8.3
DNA Replication
The student is expected to:
3E evaluate models according to their
limitations in representing biological
objects or events;
5A describe the stages of the
cell cycle, including
deoxyribonucleic acid (DNA)
replication and mitosis, and
the importance of the cell cycle
to the growth of organisms;
9C identify and investigate the
role of enzymes
TEKS 3E, 5A, 9C
8.3
DNA Replication
TEKS 3E, 5A, 9C
KEY CONCEPT
DNA replication copies the genetic information of a
cell.
8.3
DNA Replication
TEKS 3E, 5A, 9C
Replication copies the genetic information.
• A single strand of DNA serves as a template for a new
strand.
• The rules of base pairing direct
replication.
• DNA is replicated during the
S (synthesis) stage of the
cell cycle.
• Each body cell gets a
complete set of
identical DNA.
8.3
DNA Replication
TEKS 3E, 5A, 9C
During DNA replication:
1. the DNA is unwound and unzipped by the
enzyme Helicase.
• The strands are held apart by single-stranded
binding proteins (also known as ssbps)
8.3
DNA Replication
TEKS 3E, 5A, 9C
2. each original DNA strand is used as a
template (or model) to make a new
DNA strand with base pairing
New DNA strand
New DNA strand
8.3
DNA Replication
TEKS 3E, 5A, 9C
3. The enzyme Primase lays down an
RNA Primer a few base pairs long to
which the new DNA can be added.
8.3
DNA Replication
TEKS 3E, 5A, 9C
4. Another enzyme, called DNA
Polymerase, adds new
nucleotides to the RNA
Primer.
– It always reads 3’ to 5’ and
synthesizes the new strand
from 5’ to 3’.
– This occurs in the direction
following Helicase opening up
the “replication fork”. For this
reason, the new strand is
called the leading strand.
8.3
DNA Replication
5. Another DNA Polymerase also
“proofreads” the new DNA to
check for errors.
6. Meanwhile, on the other strand,
known as the lagging strand,
Primase and DNA
Polymerase synthesize DNA
from 5’ to 3’ away from the
replication fork.
•
These small spurts of
replication form what are
known as Okazaki fragments.
TEKS 3E, 5A, 9C
8.3
DNA Replication
TEKS 3E, 5A, 9C
** DNA Polymerase only works in one direction. One
strand is read and synthesized continuously while the
other is synthesized in fragments.
New strand
Original
strand
DNA polymerase
Growth
DNA
polymerase
Growth
Replication
fork
Replication
fork
New strand
Original
strand
Nitrogenous
bases
8.3
DNA Replication
TEKS 3E, 5A, 9C
7. The Okazaki fragments are joined together by the
enzyme Ligase.
8. On both strands, the RNA Primer is replaced with DNA
nucleotides by the enzyme DNA Polymerase.
8.3
DNA Replication
• When replication is complete,
each DNA molecule is made of
one old strand and one new
strand. This is described as the
semi-conservative model of
replication.
9. The new DNA molecules are
rewound by the enzyme Gyrase.
TEKS 3E, 5A, 9C
Original DNA
New DNA
(one old strand,
one new strand)
8.3
DNA Replication
TEKS 3E, 5A, 9C
8.3
DNA Replication
TEKS 3E, 5A, 9C
Replication is fast and accurate.
• DNA replication starts at many points in eukaryotic
chromosomes.
There are many origins of replication in eukaryotic chromosomes.
• DNA polymerases can find and correct errors.
8.3
DNA Replication
TEKS 3E, 5A, 9C
• Two new molecules of DNA are formed, each with an
original strand and a newly formed strand.
• DNA replication is semiconservative.
original strand
Two molecules of DNA
new strand