DNA: Structure + Replication

DNA:

• a complex molecule containing the genetic information of an organism. 
• a polymer made up of necleotides

One nucleotide ~ sugar, phosphate group & nitrogen base

• SUGAR - Deoxyribose
• NITROGEN BASES - A, T, C, G

Nucleotides join together in a condesation reation - forms polynucleotide chains

DOUBLE HELIX STRUCTURE - sugar phosphate backbone

held together by the bases + hydrogen bonds

A --> T : TWO HYDROGEN BONDS 

G --> C : THREE HYDROGEN BONDS

For cells to divide, DNA needs to replicate. It does this by the following process:

• HELICASE enzymes BREAK hydrogen bonds holding the two strand of DNA together (UNZIPS)
• Each polynucleotide strand acts a template ~ complementary nucleotides are attracted
• ENERGY used to activate nucleotides
• activated nucleotides join together ~  BY DNA POLYMERASE 
• these form the missing polynucleotide strand on each of the original strands of DNA

Process called: SEMI-CONSERVATIVE REPLICATION because:

• each NEW DNA molecule contains one of the original DNA strands
• so half the original DNA has been saved and built into each of the new DNA molecule

SEMI-CONSERVATIVE REPLICATION was proved by MENDELSOHN + STAHL:

• bacterial was collected in a medium of 'heavy' nitrogen ~ mean DNA strands would be heavy
• a sample of bacterial DNA was taken
• bacteria then transferred to a medium contain normal nitrogen and allowed to divide
• sample of bacterial DNA was taken again
• bacterial was allowed to divide again, a third sample was then taken. 

the three samples were analysed using CENTRIFUGATION ~ separates molecules by density

• parent strands ~ gave a single band in the lower region, contained only heavy nitrogen
• first gen, ~ gave a band of intermediate density, rules out conservative replication
• second gen. ~ gave two bands, one light and one intermediate, rules out dispersive replication