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Sexual reproduction begins with a specialized type of cell division, called __, which produces the male and female sex cells, or __. The cell that undergoes meiosis is __—having two copies of the genome. The gametes are __.
Each gamete has one copy of the genome. __ leads to a fertilized egg cell, which is __, having inherited one copy from each parent. Repeated __give rise to the somatic cells in the new organism.
- Fusion of a pair of gametes
Meiosis requires __
Two or more identical chromosomes present in a single nucleus is __
At the start of cell division, whether __ or __, every __ has itself replicated, the pairs of daughter chromosomes remaining attached at their __. The cell at this point is __, possessing four copies of each chromosome
- two successive cell divisions
- meiosis or mitosis
- homologous chromosome
Meiosis results in __cells because it is two successive cell divisions of the __cell, with no __ occurring during either of these divisions. They are __ and __
- DNA replication
- meiosis I and meiosis II
Meiosis I results in two cells, each containing one copy of each of the replicated homologous chromosomes. The cells resulting from meiosis I are diploid
Meiosis II involves breakage of the attachment that holds each pair of daughter chromosomes together. One daughter chromosome passes into one gamete, and the second daughter into the second gamete. These gates are therefore haploid
During meiosis I, __are formed between homologous chromosomes
As meiosis I begins, the replicated chromosomes do what?
condense and migrate to the middle of the nuclear region.
Unlike, in mitosis, where the pairs of homologous chromosomes remain separate from one another, in meiosis I, __. Instead, each chromosome does what?
the pairs of homologous chromosomes are by no means independent
finds its homolog and foms a bivalent
It is these blivalents, not the independent chromosomes, that do what?
line up in the middle of the nucleus
After formation of the __, meiosis I continues in a similar manner to a mitotic cell division. Explain
- Microtubules radiate out from the centrosomes, attach to the kinetochores, and begin to pull in opposite directions
The __ exerted on a bivalent does what, and the two homologs are __. A complete set of chromosomes is therefore assembled at __. Meiosis I is completed by __
- breaks it apart
- pulled in opposite directions
- each of the poles on the mitotic spindle
Meiosis II begins almost immediately. In each of the cells, the chromosomes again __. Each cell only has__ so the chrmosomes remain __.
- migrate to the middle of the nucleus
- one member of each pair of homologous chromosomes
- independent of one another.
Microtubules attach to the kinetochores and pull the chromosomes apart, separating the two daughters, which again move in opposite directions. A second round of __creates the four gametes.
Formation of bivalents is so exciting because it is the reason why __.
If meiosis had the same outcome as mitosis, then what?
- siblings are not exactly the same
- all the gamtes produced by an individual would be identical to one another
There are two reasons why siblings are different from one another, both of them outcomes of the __.
formation of bivalents.
What is the first?
results from the separation of the bivalents during the anaphase period of meiosis I
The two homologs in a bivalent are __. They contain the __ but will possess __. When the bivalents break apart, what happens? Which chromosome goes in which direction is entirely __, depending only on the __
same set of genes
different alleles of many of those genes
the two chromosomes are pulled toward opposite centrosomes
orientation of the bivalent with respect to the two spindle poles
Meiosis can result in __
gametes with any of four different chromosome combos depending on the directions in which the members of each homologous pair segregate during anaphase I
Each of these chromosome combinations corresponds to a gate with a __. For a cell with three chromosomes, there are 2^3=8 possible combinations. For human cells, with 46 chromosomes in the diploid set there are 2^23 possible ways in which the pairs of homologous chromosomes can be distributed during meiosis I
different set of alleles
Recombination occurs between __ within a __
Random segregation of chromosomes during anaphase I is not the only factor influencing the variability of the gametes resulting from meiosis. The __contributes to the resulting variability in a second, even more important way.
- homologous chromosomes
Within the bivalent, the chromosome arms—the chromatids—can do what? This exchange is called __. The actual crossover points are called __.
can exchange segments of DNA
crossing over or recombination
What impact does crossing over have on the genetic variability of the gametes resulting from meiosis?
Recombination increases variability of the gametes resulting from meiosis by changing the allele combos within individual chromosomes prior to the random segregation of those chromosomes during anaphase I
If the pair of homologous chromosomes in a bivalent exchange a segment of DNA, then what happens? Furthermore, the two chromosomes in each pair of homologous chromosomes can participate in __
any genes present in that segment also get exchanged and the alleles get transferred
__ on its own can give rise to any of eight million different chromosome combos in the gametes. The number of possible allele combos is a lot more