AQA A Level Biology复习笔记4.3.6 Meiosis Under a 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

• 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