DNA Replication: DNA Structure, Nucleus, Chromosomes

Contributed by:
Sharp Tutor
This presentation will give an idea about how DNA replication occurs. In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part of biological inheritance.
1. DNA Replication
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
7. DNA Replication
• Replication is semi-conservative (one strand is
old, one strand new)
8. General Process of DNA Replication
• Step 1: Initiator Proteins bind to origins of
replication to begin replication
– Attracts other enzymes involved in replication
Step 2:
DNA Helicase separates the DNA helix
9. Question…
• What kind of bonds does Helicase break?
– Hydrogen bonds
10. General Process
• Step 3: Primase builds an
RNA primer at the starting 3’ 5’
from the 5’ end of the new 5’
DNA strand
– Uses the 3’ end of existing RNA Primer
DNA strand
– Why? Because the DNA
Polymerase (main DNA
building enzyme) needs
something to build off of 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
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
14. Lagging Strand
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
16. SSBs
Helicase DNA
Polymerase III
Leading Strand
DNA DNA Ligase
Polymerase I
Primase DNA Pol
Primer III Lagging