DNA Replication DNA Structure Nucleic acid Double Helix

DNA Replication

DNA Structure • Nucleic acid Double Helix (2 strands of DNA) – Nucleotide – sugar, phosphate group and a base • Complementary strands pair up (Adenine & Thymine, Cytosine & Guanine) – Base pairs held together by hydrogen bonds • Strands are antiparallel (5’ - 3’ and 3’ – 5’)

Nucleus

Nucleus • Nuclear envelope is a double membrane and has two overlapping phospholipid bilayers with nuclear pores • Chromatin – total DNA and proteins in nucleus • Nucleolus – ribosome subunits are made here

Chromosomes – single DNA molecule wrapped around histone proteins, then coiled into chromosome cylinders. Humans have 46 chromosomes -One pair of sex chromosomes (XX – girls and XY – boys) - 22 pairs of Autosomes - all other chromosomes

DNA Replication • Basic Concept = create a new strand by matching nucleotides to an existing strand

DNA Replication • Replication is semi-conservative (one strand is old, one strand new)

General Process of DNA Replication • Step 1: Initiator Proteins bind to origins of replication to begin replication – Attracts other enzymes involved in replication process Step 2: DNA Helicase separates the DNA helix

Question… • What kind of bonds does Helicase break? – Hydrogen bonds

General Process • Step 3: Primase builds an RNA primer at the starting from the 5’ end of the new DNA strand – Uses the 3’ end of existing DNA strand – Why? Because the DNA Polymerase (main DNA building enzyme) needs something to build off of Primase 5’ 3’ 5’ RNA Primer DNA Polymerase III

General Process • Step 4: DNA Polymerase builds the new strand of DNA in a 5’ to 3’ direction by adding matching nucleotides

Problem… • How can both strands of DNA be replicated in a 5’- 3’ direction at the same time they are antiparallel? • Answer: leading and lagging strands

Leading and Lagging Strands • Leading strand is synthesized continuously in the direction of replication (goes in the same direction as helicase) • Lagging strand is synthesized in short fragments the opposite direction of replication (opposite direction as helicase) • DNA Ligase seals DNA fragments together

Lagging Strand

Finishing DNA Replication • Problem #1: There are still RNA nucleotides in the DNA (primers) • Solution = DNA Polymerase cuts out the RNA nucleotides and replaces them with DNA

Helicase SSBs DNA Polymerase III Leading Strand DNA Ligase DNA Polymerase I Primase DNA Pol Primer III Lagging Strand