Chapter 13

Part 2

Crossing Over

• Duplicated homologous chromosomes connect, this is called synapsis.

• Pieces of DNA swap, called crossing over.

• All 4 chromatids together make a tetrad.

• Each tetrad has at least one site of chiasma, where crossing over occurs.

Metaphase I

• Tetrads line up on the metaphase plate

Anaphase I

• The chromosomes begin to move to the poles.• Sister chromatids remain attached!

Telophase I and Cytokinesis

• 2 new cells form.• Each has a complete

haploid set of chromosomes (23), each chromosome is still composed of two sister chromatids.

Prophase II

• A new spindle apparatus forms, and the 2 new cells get ready to divide again.

Metaphase II

• Chromosomes line up on the metaphase plate.

• The two sister chromatids are NOT genetically identically (because crossing over happened in Prophase I)

Anaphase II

• Centromeres of each chromosome separate, and sister chromatids start moving apart.

Telophase II and Cytokinesis

• Nuclei form, 4 new daughter cells are formed.• Each of the 4 daughter cells are genetically

different.

Mitosis vs. Meiosis

Mitosis• Cells divide once• No crossing over• Two daughter cells made• Daughter cells are identical

to each other and parents• Daughter cells are 2n• Occurs in somatic cells

Meiosis• Cells divide twice• Crossing over (prophase I)• Four daughter cells made• Daughter cells are all

different from each other and parents

• Daughter cells are n• Occurs in ovaries/testes• Makes gametes

Why is crossing over important?

• During Prophase I, crossing over occurs and produces genetic variation.

• This produces recombinant chromosomes, that carry genes (DNA) from two different parents

• Powers natural selection/evolution: all individuals are different and the most fit survive to reproduce

• Makes species more “hardy,” if bad things happen, at least some will have adaptations that help them survive.

Random Fertilization

• Each egg or sperm represents 8 million possible chromosome combinations

• This is because during Metaphase I, sometime the male chromosome and sometimes the female chromosome points towards a pole

• So, each new cell has some genes from Mom and some from Dad

• Each zygote has 64 trillion possible diploid combinations, increasing genetic variation.