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Mitosis in Onion Tip cells and Meiosis in Locust Testis Squash Cells

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Introduction

Mitosis in Onion Root Tip Cells The chromosomes in onion root tip cells go through interphase where they replicate and form 92 haploid chromosomes from 46 haploid chromosomes, this happens specifically in S phase. However the chromosomes are loose, not completely tightly coiled; therefore, they are named ?chromatin? and they haven?t yet formed diploid structures. At this phase the nuclear membrane still exists. Prophase is when the 92 haploid chromosomes condense and take the shape of fat fingers, forming 46 diploid chromosomes. Each diploid structure consists of two identical sister chromatids. The centriole splits into two daughter centrioles and moves to the opposite poles of the cell in order to conduct the separation of the centromere, which is the area that binds the sister chromatids. Metaphase is when all 46 diploid chromosomes align in the equator of the cell by the manoeuvring of spindle fibres (products of centrioles). The spindle fibres hold on to the centromere of the chromosome to accomplish the task of aligning them. ...read more.

Middle

chromosomes which are in the state of chromatin. They do not form diploid chromosomes as they are not yet condensed. The nuclear membrane still exists and protects the chromosomes. In Prophase I, the 92 single stranded chromosomes become condensed and form 46 diploid chromosomes. The centriole divides into two daughter centrioles and they each move to opposite poles of the cell. The nuclear membrane disappears along with the rest of the organelles. A tetrad is formed when two homologous diploid chromosomes come together. Every tetrad consists of two homologs, each containing sister chromatids of the maternal and paternal DNA, in total there are four chromatids. These homologous pairs go through a synapsis process where they join and intertwine; they are expected to exchange alleles (pieces of corresponding genes on the DNA) and form new genetic combinations in chromosomes; however, this process called crossing over may not always occur. During Metaphase I, once the nuclear membrane completely disappears and allows the spindle fibres which grow from centrioles to hook on to the centromere that holds the homologous chromosome pairs together and align the 23 tetrads at the equator of the cell. ...read more.

Conclusion

The spindle fibres grow and attach to the centromere (that holds the sister chromatids) from both sides of the cell in order to manoeuvre the chromosomes into aligning position. In Anaphase II, this time the centromeres of each diploid chromosome are separated and the sister chromatids are pulled to opposite poles by the shortening of the spindle fibres. This results in 23 haploid chromosomes on each side of the cell. Telophase II is when the nuclear membrane re-appears around the two sets of 23 haploid chromosomes in the two daughter cells. The haploid chromosomes become un-coiled and spaghetti-like again. The spindle fibres break down and disappear. Cytokinesis occurs to divide both the cells with each containing 23 haploid chromosomes. This results in four daughter cells, each containing 23 haploid chromosomes. The four daughter cells, each containing 23 haploid chromosomes are not genetically identical to each other due to the recombination between chromosome segments of the original parent cell, which is crossing over. Random assortment during the lining up of the 23 tetrads in Metaphase I has also caused genetic variation in the daughter cells produced. ...read more.

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