Topic 12: DNA AS THE GENETIC MATERIAL (lectures 19-20)
OBJECTIVES:
1. Understand the 3-D structure of DNA, its chemical composition and its covalent and
hydrogen bonding patterns.
2. Understand what is meant by anti-parallel. Be able to identify 3’ from 5’ ends.
3. Understand what is meant by semi-conservative mechanism of DNA replication and
how the classic experiments of Messelson and Stahl proved that this mechanism
operates in organisms.
4. Know the overall mechanisms of DNA replication including the action of DNA
polymerase in the leading and lagging strands and the roles of other
macromolecules in this process.
5. Understand the role of telomeres.
Heredity- the study of the inheritance of characters from one generation to another.
Early workers concluded that some factor in cells was responsible for carrying the
hereditary information but where in the cell?
What is the chemical nature of this genetic material?- chromosomes/chromatin in
eukaryotes and nucleoidal material in prokaryotes consists of proteins and DNA
DNA- deoxyribonucleic acid (fig. 5.29); consists of polymers of four different
nucleotides (phosphate, deoxyribose sugar and one of four nitrogenous bases
(cytosine, thymine, adenine, guanine). DNA is known as a nucleic acid. Chargaff
(1948) showed that the base composition of DNA varied from species to species and
that A = T and G = C.
Watson & Crick- in mid 1950’s solved the structure of DNA
1. DNA consists of two very long strands of nucleotide polymers (fig. 16.3)
each DNA strand is a string of nucleotides connected to each other
by the phosphates at the 5 and then 3 position on the deoxyribose sugar;
strand has a 5’ end and a 3’ end
2. two strands are wrapped around each other in a double helix (fig. 16.5)
- two strands are anti-parallel; that is, they run in opposite directions
5’3’ and 3’5’ (fig. 16.12)
- the length of each helical turn is 3.5 nm (nm = 10-9 m)
- the two strands are stabilized by hydrogen bonding between the
G-C (3 H-bonds) and AT (2 H-bonds) (fig. 16.6)
DNA replication- the process by which DNA is copied prior to cell division; for DNA to
be the hereditary material, there must be a mechanism to accurately copy it so that
daughter cells contain the exact information as parent cells. The unique base-pairing
structure of DNA allows for replication (fig. 16.7).
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Messelson and Stahl showed that DNA replication takes place via a semi-conservative
mechanism (fig. 16.8).
Mechanics of replication1. origin of replication- sites along the DNA where the H-bonds of the base pairs break;
fig. 16.10; this forms a replication bubble. DNA synthesis goes in both directions.
At either end of the replication bubble is what is known as a replication fork.
Process goes on until both strands have been replicated.
2. DNA polymerase- an enzyme that creates a new DNA strand by attaching
nucleotides that have bases which are complementary to the parent (template)
strand; fig. 16.11
3. fig. 16.13- DNA is synthesized going in the 5’3’ direction only; so moving towards
the replication fork, the DNA polymerase makes a continuous strand known as the
leading strand. Going away from the replication fork, the DNA polymerase can only
make 100-200 nucleotide fragments (known as Okazaki fragments) which are joined
together by DNA ligase; this is the lagging strand.
4. there is an additional complication - namely that DNA polymerase can add
nucleotides only onto the 3’ end of an existing double stranded chain. That is, DNA
polymerase can only add nucleotides to the 3’ end on a growing double strand. A
small (10 nucleotides) primer made up of RNA (another kind of nucleic acid using
ribose instead of deoxyribose; more later) which is made by an enzyme known as
primase. Thus, the growing strand begins with an RNA primer which is eventually
replaced by DNA (fig. 16.14).
Overall process of replication (fig. 16.16)- several additional protein players in this
process1. helicase- unwinds the double helix
2. single-strand binding proteins- stabilize the single strand prior to action of DNA
polymerase
3. primase- synthesis a small 10 base RNA primer on both the leading strand and the
lagging strand
4. leading strand- continuous DNA formation; final step is replacement of RNA primer
with DNA by DNA polymerase
5. lagging strand-discontinuous DNA formation; with RNA primers being replaced with
DNA by DNA polymerase and fragments being joined together by ligase
Fig. 16.15- Key Protein Players- helicase, single-strand binding proteins, primase,
DNA polymerase and ligase.
The end replication problem.
Fig. 16.18- DNA polymerase can only add nucleotides to a 3’ end; with each round of
replication the DNA molecule gets shorter. How is damage to genes prevented? The
ends of eukaryotic DNA have telomeres which contain many repetitions (10 to 1,000) of
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a short nucleotide sequence ( like TTAGGG). Telomeres protect the coding regions of
the DNA (even though the DNA is getting shorter).
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