Mitosis And Meiosis

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Prokaryotes

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Cells divide by binary fision

Results in genetically identical cells

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Eukaryotic Cell Cycle

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• M – Mitosis (when cell is actually dividing)
• Interphase (when cell is not dividing)
S – When DNA synthesis (replication) occurs
G1 and G2 – Gap periods – contain a number of quality control checkpoints

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Chromosomes

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• Sister Chromatids – Genetically identical arms of a chromosome – The result of DNA replication in the S phase of the cell cycle.
• Homologous Chromosomes – Same genes in the same order but often not the same forms (alleles) of those genes (eg Brown eyes and Blue eyes).

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Reproduction

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• Asexual – Offspring derived from a single parent and are genetically identical to the parent.
• Sexual – Two parents contribute genetic info. needed to produce offspring. Offspring not genetically identical to either parent. Greater chances for diversity.

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Cell Types

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• Somatic Cells – Cells of the body. Reproduce by mitosis.
• Germ Cells – Cells present w/in the reproductive system that divide by meiosis to produce gametes (egg and sperm)

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Interphase Cells

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Not currently dividing. DNA-protein complex w/in nucleus called chromatin. Nuclear membrane present.

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Mitosis

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• Prophase – Nuclear membrane breaks down, chromatin condenses to form chromosomes, spindle fibers begin to form.
• Metaphase – chromosomes line up single file on equator. Homologous pairs do not associate.
• Anaphase – Spindle fibers shorten and draw chromatids (now called chromosomes) toward the poles. The genetic info heading toward each pole is identical because chromatids are genetically identical.
• Telophase – Chromosomes arrive at poles. Nuclear membrane reforms and chromosomes decondense to form chromatin. Cytokinesis or division of the cytoplasm usually takes place. Result is two genetically identical diploid cells.

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Meiosis

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• Occurs only in germ cell (cells that form gametes - egg and sperm cells)
• Results in the formation of haploid gametes (single copy of each gene)
• During fertilization, gametes fuse to form a diploid zygote which will undergo mitosis to form an embryo and ultimately a new organism.
• Prophase I of meiosis – Homologous chromosomes pair and crossing over occurs
• Metaphase I – Homologous chromosomes line up double file. Their double file orientation is random (independent assortment)
• Anaphase I – Whole chromosomes pulled toward the poles. The genetic material moving toward each pole is different (because chromosomes of homologous pairs are not genetically identical)
• Telophase I – Chromosomes reach poles. Nuclear membrane reforms. Cell divides in two. Each of these two cells will go through a second division cycle, which mechanistically is identical to mitosis.
• Interkinesis – Period between the division cycles.
• Prophase II – Nuclear membrane breaks down, chromatin forms chromosomes. (no crossing over since homologous pairs no longer present in same cell).
• Metaphase II – Chromosomes line up on equator single file.
• Anaphase II – Chromatids pulled toward poles (as soon as they separate they are called chromosomes again)

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Independent Assortment

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• During metaphase I of meiosis, the way the homologous pairs line up is random – Thus the same parents can produce genetically different gametes. This is why siblings of the same parents are different.

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Nondisjunction

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• Errors that occur during meiosis can cause an incorrect number of chromosomes to migrate toward the poles generating gametes (egg and sperm cells) with an abnormal number of chromosomes (aneuploidy).
• In most cases having an abnormal number of chromosomes (too many or too few) causes serious problems or more commonly death.
• Three copies of chromosome #21 causes Down Syndrome.
• The frequency of aneuploidy increases with the age of the mother (especially beyond the age of 35).