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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
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
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
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.
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)
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
• 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