CHAPTER 10: CELL CYCLE, MITOSIS

What is an example of a Model Organisms for Cell Cycle Research?

Saccharomyces cerevisiae yeast - common single-celled model that led to the discovery that some genes can control the eukaryotic cell cycle. 

• Prime organism for research because its many cells can be easily visualized and that the cell cycle can be paused at a specific phase. ALL CELLS CAN ALSO BE SYNCHRONIZED AT ONE PHASE, and mutations are easily generated.

What are the 2 irreversible checkpoints?

S phase (replication of genetic material), and anaphase (separation of the sister chromatids)

What are the 3 major checkpoints?

1. G1/S - primary point for external influence, checks if DNA is damaged from radiation/chemicals or if the cell is nutritionally deficient. DNA can be repaired and the cell cycle starts again

2. G2/M - COMMITS TO MITOSIS, checks if DNA has been replicated accurately

3. late metaphase (spindle) checkpoint - IREVERSIBLY COMMITED TO COMPLETING M, checks if chromosomes are attached to the spindle

How does cell division occur in prokaryotes? List the steps.

Via binary fission, where the cell divides into 2 halves.

1. Bacteria cells replicate in the origin of replication (ori).

2. Replication begins and proceeds in opposite directions, takes place in the middle of the cell where DNA replication enzymes are located

3. Two replicated origins migrate to the poles of the cell as replication continues for the rest of the chromosomes. Movement is active, but the mechanism is unknown.

4. Replication is complete & cell division begins as the plasma membrane grows inward and a new cell wall is synthesized.

5. When the new wall is built, the cell cuts into 2.

*copying begins at replication original, proceeds bi-directionally. 1 genome ends up in each daughter cell

How does genetic information appear in prokaryotes?

Exists as a nucleoid - a single, circular DOUBLE-STRANDED DNA molecule

Where is DNA stored in eukaryotes?

Chromosomes - humans have 46 chromosomes, with 23-near identical pairs (half from mom and half from dad)

What are the 3 parts of the cell cycle?

1. Cell growth and activity, replication of DNA (interphase)

2. Nuclear division

   1. Mitosis: divides replicated DNA equally, generating 2 diploid cells (daugher cells)

   2. Meiosis: produces daughter nuclei with HALF THE # OF CHROMOSOMES of the parental nucleus (4 haploid gametes)

3. Division of the cytoplasm (cytokinesis)

What is MITOSIS?

DNA synthesis replicates each DNA chromosome into 2 copies. The mitotic cytoskeleton separates replicated DNA molecules into the daughter cells.

How are chromosomes shaped, where are they located, and what are they made of?

• Eukaryotes have hereditary information WITHIN THE NUCLEUS, distributed among individual, linear DNA molecules that combine with proteins to stabilize them, assist in packaging DNA during cell division, and influence the expression of individual genes.

• Chromosomes = 1 DNA molecule + proteins

How is linear DNA packaged into chromosomes?

• Histone proteins pack DNA into a shorter length so it can fit into the nucleus

• Other nonhistone proteins also associate with DNA; chromatin is the complex of DNA & its associated proteins

• In a nucleosome, an 8-protein nucleosome core particle forms when DNA winds around the histones H2A, H2B, H3, and H4 (x2)

• A short linker segment of DNA connects nucleosomes

What packs DNA?

• Nucleosomes and linkers are like beads on a string

• 10-nm chromatin fiber is the diameter of the beads, COMPACTS DNA BY A FACTOR OF 7

• The 30-nm chromatin fiber packs the DNA further, when the nucleosome and linker are bound by the 5th histone protein, H1

What is the solenoid model?

Predicts the nucleosomes spiral helically without about 6 nucleosomes per turn.

• Chromatin packing continues at higher levels. The light band is euchromatin being loosely packed, the dark band is heterochromatin (dense packing)

  • Euchromatin regions are highly expressed compared to heterochromatin regions

What are sister chromatids? What are they held together by?

• Chromosomes must be replicated before cell division

• Sister chromatids (2 copies of the chromosome, within the replicated chromosomes) are connected at the centromere

• They are held together by sister chromatid cohesion (cohesin - a protein complex) until mitosis separates them and one of each are placed into the 2 daughter nuclei

• The equal distribution of chromosomes into each of 2 daughter nuclei is called chromosome segregation

What is ploidy? What are the different kinds?

The number of chromosome sets in a cell/species is called a ploidy.

