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Transcript
DNA Structure and Replication Genetics DNA • Can replicate • Is the hereditary material • Controls cellular activities by coding for and controlling protein synthesis Long Road to Discovery • Griffith observed transformation • DNA passed from a harmful bacteria into a harmless one, and the new bacteria killed mice in the experiment Transformation DNA • Hershey and Chase confirmed that DNA, and not protein, was the hereditary material • Labeled viruses and watched what part entered the cell • The DNA went in, the protein coat stayed out Deciphering the Structure • Watson and Crick proposed the final model • They used the research of many others. • Levene identified the three parts: a sugar, a nitrogen containing base, and a phosphorous containing part. And that they were always present in equal numbers. Four different bases were recognized. Deciphering the Structure • Chargaff showed that equal numbers of the bases adenine and thymine were always present; and that guanine and cytosine were always present in equal numbers. • Wilkins and Franklin X rayed DNA and revealed a pattern of repeating building blocks. Deciphering the Structure • From all of that research, Watson and Crick put together the double helix model. Deciphering the Structure • Each repeating unit is a nucleotide. • The sugar is deoxyribose, and together with phosphate, makes up the sides of the DNA ladder • The bases make up the rungs Nucleotide Deciphering the Structure • Hydrogen bonds hold the bases together • Two strands of DNA are joined in the middle by the bases, and then the whole structure twists to become a helix. Structure of DNA • Antiparallel: the sides of the ladder run in opposing directions, as if one strand was upside down. • Each carbon on the deoxyribose sugar is numbered; one strand runs in the 5’ to 3’ direction, the other in the 3’ to 5’ direction. Page 221 The DNA wraps around the histones; the combination of DNA and a histone is a nucleosome DNA codes for Proteins • The sequence of the bases: adenine, thymine, cytosine, and guanine determines the order of amino acids in a protein chain. DNA Replication • DNA unzips using an enzyme called helicase • Each strand makes a new side • This is semiconservative; each new molecule of DNA has one old strand and one new. Semiconservative Old strand New strand Replication • Can be done in the lab to just certain sections or fragments of DNA • Requires a PRIMER, a short segment of nucleotides recognized as a START tag by DNA polymerases • Primers are on either end of the segment you want to copy Replication • Nucleotides can only be added to the 3’ end of the sugar • Nucleotides can only be added in the 5’ to 3’ direction; the 5’ to 3’ direction refers to the NEW strand being added • This is easy on one side, but the other strand is 3’ to 5’ (antiparallel) Antiparallel structure Replication • The 5’ to 3’ side must be replicated in short fragments. • It unzips, and then fragments are added by going up toward the fork and working back down. • Discontinuous; has a leading strand and a lagging strand Replication Replication • Enzymes are involved • DNA polymerase attaches short stretches of nucleotides to the template • DNA ligases connect fragments to make a continuous strand DNA Repair • Replication is very accurate—only about one in a million base pairs has a mistake • Some genes produce repair enzymes • Discovered when fungi exposed to UV radiation was repaired by being in light Types of damage and the fix • Ultraviolet radiation damages DNA • Causes the formation of an extra bond between two bases on the same strand • Most frequently happens to thymines Types of damage and the fix • This thymine dimer causes a kink in the strand • Enzymes called photolyases use light to detect and break the extra bond • This type of repair is called photoreactivation Types of damage and the fix • Another way to fix ultraviolet radiation damage is by excision repair • The dimer is cut out by a nuclease enzyme and is replaced with completely new nucleotides Types of damage and the fix • A third type of repair is mismatch repair • Enzymes proofread • Look for areas where the bases are not aligned properly, as if the strand slipped • Most common in repeating sequences Types of damage and the fix • Disorders can occur when mutations occur in the repair genes or there are problems in the repair enzymes • Conditions such as AT, Bloom syndrome, and Hereditary nonpolyposis colon cancer