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4.3.6Genetics, Variation and Interdependence

Meiosis Under a Microscope — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 19 of 46 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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4.3.6 Meiosis Under a Microscope — AQA A Level Biology 7402 notes: Identify the stages of meiosis in micrographs and drawings, and use them to describe what is happening to the chromosomes.Meiosis can be observed in cells taken from the reproductive tissues, such as the anthers o…

  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Magnification
  • Mitotic index
  • Show the outline of the cell and the chromosomes, but do not draw every detail

Section 1 of 9Observing meiosis

Meiosis can be observed in cells taken from the reproductive tissues, such as the anthers of a flowering plant (for example lily) or the testes of a locust.

Observing meiosis
The tissue is fixed,
  • so that the cells are stopped in their stage of division,
  • and then treated with a stain that binds to DNA, such as acetic orcein or toluidine blue.
The tissue is gently squashed under a coverslip, so that a thin single layer of cells is spread out and can be viewed under a light microscope.
The cells appear in different stages, because they divide at different times, so by scanning a slide you can see all the stages.

Section 2 of 9Recognising stages

StageWhat you see
Prophase IChromosomes condensed, visible as thread-like structures; homologous pairs lie side by side, with chiasmata visible; no nuclear envelope near the end
Metaphase IPairs (bivalents) lined up along the equator of the cell
Anaphase IWhole chromosomes, each still made of two chromatids, moving to opposite poles
Telophase I / interkinesisTwo groups of chromosomes at opposite ends; the cell may be dividing into two
Metaphase IITwo cells, each with chromosomes lined up singly on the equator
Anaphase IIChromatids (now chromosomes) pulled apart to the poles in each cell, separate V shapes
Telophase IIFour groups of chromosomes; four cells forming

A key difference from mitosisin meiosis I the chromosomes are in pairs at the equator, whereas in mitosis they are single and not paired.

Section 3 of 9Drawing and labelling

Scientific drawings should be done in pencil, with clear single lines and no shading.

1
Show the outline of the cell and the chromosomes, but do not draw every detail.
2
Use label lines drawn with a ruler that do not cross.
3
Write the magnification or include a scale bar.
4
Choose cells that are clearly in a single stage, and name the stage in the title.

To calculate the actual size of a chromosome or cell: magnification = image size / actual size. For example, if a drawn cell measures 60 mm and the magnification is x400, its actual size is 60 / 400 = 0.15 mm = 150 micrometres. Make sure that the units agree before dividing.

Section 4 of 9Counting cells to estimate duration

If you count the cells in each stage in a large sample, you can estimate the relative time spent in each: the proportion of cells in a stage is approximately the proportion of the total time spent in that stage.

Counting cells to estimate duration
For instance, if 80 of 200 cells counted are in prophase I, then prophase I takes about 40 per cent of the time.
Prophase I is long in meiosis because of the pairing of chromosomes and crossing over.
A mitotic index can be calculated in the same way for mitosis.

Sources of error include counting too few cells, choosing the area of tissue where cells divide most (not representative), and difficulty in assigning cells to a stage when they are between two stages. Counting several fields of view and calculating a mean improves reliability.

Section 5 of 9Links

This lesson ties the stages of meiosis to the practical skills of microscopy and drawing. It also links to the comparison of meiosis and mitosis, as many questions give a micrograph and ask you to decide which type of division is shown, from the pairing of homologous chromosomes or their position at the equator.

Section 6 of 9Interpreting micrographs of dividing cells

In an exam you may be given a micrograph or diagram and asked to identify the stage and explain your reasoning.

1
Work through a checklist.
2
First, ask whether the chromosomes are visible as separate structures, or whether the nucleus is intact (interphase).
3
Second, ask whether chromosomes are in homologous pairs (a sign of prophase I or metaphase I) or are single.
4
Third, look at the position: on the equator (metaphase), moving apart (anaphase) or at the poles (telophase).
5
Fourth, count the number of cells and the number of chromosomes in each; for meiosis II look for two cells, each with half the original number of chromosomes.

When describing what is happening, use correct terms such as bivalent, chiasma, spindle fibre and centromere, and explain the significance, for example that crossing over at a chiasma exchanges alleles.

Section 7 of 9Exam-style questions

  1. Describe how you would prepare a slide to observe the stages of meiosis in an anther. [4]4 marks

  2. A micrograph shows cells with chromosomes in pairs on the equator. Identify the stage and state how you know it is meiosis and not mitosis. [3]3 marks

  3. In a sample of 250 cells, 100 are in prophase I and 25 in metaphase I. Calculate the percentage in each, and suggest what this shows about the duration of prophase I. [3]3 marks

Section 8 of 9Key terms

TermMeaning
FixingTreating tissue to stop division and preserve the structure.
SquashPreparation where tissue is pressed to a thin layer for viewing.
OrceinA stain that binds to DNA and colours chromosomes.
BivalentA pair of homologous chromosomes aligned in meiosis I.
MagnificationImage size divided by actual size.
Scale barA line on a diagram showing a known length.
Mitotic indexThe proportion of cells in a tissue that are undergoing mitosis.

Section 9 of 9Exam technique

1
Describing chromosomes lined up in pairs as mitosis.
2
Writing that cells are stained to kill them. Fixing does that; the stain makes chromosomes visible.
3
Calculating magnification without converting units (mm to micrometres).
4
Drawing shaded or sketchy outlines in a biological drawing.
5
Assuming the proportion of cells in a stage gives actual time without noting it assumes the stages are independent of each other and sampling is random.

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