Meiosis

Learning Objectives§ Contrast the number of chromosomes in body cells and in

gametes. § Summarize the events of meiosis. § Contrast meiosis and mitosis. § Describe how alleles from different genes can be inherited

together.

Chromosome NumberHomologous:chromosomes with the same genes, one originally from each of the organism’s parents

chromosome number = 4

Diploid: containing both sets of homologous chromosomes; 2N

Haploid: containing only a single set of chromosomes; 1N

Meiosis – A Source of DistinctionWhy do you share some but not all characters of each parent? What are the rules of this sharing game?

At one level, the answers lie in meiosis.

Reviewing Mitosis

Prophase

Metaphase

Anaphase

Telophase

Meiosis

Prophase ICentrioles

Spindle forming

Chromosomes

Nuclear envelope Centromere

Tetrad

Hilarious crossing over video: https://www.youtube.com/watch?v=4YfDq8rPDdM

Metaphase I

Pair of homologous chromosomes

During metaphase I of meiosis paired homologous chromosomes line up across the center of the cell.

Anaphase I

Sister chromatids

During anaphase I spindle fibers pull each homologous chromosome pairs toward opposite ends of the cell.

Telophase I

Nuclear envelopes reforming

In telophase I, a nuclear membrane forms around each cluster of chromosomes.

What is the end product of meiosis I?Are the cells haploid or diploid?

Prophase II

Metaphase II and Anaphase II

Anaphase II

Metaphase II How does anaphase II differ from anaphase of mitosis? How are they similar?

Telophase II and CytokinesisThe end product of meiosis is daughter cells that have the normal chromosome number. half

four

Summary of Meiosis

Comparing Mitosis and Meiosis

prophase (I)

metaphase (I)

anaphase (I)

telophase (I)

2N = 4 N = 2

Gene LinkageAlleles of different genes tend to be inherited together when those genes are located on the same chromosome.

Gene MapsAlfred Sturtevant (American geneticist) wondered if the frequency of crossing-over between genes during meiosis might be a clue to the genes’ locations. Sturtevant reasoned that the farther apart two genes were on a chromosome, the more likely it would be that crossing-over would occur between them.

#s = map units or centimorgans