Above are monads forms from meiosis. What event appear to have happened to one o
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Above are monads forms from meiosis. What event appear to have happened to one of the original homologous chromosomes? Draw the inversion loop with the crossing over that led to these monads. Which gametes would you expect to be viable and which would you expect to be not viable?
Above are monads forms from meiosis. What event appear to have happened to one of the original homologous chromosomes? Draw the inversion loop with the crossing over that led to these monads. Which gametes would you expect to be viable and which would you expect to be not viable?Explanation / Answer
In meiosis
Homologous chromosomes pair or synapse to form a bivalentBivalent gives rise to a unit called the tetrad which consists of four chromatids.
Two divisions take place:
i. reductional - Meiosis I (tetrad separates into dyad)
ii. equational - Meiosis II (dyad splits into two monads)
Chromosomes may exchange information - crossing over
Interphase I: same as mitosis
Prophase I
Leptonema (n)
Leptotene stage - chromosome begin to condense
Homology Search: initial pairing of homologues
b. Zygonema
Zygotene stage: rough pairing of the homologues
Synaptonemal Complex is formed: the paired structures are called bivalents.
c. Pachynema
Pachytene stage: intimate point by point synapsis of homologous chromosomes
Shortening and coiling of paired chromosomes occurs - bivalents thicken
Can distinguish sister chromatids Bivalent = Tetrad
Crossing over occurs
d. Diplonema
Crossing over occurs at chiasmata
Crossing over involves nonsister chromatids
Genetic variability
Chromatids begin to separate
e. Diakinesis - further shortening of chromosomes and separation. Linkage to the spindle.
Nuclear membrane and nucleolus disappear.
Terminalization: the chiasmata move toward the ends of the tetrad
END OF PROPHASE I
4. Metaphase I - chromosomes pair up in metaphase plate.
5. Anaphase I - homologous pairs (tetrads) separate to form dyads
Disjunction: the separation of tetrads into dyads
Nondisjunction: error where separation does not occur
Random segregation of dyads is the basis for Mendelian principle of independent assortment.
6. Telophase I - cytokinesis
B. MEIOSIS II - Equational Division
1. Interkinesis: no DNA synthesis. only a haploid complement (n)
2. Prophase II: dyad is composed of one pair of sister chromatids attached by a common
centromere
3. Metaphase II: centromeres directed to the equatorial plate
4. Anaphase II: sister chromatids of each dyad are pulled to opposite poles (Haploid)
5. Telophase II: monads and cytokinesis. Each chromosome is a monad (unreplicated)
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