Download DNA Structure and Replication

DNA Structure and Replication
2002
Structure
-Was determined by Watson and Crick based on an x-ray by Rosalind Franklin
-Double helix made of deoxyribose and phosphate backbone; nitrogen bases make up the
rungs
-A-T are held together by 2 H bonds
-C-G are held together by 3 H bonds
-Strands are complementary which provides a mechanism for replication
DNA Replication
-Each strand acts as a template for the formation of the new strand; semi-conservative
replication
-Is under the control of many enzymes and is a very rapid, accurate process (500
nucleotides per second in prokaryotes, only 1/1,000,000,000 is incorrectly paired)
Steps in replication
-An enzyme—DNA helicase—binds to the DNA strand at a site called the origin of
replication
-DNA opens at the origin and replication forks open in both directions creating a
replication bubble
-DNA polymerase forms the new DNA strand
-Nucleoside triphosphates are aligned to the template strand
-DNA polymerase links the new base to the growing strand by breaking off 2 phosphate
groups
-DNA polymerase links new bases to the template strand
-Always adds them in the 5’ to 3’ direction on the leading strand; is synthesized in a
continuous strand from a replication fork
-3’ to 5’ strand is the lagging strand
-It is replicated in short discontinuous pieces called Okazaki fragments
-Each piece is synthesized in the 5’ to 3’direction and is them attached together with
DNA ligase
-At each replication fork RNA primers are needed so DNA polymerase can add new
bases to the DNA strand
-Proofreading enzymes check for errors and repair them which decreases the rate of
mutation from 1/1,000,000 to 1/1,000,000,000
-At the ends of each chromosome are telomeres—non-coding regions that keep genes
from being deleted; get shorter with each replication