Biology Notes for Ib

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10.1.3 CROSSING OVER
Outline the process of crossing over in :
Meiosis 1:
• Prophase I - homologous chromosomes are paired up tightly into tetrads, then crossing over, the exchange of genetic material between the DNA in these tetrads occurs, forming a chiasmata, an x-shaped structure.
• Metaphase I - paired chromosomes line up along the equator of a cell, the metaphase plate as the spindle microtubulue apparatus pulls them.
• Anaphase I - The spindle microtubules pull homologous chromosomes to opposite sides of the cell, causing them to separate.
• Telophase I - The spindle microtubule appartus begins to disappear/disintegreate, the nucleus membrane reforms around chromosomes
• Cytokenesis I - The cell divides along the equator, creating 2 haploid daughter cells

A brief period of Interphase usually occurs between Meisois I and II, in which the cell grows, and DNA is replicated to prepare for meiosis.
Meiosis 2:
• Prophase II - sister chromatids pair up and attach to the spindle microtubule apparatus.
• Metaphase II - sister chromatids line up at equator of cell due to the movement of the spindle microtubule apparatus.
• Anaphase II - sister chromatids separate as spindle fibers pull them in opposite directions.
• Telophase II - sister chromatids are on opposite sides of cell, spindle fibers disappear.
• Cytokenesis II - cell divides along the equator, nuclei begin to reform, creating 4 daughter haploid cells.

10.1.4 STATE MENDEL’S LAW OF INDEPENDENT ASSORTMENT
This principle states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization.

10.1.5 EXPLAIN THE RELATIONSHIP BETWEEN MENDEL’S LAW OF INDEPENDENT ASSORTMENT AND MEIOSIS
During metaphase I of meiosis the homologous pairs of chromosomes align along the equator. The orientation of the chromosomes is random. This means that when the pairs of homologous chromosomes move to opposite poles during anaphase

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