A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: The Cell Cycle and Mitosis — mark scheme explained
The short answer
In multicellular organisms, not all cells retain the ability to divide. However, those that do follow a specific sequence of events known as the cell cycle. This cycle is crucial for growth, development, and tissue repair. The cell cycle consists of two main phases: interphase and mitosis (M phase).
The question
Describe the stages of mitosis in a eukaryotic cell, including the key events that occur during each stage. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the four main stages of mitosis.
- S2
The four main stages are prophase, metaphase, anaphase, and telophase.
- S3
Step 2: Describe the key events in prophase.
- S4
During prophase, chromosomes condense and become visible. The nuclear envelope breaks down, and spindle fibers form.
- S5
Step 3: Describe the key events in metaphase.
- S6
In metaphase, chromosomes align at the equatorial plane of the cell, attached to spindle fibers via their centromeres.
- S7
Step 4: Describe the key events in anaphase.
- S8
During anaphase, sister chromatids separate and are pulled to opposite poles of the cell by the spindle fibers.
- S9
Step 5: Describe the key events in telophase.
- S10
In telophase, chromosomes decondense, nuclear envelopes re-form around the separated sets of chromosomes, and the cell prepares for cytokinesis.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the four main stages of mitosis.
- S2
The four main stages are prophase, metaphase, anaphase, and telophase.
- S3
Step 2: Describe the key events in prophase.
- S4
During prophase, chromosomes condense and become visible. The nuclear envelope breaks down, and spindle fibers form.
- S5
Step 3: Describe the key events in metaphase.
- S6
In metaphase, chromosomes align at the equatorial plane of the cell, attached to spindle fibers via their centromeres.
- S7
Step 4: Describe the key events in anaphase.
- S8
During anaphase, sister chromatids separate and are pulled to opposite poles of the cell by the spindle fibers.
- S9
Step 5: Describe the key events in telophase.
- S10
In telophase, chromosomes decondense, nuclear envelopes re-form around the separated sets of chromosomes, and the cell prepares for cytokinesis.
Final answer: Prophase: Chromosomes condense, nuclear envelope breaks down, spindle fibers form. Metaphase: Chromosomes align at the equatorial plane, attached to spindle fibers via centromeres. Anaphase: Sister chromatids separate and are pulled to opposite poles by spindle fibers. Telophase: Chromosomes decondense, nuclear envelopes re-form, cell prepares for cytokinesis.
Common mistakes
- Misidentifying stages of mitosis. — Practice identifying and describing the key events in each stage of mitosis. Use diagrams and labels to reinforce understanding.
- Forgetting the role of spindle fibers. — Always include the role of spindle fibers in your descriptions of mitosis, especially during anaphase. Spindle fibers are crucial for separating sister chromatids.
- Confusing cytokinesis in animal and plant cells. — Practice comparing and contrasting cytokinesis in animal and plant cells. Animal cells use a cleavage furrow, while plant cells form a cell plate.
- Misunderstanding the process of binary fission in prokaryotes. — Review the specific steps involved in binary fission, including DNA replication and cytoplasmic division. Use diagrams to help visualize the process.
- Failing to explain how uncontrolled cell division leads to cancer. — Practice explaining the normal regulation of cell division and what happens when these mechanisms fail. Emphasize the consequences of uncontrolled cell division, such as tumour formation and metastasis.
- Confusing the roles of different phases in interphase. — Review the specific events that occur during each phase of interphase. Use a timeline or flowchart to help organize the information.
Where the marks go
- Full worked solution (all marking points)4 marks