damaged cells divide, cancer develops p53 allows cells with repaired DNA to divide. p53 protein Normal p53 DNA repair enzyme 1. DNA damage is caused by heat, radiation, or chemicals. 2. Cell division stops, and p53 triggers enzymes to repair damaged region. 3. p53 triggers the destruction of cells damaged beyond repair. Abnormal p53 protein Cancer cell Abnormal p53 2. The p53 protein fails to stop cell division and repair DNA. Cell divides without repair to damaged DNA. 3. Damaged cells continue to divide. If other damage accumulates, the cell can turn cancerous. 1. DNA damage is caused by heat, radiation, or chemicals.

Proto-oncogenes Proto-oncogenes are normal cellular genes that become oncogenes when mutated Some encode receptors for growth factors If receptor is mutated in “on,” cell no longer depends on growth factors Only one copy of a proto-oncogene needs to undergo this mutation for uncontrolled division to take place

Tumor-suppressor genes p53 gene and many others Both copies of a tumor-suppressor gene must lose function for the cancerous phenotype to develop First tumor-suppressor identified was the retinoblastoma susceptibility gene (Rb)

Key Proteins Associated with Human Cancers Proto-oncogenes Growth factor receptor: more per cell in many breast cancers. Ras protein Src kinase Cytoplasm Ras protein: activated by mutations in 20–30% of all cancers. Src kinase: activated by mutations in 2–5% of all cancers. Rb protein p53 protein Nucleus Tumor-suppressor Genes Cell cycle checkpoints Rb protein: mutated in 40% of all cancers. p53 protein: mutated in 50% of all cancers. Mammalian cell

Question 11 Injecting MPF (maturation promoting factor) into cells — Signals apoptosis Causes cell lysis Induces mitosis Moves the cell into G0 Inactivates the p53 gene Bloom’s level: Application

Question 17 If a cell’s proto-oncogenes are mutated and over expressed, which of the following is most likely to happen? The cell will grow faster The cell will not finish the cell cycle The cell will improperly replicate DNA The cell will prematurely move to G0 The cell will undergo binary fission Bloom’s level: Application

Question 14 50% of cancerous cells have nonfunctioning p53 proteins. How does p53 help prevent cells from becoming cancerous? It binds growth factors It checks DNA for damage It induces the shift from G1 to S It stops mutations from occurring None of the above Bloom’s level: Application