• Haploid (n): 1 copy of each type of chromosome in their nuclei

• Diploid (2n): 2 copies of each type of chromosome in their nuclei

  • 2 chromosomes of each pair in a diploid cell are homologous chromosomes (1 from mother, 1 from father)

  • Homologous chromosomes have the same genes in the same order of the DNA of the chromosomes

  • Sperm/ova is a 2n haploid, zygotes are 2n diploids

• Polyploid: common in plant species with 3-4+ complete sets of chromosomes in each pair

What is a karyotype?

Karyotype: Individual’s specific array of METAPHASE chromosomes, arranged by size and shape

• Diploid: Cells possessing 2 copies of each chromosomes (human BODY cells)

• Made up of sister chromatids joined at the centromere

• Haploid: Possesses 2 copy of each chromosome (human SEX cells)

How do you prepare cells for a karyotype?

1. Add a sample to the culture medium that has a stimulator so the culture can grow and divide cells (white blood cells for blood). Incubate at 37 degrees celcius

2. Stain the cells so chromosomes can be distinguished

3. View stained cells under a microscope with digital imaging systems and take a photo

Interpret the results

• FISH: “FLUORESCENT IN SITU HYBRIDIZATION” 

• 23 pairs of chromosomes, XY male, XX female

What is G1?

• Primary growth phase - longest phase & varies in length

• Many cell types stop dividing and are shunted into G0 phase - some cells reenter G1 or they never resume the cell cycle

• Internal regulatory controls trigger each phase of the cycle & controls the number of cycles of cell undergoes

• PURPOSE: The cell grows and makes proteins that are required in the S phase 

• *COPYING OF DNA HAS NOT OCCURRED

What is S phase?

• Replicates DNA - each chromosome is copied and held together to form sister chromatids

What is G2?

• Organelles replicate, microtubules organize 

• Correct DNA replication check

• Commits to mitosis

• *COPYING OF DNA HAS OCCURRED

What are the stages of mitosis based on?

The stages of mitosis are characterized by the LOCATION and BEHAVIOR of the chromosomes. Some conversions between stages correspond to cell cycle events & are irreversible.

What is prophase?

• CONDENSATION continues throughout prophase - individual condensed chromosomes become visible with light microscope

• Spindle apparatus assembles

• 2 centrosomes move to opposite poles forming spindle apparatus (- in plants, there are no centrioles or asters)

• Nuclear envelope disintegrates, nucleolus disappears

What is prometaphase?

• Spindle microtubules grow from centrosome at opposite spindle poles TOWARD THE ENTER of the cell

• Kinetochore forms on each sister chromatid at the centromere. The kinetochore microtubules bind to the kinetochore

• Nonkinetochore microtubules overlap those from opposite spindle form - do NOT bind to kinetochore, binds in the center

• Congregate chromosomes in the center of the cell

• Assembly and disassembly of microtubules

• Motor proteins at kinetochores

What is metaphase?

1. Spindle microtubules move chromosomes to the spindle midpoint (metaphase plate)

   • Condensation gives each chromosome a characteristic shape, determined by length and centromere location

What is anaphase?

*REMOVAL OF COHESIN PROTEINS FROM ALL CHROMOSOMES

• Centromeres split

• Microtubules shorten, pulling sister chromatids to opposite sides, separated via original and copy (now just called chromosomes)

• Anaphase A: Kinetochores are pulled toward poles by motor proteins, disassembling microtubules into tubulin subunits

• Anaphase B: Poles move apart; nonkinetochore microtubules elongate the cell.

  • Motor proteins walk in opposite directions on overlapping nonkinetochore microtubules

  • Nonkinetochore microtubules push the poles apart by growing in length as they slide along

What is telophase?

• Spindle apparatus disassembles, chromosomes return to extended state

• Nuclear envelope forms around each set of chromosomes whilst they uncoil

• Nucleolus reappears in each new nucleus, RNA transcription resumes

What is cytokinesis?

• PRODUCES 2 DAUGHTER CELLS, each with 1 of the 2 daughter nuclei

  1. Furrow (contractile ring): Encircles the cell and deepens until it cuts the cytoplasm into 2 parts - common in animals, protists, and fungi

  2. Cell plate: forms between the daughter nuclei and grows laterally until it divides the cytoplasm - common in PLANTS

• Plane of cytoplasmic division depends on the layer of microtubules that persists at the former spindle midpoint

Explain mitosis in a plant cell

Tell apart structures

What is the amount of time a cell spends in each stage?

1. ~10 hours for G1 *longest

2. ~9 hours for S

3. ~4 hours for G2

overall: 24 hours

What are animal spindles?

• Centrosome: the main MICROTUBULE ORGANIZING CENTER (MTOC) of animal cells & protists - Contains a pair of centrioles that are duplicated and divided

• Microtubules extending from centrosomes produce asters at the spindle tips that form the poles of spindle in prophase