Meiosis Under a Microscope (AQA A Level Biology)

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

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