Discovering the structure of DNA DNA Deoxyribose nucleic

Discovering the structure of DNA • DNA = Deoxyribose nucleic acid • Made out of sugars (deoxyribose), phosphates and nitrogen bases

DNA Nucleotide Phosphate Group O O=P-O O 5 CH 2 O N C 1 C 4 Sugar (deoxyribose) C 3 C 2 Nitrogenous base (A, G, C, or T)

Discovering the structure of DNA • Structure was discovered in 1953 by James Watson and Francis Crick

Discovering the structure of DNA Rosalind Franklin’s DNA image “Chargoff’s rule” A=T & C=G

Cell division and DNA replication • Cells divide Growth, Repair, Replacement • Before cells divide they have to double cell structures, organelles and their genetic information

DNA replication Double helix unwinds • Enzymes, called DNA helicase, breaks the H bonds • 2 strands separate and form replication fork • Other enzymes and proteins hold them apart

DNA replication • Enzymes, called DNA polymerase, move along the separated strands and add nucleotides to the exposed bases by the base pairing rule • As DNA polymerase moves along the strands, 2 new double helixes are formed • Enzymes continue adding nucleotides until it reaches a signal for it to detach

DNA replication Two complementary polynucleotide strands are joined by hydrogen bonds To make a DNA molecule.

During replication the following steps are occurred. 1. DNA molecule unwind 2. The hydrogen bonds between complementary base pairs break 3. Two strands are separated from each other 4. Each single strand act as a template for a new complementary strand. 5. Complementary nucleotides are assembled alongside each strand. 6. Sugar-phosphate backbone is formed by phosphodiester bonds between adjacent nucleotides 7. Once this had occurred along the whole length of the each strand two identical molecules of DNA are formed. W. B. K. HETTIARACHCHI

1 Hydrogen bonds break 2 Two strands separate W. B. K. HETTIARACHCHI

DNA replication Let us animate the process of replication

W. B. K. HETTIARACHCHI

H bonds break Two strands seperate W. B. K. HETTIARACHCHI

Nucleotides with Complementary bases are assembled alongside each strands Sugar phosphate backbone is made by joining the adjacent nucleotides ( DNA polymarase enzyme( ) ) W. B. K. HETTIARACHCHI

Two identical DNA molecules are formed W. B. K. HETTIARACHCHI

Proof Reading – DNA polymerase cannot add a new nucleotide unless the previous nucleotide is correctly paired – The enzyme can also remove mismatched enzymes and add the correct ones – Semi-conservative Model of Replication- each new strand has one old strand from the parent and one newly synthesized strand.

Nucleotides as Language • We must start to think of the nucleotides – A, G, C and T as part of a special language – the language of genes that we will see translated to the language of amino acids in proteins

DNA Replication Video Clip

• What would the complementary strand be? • T C G A T A C G C T T A

A strand of DNA that contains the bases TACGAT replicates. Which base sequence is in the new strand produced during replication? A. B. C. D. ATGCTA AUGCUA TACGAT CGTGGC