Chapter 12: The Cell Cycle

12.1 How Does Cell Division Take Place?

• Mitosis
• Cytokinesis

12.2 Control of the Cell Cycle

12.3 Cancer: Out-of-Control Cell Division

Introduction to the Cell Cycle

• Cells arise through cell division of preexisting cells.
• Embryo observations confirm that plants and animals:
  • Start life as single-celled embryos.
  • Grow through a series of cell divisions.
• Meiosis produces reproductive cells called gametes.
• Mitosis produces all other cell types = somatic cells.
• Mitosis and meiosis are usually accompanied by cytokinesis
  • Division of the cytoplasm into two daughter cells.
• Mitosis: Genetic material is copied and divided equally between two cells.
  • Daughter cells are genetically identical to the parent cell and to each other.
• Meiosis: Produces cells with half the amount of hereditary material as the parent cell.
  • Daughter cells are genetically different. Having half the genetic material makes sense for meiosis but not mitosis

How Do Cells Replicate?

• Basic steps in cellular replication:
  • Copy the DNA.
  • Separate the copies.
  • Divide the cytoplasm to create two complete cells.

The Cell Cycle

• Orderly sequence of events.
• Describes the process by which a cell replicates to make two cells.
• Involves duplication of a cell’s chromosomes and other components.
• Ends with cell division.

Cells Alternate between M Phase and Interphase

• M (mitotic) phase: When the cell is dividing.
  • Chromosomes are condensed into compact structures.
• Interphase: Nondividing phase.
  • Chromosomes are uncoiled.
  • Cells are growing and preparing for mitosis or are fulfilling their specialized functions.
  • Cells spend most of their time in interphase.

Cell Cycle Phases

• Interphase can be broken down into three stages: S, G1, and G2.
• S phase:
  • DNA replication occurs.
  • S = synthesis.
• The gap phases allow cells to grow and replicate organelles.
• G1 phase:
  • Occurs between the M phase and the S phase.
• G2 phase:
  • Occurs between the S phase and mitosis.

What Happens during M Phase?

• Two distinct events:
  • Mitosis: Division of the replicated chromosomes.
  • Cytokinesis: Division of the cytoplasm.

Chromosomes

• Chromosome: Single long double helix of DNA wrapped around histones.
• Every species has a characteristic number (i.e. 46 in humans).
• Before mitosis, each chromosome is replicated.
• Each double-stranded DNA copy is called a chromatid.
• Chromatids have the same genetic information.
• After replication: Sister chromatids are attached to each other along their entire length by cohesins.
• Once mitosis begins: Attached only at the centromere.
• Two attached sister chromatids are still considered a single chromosome.

Chromosomes during Mitosis:

• Consists of DNA condensed around its associated proteins, resulting in a compact chromosome that is 10,000 times shorter than its original length.

Events in Mitosis

• Mitosis begins when chromatin condenses.
  • Chromatin = DNA + histone complex that makes up chromosomes.
• During mitosis:
  • Sister chromatids separate to form independent daughter chromosomes.
  • One copy of each chromosome goes to each of the two daughter cells.

Cell Cycle Overview

• Interphase
  • G1 phase: Parent cell with 4 unreplicated chromosomes (chromosomes are shown partially condensed to make them visible)
  • S Phase: 4 replicated chromosomes, each consisting of two sister chromatids
  • G2 phase: Parent cell
• M Phase
  • At start of mitosis, replicated chromosomes condense
  • During mitosis, sister chromatids separate
  • Two daughter cells are formed by cytokinesis

Interphase

• After chromosome replication, each chromosome is composed of two sister chromatids.
• Centrosomes have replicated.
• Chromosomes replicate

Mitosis

• Continuous process with five subphases:
  • Prophase
  • Prometaphase
  • Metaphase
  • Anaphase
  • Telophase

Prophase

• Chromosomes condense.
• Become visible in the light microscope.
• Spindle apparatus forms - produces mechanical forces that:
  • Move replicated chromosomes during early mitosis.
  • Pull chromatids apart in late mitosis.

Prophase - Spindle Apparatus

• Spindle apparatus is made of microtubules
• Forms from microtubule-organizing centers (MTOCs)
• Define the two poles of the spindle apparatus
• (+) ends grow from each pole
• Polar microtubules extend from each spindle pole and overlap with each other
• In animal cells, MTOCs are centrosomes, each containing a pair of centrioles.

