IB DP Biology: SL复习笔记3.2.5 Skills: Meiosis

• Cells undergoing meiosis can be observed and photographed using specialized microscopes
• The different stages of meiosis have distinctive characteristics meaning they can be identified from photomicrographs
• Being able to identify the stages of meiosis from photomicrographs and diagrams is an important skill for a biologist

Step 1: Identifying if meiosis I or meiosis II is occurring

• 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 also help identify whether meiosis I or II is occurring
  • If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II

Identifying which stage of meiosis I is occurring

• Prophase I: Homologous pairs of chromosomes are visible in diploid cell (2n). Crossing over occurs
• Metaphase I: Spindle fibres pull homologous pairs so they are lined up side by side along the equator of the cell. Orientation of homologous chromosomes is random
• 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 the cell membrane is pinching inwards to form two cells with haploid chromosome numbers (n)

Prophase I, Metaphase I , Anaphase I and Telophase I as seen in photomicrographs

Identifying which stage of meiosis II is occurring

• Prophase II: Single whole chromosomes are visible in haploid cells
• Metaphase II: Single whole chromosomes are lined up along the equator of the cell in a single file
• 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

Prophase II, Metaphase II , Anaphase II and Telophase II as seen in photomicrographs

Drawing the stages of meiosis

• The distinguishing features mentioned above can also be used by biologists to draw scientific diagrams of meiosis I and meiosis II
• The conventions for drawing are:
  • The drawing must have a title
  • A sharp HB pencil should be used (and a good eraser!)
  • Drawings should be on plain white paper
  • Lines should be clear, single lines (no thick shading)
  • No shading
  • The drawing should take up as much of the space on the page as possible
  • Well-defined structures should be drawn
  • The drawing should be made with proper proportions
  • Label lines should not cross or have arrowheads and should connect directly to the part of the drawing being labelled

Drawing the different stages or phases of meiosis