Week #2 Readings - 11.1, 11.4, 11.5 (MITOSIS and CELL CYCLE)

Cell Division

• Cell division is the process by which a single cell produces two daughter cells.

• Must be large enough to divide in two and contribute sufficient nuclear and cytoplasmic components to each daughter cell

• Key cellular components are duplicated -DNA

What does Cell Division Occur For?

• Growth (Single to Multi)

• Cell Replacement

• Healing (Ex. Wound → New skin cells)

• Reproduction (Asexual and Sexual)

Cell Cycle

• The series of steps that take place as a eukaryotic cell

  • Grows

  • Replicates its DNA

  • Divides to produce daughter cells

Life cycle of a cell

Prokaryotic vs Eukaryotic Cell Division

Chromosomes

• Packaged genetic material

• Consist of a single DNA molecule and associated proteins

• Prokaryotic → Organized as a single, relatively small, circular chromosome

• Eukaryotic → Larger and is organized into one or more linear chromosomes

Binary Fission

• The process by which cells of bacteria and archaea divide to form two daughter cells

• A cell replicates its DNA, increases in size, and divides into two daughter cells

• Each daughter cell receives one copy of the replicated parental DNA.

• Red - Origin of Replication

• Important gene - FtsZ encodes a protein that forms a ring at the site of constriction where the new cell wall forms between the two daughter cells

Eukaryotic Cell Cycle

• More complicated

• Two distinct phases

• M Phase → Divides into two daughter cells

  • First divide the nucleus by mitosis (chromosomes separated into two nuclei)

  • Then divide the cytoplasm into two daughter cells by cytokinesis

• Interphase occurs between two
  successive M phases

What occurs during Interphase?

• Preparations for division.

• DNA replication and cell growth.

• DNA in the nucleus first replicates so that each daughter cell receives a copy of the genetic material.

• Cell then increases in size so that each daughter cell receives enough of cytoplasmic and membrane components to allow it to survive on its own.

What are the Three (4?) Phases of Interphase?

• G1 Phase (Gap 1) → Cell growth and expression of regulatory proteins

  • Ex…Kinase, activate enzymes that synthesize DNA

• S Phase (Synthesis)→ Replication of DNA

• G2 Phase (Gap 2) → Size and protein content of the cell increase in preparation for M-phase

• S phase does not immediately precede or follow mitosis but is separated from it by two gap phases

G0 Phase (“Fourth Phase”)

• The gap phase of the cell cycle in which cells pause in the cell cycle between M phase and S phase; it may last for periods ranging from days to more than a year.

• These cells are said to be quiescent.

• Ex. A neuron, axon, and dendrites would not re-enter the cell cycle

• Ex. Liver remains in G0 for a year than re-enters

• Still perform their specialized functions

How is DNA stored?

• Length of DNA 1-2 meters → Heavily condensed!

• In eukaryotic cells, DNA is organized with histones and other proteins into chromatin, which can be looped and packaged to
  form chromosomes

• Further condensed during cell division so it does not tangle as it separates into daughter cells

Karyotype

• A standard arrangement of chromosomes, showing the number and shapes of the chromosomes representative of a species.

• Most cells of the human body has 46 chromosomes (23 pairs).

• There are 22 pairs of homologous chromosomes, 1 pair of sex
  chromosomes.

• Homologous chromosomes carry the same set of genes + match in size and appearance

• One from the mother and one from the father.

• The sex chromosomes are X and Y

  • Two X chromosomes is a female

  • An X and a Y chromosome is a male.

Sets of Chromosomes

• The number of complete sets of chromosomes in a cell is known as its ploidy.

• One complete set of chromosomes is haploid (n)

• Two complete sets of chromosomes is diploid (2n)

• Plants can have polyploids! (4n)

S Phase and Sister Chromatids

• Duplication occurs during S phase

• Every chromosome in the parent cell must be duplicated so that each daughter cell receives a full set of chromosomes

• Sister chromatids – The two identical copies that are made after duplication

• Do not separate

• Are held together at the
  centromere.

• Cell still contains 46 chromosomes, count each pair as 1!

