GENETICS (9-Feb-1998)

Biology 210

GENETICS

9 February, 1998

Chapter5b
The Molecular Structure
and Replication
of the Genetic Material

A Brief Outline of the last part of Chapter 5
5.4 The Replication of DNA
5.5 DNA Synthesis
5.6 Discontinuous Replication
5.7 The Isolation & Characterisation of
Particular DNA Fragments
5.8 The Polymerase Chain Reaction
5.9 Determination of the Sequence of
Bases in DNA

5.4 The Replication of DNA

Watson & Crick proposed a simple model - but the question was HOW does this work?

Figure 5.7, page 182 from your text. Figure 5_7

DNA is replicated "semi-conservatively" (See Figure 5.9 on page 186, or check out the "MIT Hypertext" link below.)

Click here for a link to the MIT Hypertext chapter on DNA Replication

5.5 DNA Synthesis

There are several enzymes that are involved in replication, as can be seen from the following diagram:

Here's a list of a few of the enzymes involved in DNA replication in Escherichia coli. (This is not complete, but representative - believe me - there's lots more where these came from!)

1. E.coli DNA polymerase I - the best characterised part is the large fragment, known as "Klenow".

2. E.coli DNA polymerase III - this is actually the main enzyme, but was the THIRD polymerase discovered in E.coli - hence its name is more a historical accident than a true representation of its importance.

3. E.coli DNA topoisomerase I - this can relax the DNA and facilitate the opening of the helix.

4. E.coli DNA Ligase - this enzyme is responsible for linking up the Okazaki fragments.

5. E.coli RNA primase is necessary to start the synthesis, since DNA polymerase needs a template. The RNA is later chewed up and replaced with DNA, once a longer chain of double-stranded DNA has been made.

6. E.coli DNA Single-Stranded Binding proteins (SSB) bind and stablise single-stranded DNA. This is necessary to keep the DNA from folding back on itself and also to help keep the unwound strands from rewinding.

Figure 5.27 from page 199 in your text. Notice that most of the enzymes mentioned above should be in this figure....
Figure 5.27 from Hartl & Jones

Here's a few other proteins associated with replication in other organisms. Also, I've included a link from the University College in London's Biochemistry department - more information about all of these proteins is avaliable at this site.

Bacteriophage T7 DNA Ligase

E.coli DNA Polymerase I - Klenow Fragment

DNA PolIII Beta-Subunit & PCNA

Rat DNA Polymerase Beta

HIV-1 Reverse Transcriptase

5.6 Discontinuous Replication

The DNA is always replicated from the 5' end to the 3' end. Because the two strands are antiparallel, this means there's a "leading strand", which just keeps on going, and a "lagging strand" which consists of small fragments that must later be linked together (by DNA Ligase enzyme).

Figure 5.21 (from page 195 from your text).
Figure 5_21 from Hartl & Jones

5.7 The Isolation & Characterisation of
Particular DNA Fragments

5.8 The Polymerase Chain Reaction

5.9 Determination of the Sequence of
Bases in DNA

Alaskan Victim of 1918 Flu Yields Sample of Killer Virus
(New York Times, 9-Feb-98)

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Last modified on: 1 February, 2000 by Dave Ussery