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Why is meiosis so important.
It creates cells that have half the regular number of chromosomes. These haploid cells only contain one set of chromosomes, as compared to regular body cells that are diploid, containing two sets of chromosomes.
Germ-line cells are gamete-forming cells, located in the reproductive organs of males and females.
lSomatic cells are the rest of the cells of the body with the exception of germ-line cells.
- lGerm-line cells of an organism are diploid but produce haploid gametes (eggs or sperm)
- through meiosis. Whereas the somatic cells
- are diploid but undergo mitosis to form genetically identical, diploid daughter cells.
- Before cells divide, the DNA replicates, forming two identical copies of each chomosome.
-The cell replicates its chromosomes
-Each chromosome has two sister chromatids held together by a centromere.
- lIn prophase I the chromosomes become visible. The two chromosomes combine or
- synapse to form tetrads. Tetrads are also known as bivalents because they contain two pairs of chromosomes. At this point the chromosomes cross over. As in the prophase of mitosis the nuclear envelope disperses, the spindle fibers form moves.
lSynapsis- a process by which two homologous chromosomes pair up.
lCrossing Over- The process by which nonsister chromatids exchange portions of their DNA strands.
lIn metaphase I the tetrads are again arranged across the center by the movements of the kinetochores with the two centromeres opposite each other, but this time the sister chromatids will not be pulled apart as in mitosis.
- lIndependent Assortment- This is the random orientation of homologous pair of chromosomes on the metaphase plate. This process results in gametes with different
- combinations of parental chromosomes.
lHomologous chromosomes are pulled apart and move toward the opposite poles. At the end of anaphase I each pole has half as many chromosomes as were present in the cell.
lIn this phase, like in mitosis the chromosomes are moved into opposite poles and the nuclear envelope reforms and the spindle is broken down. Remember that there are two chromosomes, not one as in mitosis.
- meiosis the cell goes directly from telophase I to prophase II without the interphase. In prophase II the nuclear envelope is again dissolved and the spindle is set up again. Prophase II is identical to prophase of mitosis except that there is half the amount of chromosomes.
Again the chromosomes move into the center and line up. Now there are two chromosomes, instead of two tetrads, so that the chromatids will split off this time.
- lThe spindle fibers shorten, splitting up the sister chromatids and they move to the
- opposite poles.
lIn telophase II the chromatids concentrate in the poles and the nuclear envelope is reformed and the spindle again is dissolved. The cells divide for the last time, leaving a total of four daughter cells (haploid cells), which have half the chromosomes of a diploid cell. Unlike the daughter cells from mitosis, the daughter cells produced here cannot immediately cycle back to interphase.
How Meiosis differs from Mitosis
- in Humans, animals, plants, fungi
- cells are genetically different
- of division is two
- daughter cells produced
- number reduced by half
- centromeres do not separate during anaphase I, but during anaphase II
- cells only
- cells are genetically identical
- pairing of homologues
- of division is one
- daughter cells produced
- number stays the same.
- centromeres split during Anaphase
- reproductive cells called Somatic cells