DNA = Deoxyribonucleic Acid Carries coded information that controls the traits of living things

How can DNA be seen? X-ray Crystallography: tool used for identifying the atomic and molecular structure of a crystal

The First X-Ray Crystallography 1952 Taken by Rosalind Franklin Revealed that DNA: Is double stranded Has a helix structure Made of repetitive pieces Is long and thin

James Watson and Francis Crick Awarded Nobel Prize in 1953 for “discovering” the structure of DNA

What is DNA built from?

Monomer: Nucleotide Nitrogen base Phosphate group Deoxyribose sugar

The name of nucleotide is determined by base it contains.

How is DNA put together?

Nucleotides are joined in any order Sequence creates a code Backbone/ sides- sugar and phosphate

Bases: point inward Hydrogen bonds hold the bases together. Sides are antiparallel.

Strands are identified by a numbering system… based on orientation (position) of sugar 5’ 3’ 5’ 3’

Importance of the Sequence of Bases The order determines the information it stores Change in the order = change in the traits

Summary of DNA Structure Double stranded Helical (helix) shape Sides are anti-parallel Complementary bases Repeating monomers (nucleotides)

DNA REPLICATION

How (and WHY) does DNA make copies of itself? Process: Replication Purpose: to make exact copies of DNA (must occur before a cell divides) Each cell needs a copy of the info in DNA in order to grow Occurs: in the nucleus

Step 1: DNA “Unzips” Weak hydrogen bonds break Two strands separate (each strand is a template) The enzyme helicase uncoils and unzip the double helix by separating the base pairs

Step 2: Pairing of Nucleotides Free DNA nucleotides pair up with complementary base (A-T and C-G) Uses DNA polymerase to pair them

Step 3: Binding Nucleotides bind to each other (H-bonds reform) Uses DNA ligase (like a molecular “glue”) Results in 2 new DNA molecules – each has a strand from the parent molecule and a new complimentary strand

How is accuracy insured?

Accuracy Original side  template Each new DNA molecule will have One strand that is the original One strand that is new Both are identical to the template strand DNA polymerase also “proofreads” DNA polymerase initially makes about 1 in 10,000 base pairing errors Enzymes proofread and correct these mistakes The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors Certain chemicals & ultraviolet radiation damage the DNA in our body cells Cells must continuously repair DAMAGED DNA Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA DNA polymerase and DNA ligase replace and bond the new nucleotides together

Called Semi -Conservative Replication: “one DNA strand is “conserved” (saved)

How is DNA copied so quickly? Both strands are copied at almost the same time Copied in opposite directions

Faster strand is leading strand One strand is copied slightly slower …lagging strand

More Replication Details….. Add additional terms in red to your notes.

DNA polymerase adds new complementary nucleotides to the 3’ end.

The leading strand is built continuously. The lagging strand is built in fragments and then glued together by ligase.

Final product! Two new double stranded DNA molecules are produced by semi-conservative replication.