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 and Mitosis
Chromosome diagrams can be confusing because they show DNA at its most tightly packed. A replicated chromosome has two matching sister chromatids. Each chromatid contains one complete DNA molecule.
The sisters remain connected by proteins at the centromere until the moment of separation. Scientists usually count chromosomes by centromeres. This means a cell can contain duplicated DNA without having twice its usual chromosome count.
Once sister chromatids separate, each is counted as an individual chromosome. Keeping these definitions clear prevents a common mistake in cell cycle diagrams and exam questions.
Accurate division depends on a moving structure called the mitotic spindle. It is built from microtubules, which are protein tubes that can grow or shorten quickly. In many animal cells, microtubules grow outward from two centrosomes positioned at opposite ends of the cell.
They connect to protein structures called kinetochores at each centromere. A chromosome must attach to microtubules from opposite sides. This creates tension across the chromosome.
If only one side is attached, the cell holds back separation. The spindle does not simply pull chromosomes randomly. It uses attachment, tension, and controlled shortening of microtubules to send one complete set of genetic instructions to each new cell.
Cell cycle controls rely on chemical signals made by proteins. Cyclins rise and fall in amount during the cycle. They work with enzymes called cyclin dependent kinases to switch important cell activities on or off.
When DNA is damaged, proteins can stop the cycle and allow repair enzymes time to work. If the damage is too serious, a cell may be directed to die through programmed cell death.
This protects the body from cells that could pass harmful mutations to later generations of cells. Cancer can develop when genes controlling these signals are changed, allowing damaged cells to keep dividing.
Mitosis matters throughout life. Skin cells use it to replace cells lost from the surface. Cells near a cut divide to help repair tissue.
Bone marrow cells divide continuously to make new blood cells. Plant root tips are another useful place to observe mitosis because growth is active there. Cytokinesis differs between major groups of organisms.
Animal cells form a pinching ring that pulls the cell membrane inward. Plant cells build a cell plate in the middle, which develops into a new cell wall.
Mitosis is different from meiosis, the division process that produces eggs and sperm with half the usual chromosome set. When studying images, track one chromosome pair through each stage and focus on where the centromeres move.
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.