Prometaphase

• Nuclear envelope breaks down
• Microtubules attach to chromosomes at kinetochores
  • Protein structures that form at the centromere
• Microtubules attach on both sides of each chromosome
  • These are the kinetochore microtubules
• Chromosomes are pushed and pulled by microtubules until they reach the middle of the spindle

Metaphase

• Formation of the mitotic spindle is completed
• Chromosomes are lined up on the metaphase plate
  • Imaginary plane between the two spindle poles
• Each chromosome is held by kinetochore microtubules from opposite poles
• Astral microtubules hold spindle poles in place by interacting with proteins at the PM

Anaphase

• Cohesins holding sister chromatids together split
• Sister chromatids are pulled by the spindle fibers toward opposite poles of the cell
• Two forces pull:
  • Kinetochore microtubules shrink
  • Motor proteins of the polar microtubules push the two poles of the cell away from each other
• Creates two identical sets of daughter chromosomes

Telophase

• New nuclear envelope begins to form around each set of chromosomes
• Chromosomes begin to decondense
• Mitosis is complete when two independent nuclei have formed

Cytokinesis

• Typically occurs immediately after mitosis
• Cytoplasm divides to form two daughter cells
• Cell division begins: Actin–myosin ring causes plasma membrane to begin pinching in.
• Cell division is complete: Two daughter cells form.

How Do Chromosomes Move during Anaphase?

• The kinetochore attaches the centromere to microtubules
• Kinetochore microtubules
  • Initially stationary during anaphase
  • Shorten because tubulin subunits are lost from their plus ends
• Proteins from the kinetochore attach to a ring that surrounds the kinetochore microtubule
• As the plus end disassembles, the ring moves along the microtubule

Cytokinesis in Plants vs. Animals

• Plants:
  • Vesicles from the Golgi apparatus bring membrane and cell wall components to the middle of the cell
  • Vesicles fuse to form a cell plate
  • Fuses with the PM to make two cells
• Animals/Eukaryotes:
  • A ring of actin and myosin filaments contracts inside the cell membrane
  • Pinches inward to form a cleavage furrow
  • Ring shrinks and tightens until division is complete

Structures Involved in Mitosis:

• Chromosome: A structure containing genetic information in the form of genes.
• Chromatin: The material that makes up eukaryotic chromosomes; consists of a DNA molecule complexed with histone proteins.
• Chromatid: One double-stranded DNA copy of a replicated chromosome with its associated proteins.
• Sister chromatids: The two attached, double-stranded DNA copies of a replicated chromosome. When chromosomes are replicated, they consist of two sister chromatids. The genetic material in sister chromatids is identical. When sister chromatids separate during mitosis, they become independent chromosomes.
• Centromere: A specialized region of a chromosome where sister chromatids are most closely joined to each other
• Kinetochores: The structures on sister chromatids where microtubules attach
• Microtubule-organizing center: Any structure that organizes microtubules
• Centrosome: The microtubule-organizing center in animals and certain plants and fungi
• Centrioles: Cylindrical structures consisting of microtubule triplets, located inside animal centrosomes

Bacterial Cell Replication

• Bacteria divide via binary fission.
• Similar to eukaryotic M phase.
• Bacterial chromosomes are replicated.
• Proteins attach to chromosomes and separate them.
• Other proteins divide the cytoplasm.

Amount of DNA & Chromosomes Variation During the Cell Cycle

• A cell in G1 of interphase has 60 pg of DNA and 22 chromosomes.
  • During prophase: 120 pg of DNA and 22 chromosomes
  • During anaphase: 120 pg of DNA and 44 chromosomes
  • One of the resulting daughter cells immediately after cytokinesis: 60 pg of DNA and 22 chromosomes

Control of the Cell Cycle

• Cell-cycle length can vary greatly among cell types.
• Mostly due to variation in the length of G1 phase.
• Rapidly dividing cells essentially eliminate G1.
• Nondividing cells get permanently stuck in G1.
  • Arrested state = G0.
• Division rate varies in response to changing conditions.

Cell Cycle Regulation

• First evidence in the 1970s.
• Scientists fused two cells in different stages of the cell cycle.
• Fusion caused one of the cells to change phases.
• Conclusion: A regulatory molecule was initiating the change.

M Phase-Promoting Factor (MPF)

• Present in the cytoplasm of M-phase cells.
• Induces mitosis in all eukaryotes.
• MPF is composed of two distinct subunits:
  • Protein kinase (Cdk): Enzyme that transfers a phosphate group from ATP to a target protein.
  • Cyclin: Protein present in different concentrations throughout the cell cycle.

MPF - Protein Kinase and a Cyclin

• Concentration of MPF cyclin increases during interphase.
• Peaks in M phase before decreasing again.
• MPF protein kinase is a cyclin-dependent kinase (Cdk).
  • Active only when bound to the cyclin subunit.
• When [cyclin] is high, MPF is active.
• Target proteins are phosphorylated, initiating mitosis.
• Target activity is temporary

MPF Turned On

• MPF’s Cdk subunit has 2 phosphorylation sites.
• Both phosphorylated after cyclin binds.
  • 1 inhibits kinase activity.
• Late in G2 phase, the inhibitory phosphate group is removed.
  • Activates the kinase.
• Leads to:
  • Chromosome condensation
  • Formation of the mitotic spindle

MPF and Phosphorylation

• Allows the [cyclin-Cdk complex] to increase…
• Without prematurely starting M phase
• As soon as MPF is active…
• Downstream proteins are activated rapidly