Stages of Mitosis

• Is divided into 5 phases

  • Prophase

  • Prometaphase

  • Metaphase

  • Anaphase

  • Telophase

• These 5 stages are followed by Cytokinesis

• Each of the five stages of mitosis can be determined using a microscope depending on the position of the chromosomes.

• Please Pass Me A Taco, Chef

Prophase

• Characterized by the appearance of visible chromosomes

• Chromosome Condensation → The progressive coiling of the chromatin fiber, an active, energy-consuming process requiring the participation of several types of proteins.

• Chromosomes change from long, thin, threadlike structures to short, dense forms (visible under microscope)

• Centrosomes radiate microtubules

• Migrate to opposite poles

What happens to the cytoskeleton in this phase?

• Microtubules assemble into the mitotic spindle, a structure that pulls the chromosomes to opposite ends of the dividing cell.

• These spindles radiate from the centrosome, a compact structure that is the microtubule organizing center for animal cells.

• These centrosomes define the future daughter cell poles and organize the microtubules that guide chromosome movement.

Prometaphase

• Nuclear envelope breaks down

• The microtubules of the mitotic spindle attach to chromosomes

• Attach to the chromosomes at their centromeres

• Kinetochores → Protein complexes that are sites for spindle attachment

• Ensures that each sister chromatid travels to an opposite pole

Metaphase

• Once each chromosome is attached to the mitotic spindles from both poles of the cell, the microtubules of the mitotic spindle lengthen or shorten to move the chromosomes to the middle of the cell.

• There the chromosomes are lined up in a single plane that is roughly equidistant from both poles of the cell.

Anaphase

• The centromere holding a pair of sister chromatids together splits, allowing the two sister chromatids to separate from each other.

• After separation, each chromatid is considered to be a full-fledged chromosome.

• Anaphase ensures that one chromatid from each pair of sister chromatids goes to opposite poles of the cell

Telophase

• A complete set of chromosomes is now at each pole of the cell.

• The microtubules break down completely and the nuclear envelope re-forms.

• Chromosomes decondense, marking the end of telophase and mitosis.

Cytokinesis: In Animal Cells

• As mitosis ends, cytokinesis begins when actin filaments form a contractile ring at the equator of the cell, perpendicular to the spindle

• Motor proteins (FtsZ) cause the ring to constrict like a drawstring, pinching the cytoplasm and dividing the parent cell into two daughter cells, each with its own nucleus, which then enter G₁ phase.

Cytokinesis: In Plant Cells

• Since plant cells have a cell wall, the cell divides in two by constructing a new cell wall.

• During telophase, a phragmoplast of microtubules guides vesicles to the cell’s center, where they fuse to form a cell plate that grows outward and joins the original cell wall, separating the cell into two daughter cells.

How do cells know when to divide?

• In response to external signals

• When internal requirements are met - does not divide unless ready

  • DNA replication is complete

  • The cell is large enough to divide into two

• Uncontrolled division is dangerous and can lead to cancer

Regulation of the Cell Cycle

• Progression through the cell cycle is
  controlled by cyclins

• Activate kinases (cyclin-dependent kinases (CDKs), which target proteins that promote cell division

• Are always present within the cell but are active only when bound to the appropriate cyclin.

• KINASE- Enzymes that phosphorylate other
  molecules

CDK/Cyclin: Regulation of the Cell Cycle

• Cyclins appear and disappear throughout the cell cycle.

• Cyclins activate cyclin-dependent kinases (CDKs).

• Active CDKs phosphorylate target proteins

• The phosphorylated target proteins
  promote cell division and progression through the stages of the cell cycle.

Cyclin-CDK complexes and Different stages of the cell cycle

• There are 3 specific time points that are regulated.

  • G1 → S

  • S → G2

  • G2 → M

• Different cyclins and CDKs act at these specific steps of the cell cycle.

