GN 311 (003) - Meiosis + Mitosis

Meiosis significance

Maintain a constant # of chromosomes (gen to gen), variability, shuffling of maternal and paternal chromosomes (Independent Assortment)

Homologous chromosomes

separate matching pairs, one from each parent that carry the same genes but may have different versions (alleles)

Sister chromatids

identical, joined copies of a single replicated chromosome

Mitosis Goal (in an Organism Described as 2n = 4)

make identical copy of cell (end w/ 2)

Cohesin (protein)

holds sister chromatids together in mitosis and meiosis until anaphase. It also acts at chiasmata in meiosis to hold homologs together

Shugoshin (Guardian Spirit)

protects Cohesin at the centromere in anaphase I of meiosis, but is degraded by anaphase II

Meiosis Goal

Process by which gamete-producing cell divides to produce 4 meiotic products each with half of the OG chromosome #

Interphase (Mitosis)

G1 phase, S phase, G2 phase

S phase: Cell copies all of its DNA, forms 2 identical copies of each chromosome (sister chromatids)

Prophase (Mitosis)

1. Spindle fibers start to form

2. Nuclear envelope dissolving

3. Condensing chromosomes, see under microscope 

4. Homologous, each 2 chromatids 

Prophase (Mitosis) Image

Metaphase (Mitosis)

1. Chromosomes (homologs up + down) line up in the middle

2. Spindle fibers attached to centromere and pulling

3. Check pnt for line up

Metaphase (Mitosis) Image

Anaphase (Mitosis)

Centromeres snipped apart and chromosomes (sisters) pulled apart

Anaphase (Mitosis) Image

Telophase (Mitosis)

1. Reform nuc envelope 

2. Cytokinesis sep cells

3. Relaxed, can’t tell # chromosomes

Telophase (Mitosis) Image

Order of Mitosis

Interphase, prophase, metaphase, anaphase, telophase

Meiosis Parts

1) Reductional Division (I) 

2) Equational Division (II)

Meiosis Reductional Division (I) 

• # of chromosomes in the daughter cells is 1/2 the # in the OG cell 

• One of each homologous pair is in each daughter cell.

• Separation of Homologs

Equational Division(II) 

• # of chromosomes at the beginning and end of the division cycle is the same, but the daughter cells only have one chromatid

• Separation of Sister Chromatids

Prophase 1 (Meiosis 1)

1. Homologous chromosomes come together 

2. Centromeres repel

3. Chiasmata (x-shaped link) visible 

4. 2 homologs connected

Prophase 1 (Meiosis 1) Image

Metaphase 1 (Mitosis 1)

1. Line up vertical 

Sis            homolog

Sis            homolog

Metaphase 1 (Mitosis 1) Image

Anaphase 1 (Meiosis 1)

1. Pull apart homologs

Anaphase 1 (Meiosis 1) Image

Telophase 1 (Meiosis 1)

1. Break cell envelope, cytokinesis

Telophase 1 (Meiosis 1) Image

Prophase 2 (Meiosis 2)

1. Break nuclear envelope

Prophase 2 (Meiosis 2) Image

Metaphase 2 (Meiosis 2)

1. One of each homolog

Metaphase 2 (Meiosis 2) Image

Anaphase 2 (Meiosis 2)

Sis chromatids sep

Anaphase 2 (Meiosis 2) Image

Telophase 2 (Meiosis 2)

1. 4 daughter cells w/ 1/2 OG chromosome #

Telophase 2 (Meiosis 2) Image

Mitosis - Prophase (# things 2n = 4)

4 chromosomes per cell, 2 chromatids per chromosome (1 cell)

| |

AA (2 sis chromatids)

 | |

 | |

aa (2 sis chromatids)

 | |

Mitosis - Metaphase (# things 2n = 4)

4 chromosomes per cell, 2 chromatids per chromosome (1 cell)

• Both pairs align individually along the center of the cell

Mitosis - Anaphase (# things 2n = 4)

8 chromosomes per cell, 1 chromatids per chromosome (1 cell)

Sister chromatids pull apart.

• One (A) chromatid goes to each side.

• One (a) chromatid goes to each side.

 |                   |

A                  A 

 |                   |

 |                   |

a                  a 

 |                   |

Mitosis - Telophase (# things 2n = 4)

8 chromosomes per cell, 1 chromatids per chromosome (1 cell)

Two new cells form.

• Each new cell has genotype (Aa)

1 Aa (1 cell) -mitosis-> 2 Aa (2 cells)

Meiosis 1 - Prophase 1 (# things 2n = 4)

4 chromosomes per cell, 2 chromatids per chromosome (1 cell)

Meiosis 1 - Metaphase 1 (# things 2n = 4)

4 chromosomes per cell, 2 chromatids per chromosome (1 cell)

• Homologous chromosomes pair up side-by-side in the center of the cell.

( | |    -    | | )

(AA   -    aa)

( | |    -    | | )

   ↑          ↑

  Homologs

Meiosis 1 - Anaphase 1 (# things 2n = 4)

4 chromosomes per cell, 2 chromatids per chromosome (1 cell)

Homologous pairs separate, but sister chromatids stay joined together.

• One side gets both (A) sister chromatids.

• The other side gets both (a) sister chromatids

( | | )       ( | | )

(AA)       (aa)

( | | )       ( | | )

Meiosis 1 - End (# things 2n = 4)

2 chromosomes per cell, 2 chromatids per chromosome (2 cell)

Two cells form. (haploid)

• One cell has 2 (A) chromatids; the other has 2 (a) chromatids.

Meiosis 2 - Prophase 2 (# things 2n = 4)

2 chromosomes per cell, 2 chromatids per chromosome (2 cell)

( | | )                     ( | | )                          

(AA)                     (aa)

( | | )                     ( | | )

Meiosis 2 - Metaphase 2 (# things 2n = 4)

2 chromosomes per cell, 2 chromatids per chromosome (2 cell)

In both new cells, individual chromosomes line up in the center.

X -> / /

Meiosis 2 - Anaphase 2 (# things 2n = 4)

4 chromosomes per cell, 1 chromatids per chromosome (2 cell)

Sister chromatids pull apart in both cells (double chromosomes, 1/2 chromatids)

Meiosis 2 - Telophase 2 (# things 2n = 4)

2 chromosomes per cell, 1 chromatids per chromosome (4 cell)

1 Aa (1 cell) -meiosis 1-> 1AA, 1aa (2 cells) -meiosis 2-> 2A, 2a (4 cells)

( | )      ( | )                     ( | )      ( | )                          

(A)       (A)                     (a)       (a)

( | )       ( | )                    ( | )      ( | )

4 haploid cells (2 A, 2 a)