MPF Turned Off

• Enzymes begin degrading MPF cyclins during anaphase.
• MPF deactivation illustrates two key concepts about regulatory systems in cells:
  • Negative feedback: When a process is slowed or shut down by its products.
  • Destroying specific proteins: Common way to control cell processes.
• An enzyme complex that is activated during anaphase attaches ubiquitin proteins to the cyclin subunit.
  • Marks the subunit for destruction (Ub-tag)
  • Leads to deactivation of MPF

Cell-Cycle Checkpoints

• If there are problems with the cell cycle…
• The cell needs to be able to stop (“arrest”) the cell cycle
• Many proteins are involved in regulation
• Four main cell-cycle checkpoints:
  • Critical points in the cell cycle that are regulated
  • Regulatory molecules at each checkpoint allow a cell to “decide” whether to proceed with division
  • Prevent the division of cells that are damaged or are in G0
  • If regulation is defective, the checkpoint may fail
  • Cells that divide without control may form a tumor

Cell-Cycle Checkpoints Details

• G1 Checkpoint
  • Pass checkpoint if:
    • cell size is adequate
    • nutrients are sufficient
    • social signals are present
    • DNA is undamaged
  • Mature cells do not pass this checkpoint (they enter G0 state)
• G2 Checkpoint
  • Pass checkpoint if:
    • chromosomes have replicated successfully
    • DNA is undamaged
    • activated MPF is present
• M-Phase Checkpoints
  • Pass checkpoints if:
    • chromosomes have attached to spindle apparatus (metaphase to anaphase transition)
    • chromosomes have properly segregated and MPF is absent (anaphase to telophase transition)

DNA Damage at Checkpoint

• If DNA is physically damaged, the p53 protein either:
  • Activates proteins that pause the cell cycle until damage can be repaired
  • Or initiates apoptosis = programmed cell death
• p53 is an example of a tumor suppressor.
• Damage to the p53 gene can lead to uncontrolled cell division

Cancer: Out-of-Control Cell Division

• Cancer
  • 40% of Americans will develop cancer
• Complex family of diseases caused by cells that:
  • Grow in an uncontrolled fashion
  • Invade nearby tissues
  • Spread to other sites in the body

Properties of Cancer Cells

• Over 200 types of cancer
• All arise from cells in which cell-cycle checkpoints have failed
• Cancerous cells have two types of defects:
  • Activate proteins required for cell growth when they should not be active
  • Prevent tumor suppressor genes from shutting down the cell cycle

Types of Tumors

• Two types of tumors:
  • Benign: Noncancerous and noninvasive
  • Malignant: Cancerous and invasive
  • Can spread throughout the body via the blood or lymph (metastasis)
  • May initiate secondary tumors
• Many cancers are thought to arise from cells with defects in the G1 checkpoint
  • Divide when they should not

Benign vs. Malignant Tumors

• Benign Tumor
  • Normal cells
  • Blood vessel
  • Benign tumor cells may continue to divide but are not invasive (they do not spread from tumor)
  • Lymphatic vessel
• Malignant Tumor
  • Malignant tumor cells divide and spread to adjacent tissues and to distant tissues through lymphatic vessels and blood vessels.
  • Lymphatic vessel
  • Blood vessel
  • New tumor that has formed in distant tissue by metastasis

Social Control in Multicellular Organisms

• Cells respond to signals from other cells
• Divide only when it benefits the whole organism
• Social control is based on growth factors (GFs):
  • Small proteins that stimulate division
• Cells in culture will not grow unless serum is added
  • Serum: Liquid portion of blood
  • GFs in serum allow cells to pass the G1 checkpoint
• Many cancer cells can divide without growth factors

G1 Checkpoint Work

• Growth factors stimulate production of E2F protein and G1 cyclins
• Rb protein is a tumor suppressor.
• It suppresses E2F activity until the appropriate time.
• Keeps the cell in G0
  1. Growth factors arrive from other cells.
  2. Growth factors cause increase in cyclin and E2F concentrations.
  3. Cyclin binds to Cdk; Cdk is phosphorylated.
  4. Inactivating phosphate is removed, and active Cdk phosphorylates Rb.
  5. Phosphorylated Rb releases E2F.
  6. E2F triggers production of S-phase proteins.
• GFs are the social signals that say, “it’s OK to override Rb. Go ahead and pass the G1 checkpoint and divide.”

Social Controls and Cell-Cycle Checkpoints Failures

• In some cancers, the G1 cyclin is overproduced
  • Permanently activates Cdk
  • Continuously phosphorylates Rb so it can’t bind E2F
• In other cancers, Rb is missing or defective
  • Doesn’t bind to E2F
  • E2F activates genes to start S phase

Cancer Treatment

• Since every cancer is basically a different disease, some therapies will work for some cancers but not others
• Understanding and targeting the cell cycle is a big focus of how many cancer therapies work
• Ideally want to kill cancer cells that are not behaving properly, while leaving healthy cells alone
  • Targeted therapy
• We are learning more all the time…
• Hopefully will not have to use less specific treatments like chemotherapy and radiation for too much longer