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Mitosis and Meiosis
Transcript of Mitosis and Meiosis
Nuclear membrane breaks down, nucleolus disappears
Spindle fibers and centrosomes form the spindle apparatus as they move to opposite poles of the cell The cellular DNA is replicated, existing as uncondensed fibers called chromatin The cell synthesizes a variety of proteins necessary to replicate DNA The cell synthesizes more proteins, particularly microtubules, proteins that are required during mitosis Metaphase: The spindle fibers from the opposite poles attach to the centromere of each chromosome, guiding them to the equator of the cell Anaphase: The centromere splits apart separating the sister chromatids into chromosomes
The spindle fibers shorten pulling the chromosomes to opposite poles Telophase: The chromosomes begin to unwind into strands of chromatin and the spindle fibers breakdown
Nuclear membrane forms around each new set of chromosomes and a nucleolus forms within each Cytokinesis: The cytoplasm divides to create the two new daughter cells Meiosis Meiosis Mitosis and Interphase: Growth 1 (G1) The cell synthesizes a variety of proteins necessary to replicate DNA Synthesis (S) The cellular DNA is replicated, existing as uncondensed fibers called chromatin Prophase I: Each pair of homologous chromosomes line up side by side, this is called synapsis and is when crossing over (exchange of chromosome segments) can occur
The centrosomes move to opposite poles of the cell and the spindle apparatus forms Metaphase I: The pair of homologous chromosomes line up along the equator of the cell
Spindle fibers attach to the centromere of each homologous chromosome Anaphase I: The homologous chromosomes seperate and move to opposite poles
The sister chromatides are still held together, resulting in a single chromosome from each homologous pair moving to each pole, reducing the chromosome number from 2n (diploid) to n (haploid) Telophase I: The homologous chromosomes begin to uncoil and spindle fibers disappear
Cytokenesis takes place and two new haploid cells form Prophase II: The nuclear envelope breaks down and the spindle fibers reform in the haploid daughter cells Metaphase II: A haploid number of chromosomes line up along the equator Anaphase II: The sister chromatids are puled apart at the centromere by the spindle apparatus
Chromosomes move towards opposite poles of the cell Telophase II: The nuclear membrane and nuclei begin to reform as the spindle apparatus disappears Cytokinesis: Results in 4 haploid daughter cells In Mitosis: Errors That Can Occur: Errors in mitotic chromosome segregation (such as lagging chromosomes) generate DNA breaks via the formation of structures called micronuclei
Micronuclei undergo defective and asynchronous DNA replication, resulting in DNA damage and often extensive fragmentation of the chromosome in the micronucleus
As micronuclei can persist in cells over several generations and chromosomes in the micronucleus be distributed to daughter nuclei, chromosome segregation errors potentially lead to mutations and chromosome rearrangements that can integrate into the genome
Pulverization of chromosomes in micronuclei may also be one explanation for 'chromothripsis' in cancer and developmental disorders, where isolated chromosomes or chromosome arms undergo massive local DNA breakage and rearrangement. In Meiosis: Changes in Chromosome Structure: Changes in Chromosome Number: During crossing over the chemical bonds holding the DNA together in the chromosomes are broken and reformed, they don't always reform properly
Non-homologous pairs may also cross over, producing chromosomes that contain genes not normally on that chromosomes
Errors to the chromosomal structure include; deletion (a piece of a chromosome is deleted), duplication (a section of a chromosome appears 2 or more times in a row), inversion (a section of a chromosome is inverted), and/or translocation (a segment of one chromosome becomes attatched to a different chromosome Sometimes homologous chromosome pair do not seperate as they should during meiosis, this is called non-disjunction
Non-disjunction can occur in anaphase I (when homologous pairs do not seperste to opposite poles, and/or anaphase II (when sister chromatids do not seperate to opposite poles
Non-disjunction occurs as trisomies (the gain of an extra chromosome) and monosomies (the loss of a chromosome, most of these mutations are fatal)
Some of these chromosomal abnormalities include trisomy 21 (Down syndrome) and Turner syndrome (XO, the loss of the second female sex gene)