Cell Cycle: Interphase and Cell Division

• Cells grow to a size limit then either stop or divide.
• Most cells will eventually divide.
• Smaller cells are better as energy requirements are lower.
• Smaller cells allow better transport of substances, movement, and communication.

Cell Cycle

• Cells reproduce via growth/division, otherwise called the cell cycle.
• One cell will divide into two
• Cells divide for growth, development, and to replace old/damaged cells.
• The three main stages are interphase, mitosis, and cytokinesis.

Cell Cycle - Interphase

• Interphase is the stage where the cell grows, performs regular functions, and prepares for cell division by copying its DNA
• Interphase has G1, Synthesis, and G2 phases.

Phases of Interphase

• G1 Phase happens right after cell division and is the longest stage, known as "the first growth phase."
• Cells grow and perform regular functions during G1 phase.
• Muscle and nerve cells leave the cell cycle during G1 and enter G0 phase where they don't divide.
• S Phase is when a cell copies its DNA to prepare for division
• Chromatin is the relaxed form of DNA in the nucleus.
• Chromosomes are the condensed form of DNA in the nucleus.
• A checkpoint at the end of the S phase ensures no major mistakes, so it goes to the next stage.
• G2 Phase is the second growth phase when the cell prepares to divide its nucleus.
• Centrioles are synthesized in G2.
• Organelles are duplicated in G2.
• Another checkpoint occurs at the end of G2 to ensure there are no mistakes before proceeding.

Mitosis & Cytokinesis

• Mitosis is the nuclear division in eukaryotic cells.
• The purpose of mitosis is to accurately divide the cells replicated DNA
• Two genetically identical daughter cells are produced during mitosis
• Purpose of mitosis is growth/development, and repair of dead/damaged cells.
• Cytokinesis is the cytoplasmic division at the end of mitosis

Stages of Mitosis

• Mitosis has four stages, followed by cytokinesis.
• These stages in order are prophase, metaphase, anaphase, and telophase.
• Cytokinesis can sometime be separated from mitosis

Mitosis - Prophase (1st stage)

• Prophase is the first and longest phase of mitosis.
• Chromatin condenses into visible chromosomes
• Chromosomes exists at sister chromatids, which contain identical DNA copies held together by a centromere
• Organization is the importance of the centromere
• Centrioles migrate to opposite poles
• Spindle fibers start forming and attach to sister chromatids
• Spindle apparatus forms during prophase
• The nuclear membrane breaks down/disintegrates

Mitosis - Metaphase (2nd stage)

• Metaphase is the second and shortest stage of mitosis.
• In metaphase, sister chromatids are pulled by the spindle to the cell's center.
• This ensures new cells have the correct chromosome number and amount.

Mitosis - Anaphase (3rd stage)

• Anaphase is the third mitosis stage when chromatids are pulled apart.
• Spindle microtubules shorten and pull at centromeres.
• The sister chromatids separate into two chromosomes.

Mitosis - Telophase (4th stage)

• Telophase occurs when chromosomes arrive at the poles and decondense back into chromatin
• Nuclear membrane begins reforming
• The spindle apparatus & centrioles disassemble
• The cell begins pinching into two cells.
• The organelles fully migrated to each pole

Cytokinesis

• Cytokinesis is the division of the cytoplasm
• In animal cells microfilaments constrict and pinch the cytoplasm, forming a cleavage furrow
• Eventually, the cell will split in two.
• In plant cells a cell plate forms between the two daughter nuclei to split the cell.

Cell Cycle Checkpoints

• Checkpoints are surveillance mechanisms that monitor the order, integrity, and accuracy of stages in the cell cycle.
• Checkpoints ensure correct growth, DNA replication, chromosome integrity, and accurate segregation.