AP BIOLOGY ESSAY QUESTION DNA (TAB 4) – Flashcards

In protein synthesis, the first step is the transcription of mRNA from a DNA gene in the nucleus. Messenger RNA is read from the gene for that protein by base pairing. The order of nucleotides in mRNA determine the order of amino acids, which control what the gene codes for. Transcription occurs in the nucleus of eukaryotic cells. For every gene there is a coding strand of DNA and a "dummy" strand. Both strands are located randomly on the DNA molecule. The locations of these strands are determined by the evolutionary origins of their chromosomes. Only the coding strand will be translated into RNA base pairing. RNA polymerase is the enzyme that arranges DNA into RNA. The RNA polymerase builds the RNA strand from the 5' to the 3' end. Eukaryotes have 3 types of RNA polymerase. RNA polymerase recognizes an area in DNA called the promoter and binds to it. The promoter is a string of nucleotides in DNA to which RNA polymerase binds. Promoters signal the beginning of a gene. When the RNA polymerase binds to a promoter, its starts transcribing DNA into RNA. The mRNA is then read into protein on the ribosomes. Protein is made on a ribosome. A ribosome has a large subunit and a small subunit. The large subunit contains 2 RNA molecules ( one large, one small), while the small subunit contains 1 RNA molecule. The large subunit has three sites that can hold an amino acid: the A site, the P site, and the E site. The next step is initiation, this is when the small subunit of the ribosome attaches to an mRNA molecule. The large ribosomal subunit attaches to the mRNA-formyl methionine complex with the formylmethionine tRNA going into the P site of the large subunit. Next proteins bring all the components together. Elongation is the next process to occur. A second tRNA with its amino acid goes into the A site of the large ribosomal subunit.A peptide bond is formed between the first two amino acids. The tRNA in site P, moves to the E site and exits the cell. The tRNA in the A site moves into the P site. The next tRNA moves into the A site and another peptide bond is formed. This process continues adding one amino acid at a time. Finally termination occurs, this happens when the ribosome reaches one of the three termination codons which are : UAA, UAG, UGA. At this point the protein is released from the ribosome and the ribosomal subunits separate and move to find more mRNA.

DNA is deoxyribonucleic acid, which is found in chromosomes, contains inherited information, they are made up of nucleotides, and are what make up genes. A nucleotide is composed of a sugar (deoxyribose), a phosphate group, and a base. There are 4 bases found in DNA, Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). Adenine and guanine are double ring bases while thymine and cytosine are single ring bases. Nucleotides are joined to each other by covalent bonds between the phosphate group of one nucleotide and the 3' carbon atom of the deoxyribose (sugar) of the next nucleotide. Each DNA molecule is unique because the order of nucleotides is unique. The order of nucleotides determines the order of amino acids in a protein. In order for DNA to replicate the initiator proteins must recognize a DNA sequence. Helicases break the hydrogen bonds between the two strands of nucleotides so that the strands can be separated. Then DNA polymerase adds new nucleotides to each strand. DNA needs to be very accurate or else there will be errors in the proteins. There are 50 known repair enzymes and they will repair the DNA after it replicated. Point mutation is when there is a change in a single base pair. These changes (depending on the type) can have no to a very drastic effect on the resulting protein. But the repair enzymes correct these mutations throughout the process.

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