OCR A Level Biology:复习笔记2.6.5 The Stages of Meiosis

• Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
• It produces gametes in plants and animals that are used in sexual reproduction
• It has many similarities to mitosis however it has two divisions: meiosis I and meiosis II
• Within each division there are the following stages: prophase, metaphase, anaphase and telophase

Prophase I

• DNA condenses and becomes visible as chromosomes
• DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
• The chromosomes are arranged side by side in homologous pairs
  • A pair of homologous chromosomes is called a bivalent

   

• As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
• In this stage centrioles migrate to opposite poles and the spindle is formed
• The nuclear envelope breaks down and the nucleolus disintegrates

Metaphase I

• The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
• The maternal and paternal chromosomes in each pair position themselves independently of the others; this is independent assortment
  • This means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is due to chance

   

Anaphase I

• The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
• The centromeres do not divide

Telophase I

• The chromosomes arrive at opposite poles
• Spindle fibres start to break down
• Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
• Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I

Cytokinesis

• This is when the division of the cytoplasm occurs
• Cell organelles also get distributed between the two developing cells
• In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
• In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
• The end product of cytokinesis in meiosis I is two haploid cells

Second division of Meiosis : Meiosis II

• There is no interphase between meiosis I and meiosis II so the DNA is not replicated
• The second division of meiosis is almost identical to the stages of mitosis
• Prophase II
  • The nuclear envelope breaks down and chromosomes condense
  • A spindle forms at a right angle to the old one

   

• Metaphase II
  • Chromosomes line up in a single file along the equator of the spindle

   

• Anaphase II
  • Centromeres divide and individual chromatids are pulled to opposite poles
  • This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell

   

• Telophase II
  • Nuclear membranes form around each group of chromosomes

   

• Cytokinesis
  • Cytoplasm divides as new cell surface membranes are formed creating four haploid cells