9.5 and 9.6

Section 5


Objectives:

• Describe how homologous chromosomes are alike and how they differ.
• Contrast haploid and diploid cells.
• Summarize the process of meiosis.

1. Describe how homologous chromosomes are alike and how they differ.

Homologous chromosomes are similar in the fact that they carry the same sequence of genes controlling the same characteristics. One difference is that they have different forms of the same gene. One might have the traits for brown eyes and the other for blue.

2. Contrast haploid and diploid cells.

Diploid cells carry two homologous sets of chromosomes (the exceptions are egg and sperm). A cell with a single set of chromosomes is a haploid cell. They are produced through meiosis. 

3. Summarize the process of meiosis.

Interphase 1: The cell duplicates the DNA.

Prophase 1: Proteins cause the homologous chromosomes to stick together (tetrads). Tetrads attach to the spindle . Sister chromatids in the tetrads exchange genetic information (crossing over).

Metaphase 1: Tetrads move across the middle and line up across the spindle. 

Anaphase 1: Homologous chromosomes separate and move to opposite sides of the spindle.

Telophase 1 and Cytokinesis: The nuclear envelope reforms as chromosomes are in opposite poles, and the cell starts to separate.

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Prophase 2: A spindle forms, attaches to centromeres, and chromosomes are moved to the middle of the cell.

Metaphase 2: Chromosomes line up in the middle, attached to the spindle.

Anaphase 2: Sister chromatids separate and move to opposite ends.

Telophase 2 and Cytokineis: Chromatids arrive at the ends, and the cells are split one more time.

Section 6

Objectives:

• Describe how chromosome assortment during meiosis contributes to genetic variation.
• Explain how crossing over contributes to genetic variation.
• Compare and contrast mitosis and meiosis. 

1. Describe how chromosome assortment during meiosis contributes to genetic variation.

The way chromosomes line up is at random, so the assortment that ends up in cells is completely by chance.  Four combinations are possible for each chromosome, but there are 8 million combinations for the entire cell.

2. Explain how crossing over contributes to genetic variation.

Crossing over can produce a new chromosome that contains new combination of genetic material from different parents. 

3. Compare and contrast mitosis and meiosis.