Cell Division, Reproduction & Development — Problem set, Grade 10

Connect chromosome continuity, variation, regulated growth, and specialization.

Review the key ideas

Mitosis & the Cell Cycle

In the standard cycle, S phase copies nuclear DNA before mitosis separates sister chromatids. Cytokinesis partitions the cell. Daughter nuclei normally retain corresponding chromosome sets; this is distinct from a guarantee of identical cells or error-free copying.

Meiosis & Sexual Reproduction

Meiosis separates homologs and then sisters after one replication, normally reducing chromosome-set count in four nuclear products. Gamete maturation and life cycles vary. Recombination, assortment and fertilization contribute to varied inherited combinations.

Differentiation, Growth & Cell-Cycle Control

Growth requires regulated division and differentiation; differences in gene expression help produce specialized cells.

Reproduction & Development

Reproduction transmits genetic information; development uses regulated division, expression, and interactions to build an organism.

Problem 1

What is the main purpose of mitosis in a multicellular organism like a human?
Answer and explanation

To produce new, genetically identical cells for growth, repair, and replacing worn-out cells.

Mitosis makes two daughter cells that are exact genetic copies of the parent cell. This lets the body grow, heal wounds, and replace cells (like skin and blood cells) that wear out, all with matching, correctly working cells.

Problem 2

A cell with 30 chromosomes copies its DNA during interphase and then divides by mitosis. (a) How many chromatids are present just before the cell divides? (b) How many chromosomes are in each daughter cell?
Answer and explanation

(a) 60 chromatids; (b) 30 chromosomes each.

(a) Copying the DNA turns each of the 30 chromosomes into 2 sister chromatids, so there are 30 x 2 = 60 chromatids before division. (b) Mitosis separates the chromatids evenly and preserves the chromosome number, so each daughter cell ends up with the full 30 chromosomes.

Problem 3

A fruit fly body cell has 8 chromosomes. How many chromosomes are in a fruit fly gamete, and which process makes gametes?
Answer and explanation

4 chromosomes, produced by meiosis.

Gametes are haploid — they carry half the diploid number. Half of 8 is 4, so each fruit fly gamete has 4 chromosomes. Meiosis is the division that halves the chromosome number to make gametes.

Problem 4

List and briefly describe two ways meiosis creates genetic variation among gametes.
Answer and explanation

Crossing over (homologous chromosomes swap segments) and independent assortment (homologous pairs separate randomly).

Crossing over: during the first division, homologous chromosomes exchange matching segments, creating chromosomes with new mixes of alleles. Independent assortment: each pair of homologous chromosomes lines up and separates independently of the others, so gametes get many different combinations of the parents' chromosomes. (Random fertilization then adds even more variety.)

Problem 5

Of 400 cells examined in a growing tissue, 24 are in anaphase. If the cycle lasts 25 hours, estimate anaphase's duration in hours.
Answer and explanation

1.5 hours

24/400 = 0.06 of the cycle; 0.06 × 25 = 1.5 hours.

Problem 6

A student concludes that interphase is "the resting stage" because the cell looks unchanged under the microscope. Correct this.
Answer and explanation

Interphase is the most active part of the cycle, not a rest. The cell grows, builds organelles and proteins in G1, replicates its entire DNA in S, and prepares the structures needed for division in G2. The appearance of inactivity reflects the limits of light microscopy, which cannot resolve replication or protein synthesis, so a static image is being mistaken for a static cell.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 7

Chapter review: A diploid model starts with 10 chromosomes. Track chromosomes, chromatids and daughter-cell chromosome number through replication and normal mitosis/cytokinesis.
Answer and explanation

After S phase there are 10 replicated chromosomes and 20 chromatids; during separation each former chromatid becomes a chromosome. Each daughter receives 10 chromosomes, preserving diploidy in the model.

After S phase there are 10 replicated chromosomes and 20 chromatids; during separation each former chromatid becomes a chromosome. Each daughter receives 10 chromosomes, preserving diploidy in the model.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 8

An organism has 6 pairs of homologous chromosomes. Independent assortment alone allows how many gamete combinations?
Answer and explanation

64

2⁶ = 64 combinations, and crossing over increases the actual variety well beyond this.

