Meiosis: Comes From The Greek Meioun, Meaning "to Make Smaller,"

Meiosis Comes from the Greek meioun, meaning "to make smaller," Reduction-Division Process

Meiosis 



Transmitting copies of genetic material from parent to offspring Gamete formation – haploid 1n  Half

the normal chromosome number  Horse = 32 chromosomes 

Occurs in specialized cells  Testes

– Spermatogenesis  Ovary – Oogenesis

Spermatogenesis 

Originates within spermatogonium – specialized cells in the seminiferous tubules of the testes  Formation 



Spermatogonium make more layers of similar cells inside the walls of the seminiferous tubules Mitosis – spermatogonium produce primary spermatocytes from puberty on.

 Formation 





of Primary Spermatocytes

of Secondary Spermatocytes

Primary spermatocytes divide to form secondary spermatocytes (Meiosis I) Each secondary spermatocyte divides into two spermatids (Meiosis II) Meiosis – sperm production – two step process

Meiosis 

2 Stage process  Stage 

Duplication and exchange of genetic material in Homologous Chromosomes 



Tetraploid

Migration of Homologous Chromosomes to opposite daughter cells 

 Stage 

I

Diploid

II

Separation of Sister Chromatids 

Formation of hapliod cells

Prophase I 

Characterized by a Synapse  attraction

between homologous chromosomes  process of linking homologous chromosomes 

Exchange of genetic material between paired chromosomes may occur  Formation

of a tetrad – thick 4-stranded structure  Crossing over – rearranging genetic material  Increases genetic variation  Offspring inherit new gene combinations from their parents

Chiasma 





The site where the exchange of chromosome segments between homologous chromosomes takes place (crossing-over). Points of contact of the arms of adjacent chromosomes Characteristic of very late Prophase I – early Metaphase I

Prophase I Centrioles and spindle fibers form  Homologous chromosomes separate 

 Loss

of nuclear membrane  Move into the cytoplasm  Migrate to equator Prometaphase I

Metaphase I 



Homologous chromosomes are completely separated Sister chromatids united by centromere (kinetochore) attach to spindle fibers at the equator

Anaphase I 

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Movement of the homologous chromosomes to opposite poles No separation of chromatids

Telophase I 

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 

Separation of the cytoplasm to form sister cells Nuclear membranes enclose respective chromosomes Chromosomes disperse Enters interphase II – no DNA replication occurs Cytokinesis

Prophase II  

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Shortening and thickening of the chromatids. Disappearance of nucleoli and nuclear membrane. Centrioles move to the polar regions and are arranged by spindle fibers. The new equatorial plane is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plane

Metaphase II 

Chromatid pairs attach to spindle fibers at equator by kinetochores (centromeres).

Anaphase II Sister chromatids move to opposite ends of the cell  Separation at the kinetochore 

Telophase II 



 

Division of the cytoplasm Formation of nuclear membrane Two sister cells Spermatids – premature sperm cells

http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.html

Spermatogenesis Review 

One primary spermatocyte forms four sperm cells, each of approximately equal size 32 32 32 64

64 32 32 32

Oogenesis 

The process in which the primary oocyte turns to an ovum undergoing meiosis.  During

embryonic development specialized cells oogonium within the ovary divide by mitosis, producing primitive egg cells.  Each egg cell begins oogenesis as a primary oocyte.  



Born with a specific number of primary oocytes Meiosis comes to a rest during prometaphase I, known as the dictyate stage. Puberty and the estrous cycle trigger continuation of oogenesis.

Oogenesis – Meiosis I  Synapsis  Tetrads

form  Crossing over - Chiasma 

Unequal division of cytoplasmic material  Result

body

= secondary oocyte and the first polar

Oogenesis – Meiosis II 

Secondary oocyte is arrested in Metaphase II until ovulation  Sperm



penetrates oocyte, rests head in the cytoplasm

Sperm triggers completion of Meiosis II  Does

not divide until after fertilization (telophase II)  Second mitotic division forms ootid and a second polar body  Ootid undergoes maturation and turns to the ovum.

Polar bodies 



In the horse may retain one polar body (secondary) within the larger cell (egg) just below its outer membrane. Other polar bodies disintegrate.

Spermatogenesis vs. Oogenesis Both start as primordial germ cells.  The resulting gamete: 

1

primitive sperm cell (spermatogonia)



1

4 mature sperm

primary oocyte (oogonia)

1 mature egg (ovum)  Polar bodies 

Meiosis I

Meiosis II

Mitosis vs. Meiosis 

http://www.pbs.org/wgbh/nova/baby/divi_fl ash.html