Meiosis November 2014

Chromosome Number Diploid- 2 sets of chromosomes –In somatic (body) cells; One comes from mother and one from father –Also referred to as “2n” –Homologous Chromosomes - each set of two chromosomes; One comes from father and one from mother Haploid- only one set of chromosomes –Occurs in gametes (egg and sperm) –Also referred to as “n”

Meiosis Meiosis is the process which produces gametes containing half the number of chromosomes as a parent’s body cell Two distinct divisions –Meiosis I –Meiosis II

Why Meiosis? Process is necessary to produce gametes (sex cells-egg & sperm) with half the number of chromosomes normally present in body cells. Why is this necessary? Gametes with (each with 1 set of chromosomes) meet during reproduction to form a fertilized egg (zygote) with 2 sets of chromosomes

Meiosis I-Interphase DNA is replicated After replication, each chromosome consists of two identical sister chromatids, held together by a centromere

Meiosis I-Prophase I Tetrads form when homologous pairs come together Four chromatids (2 homologous chromosomes) is called a tetrad.

Crossing Over during Prophase I Crossing over (exchange of genetic material) occurs during Prophase I. It may occur at several locations on the chromosome at one time. This exchange of genetic information leads to genetic diversity.

Meiosis I-Metaphase I Centromeres attach to spindle fibers Spindle fibers pull the tetrads into the middle or equator, of the spindle Homologous pairs line up side by side (tetrads) in center (Remember in mitosis they line up on the spindle’s equator independently of each other.)

Meiosis I-Anaphase I Homologous chromosomes get pulled to opposite poles Occurs because centromeres holding the sister chromatids together do not split as they do during anaphase in mitosis

Meiosis I-Telophase I Spindle is broken down Chromosomes uncoil Cytoplasm divides to produce two new cells Each new cell has half the genetic material of the original cell because it has only one chromosome from each homologous pair

Second Division: Meiosis II There is a brief Interphase but DNA is NOT replicated during this time Meiosis II starts with a haploid cell Prophase II –a new spindle forms in each new cell

Metaphase II Chromosomes (made up of sister chromatids) line up randomly along the cell’s equator Each cell similar to Metaphase in mitosis (except with exchanged genetic info due to crossing over)

Anaphase II Centromeres split & sister chromatids (which are now individual daughter chromosomes) separate and move to opposite poles

Telophase II Nuclear membranes appear in each new cell Cytokinesis occurs Formation of 4 haploid, genetically unique cells

Gamete Formation Meiosis results in 4 equal size sperm cells in males Females get 1 egg and 3 polar bodies (cytoplasm does not split equally)

Comparing Mitosis and Meiosis Mitosis 2 genetically identical cells Diploid cells produced Body cells One division Meiosis 4 genetically different cells Haploid cells produced Gametes: egg and sperm Two divisions

So…Let’s review!

What phase of meiosis is this?

And this?

What about this?

This?

And this?

The process shown below is called…?

Our cells have 46 chromosomes. The cell’s haploid number is ? The cell’s diploid number is ?

What do these chromosomes have in common?

Same size Same shape Same genes

How are they different?

One came from one parent and one came from the other.