DNA Replication Advanced Biology Replication n Replication the

DNA Replication Advanced Biology

Replication n Replication- the process of copying DNA in a cell (specifically in the nucleus) n n End result is two identical copies of DNA Use for cell division (one copy will end up in each cell) DNA replication is called semi-conservative replication because one of the original strands is present or conserved in each new molecule of DNA Can also be used to repair DNA that has been damaged from heat, radiation, or chemicals

Replication Vocabulary n Template strand n n Original DNA strand Non-template strand n Newly formed DNA strand

Process of Replication n DNA strands will unwind n n Allows for more room to work on the DNA strands The entire strand will not unwind all at once; only a few parts will unwind

Process of Replication n DNA strands “un-zip” n n n Enzyme called helicase “un-zips” the strands at the complementary bases Two chains of nucleotides separate (template strands) Hydrogen bonds between two nitrogen bases are broken Adenine and thymine have a double bond n Cytosine and guanine have a triple bond n

Process of Replication n “Extra” nucleotides attach to each DNA strand by DNA polymerase (forms non-template strand) n n n RNA polymerase lays down a “primer” of RNA so that the DNA has something to attach to, since DNA polymerase can’t start the synthesis of DNA Binding proteins support the original strand (just after it separates from the other strand) Strands will be built from the 5’ 3’ end

Replication n Since replication occurs in one direction, one strand will be working towards the replication fork (leading strand) and the other will be working outwards from the replication fork (lagging strand) n n As helicase will be moving along the origianl DNA, one side will be discontinous Will have fragments of DNA Okazaki fragments

Replication n RNA primers are taken off of the non-template strand DNA ligase will connect the Okazaki fragments Covalent bonds attach the backbone together Hydrogen bonds attach the strands back together

Replication n n Does not start at one end of each DNA strand, it will simultaneously start at various points on the strand Enzymes (DNA polymerase) “proofread” the strand before completion to minimize mutations n n Mutation: a change in the nucleotide sequence If a mutation occurs early in development of a fetus, that mutation can be transferred as the DNA is replicated during its growth and development

Replication n Cancer is usually associated with rapidly dividing cells n n Sometimes cancerous cells are treated with chemotherapeutic drugs Act like the four different nucleotides found in DNA n If the cancerous cells mistake the drugs for their DNA, replication stops and the cells die

Semi-Conservative Replication n n In 1958, Matthew Meselson and Franklin Stahl confirmed theory of semi-conservative replication Grew E. coli in a medium that had 15 N (heavy isotope of nitrogen) n n n Nitrogen isotope was used by the DNA during protein synthesis and replication Bacteria grown in “normal” (14 N) media have a light density DNA grown in 15 N rich medium had very heavy DNA (very dense)

Semi-Conservative Replication n Bacteria (with nitrogen heavy DNA) were transferred to a “normal” growth medium and were allowed to reproduce once n n DNA after these replications had a density that was between the “normal” DNA and the “heavy” DNA Tells us that half of the DNA has the 15 N and the other half (non-template strand) has 14 N or has an “intermediate” density

Semi-Conservative Replication n When the bacterial DNA was allowed to replicate again (for a third time), there were two versions (densities) of DNA n n Half the DNA had the “normal” density (both strands used the 14 N) Half the DNA had the “intermediate” density (one strand—non-template—using the 14 N and the other strand—template—used the 15 N)