Bellwork: Describe the discoveries that lead to the modeling of DNA

DNA replication Section 12.3

How does DNA duplicate? Each strand on the double helix has the ability to replicate the other strand due to the complementary base pairing Before a cell divides, DNA will replicate Two strands of helix unzip, producing two replication forks Complementary base pairs are added Each resulting DNA molecule has one original strand and one new strand

Enzymes and DNA replication Enzymes carry out DNA replication Enzyme are proteins with highly specific functions An enzyme unzips DNA helix by breaking the hydrogen bonds - Helicase Two strands unwind DNA polymerase joins individual nucleotides to produce a new strand of DNA Also checks – or proofreads to ensure that each molecule is a near perfect copy of the original

Telomeres The tips of the Eukaryotic chromosomes are called telomeres Ends of DNA are hard to replicate Special enzyme – telomerase adds short repeated DNA sequences to telomeres as chromosome are replicated Helps prevents genes from becoming damaged or lost during replication Often switched off in adult cells Often turned on in cancer cells, explaining why the replicate so rapidly

DNA replication Process different in prokaryotic and Eukaryotic cells Occurs during the S phase Carefully regulated Completed before mitosis/meiosis Prokaryotes – single circular DNA molecule Eukaryotic cells have 1000 times more DNA Structures more complicated Histones – proteins around which chromatin is tightly coiled

Prokaryotic DNA replication Does not start until regulatory proteins bind to a single starting point on the chromosome Proteins trigger S phase and DNA replication Replication start sat one point and proceeds in two directions until the chromosome has been copied Often two membranes are attached to different parts inside the cell membrane and separate when cell splits in two

Eukaryotic DNA replication Chromosomes are bigger than prokaryotes Replication may begin at dozens or even hundreds of places, and proceed in both directions until each chromosome is copied A number of proteins check for damage and/or base pair mismatches Not foolproof – damaged areas are sometimes replicated leading to serious consequences Two strands are closely related until prophase Chromosome condense and chromatids in each chromosome become visible and separate

Writing assignment – How has our understanding of DNA developed since it was first identified as the carrier of genetic material in Avery’s experiments in the 1940s? How does it replicate?