The G1/S cyclin–CDK complex

• Prepares the cell for S phase (Cyclin D and E rise)

• Promotes the expression of histone proteins needed to package DNA

• Activate transcription factors that lead to the expression and activation of DNA polymerase and other enzymes

The S cyclin–CDK complex

• Initiation of DNA synthesis (Cyclin A rise and activate specific CDKs)

• Inhibit the activity of DNA synthesis enzymes once replication is over (prevent replication proteins from reassembling)

The M cyclin–CDK complex

• Cyclin B binds to CDKs that activate enzymes that initiate multiple events associated with mitosis

• The breakdown of the nuclear
  envelope during prophase

• The formation of the mitotic spindle

Cell Cycle Checkpoints

• Checkpoints in the cell cycle ensure
  the cell is prepared at different stages to proceed to the next stage.

• Mechanisms that block the cyclin–CDK activity required for the next step, pausing the cell cycle until preparations are complete or the damage is repaired

What are the Three Major Checkpoints?

1. DNA damage checkpoint

• Presence of damaged DNA arrests the cell at the end of G1 before DNA synthesis

• Ex. Damaged DNA (radiation) → double-stranded breaks → checkpoint delays progression until repaired

2. DNA replication checkpoint

• Presence of unreplicated DNA arrests the cell at the end of before the cell enters mitosis

3. Spindle assembly checkpoint

• Abnormalities in chromosome attachment to the spindle arrest the cell in early mitosis

DNA Damage Checkpoint: p53 Regulation

• When DNA is damaged by radiation, a protein kinase is activated that phosphorylates a protein called p53

• p53 activates the synthesis of
  proteins that block G1/S Cyclin-CDK complex

• p53 arrests the cell at the G 1 /S transition → time to repair DNA

• “Guardian of the genome”

How does Apoptosis Occur?

• When DNA is damaged, phosphorylated p53 increases transcription of Bax and represses Bcl-2.

• This shifts the balance from Bax/Bcl-2 dimers to Bax/Bax complexes

• Trigger apoptosis, a controlled and orderly form of programmed cell death.

• Responsible for eliminating cells that are unneeded, damaged, or harmful

Cancer

• Characterized by uncontrolled cell division

• Occurs when mechanisms that promote cell division are activated inappropriately or the usual checks on cell division are lost

Peyton Rous and His Research

• Studied cancers called sarcomas in chickens

• Discovered oncogenes; cancer-causing genes

• First discovered in viruses → discovery that they weren’t just in viruses

• Ex. Rous Sarcoma Virus has a gene that promotes uncontrolled cell division

• Encodes an overactive protein kinase that functions as a signal

• This gene is an example of an oncogene

Proto-Oncogenes

• Normal genes important in cell division that have the potential to become cancerous if mutated

• Various proteins that performs roles in signalling cascades leading to cell division can be a product of a protooncogene

  • Growth factors

  • Cell surface receptors

  • G proteins

  • Protein kinases

• If mutated, any of these can become an oncogene

• Environmental agents (cigarate smoke) can damage and mutate DNA

p53 and Cancer

• Cycle checkpoints that halt the cell cycle until the cell is ready to divide → p53 (stops due to DNA damage)

• When the p53 protein is mutated or its function is inhibited, the cell can divide before the DNA damage is repaired.

• The p53 protein is mutated in many types of human cancer, highlighting its critical role in regulating the cell cycle.

Tumor Suppressors and Proto-Oncogenes

• Tumor suppressors: genes that encode proteins whose normal activities inhibit cell division

• Ex. p53

• Act in opposition to proto-oncogenes

• Proto-oncogenes must be turned on and tumor suppressors must be turned off for a cell to divide

• Two counterbalancing systems that must be in agreement before the cell divides

Multiple Mutation Model for Cancer Development

• Require the accumulation of mutations in multiple genes (noy just one)

• When several different cell cycle regulators fail, leading to both the overactivation of oncogenes and the loss of tumor suppressor activity, cancer will likely develop

• Benign: relatively slow growing and does not invade the surrounding tissue

• Malignant: grows rapidly and invades surrounding tissues

• Ex. Colon Cancer → Tumor cells contain at least one overactive oncogene and several inactive tumor suppressor genes

• A cancer cell is one that no longer plays by the “rules” of a stable cellular community.