Topic 10.1 – Meiosis HL

10.1.1 Prophase I DNA condenses and chromosomes become visible Homologous chromosomes are attracted to each other and so pair up Due to similar chromosome structure and allele alignment, crossing over occurs Spindle fibers made from microtubules form at the poles of the parent cell 10.1.1 - Describe the behaviour of the chromosomes in the phases of meiosis. 10.1.1

Metaphase I Homologous chromosomes (aka bivalents aka homologues) align along the cell’s equator The nuclear membrane disintegrates 10.1.1 - Describe the behaviour of the chromosomes in the phases of meiosis. 10.1.1

Anaphase I Spindle fibers from the pole separate the homologues and pull them towards the poles Telophase I Spindles/fibers disintegrate Chromosomes uncoil and nuclear membrane reforms Cytokinesis occurs afterwards, resulting in two haploid cells. 10.1.1 - Describe the behaviour of the chromosomes in the phases of meiosis. 10.1.1

10.1.1 Prophase II DNA condenses into visible chromosomes again New spindle fibers are produced Metaphase II Nuclear membrane disintegrates Chromosomes align randomly along the equator (random orientation) Spindle fibers attach at centromeres 10.1.1 - Describe the behaviour of the chromosomes in the phases of meiosis. 10.1.1

Anaphase II Sister chromatids split at the centromere and are pulled to opposite poles by the spindle fibers New membranes (animals) or cell plates (plants) form between new cells Telophase II Chromosomes unwind New nuclear membranes form around the four haploid cells before cytokinesis 10.1.1 - Describe the behaviour of the chromosomes in the phases of meiosis. 10.1.1

10.1.1 - Describe the behaviour of the chromosomes in the phases of meiosis.

10.1.1 - Describe the behaviour of the chromosomes in the phases of meiosis.

During prophase I, homologous chromosomes are able to swap genes through a process known as crossing over. When this occurs, a chiasma (pl chiasmata) forms between the chromosomes. It is where the two chromosomes connect and exchange genetic material. 10.1.2 - Outline the formation of chiasmata in the process of crossing over. 10.1.2

When chromosomes form chiasmata, they are oriented in the same direction with their loci mostly aligned. This means that alleles on each chromosome are lined up. When DNA is exchanged and ‘crossed over,’ alleles for the same genes are swapped. The exchange is NOT random. A a 10.1.2 - Outline the formation of chiasmata in the process of crossing over. 10.1.2

Chromosomes that are a result of crossing over have both paternal and maternal DNA. Note that both chromosomes have the same loci and orientation, but not always alleles. This crossing over during prophase I combined with random orientation during metaphse I effectively results in infinite genetic variability (223 without crossing over!) 10.1.3 - Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I. 10.1.3

Crossing over can occur multiple times between homologous chromosomes with either one or both chromatids. The possibilities are endless! 10.1.3 - Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I. 10.1.3

Mendel’s Law of Independent Assortment states that during gamete formation allele pairs separate independently from other pairs. For example, the alleles that determine hair color are not bound to the alleles for skin color. 10.1.4 - State Mendel’s law of independent assortment. 10.1.4

Random orientation during metaphase I and random crossing over during prophase I support explain why alleles are able to segregate independently of each other. 10.1.5 - Explain the relationship between Mendel’s law of independent assortment and meiosis. 10.1.5

10.1.5 - Explain the relationship between Mendel’s law of independent assortment and meiosis.

10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis. 10.1.2 Outline the formation of chiasmata in the process of crossing over. 10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I. 10.1.4 State Mendel’s law of independent assortment. 10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis. 11.2.1 – State the roles of bones, ligaments, muscles, tendons and nerves in human movement. 8.2