9a Asymmetry

Question 1

How cells become different?

Answer 1

Asymmetry can be broken before or after fertilization.
Before: Xenopus, Drosophila
At: C.elegans
After: Mammals

Question 2

Polar Lobe Formation in Molluscs

Answer 2

• polar lobe secluded to CD blastomere
• called trifoliated because looks like 3 but only 2
• CD absorbs contents of polar body
• 2nd polar lobe excursion ehich remains excluded to the D blastomere
• if polar lobe excised from embryo it will lack mesodermal structures
• if forced into A then twin larvae is formed

Question 3

Removal of Polar lobe

Answer 3

Removal of polar lobe (or the D blastomere)
results in:
• normal cell division
• incomplete larva, lacking mesodermal
organs, such as shell gland, mouth,
muscles, foot,

• If polar lobe materials were forced into the AB
blastomere at first cleavage, twin larvae are
formed, joined by their ventral surfaces-thus the
polar lobe seems to have a role in dorsal-ventral
axis determination.

Question 4

Germ plasm of Ascaris

Answer 4

• equitorial. Veg has germ plasm
• cells in animal hem undergo chromosomal diminution loss of genes in cell of A.hem
• No chromo diminution in Veg.H
• Meridional in A.hem, equator in Veg H.
• Germ plasm now further down, no cell diminution seen there
• Centerfugation displaces first clevage by 90º resulting in both cells have vegital germ plams
• meridonal cleavage GP evenly distributed
• now no chromo degradation
• somatic cells are at rest
• germ cells give rise to sperm/egg
• info is needed to pass from 1 sen to the other. GP has factors that can prevent uncle from degradation.

Question 5

Distribution of Germ plasm

Answer 5

•In insects, nematodes, and frogs, a maternally derived
germ plasm is deposited in a certain region of the egg.
• Germ plasm contains granules, RNA molecules, and
proteins due to which the cells derived from this region to
become germ cells.

Simplest example of this phenomenon is in the nematode Ascaris:
• Germ plasm is segregated into a “stem cell” at cleavage
• This cell retains its full chromosome complement, and
becomes the germ cells.
• The remaining blastomeres undergo “chromosomal
diminution” they lose large portions of their
chromosomes and become somatic cells

Question 6

chromosomal diminution

Answer 6

elimination of certain chromosomes from cells that form somatic tissues

Question 7

(C. elegans)

Answer 7

• sperm are circular made by epiboya
• they roll over till they reach the spermatheca for fertilization
• PAR = partican defective proteions

Question 8

Segregation of P-granules in C. elegans

Answer 8

•Germ plasm is called P granules and is asymmetrically localized in a germ line precursor cell called P4
blastomere.
•P granule has transcription
repressors, RNA binding proteins Vasa, Nanos,etc.
•Germ plasm also contains PIE-1 protein that prevents RNA polymerase II so that it doesn’t become a somatic cell

Question 9

Asymmetry at Fertilization in C. elegans

Answer 9

•Par 1 and 2: inner cytoplasmic whereas Par 3,6 and PKC are present in the cortical cytoplasm.
•Par 1/2 are posterior, Par 3/6 are anterior
•Germ Plasm in P4 blastomere
• Upon fertilization the microtubule of sperm
centrioles organize the Par 2 along with its
binding partner Par 1 proteins in the cortical
cytoplasm of the posterior region.
• Par 3 and 6 are secluded to anterior region
•Transciption repressors prevent RNA polymerase II binding.