The cell cycle is the ordered sequence of events by which a cell duplicates its contents and divides into two daughter cells. It consists of interphase (G1, S, and G2 phases) and the mitotic phase. During interphase, the cell grows, replicates its DNA, and prepares for division.
The S phase is when DNA synthesis doubles the chromosome number from diploid to tetraploid before segregation.
Mitosis itself has four stages: prophase, metaphase, anaphase, and telophase. Chromosomes condense and become visible in prophase, align at the cell equator in metaphase, are pulled to opposite poles in anaphase, and decondense as new nuclei form in telophase. Cytokinesis then splits the cytoplasm to produce two genetically identical daughter cells.
Cell cycle checkpoints at G1, G2, and the spindle assembly checkpoint ensure accuracy - errors at these points can lead to cancer.
Understanding Cell Cycle & Mitosis
DNA is not copied as one long loose thread. It is packaged around proteins called histones, which help it fit inside the nucleus. During copying, the two DNA strands separate and each acts as a template for a new matching strand.
Enzymes check much of this work, but small mistakes can still occur. After copying, each chromosome consists of two sister chromatids carrying nearly identical DNA.
A useful detail is that chromosome number is usually counted by centromeres. Therefore, copying DNA doubles the amount of DNA without necessarily changing the chromosome count by that method.
The spindle is a moving machine built from protein tubes called microtubules. These tubes grow from opposite sides of the cell and connect to protein structures called kinetochores at each centromere. Correct attachment means that the two sister chromatids are linked to opposite sides.
The cell senses tension created by this arrangement. If a chromosome is attached incorrectly, the checkpoint can delay separation.
This matters because an uneven split can give one new cell extra chromosomes while the other receives too few. Such chromosome errors can disrupt normal cell function.
Division involves more than separating DNA. Each new cell needs enough cytoplasm, cell membrane, and organelles to survive. In animal cells, a ring of protein filaments tightens around the middle of the cell.
It pinches the membrane inward until two cells separate. Plant cells cannot pinch in the same way because of their rigid cell walls. Instead, membrane materials build a cell plate across the center.
The plate develops into a new section of cell wall. This difference is often shown in diagrams and helps explain why plant and animal cell division look different.
Cell division is controlled by proteins called cyclins and cyclin dependent kinases. Their levels rise and fall, giving the cell signals to move forward, pause, or stop. A cell with severe DNA damage may repair the damage, enter a nondividing state, or die by a controlled process called apoptosis.
These controls are important in real life during growth, wound healing, and replacement of worn out skin cells. When studying this topic, keep chromosome, chromatid, and DNA amount separate in your mind.
Trace one chromosome through a diagram and note what happens to its copies. This prevents many common mistakes, especially when comparing mitosis with meiosis.
Key Facts
- Interphase = G1 (growth) + S (DNA replication) + G2 (prep for division)
- Mitosis phases: Prophase, Metaphase, Anaphase, Telophase (PMAT)
- Sister chromatids are joined at the centromere; spindle fibers pull them apart in anaphase
- Mitosis produces 2 genetically identical diploid daughter cells
- G1 checkpoint: checks cell size and DNA integrity before committing to division
- Spindle assembly checkpoint: ensures all chromosomes are attached before anaphase
Vocabulary
- Chromatin
- The complex of DNA and proteins that makes up chromosomes; decondensed during interphase and condensed during mitosis.
- Centromere
- The constricted region of a chromosome where sister chromatids are joined and where spindle fibers attach.
- Spindle fiber
- A microtubule that forms during cell division and moves chromosomes toward opposite poles of the cell.
- Cytokinesis
- The division of the cytoplasm that follows mitosis, producing two separate daughter cells.
- Checkpoint
- A regulatory control point in the cell cycle that ensures conditions are suitable before the cell proceeds to the next phase.
Common Mistakes to Avoid
- Confusing mitosis with meiosis. Mitosis produces 2 identical diploid cells (for growth and repair); meiosis produces 4 genetically diverse haploid cells (for reproduction).
- Thinking DNA replication occurs during prophase. DNA is replicated during S phase of interphase, well before mitosis begins.
- Assuming cytokinesis is part of mitosis. Cytokinesis is a separate process that divides the cytoplasm; mitosis only refers to nuclear division.
- Forgetting that 'tetraploid' is a transient state. After S phase the cell has 4n DNA content, but after mitosis each daughter has 2n again - the ploidy level is restored.
Practice Questions
- 1 A cell begins mitosis with 46 chromosomes (human). How many chromosomes does each daughter cell contain after mitosis and cytokinesis?
- 2 During which phase of the cell cycle does DNA replication occur, and what enzyme is responsible for synthesizing the new DNA strand?
- 3 Explain what would happen if the spindle assembly checkpoint failed and a chromosome did not attach to a spindle fiber before anaphase.