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14 15 a WorLd of variation how it all gets mixed up Darwins theory depended on a mechanism that could create small variations in offspring. The answer was hidden deep within the way gametes sperm, eggs, or plant spores are produced. As Mendel had guessed, a complete copy of all the DNA needed to build an entire organism is held in every cell nucleus, organised into discrete chromosomes, with one homologous similarly mapped but different copy from each parent. During meiosis opposite, genes are snipped out and swapped between the parental pairs, the reshuffling and recombining creating new chromosomes for the gametes, each with different trait potentials crossover shown below. This was one source of the variation Darwin had been looking for. Indeed, shuffling is all very well, but wild cards also play their part. During meiosis all sorts of things can go awry copying errors, occasional deletions, duplications, sometimes inversions of sections of DNA, sometimes in genes, but more often in noncoding DNA. Often subtle the tiniest change to a single letter or protein, they occasionally turn out to be advantageous, but can also be fatal. There is a gene on chromosome 4 which simply contains the word CAG, repeated again and again. Most people have it repeated from anywhere between 6 and 30 times, but if you have it repeated over 35 times you will slowly die of WolfHirschhorn syndrome. Another example A singleletter change in a 253word gene on chromosome 20 will give you mad cow disease. PROPHASE DNA in nucleus. Aster and spindles develop from centrioles. PROPHASE I Maternal and paternal copies of chromosomes pair up. Each chromosome replicates, creating a chromatid. CROSSOVER occurs between maternal and paternal versions METAPHASE I Shuffled chromosome chromatid pairs are separated TELOPHASE I Division into two PROPHASE II Division METAPHASE II ANAPHASE II Further separation of pairs TELOPHASE II Creation of four haploid cells, each with one copy of each chromosome M IT O S IS C EL L D IV IS IO N M E I O S I S P R O D U C T I O N O F S P E R M O R E G G PROMETAPHASE Nuclear envelope breaks down. Chromosomechromatid pairs exact copies of chromosomes attach to spindles. METAPHASE Chromosomechromatid pairs line up at the equator of the cell. ANAPHASE The spindles pull the chromosomes and chromatids apart. In humans, 46 chromosomes are pulled into each half. TELOPHASE Chromosomes reach mitotic poles and cell starts to pinch. CELL DIVISION followed by INTERPHASE chromatids created for next division.
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