Chromosomes Each organism has its own unique set of DNA molecules. It

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reproduce within in its own species because those organisms DNA that is similar. In eukaryotic organisms, the DNA is

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contained within the nucleus. A chromosome is a long DNA molecule with part or all of the genetic material of an organism. The number of chromosomes varies from species to species. The male Australian ant, Myrmecia pilosula, has only one chromosome in its nucleus. The female of the species has two chromosomes. This species of ant, the Jack Jumper ant, has the least number of chromosomes in a eukaryotic organism. In contrast, some ferns can have up to 1260 chromosomes. The number of chromosomes does not have any other significance other than the number of divisions within the organism’s genome. A genome is all genetic material of an organism. Having the same number of chromosomes is one way a species is limited to reproducing only with others of the same species. The number of chromosomes can be the barrier of closely related species reproducing with each other. For instance, a horse has 64 chromosomes and a donkey has 62. You can breed a horse and a donkey to create a mule, but the hybrid offspring is unable to reproduce. With 63 chromosomes it is much harder for a mule to go through the process of meiosis to create gametes. They cannot create their own offspring. Humans have 46 chromosomes or 23 pairs. Each chromosome has a unique set of genes and the chromosomes are numbered by their size. Chromosome number 1 is the largest and has the most genes and chromosome 22 is the smallest of the autosomes. Autosomes are chromosome that are not related to the sex of the organism. Chromosomes 1-22 are autosomes in humans. The 23rd chromosome is the sex chromosomes. Sex chromosomes are chromosomes involved with determining the sex of an organism. You get half of your chromosomes from your mom and half from your dad. Although the genes on each chromosome are the same, the form or allele may be different.

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Most of the time, DNA is loose within the nucleus and kind of looks like spaghetti. This is called chromatin. Chromatin is a substance within a chromosome consisting of DNA and proteins that is present during interphase. Cells carry out normal cell functions during interphase. The DNA must be accessible in order for these processes to continue. When it is time to move chromosomes to new daughter cells, created from the parent cell, they must be in a form that prevents loss of genes. This form is highly coiled around histones. Histones are proteins that pack and order the DNA into the structural units called chromosomes. I like to think of dividing the nucleus like when you are moving to a new home. You don’t just dump the contents of every cupboard, closet or drawer loosely in the moving van. This would lead to loss of items and chaos. Instead, you create different boxes that contain closely related items or items that go to specific rooms. This way, nothing is lost during the transition. Histones help to prepare the chromosome for moving. In order to go through mitosis, chromosomes are duplicated during the S phase of interphase. After replication, the chromosomes are made up of two identical sister chromatids attached by a centromere. The histones in the chromatin have coiled further and further to create a tightly coiled chromatid. A chromatid is one half of a duplicated chromosome that is held together by the centromere. The centromere is a specialized DNA sequence of a

chromosome that links a pair of sister chromatids and where spindle fibers connect. The chromosomes resemble an X at this stage. At the end on each chromatid there is a portion called the telomere. Telomeres are regions of repetitive nucleotide sequences at each end of a chromosome that protects the end of the chromosome from deterioration. They are like the aglets at the ends of your shoelaces. They keep the DNA from losing nucleotides that are associated with genes. Telomeres do wear with time and this is thought to be part of the aging process.

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