4.1_Gene_Expression_I(1) - Last week, we looked at how...

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Last week, we looked at how cells divide and pass on genetic material to daughter cells. Further, we saw that through meiosis, genetic information is re-arranged and combined in unique combinations to create genetic diversity in the offspring of sexual reproduction. The principles of how this genetic information is passed on was explained when we examined inheritance. This week, we are going to see how this genetic information is used by individual organisms. 1
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First, we need to review this structure of DNA and RNA and examine how the structure contributes to replication and genetic expression. The expression of DNA through transcription and translation to form proteins is such an important concept in biology that it has been termed the central dogma of molecular biology. 2
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Deoxyribose nucleic acid or DNA for short is a complex molecule made of repeating units of nucleotides. There are four types of nucleotides that are incorporated into DNA and they vary based on the type of nitrogenous base they contain. Otherwise, all nucleotides are the same in that they contain a phosphate group and a deoxyribose sugar molecule. The supporting backbones of the molecule are covalently bonded phosphate groups and sugars. Two complementary sequences of nucleotides form the helical staircase by means of hydrogen bonds of specific nitrogenous bases: adenine with thymine and guanine with cytosine. 3
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4 The four nitrogenous bases are of two categories: The pyrimidines, cytosine and thymine, consist of a single ring. The purines, adenine and guanine, are double-ringed structures.
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5 These figures further illustrate the bonding between DNA strands. It’s important to note several things: firstly, the hydrogen bonds between bases are very specific; that is, purines bond with pyrimidines such that adenine bonds with thymine and cytosine bonds with guanine. This allows a consistent width of the helix. Second, the two strands run in opposite directions. You can see this with the orientation of the sugar and phosphate group backbones. This has significance relative to enzyme attachment for replication.
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6 Now looking further at the orientation. This figure emphasizes the opposite polarity of the DNA strands. The 3 end has a free hydroxyl group attached to the 3 carbon of the sugar, while the 5 end has a free phosphate attached to the 5 carbon of the sugar. The term prime refers to the apostrophe notation. DNA polymerase enzymes elongate the chain by adding to a free hydroxyl group so that synthesis occurs in the 5 3 direction.
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This note was uploaded on 01/29/2012 for the course BIOL 212 taught by Professor Rockhill during the Spring '08 term at Seattle Central Community College.

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4.1_Gene_Expression_I(1) - Last week, we looked at how...

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