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.
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.
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.
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.
To produce reproductive cells (gametes).
For organism growth.
To repair.
To maintain tissue.
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.
PPMAT: Prophase, Prometaphase, Metaphase, Anaphase, Telophase, and Cytokinesis
Chromosomes condense.
Genetic information is packed into chromosomes.
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).
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.
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: Tightly wound DNA and proteins.
Sister chromatid: Each arm of the duplicated chromosome.
Two sister chromatids make up the duplicated chromosome.
Centromere: Area linking two sister chromatids together.
Kinetochore: Area on the centromere where mitotic spindle fibers attach.
Mitotic spindle is made of microtubules.
Microtubules come from the centriole.
Centrioles originate from the centrosome (home of the centriole).
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.
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.
Chromosomes line up in the middle of the cell on the equatorial plate.
Separated chromosomes (sister chromatids) move to opposite ends/poles of the cell.
Nuclear membrane reforms (disappeared in prometaphase).
Chromosomes have moved to opposite ends.
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.
Euchromatin:
Active form.
Replicating, transcribing, translating.
Not wrapped around nucleosomes, so it is available.
Heterochromatin:
Inactive form.
Wrapped around nucleosomes.
Can change heterochromatin to euchromatin, and vice versa, via histone protein modifications.
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.
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.
See lecture 13 video for more details.
Cell Cycle and Mitosis Notes