🧬 Biology · Genetics

Mitosis and the Cell Cycle practice questions

Questions start easy and step up. Before you answer, rate how sure you are. It trains you to know what you really know.

Answer key with explanations

1. In which part of the cell cycle is a cell's DNA copied?

   Answer: S phase. The S in S phase stands for synthesis: the DNA is copied then. G1 and G2 are gap phases for growth and preparation, and prophase comes after the copying is already done.

2. Which list shows the four stages of mitosis in the correct order?

   Answer: Prophase, metaphase, anaphase, telophase. The order is prophase, metaphase, anaphase, telophase (PMAT). The chromosomes must be lined up (metaphase) before the sister chromatids can be pulled apart (anaphase), and the new nuclei form last (telophase). The other lists swap stages out of order.

3. Which statement about the cell cycle is correct?

   Answer: Interphase includes G1, S and G2, and in a typical dividing cell it takes up more of the cycle than M phase does.. Interphase is made of G1, S and G2, and it is the longest part of the cycle for a typical dividing cell. Mitosis is only part of M phase, interphase is a time of growth and DNA copying, and the cycle includes interphase as well as PMAT.

4. A cell is viewed under a microscope. Its chromosomes are in a single line across the middle of the cell, and each one is attached to spindle fibers coming from both poles. Which stage is this?

   Answer: Metaphase. Chromosomes lined up along the middle of the cell, attached to spindle fibers from both poles, are the signature of metaphase. In prophase they are not yet lined up. In anaphase they have already separated, and in telophase they are at the poles in two new nuclei.

5. Which statement correctly describes the two daughter cells made by mitosis and cytokinesis?

   Answer: They are genetically identical to each other, and each has the same chromosome number as the parent cell.. S phase copies every chromosome, and anaphase sends one sister chromatid of each chromosome to each pole. So each daughter cell has a full set, identical to the other and to the parent. Halving the number, and making four cells, describe meiosis, not mitosis.

6. An onion root-tip cell has 16 chromosomes in G1. How many sister chromatids does it have at metaphase?

   Answer: 32. DNA is copied in S phase, so by metaphase each of the 16 chromosomes has two sister chromatids: 2 × 16 = 32. The number 16 is the number of chromosomes, which does not change at metaphase. The number 8 would be half, and 64 would double the 32 a second time.

7. In a dividing plant cell, what builds the new wall between the two daughter nuclei?

   Answer: A cell plate formed from vesicles. A plant cell has a stiff wall, so vesicles carrying wall material fuse in the middle to form a cell plate, which grows outward into a new wall. A cleavage furrow is how animal cells pinch in two. The metaphase plate is only an imaginary plane where chromosomes line up earlier. A nuclear envelope surrounds a nucleus and is not a cell wall.

8. In late anaphase, an imaginary cell has 6 separated chromosomes at each pole (count one chromosome per centromere). How many chromosomes did this cell have in G1?

   Answer: 6. Each pole receives one copy of each of the cell's chromosomes, so the number at each pole equals the number the cell had in G1. The cell holds 12 chromosomes in total at this moment (6 at each pole), but that is twice its G1 number. The number 3 would be half, and 24 would double the total again.

9. A cell has its usual number of chromosomes in G1. At which point does the cell hold twice that number of chromosomes (count one chromosome per centromere)?

   Answer: At the start of anaphase. Counting by centromeres, the number doubles only when the centromeres split at the start of anaphase. At the end of S phase the DNA and the number of chromatids have doubled, but the sister chromatids still share centromeres, so the chromosome number is unchanged. In prophase the counts are the same as at the end of S phase. After cytokinesis is complete, each daughter cell is back to the usual number.

10. Which statement about cytokinesis is correct in the strict textbook model?

    Answer: It is the division of the cytoplasm, and it usually begins in late anaphase or telophase, so it overlaps the end of mitosis.. Cytokinesis divides the cytoplasm so that the two nuclei end up in separate cells, and it overlaps late anaphase and telophase. It is not part of interphase, it does not come between prophase and metaphase, and animal cells use a cleavage furrow to do it.