56d ago

Cell Cycle and Mitosis Notes

Cell Cycle

Overview

  • The cell cycle is how two cells become one.

  • Two phases:

    • Mitosis: Active cell division.

    • Interphase: Time when cell grows and prepares for cell division.

Interphase

  • Subdivided into G1, S phase, and G2.

  • G1 Phase:

    • Cell is "chilling", carrying out normal metabolic functions, and growing.

  • S Phase:

    • DNA replication occurs.

  • G2 Phase:

    • Cell readies for mitosis, and continues to grow.

Cell Cycle Schematic

  • M phase: Active cell division.

  • Interphase: G1, S, and G2 phases.

  • G1: Normal metabolic function and growth.

  • S phase: DNA synthesis/replication.

  • G2: Getting ready for mitosis and more growth.

G0 Phase

  • When a cell enters G0, it is not dividing or thinking about dividing.

  • Examples: Muscle cells and neurons.

  • These cells have "left the cell cycle", typically from G1.

Why Divide?

  • To produce reproductive cells (gametes).

  • For organism growth.

  • To repair.

  • To maintain tissue.

How: Organizing a Signal

  • A signal tells the cell it's time to divide.

  • Examples:

    • Growth factors.

    • Interleukins (IL).

    • Erythropoietin.

    • Hormones (e.g., estrogen).

  • The signal stimulates movement from G1 into S phase.

  • S phase: Duplication of DNA.

  • G2: Getting ready for mitosis.

    • Duplicating organelles.

    • Duplicating centrosomes.

    • Producing microtubules for the mitotic spindle.

  • M phase: Active cell division.

M Phase (Mitosis) - Detailed

  • PPMAT: Prophase, Prometaphase, Metaphase, Anaphase, Telophase, and Cytokinesis

Prophase
  • Chromosomes condense.

  • Genetic information is packed into chromosomes.

Chromosome Composition
  • Chromosomes are made up of chromatin.

  • Chromatin is comprised of DNA and proteins.

  • DNA and proteins make up nucleosomes (beads on a string).

  • Nucleosomes are made of DNA and histones.

  • Histones:

    • H1 (linker histone).

    • H2A, H2B, H3, and H4 (core histones).

Nucleosome Structure
  • Core histones: Two of each (H2A, H2B, H3, H4) = 8 total.

  • DNA wrapped around core histones: 146 base pairs.

  • H1 linker histone connects everything together.

Chromatin Compaction
  • Multiple nucleosomes compact further.

  • H1 histones link together to form a solenoid.

  • Solenoid compacts into a 300 nm structure.

  • Further compaction into loop structures.

  • Final structure: Duplicated chromosome (after S phase).

Chromatin, Sister Chromatids, and Duplicated Chromosomes
  • Chromatin: Tightly wound DNA and proteins.

  • Sister chromatid: Each arm of the duplicated chromosome.

  • Two sister chromatids make up the duplicated chromosome.

Centromere and Kinetochore
  • Centromere: Area linking two sister chromatids together.

  • Kinetochore: Area on the centromere where mitotic spindle fibers attach.

Mitotic Spindle Formation
  • Mitotic spindle is made of microtubules.

  • Microtubules come from the centriole.

  • Centrioles originate from the centrosome (home of the centriole).

Nucleolus Disappearance
  • Nucleolus disappears during prophase.

  • The nucleolus is the area in the nucleus where ribosomes are assembled.

  • Because chromosomes are condensed, transcription and translation are not occurring, so ribosomes are not needed.

Prometaphase
  • Nuclear envelope/membrane breaks down and disappears.

  • Mitotic spindle is fully formed and starts to attach to chromosomes, not fully attached.

  • Kinetochore microtubules attach to kinetochores on the centromeres.

Metaphase
  • Chromosomes line up in the middle of the cell on the equatorial plate.

Anaphase
  • Separated chromosomes (sister chromatids) move to opposite ends/poles of the cell.

Telophase
  • Nuclear membrane reforms (disappeared in prometaphase).

  • Chromosomes have moved to opposite ends.

Cytokinesis
  • Division of the cytoplasm.

  • Two cells become one.

  • Uses contractile microfilaments called actin and myosin.

  • Contractile ring forms in the center and pinches the cell in two.

Additional Important Vocabulary

Euchromatin vs. Heterochromatin
  • Euchromatin:

    • Active form.

    • Replicating, transcribing, translating.

    • Not wrapped around nucleosomes, so it is available.

  • Heterochromatin:

    • Inactive form.

    • Wrapped around nucleosomes.

Histone Protein Modifications
  • Can change heterochromatin to euchromatin, and vice versa, via histone protein modifications.

Acetylation
  • Addition of a two-carbon molecule (acetyl group) to histones.

  • Use HATs (histone acetyltransferases) to add an acetyl group to heterochromatin.

  • HATs loosen the DNA-histone interaction, increasing transcription.

  • Heterochromatin to euchromatin.

Deacetylation
  • Removal of an acetyl group from euchromatin.

  • Use HDACs (histone deacetylases) to remove the acetyl group.

  • HDACs tighten the DNA-histone interaction, decreasing transcription.

  • Euchromatin to heterochromatin.

Chromosomal Mutations

  • See lecture 13 video for more details.


