The
Molecular
Basis of
Inheritance
26 October, 2005
Text Chapter 16
What is the genetic material?
It could be DNA or protein. Transformation of bacterial cells must
involve transfer of genetic material.
Since bacterial DNA, but not bacterial protein is capable of
transforming R to S, DNA must be the genetic material.
DNA and not protein in injected into cells by phage.
Hershey and Chase, 1952
DNA Structure
Each DNA molecule is a linear
polymer of nucleic acid monomers.
The DNA backbone consists of
ribose and phosphate groups. This
part of the molecule is the same for
every monomer.
The variable part of DNA consists
of four different nitrogenous bases
attached to the C1 of the ribose
ring.
Base Pairing
Two DNA molecules with
complementary sequences can form
a double helix. This structure is
only possible when the sequence of
bases on the two strands allow base
pairing over an extended region.
The two strands (molecules) in a DNA duplex are antiparallel.
Base Pairing
The complementary nature of a DNA duplex allows each strand to serve
as the template for the synthesis of its partner.
Semiconservative Replication
In living systems, DNA replication is semiconservative. Each daughter
duplex contains one old and one newly synthesized strand.
Semiconservative Replication
Replication Origins, Forks, and Bubbles
DNA replication begins at each of several origin sequences.
Replication proceeds until the entire double-stranded molecule
has been replicated.
DNA monomers are triphosphates that are hydrolyzed in the process of
polymerization.
This means that DNA
molecules can only
grow in the 5’ to 3’
direction.
DNA Synthesis
At each replication fork,
there are two strands of
DNA to be synthesized.
From the point of view of
the leading strand, the
fork moved in the 5’ to 3’
direction.
From the point of view of
the lagging strand, fork
movement is in the 3’ to
5’ direction.
Priming
Since DNA synthesis
requires a free 3’ OH
group, DNA synthesis
begins with the synthesis
of an RNA primer.
This primer is replaced
by DNA later.
DNA Synthesis