PART IV: REVIEWING PCAT BIOLOGY in .NET framework

Generation ECC200 in .NET framework PART IV: REVIEWING PCAT BIOLOGY

glycolysis to continue in the absence of a functional electron transport chain It is important to note that fermentation itself produces no ATP The only ATP created during anaerobic respiration (glycolysis and fermentation) is from the glycolysis step Therefore it is absolutely critical to regenerate the NAD+ needed to continue with glycolysis The total net gain of ATP from anaerobic respiration is 2 ATP as compared to 36 from complete aerobic respiration

Lactic acid fermentation occurs in some types of bacteria and fungi, as well as in the muscle cells of animals when oxygen levels are not suf cient to meet the demands of aerobic respiration In this step, pyruvate is reduced to lactic acid, thus regenerating the NAD+ needed to continue glycolysis In humans, large amounts of lactic acid are responsible for muscle fatigue after major exertion

Some organisms, such as certain bacteria and yeast, use alcoholic fermentation In this step, pyruvate is decarboxylated, which produces CO2 gas, and then reduced to form ethanol As with lactic acid fermentation, NAD+ is recycled so that glycolysis may continue

Deoxyribonucleic acid (DNA) is the genetic material of the cell The information encoded in DNA ultimately directs the synthesis of particular proteins within cells These proteins determine all of our biological characteristics When a cell divides, DNA will selfreplicate to ensure that progeny cells receive the same DNA instructions as the parent cell

DNA is a nucleic acid polymer consisting of the nucleotide monomers A nucleotide is a hybrid molecule consisting of deoxyribose (a sugar), a phosphate group (PO4), and a nitrogenous base There are four nitrogenous bases used to make nucleotides: adenine (A), thymine (T) cytosine (C), and guanine (G) Each nucleotide differs only by the nitrogenous base used In total, there are four possible nucleotides used in DNA The nitrogenous bases of each nucleotide can be classi ed as a purine or pyrimidine based on their chemical structure A purine is a double-ringed structure while pyrimidines are single-ring structures The nitrogenous bases A and G are purines while C and T are pyrimidines

James Watson and Francis Crick proposed their famous model for DNA structure in 1953 By using information from other studies, they knew that DNA existed in a double-stranded conformation and that the amount of A and T in a DNA molecule was always the same, as was the amount of C and G Using this information, they developed the model of DNA structure seen in Figure 712 A single stand of DNA has a sugarphosphate backbone Two strands of DNA are hydrogen-bonded together via their nitrogenous bases The idea of complementary base pairing is essential to this model This means that a purine must pair with a pyrimidine An A on one strand of DNA will always bond to a T on another strand of DNA, using two hydrogen bonds A C on one strand will always bond to a G on another strand, using three hydrogen bonds This base pairing holds together the two strands of DNA, which then twist around themselves to take on the double helix conformation

FIGURE 712: DNA double helix The Watson and Crick model of DNA structure shows two strands of DNA that run antiparallel to each other

Each strand of DNA has a speci c polarity or direction in which it runs This polarity is referred to as 5' and 3', referring to particular carbon atoms in the ribose molecule The complementary strand of DNA always runs antiparallel, in the opposite direction of the original strand So if one DNA strand runs 5' to 3', the other strand of the double helix runs 3' to 5'