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Cell Cycle and Mitosis Notes

Cell Cycle

Overview

  • The cell cycle is how two cells become one.
  • Two phases:
    • Mitosis: Active cell division.
    • Interphase: Time when cell grows and prepares for cell division.

Interphase

  • Subdivided into G1, S phase, and G2.
  • G1 Phase:
    • Cell is "chilling", carrying out normal metabolic functions, and growing.
  • S Phase:
    • DNA replication occurs.
  • G2 Phase:
    • Cell readies for mitosis, and continues to grow.

Cell Cycle Schematic

  • M phase: Active cell division.
  • Interphase: G1, S, and G2 phases.
  • G1: Normal metabolic function and growth.
  • S phase: DNA synthesis/replication.
  • G2: Getting ready for mitosis and more growth.

G0 Phase

  • When a cell enters G0, it is not dividing or thinking about dividing.
  • Examples: Muscle cells and neurons.
  • These cells have "left the cell cycle", typically from G1.

Why Divide?

  • To produce reproductive cells (gametes).
  • For organism growth.
  • To repair.
  • To maintain tissue.

How: Organizing a Signal

  • A signal tells the cell it's time to divide.
  • Examples:
    • Growth factors.
    • Interleukins (IL).
    • Erythropoietin.
    • Hormones (e.g., estrogen).
  • The signal stimulates movement from G1 into S phase.
  • S phase: Duplication of DNA.
  • G2: Getting ready for mitosis.
    • Duplicating organelles.
    • Duplicating centrosomes.
    • Producing microtubules for the mitotic spindle.
  • M phase: Active cell division.

M Phase (Mitosis) - Detailed

  • PPMAT: Prophase, Prometaphase, Metaphase, Anaphase, Telophase, and Cytokinesis

Prophase

  • Chromosomes condense.
  • Genetic information is packed into chromosomes.
Chromosome Composition
  • Chromosomes are made up of chromatin.
  • Chromatin is comprised of DNA and proteins.
  • DNA and proteins make up nucleosomes (beads on a string).
  • Nucleosomes are made of DNA and histones.
  • Histones:
    • H1 (linker histone).
    • H2A, H2B, H3, and H4 (core histones).
Nucleosome Structure
  • Core histones: Two of each (H2A, H2B, H3, H4) = 8 total.
  • DNA wrapped around core histones: 146 base pairs.
  • H1 linker histone connects everything together.
Chromatin Compaction
  • Multiple nucleosomes compact further.
  • H1 histones link together to form a solenoid.
  • Solenoid compacts into a 300 nm structure.
  • Further compaction into loop structures.
  • Final structure: Duplicated chromosome (after S phase).

Chromatin, Sister Chromatids, and Duplicated Chromosomes

  • Chromatin: Tightly wound DNA and proteins.
  • Sister chromatid: Each arm of the duplicated chromosome.
  • Two sister chromatids make up the duplicated chromosome.

Centromere and Kinetochore

  • Centromere: Area linking two sister chromatids together.
  • Kinetochore: Area on the centromere where mitotic spindle fibers attach.

Mitotic Spindle Formation

  • Mitotic spindle is made of microtubules.
  • Microtubules come from the centriole.
  • Centrioles originate from the centrosome (home of the centriole).

Nucleolus Disappearance

  • Nucleolus disappears during prophase.
  • The nucleolus is the area in the nucleus where ribosomes are assembled.
  • Because chromosomes are condensed, transcription and translation are not occurring, so ribosomes are not needed.

Prometaphase

  • Nuclear envelope/membrane breaks down and disappears.
  • Mitotic spindle is fully formed and starts to attach to chromosomes, not fully attached.
  • Kinetochore microtubules attach to kinetochores on the centromeres.

Metaphase

  • Chromosomes line up in the middle of the cell on the equatorial plate.

Anaphase

  • Separated chromosomes (sister chromatids) move to opposite ends/poles of the cell.

Telophase

  • Nuclear membrane reforms (disappeared in prometaphase).
  • Chromosomes have moved to opposite ends.

Cytokinesis

  • Division of the cytoplasm.
  • Two cells become one.
  • Uses contractile microfilaments called actin and myosin.
  • Contractile ring forms in the center and pinches the cell in two.

Additional Important Vocabulary

Euchromatin vs. Heterochromatin

  • Euchromatin:
    • Active form.
    • Replicating, transcribing, translating.
    • Not wrapped around nucleosomes, so it is available.
  • Heterochromatin:
    • Inactive form.
    • Wrapped around nucleosomes.

Histone Protein Modifications

  • Can change heterochromatin to euchromatin, and vice versa, via histone protein modifications.
Acetylation
  • Addition of a two-carbon molecule (acetyl group) to histones.
  • Use HATs (histone acetyltransferases) to add an acetyl group to heterochromatin.
  • HATs loosen the DNA-histone interaction, increasing transcription.
  • Heterochromatin to euchromatin.
Deacetylation
  • Removal of an acetyl group from euchromatin.
  • Use HDACs (histone deacetylases) to remove the acetyl group.
  • HDACs tighten the DNA-histone interaction, decreasing transcription.
  • Euchromatin to heterochromatin.

Chromosomal Mutations

  • See lecture 13 video for more details.