2. 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’)
4. 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
5. 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
6. DNA Replication
• Basic Concept = create
a new strand by
matching nucleotides
to an existing strand
8. 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
10. 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
3’ 5’
5’
Primase
RNA Primer
DNA Polymerase III
11. General Process
• Step 4: DNA
Polymerase builds the
new strand of DNA in
a 5’ to 3’ direction by
adding matching
nucleotides
12. 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
13. 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
15. 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
