FOR A PRINT-FRIENDLY VERSION OF THESE NOTES, PLEASE CLICK HERE. DO NOT print these pages! BIL 104 - Lecture 6

FOR A PRINT-FRIENDLY VERSION OF THESE NOTES, PLEASE CLICK HERE. DO NOT print these pages!

THE FUNCTION OF DNA

As you now know, DNA is the permanent "blueprint" of instructions for manufacturing and metabolically "running" the organism in which it occurs.
But DNA's mere presence does not magically cause the organism to grow and operate. Rather, DNA forms the manufacturing instructions for a number of "team players" that cooperate to form the moving machinery that builds and runs the organism.
Three major FUNCTIONS of the DNA system:

REPLICATION (manufacture of new DNA from original DNA)
TRANSCRIPTION (manufacture of RNA from DNA)
TRANSLATION (manufacture of protein from RNA)

Let's have a look at each of these processes, one at a time.

Before we do, however, let's have a brief overview of just what is an ENZYME.

a catalyst is a substance that speeds up a chemical reaction, but is not used up or permanently changed in the reaction.
An enzyme is a biological catalyst
Recall the structures of proteins
The pockets formed by tertiary and quaternary structure can hold specific substances (SUBSTRATES).
These pockets are called ACTIVE SITES.
When all the proper substrates are nestled in a particular enzyme's active sites, the enzyme can cause them to react quickly
Once the reaction is complete, the enzyme releases the finished products and goes back to work on more substrate.
ALL THE PROCESSES YOU WILL SEE TODAY ARE MEDIATED BY SPECIALIZED ENZYMES WHICH HAVE VERY SPECIFIC JOBS TO DO! Each one is a "specialist" which drives one reaction and one reaction only.
Enzymes almost always have names ending in "ase", and preceded by a word which describes what they do. For example:
- lipases - break down fats
- proteases - break down proteins
- DNAse - breaks down DNA
- nucleases - break down nucleic acids
- ...and there may be several types of each of the above

And now, we return to our regularly scheduled processes.

DNA REPLICATION

This process takes place only in the nucleus.
It is the manufacture of a new strand of DNA from the originally existing ("template") strand of DNA.
The process is necessary before a cell can divide, since if it is to make an identical copy of itself, it must first duplicate its own "instructions!"

In 1953, Watson and Crick published their two-page paper in the journal Nature, entitled: "Molecular structure of Nucleic Acids: A Structure for Deoxyribonucleic Acid". Their now-famous model

explained Chargaff's Rule

provided a mechanism for replication of the code

And was largely based on the x-ray crystallography of ROSALIND FRANKLIN who never got to share in the Nobel Prize won by the colleagues who depended so heavily on her research.

Recall:

Adenine-Thymine are joined by 2 hydrogen bonds

Guanine-Cytosine by 3 hydrogen bonds.

Regions of the molecule with more G-C areas are more stable.

Regions of the molecule with more T-A areas are less stable.

The two halves of the DNA double helix are antiparallel: they run in opposite directions, 5' to 3' running across from 3' to 5'.

In 1958 - Meselson & Stahl published their work on DNA replication. They used ¹⁵N to label DNA molecules and hybridized the strands in vitro and then allowed their samples to replicate.

They found that DNA replication is SEMI-CONSERVATIVE, and that each newly replicated strand consists of half the original template and half new material.

The enzymes responsible for this miracle are:

Topoisomerase (which unwinds the helix in front of the "unzip")
DNA helicase (which "unzips" the DNA at the hydrogen bonds)
SSB's (single strand binding proteins, which hold the DNA strands apart while they are replicated
RNA primase (which lays down RNA nucleotide "primers" on the template)
DNA polymerase (which adds new dATP, dTTP, dGTP and dCTP to the growing strand
DNA ligase (which joins the fragments on the lagging strand.)

Have a look at this swell diagram of REPLICATING DNA while I explain. Lagging strand? Huh?

Allow me to explain.

Remember how the strands run antiparallel?

DNA polymerase can add nucleotides only in ONE DIRECTION: 3' --> 5'

This means that one strand (the LEADING STRAND) gets copied in one long piece

But the other strand (the LAGGING STRAND) gets copied in little pieces that must be joined together at the end (by DNA ligase).

Now that you're completely befuddled, let's have a look at some MOVIES of this process, so you can see it in action.

This shows DNA replication close up, at the level of the dATP, dCTP, dGTP and dTTP being added to the template strand where it has been "unzipped".

This one shows a longer view of DNA replication, and illustrates how only the LEADING STRAND is copied in one continuous piece. The LAGGING STRAND is copied in pieces (called Okazaki fragments) which are joined later by the enzyme named DNA LIGASE. (The reason for this: DNA polymerase can add new nucleotides only in one direction: 3' to 5'.)

This one shows how DNA replication relates to the duplication of an entire chromosome. Note how one chromosome copies itself to form two identical SISTER CHROMATIDS. And note that "SISTER CHROMATIDS" ARE NOT THE SAME AS "HOMOLOGOUS PAIRS". Each member of a homologous pair undergoes replication, and each member of that pair becomes a pair of identical, twin sister chromatids.

FOR A PRINT-FRIENDLY VERSION OF THESE NOTES, PLEASE CLICK HERE. DO NOT print these pages! BIL 104 - Lecture 6