MCAT Biology > Meiosis
Meiosis is the process by which sexual cells replicate, creating new, genetically diverse haploid (n) cells through each division. The process of meiosis is divided into two distinct phases, where upon 4 new cells are created from one original cell.
In prophase I, the chromatin condenses into chromosomes called tetrads. Next the spindle apparatus appears which provides structure to the cell during meiosis. After this, the nucleoli and the nuclear membrane disappears. Finally, the chromosomes mix and intertwine in synapsis where they may exchange DNA in a process called crossing over. This occurs at the chiasma
In metaphase I, the homologous pairs (tetrads) align at the metaphase plate where they attach to the spindle apparatus via the kinetochores.
During anaphase I, the homologous pairs separate by being pulled apart at opposite ends by the centrioles. Next, the mother and father chromosomes separation causes genetic variation through the process of disjunction. This follows in line with Mendel's Law of Independent Assortment, which creates a random genetic code based on alignment of chromosomes.
During telophase I, the cells split and create haploid (n = 23 chromosomes) cells which only have half of the normal genetic material. Each of these cells has 46 chromatids (2 per chromosomes) for a total of 23 chromosomes. After this, the nuclear membrane reforms and the cells divide via the process of cytokinesis.
During prophase II the centrioles migrate to opposite poles and the spindle apparatus reforms.
Metaphase II involves the chromosomes lining up along the metaphase plate to resume division. At this point the centromeres divide and separate the chromosomes.
During anaphase II the sister chromatids are pulled to the opposite poles by the spindle fibers.
During telophase II the cell divides once again and undergoes reductional division which creates four haploid cells (n) from the previous two diploid (2n) cells through cytokinesis.
Asexual vs. Sexual Reproduction
|Feature||Asexual Reproduction||Sexual Reproduction|
|Parents Required||One Parent||Two Parents|
|Type of Replication||Binary Fission, Budding, Regeneration, Parthenogenesis||Meiosis|
|Occurs In||Lower Organisms||Invertebrates and Vertebrates|
|Genetic Variation||Identical (only some mutations)||Unique Genetic Material (Fundamental for Evolution)|
|Sex Cells Required or Formed?||No||Yes (Gametes)|
|Energy Required||Small Amount||More Energy Required|
|Number of Offspring Produced||2+||1+|