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

Drawing the Stages of 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)

Photomicrograph-showing-stages-of-Meiosis-I

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

Photomicrograph-showing-stages-of-Meiosis-II

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-stages-of-meiosis-1Drawing-the-stages-of-meiosis-2

Drawing the different stages or phases of meiosis

Exam Tip

For metaphase remember M for the middle of the 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.When drawing the stages of meiosis you do not have to show crossing over occurring.

 

 

 

转载自savemyexams

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