DNA Replication Lecture #23 Ms. Day Honors Biology

DNA Replication (Part 3A-initiation)

DNA Replication DNA Replication = DNA  DNA –Parent DNA makes 2 exact copies of DNA –Why?? Occurs in Cell Cycle (S phase)

DNA Replication: a closer look ication.mov

DNA Replication Steps: Initiation –involves assembly of replication fork (bubble) at origin of replication sequence of DNA found at a specific site Elongation –Parental strands unwind and daughter strands are synthesized. –the addition of bases by proteins Termination: –the duplicated chromosomes separate from each other. Now, there are 2 IDENTICAL copies of DNA.

Segments of single-stranded DNA are called template strands. Copied strand is called the complement strand (think “c” for copy) BEGINNING OF DNA REPLICATION (INITIATION) DNA helicase (think “helix”) –binds to the DNA at the replication fork –untwist (“unzips”) DNA using energy from ATP –Breaks hydrogen bonds between base pairs THE REPLICATION FORK and BUBBLE IS NOW CREATED!

base pairs 5’ 3’ DNA is unzipped by helicase using ATP Helicase ATP

base pairs 5’5’ 5’ 3’ 3’3’ Helicase ATP DNA polymerase –A DNA chain making enzyme –adds nucleotides to make daughter (new) DNA strands –Adds 1 nucleotide AT A TIME  matches up to opposite nucleotide on template (parent) strand DNA Replication (Elongation) DNA Polymerase

DNA Replication: Elongation on16.html animations.asp

1.) Make telomeres Telomeres –Short repeats of “G” base found at END of LINEAR chromosomes in e ukaryotes –protect ends of linear chromosomes Telomerase is the enzyme that makes telomeres DNA Replication (Termination)

Telomeres, Aging & Cancer Telomeres get shorter as cell divides  leads to aging??? Most cancers come from body cells –Cancer cells divide indefinitely. –Normal cells limited to ~50-75 divisions  stop making telomerase. –85–90% cancer cells continue to LOTS of telomerase & are able to prevent further shortening of their telomeres. Leads to “immortality”

Mistakes Made during DNA Replication Mutation –Change in DNA (genetic material) 2 Main Types of Mutations: –Frameshift(s) »extra or missing base(s). –Substitutions »when the wrong nucleotide is incorporated (mismatch mutation).

2.) Fix Mutations Mismatch repair (wrong nucleotide used) –DNA polymerase “Proofreads” new DNA as it makes it If mistake is missed my DNA polymerase, excision (“cut out”) repair needs to happen –Nuclease does this DNA Replication (Termination)

DNA Repair bio3/full/content/ch12/animations.asp