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 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 Primase RNA Primer 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 3’ 5’ 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

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