Problem 9

A gamete is found to carry two copies of chromosome 21 instead of one. Explain what went wrong and at which stage.
Answer and explanation

A pair failed to separate correctly during meiosis — nondisjunction — so one gamete received both members of that pair and another received none. If it happened in meiosis I the homologous pair did not separate; if in meiosis II the two copies of one chromosome did not. A zygote formed from such a gamete would have three copies of chromosome 21 rather than two, which is why correct separation at both divisions matters.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 10

Chapter review: A diploid model has 2n=6. Give chromosome and chromatid counts after S phase, after meiosis I per nucleus and after meiosis II per nucleus.
Answer and explanation

After S: 6 chromosomes and 12 chromatids. After I: 3 replicated chromosomes and 6 chromatids per nucleus. After II: 3 unreplicated chromosomes per nucleus. The standard model includes one replication before two divisions.

After S: 6 chromosomes and 12 chromatids. After I: 3 replicated chromosomes and 6 chromatids per nucleus. After II: 3 unreplicated chromosomes per nucleus. The standard model includes one replication before two divisions.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 11

Explain why “all cells have the same DNA, so all cells must do the same job” fails.
Answer and explanation

Expression and regulatory conditions differ; DNA presence alone does not mean every gene is used in the same way.

Problem 12

A muscle cell and a nerve cell in the same person contain the same genes yet look and behave differently. Explain how, and name what would have to be measured to support your explanation.
Answer and explanation

Both cells carry the same genome, but different subsets of genes are transcribed in each, so each builds a different set of proteins and therefore has different structures and capabilities. Differentiation is a difference in gene expression rather than in gene content. Supporting this requires measuring which messenger RNAs or proteins are present in each cell type: the same genes should be detectable in both genomes, while the transcripts and proteins should differ substantially.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 13

A population of cells doubles every 18 hours. Starting from 400 cells, how many are present after 54 hours?
Answer and explanation

3200

54/18 = 3 doublings, so 400 × 2³ = 400 × 8 = 3,200 cells.

Problem 14

Chapter review: Two samples share the same DNA sequence at a tested gene but show different RNA amounts and cell functions. Explain a possible mechanism and one limit of the evidence.
Answer and explanation

Different regulation can change expression without changing that tested sequence. Other genes, processing and cellular conditions can contribute; matching one gene does not establish complete genome identity or a unique cause.

Different regulation can change expression without changing that tested sequence. Other genes, processing and cellular conditions can contribute; matching one gene does not establish complete genome identity or a unique cause.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 15

Connect meiosis, fertilization, and development in three sentences.
Answer and explanation

Meiosis produces haploid gametes in the animal model. Fertilization combines gametes to form a diploid zygote. Regulated division, differentiation, and organization develop tissues and organs.

Problem 16

A zygote divides every 12 hours with no cell loss. How many cells are present after 3 days?
Answer and explanation

64

Three days is 72 hours, which is 6 doublings: 2⁶ = 64 cells.

Problem 17

Explain why a zygote's first few divisions produce a cluster of cells without the embryo growing much larger, and what that tells you about the cell cycle at that stage.
Answer and explanation

The early divisions divide the existing cytoplasm among more cells rather than adding new material, so cell number rises while total size changes little and each cell becomes smaller. This indicates that the growth portions of the cell cycle are brief or largely skipped at this stage, with replication and division following each other rapidly. Growth in overall size comes later, once the embryo takes in materials from its surroundings.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 18

Chapter review: A supplied embryo model divides from 1 to 8 cells while total material is initially comparable. Explain cell number versus size, and how different cell fates could arise without new mutations.
Answer and explanation

Cleavage partitions material among more, smaller cells rather than requiring eightfold growth. Positional signals and gene regulation can change cell fate without new DNA-sequence changes; structure/function evidence must test those outcomes.

Cleavage partitions material among more, smaller cells rather than requiring eightfold growth. Positional signals and gene regulation can change cell fate without new DNA-sequence changes; structure/function evidence must test those outcomes.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the biological mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
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