1/27/2012 Genetics

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Cytokinesis (Division of cytoplasm)

  • animal cell
  • begins shortly after anaphase
  • contractile ring
    • myosin motor proteins
    • actin filaments squeezed by the myosin motor proteins tighter and tighter to make the contractile ring

Golgi-derived vesicles carry cell wall material to cell plate (figure 8-8)

Vesicles follow tracks or microtubules and actin to site

Fusion of vesicles forms plate toward outer walls

Meiosis terminology

  • Gamete (n): egg or sperm/haploid
  • Fertilization: fusion of gametes
  • Zygote (2n): 1st cell resulting from union of gametes
  • Isogamous: gametes of equal size, structure and motility
  • Heterogamous: gametes different in size, structure and motility
    • Oogamous: larger non-motile egg; smaller motile sperm (humans)

Object of Meiosis

Diploid cell (2n) to Haploid cell (n)

Not just any half will do!  One homologous chromosome from each set

Superficially similar to mitosis, but involves two successive divisions after G1, S, G2: Phase I & Phase II

Focus on Prophase I (5 subdivisions)

1. Leptotene

-Sister chromatids condense and become visible

-Often take on a horse shoe shape; ends associated with proteins on the nuclear lamina

2. Zygotene

Synapsis: homologous sister chromatids join along their lengths

-The two joined sister chromatids are now knows as “bivalents”

2 pair of sister chromatids

4 total chromatids

Also known as “tetrad”

-Joined by synaptonemal complex

can begin anywhere

homologous genes must be aligned

3. Pachytene

-Crossing over occurs; recombinant DNA- major place where exchange occurs

-breakage and rejoining of chromosomal strands

-Specific Enzymes Involved:

-endonucleases (cut)

-DNA ligases (glue ends)

-DNA polymerases (repair)

Chiasma: site of crossing; shaped like the greek letter chi (X)

4. Diplotene

-Synaptonemal Complex dissolves

-Bivalents come apart

-In females, meiosis is arrested at this stage when female embryos are 5 months and will not resume process until puberty

5. Diakinesis

-All that remains of Synaptonemal Complex is gone

-Nuclear membrane disappears completely

-Nucleolus (dark stained area of nucleus, where ribosomes are produced) disappears

-Official end of Prophase I

Meiosis 2n=46

Meiotic Division, DNA replication and recombination     Bivalents or tetrads


Cell division 1 Separation of homologous bivalents (2n=46)

Meiotic division 2 No replication

Cell division 2 Separation of sister chromatids (n=23)

Gametes (n) not genetically identical

We call this gametogensis because general production of a gamete

Oogenesis in Humans

Dictyate: arrested at primary oocyte (diploid) will not continue until female enters puberty


Polar Bodies:  Genetic material viable


Polar Body Twins? 50% identical

monozygotic twin (identical) dizygotic twin

PGD & PGS (preimplantation genetic diagnostics / screening)

Posted on January 27, 2012, in Genetics. Bookmark the permalink. Leave a comment.

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