AQA A Level Biology

Revision Notes

4.3.6 Meiosis Under a Microscope

Apparatus & Techniques: Using an Optical Microscope

  • Many biological structures are too small to be seen by the naked eye
  • Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied
  • For example, the movement of chromosomes during mitosis can be observed using a microscope

How optical microscopes work

  • Light is directed through the thin layer of biological material that is supported on a glass slide
  • This light is focused through several lenses so that an image is visible through the eyepiece
  • The magnifying power of the microscope can be increased by rotating the higher power objective lens into place

Apparatus

  • The key components of an optical microscope are:
    • The eyepiece lens
    • The objective lenses
    • The stage
    • The light source
    • The coarse and fine focus
  • Other tools used:
    • Forceps
    • Scissors
    • Scalpel
    • Coverslip
    • Slides
    • Pipette

Method

  • Preparing a slide using a liquid specimen:
    • Add a few drops of the sample to the slide using a pipette
    • Cover the liquid/smear with a coverslip and gently press down to remove air bubbles
    • Wear gloves to ensure there is no cross-contamination of foreign cells
  • Preparing a slide using a solid specimen:
    • Use scissors to cut a small sample of the tissue
    • Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps)
    • Some tissue samples need be treated with chemicals to kill/make the tissue rigid
    • Gently place a coverslip on top and press down to remove any air bubbles
    • A stain may be required to make the structures visible depending on the type of tissue being examined
    • Take care when using sharp objects and wear gloves to prevent the stain from dying your skin
  • When using an optical microscope always start with the low power objective lens:
    • It is easier to find what you are looking for in the field of view
    • This helps to prevent damage to the lens or coverslip incase the stage has been raised too high
  • Preventing the dehydration of tissue:
    • The thin layers of material placed on slides can dry up rapidly
    • Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration
  • Unclear or blurry images:
    • Switch to the lower power objective lens and try using the coarse focus to get a clearer image
    • Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly
    • There could be cross-contamination with foreign cells or bodies
  • Using a graticule to take measurements of cells:
    • A graticule is a small disc that has an engraved ruler
    • It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
    • As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
    • By using the two scales together the number of micrometers each graticule unit is worth can be worked out
    • After this is known the graticule can be used as a ruler in the field of view

Limitations

  • The size of cells or structures of tissues may appear inconsistent in different specimen slides
    • Cell structures are 3D and the different tissue samples will have been cut at different planes resulting in inconsistencies when viewed on a 2D slide
  • Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen

Exam Tip

Remember the importance of calibration when using a graticule. If it is not calibrated then the measurements taken will be completely arbitrary!

Apparatus & Techniques: Looking at Meiosis Under a Microscope

  • Cells undergoing meiosis can be observed and photographed using specialised microscopes
  • The different stages of meiosis have distinctive characteristics meaning they can be identified from photomicrographs or diagrams

Meiosis I or Meiosis II

  • Homologous chromosomes pair up side by side in meiosis I only
  • This means if there are pairs of chromosomes in a diagram or photomicrograph meiosis I must be occurring
  • The number of cells forming can help distinguish between meiosis I and II
  • If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II

The distinguishing features at each stage of Meiosis I

  • Prophase I: Homologous pairs of chromosomes are visible
  • Metaphase I: Homologous pairs are lined up side by side along the equator of spindle
  • Anaphase I: Whole chromosomes are being pulled to opposite poles with centromeres intact
  • Telophase I: There are 2 groups of condensed chromosomes around which nuclei membranes are forming
  • Cytokinesis: Cytoplasm is dividing and cell membrane is pinching inwards to form two cells

The distinguishing features at each stage of Meiosis II

  • Prophase II: Single whole chromosomes are visible
  • Metaphase II: Single whole chromosomes are lined up along the equator of the spindle in single file (at 90 degree angle to the old spindle)
  • Anaphase II: Centromeres divide and chromatids are being pulled to opposite poles
  • Telophase II: Nuclei are forming around the 4 groups of condensed chromosomes
  • Cytokinesis: Cytoplasm is dividing and four haploid cells are forming

Identifying the Stages of Meiosis Table

Identifying the Stages of Meiosis Table, downloadable AS & A Level Biology revision notes

Exam Tip

For metaphase remember M for the middle of the spindle and cell which is where the chromosomes will be lined up.

For anaphase remember A for away from the middle to the poles, which is where the chromosomes / chromatids are being pulled.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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