The Cell Cycle & Mitosis
- Describe the events of interphase (G1, S, G2) and identify when DNA is copied
- Order the four phases of mitosis and the key event of each
- Distinguish mitosis (nuclear division) from cytokinesis (cell division)
The cell cycle: interphase then division
A dividing cell runs a repeating cell cycle with two big parts. Interphase is the long growth-and-preparation stretch, split into three sub-phases: G1 (the cell grows and makes proteins and organelles), S (the cell synthesizes DNA — every chromosome is copied), and G2 (the cell grows more and readies the machinery for division). Then comes the M phase (mitosis + cytokinesis), the brief period when the copied contents are split into two cells. A cell spends most of its life in interphase; division is quick by comparison.
What S phase produces: sister chromatids
The single most important event of interphase happens in S phase: the cell replicates all of its DNA. Afterward, each chromosome consists of two identical copies — sister chromatids — joined together at a constricted region called the centromere. Note the count: replication does not change the number of chromosomes, only how many chromatids each one has. A human cell still has 46 chromosomes after S phase; each is now a doubled X-shape held together at its centromere, waiting to be pulled apart during division.
The four phases of mitosis
Mitosis divides the nucleus in four ordered phases, easy to remember as PMAT. Prophase — chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and the spindle (made of microtubules) begins to form. Metaphase — chromosomes line up single-file along the cell’s metaphase plate (the middle). Anaphase — sister chromatids are pulled apart to opposite poles, so each pole gets one full copy of every chromosome. Telophase — chromosomes arrive at the poles, two new nuclear envelopes form, and the chromosomes begin to de-condense. Mitosis is nuclear division; the cytoplasm has not yet split.
Cytokinesis: finishing the split
Cytokinesis is the division of the cytoplasm that produces two separate daughter cells, usually overlapping with telophase. It looks different in the two cell types. In animal cells, a ring of protein filaments pinches the cell inward, forming a cleavage furrow that tightens until the cell splits in two. In plant cells, the rigid cell wall cannot pinch, so vesicles gather in the middle and fuse to build a new cell plate that grows outward into a dividing wall. Either way, the result is two genetically identical daughter cells, each with a full chromosome set.
A skin cell in a person has 46 chromosomes. Track the chromosome and chromatid count as it goes through S phase and then completes mitosis and cytokinesis. How many chromosomes does each daughter cell have?
- 1.Before S phase: 46 chromosomes, each a single chromatid.
- 2.After S phase: DNA is replicated, so still 46 chromosomes, but each now has 2 sister chromatids joined at the centromere.
- 3.During anaphase: sister chromatids separate to opposite poles, so each pole receives 46 chromatids — now counted as 46 chromosomes.
- 4.After cytokinesis: the cell splits into two, and each daughter cell ends with 46 single-chromatid chromosomes, identical to the original.
During which part of interphase is a cell’s DNA replicated so that each chromosome ends up with two sister chromatids?
Replication does not double the chromosome number. After S phase a human cell still has 46 chromosomes — each simply now has two sister chromatids. Chromatids are not counted as separate chromosomes until they split apart at the centromere in anaphase.
In which phase of mitosis do sister chromatids separate and move to opposite poles of the cell?
Remember mitosis as PMAT and pair each phase with its one signature event: Prophase = condense, Metaphase = line up in the Middle, Anaphase = Apart, Telophase = Two nuclei. Then cytokinesis makes two whole cells.
Answer the 2 checkpoints as you read.
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