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Day 7: Jumpin' Genes!

Mitosis

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The genetic material of a species is the blueprint from which an entire organism is made. It consists of long strands of DNA, called chromatin, which can be read by the cell like Morse code. Sequences of DNA along the strand which carry the code for a particular trait are called genes. In general most of the code is the same from one individual to the next within a species. For example: All humans (with a few rare exceptions) have two legs, two arms, two eyes, and they can stand upright and think. They all need to eat and breathe and these processes are all carried out in the same way. The details, however, vary: the color of your eyes, hair, and skin, your fingerprints, your size and shape. Even your talents and personality are at least partially determined by the genetic code that you carry in every cell of your body.

Each cell contains almost 200 meters of DNA strands which are organized and packaged into chromosomes. Each chromosome consists of one long molecule of DNA. The human nucleus contains 46 chromosomes. Each chromosome is a partner in a pair, so that there are 23 pairs of homologs. Homologs are two chromosomes who look the same under a microscope and whose genes code for the same inherited traits; one of the chromosomes of each pair comes from your father and one comes from your mother. 22 of the 23 pairs of human chromosomes are homologs - thier genes code for the exact same traits and they look exactly alike under a microscope. The remaining pair of chromosomes are known as the sex chromosomes. Females posess two X chromosomes which look exactly alike and code for the same traits. Males posess an X chromosome and a Y chromosome which look different and code for male traits.

DNA Replicates Unwinding Chromosomes

When the cell is not dividing, the chromosomes unwind. When a cell needs to make a particular protein, the portion of the chromosome which codes for that protein unwinds completely, so that the cell has access to the protein's DNA code. Those portions of the chromosome not in use remain tightly coiled and folded. Just before the cell divides by mitosis, each chromosome unwinds completely and a copy of the DNA molecule is made. At this point, each chromosome is composed of two identical DNA molecules, joined to each other near their midpoints by a centromere. The two identical molecules of DNA are referred to as chromatids. Each side of the centromere posesses an attachment site called a kinetochore, for each of the two identical DNA molecules.



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