Biology & life sciences · 01 / 12
Cells and DNA: levels of organisation
Middle school · Worked example and 4 exercises with solutions, at your pace.
The principle, simply
A cell is a unit of life. Eukaryotic cells contain much of their DNA in a nucleus; prokaryotes have no nucleus. Chromosomes organise DNA with proteins. A gene is a region of DNA; alternative versions are alleles. Cells, chromosomes and genes describe different scales.
A simplified eukaryotic cell. Most DNA is nuclear; mitochondria also contain DNA.
- 01Organism
- 02Cell
- 03Chromosome
- 04Gene and alleles
WORKED EXAMPLE
From principle to reasoning
Think of nested folders: organism → cell → nucleus → chromosome → DNA region. This is a scale map, not a rule for every organism.
Solve a problem in this subject
- Frame a question
State the relationship to explain and its scale: molecule, cell, organism, population or geological time.
- Describe evidence
Give a measurable observation, unit, comparison and conditions before explaining it. Observation and inference do different jobs.
- Connect a mechanism
Use a precise concept and label arrows as production, transfer, action or transformation. Sequence alone does not prove causation.
- Answer with a limit
Connect evidence and knowledge to the question. Distinguish the simplified model from living systems and identify a missing condition.
YOUR TURN
01 · Try before checking
Are two variants of a gene two different chromosomes?
One hint
Separate an information location from its versions.
Reveal and understand the solution
No: they are alleles, potentially occupying the same locus on homologous chromosomes.
Compare method, conditions and answer; identify the exact part to revisit.
IMPROVE THROUGH PRACTICE
Three additional challenges with solutions
Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.
Does a plant cell have a wall instead of a membrane?
A hint to get started
Distinguish the living boundary from the outer layer.
Step-by-step solution
- It has both a plasma membrane and an outer wall.
- The membrane regulates exchange; the wall supports the cell.
- Animal cells have a membrane without a plant wall.
Which step did you miss? Retry without help, then explain why the method works.
Order cell, nucleus, chromosome and nucleotide from largest to smallest.
A hint to get started
A chromosome contains DNA and proteins.
Step-by-step solution
- Cell → nucleus → chromosome → nucleotide.
- In a eukaryotic cell, the nucleus contains chromosomes.
- A nucleotide is a DNA unit, not a tiny chromosome.
Which step did you miss? Retry without help, then explain why the method works.
Do all cells express the same genes at the same level?
A hint to get started
Separate stored information from used information.
Step-by-step solution
- Many cells share broadly the same genome.
- They express different gene sets and amounts.
- Regulation contributes to specialisation; there are exceptions to the shared-genome pattern.
Which step did you miss? Retry without help, then explain why the method works.
The trap to avoid
Not all DNA is nuclear and not every trait depends on one gene.
Without notes, can you…
- Explain the principle in your own words?
- Rebuild the diagram and its connections?
- Solve the exercise and justify the method?
Choosing a state saves it in this browser. This is your assessment, not a grade.