• 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

• 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