Chapter 10
Molecular Biology of the Gene
Overview
In this chapter we will examine the structure and function of nucleic acids. DNA is responsible for pretty much
everything in living organisms by giving instructions for the creation of various proteins. We will begin by
learning how it is structured look at how DNA is copied. Then we will study how the code of the DNA becomes
proteins in the body through the linked processes of transcription and translation.
Assigned Reading
Text, Pages 184-199
PowerPoint Presentation
Chapter Review, Page 206-207
Testing Your Knowledge, Page 207
Key Terms
 Double helix
 Nucleotide
 Thymine, cytosine, adenine, guanine, uracil
 Semiconservative replication
 DNA polymerase
 DNA ligase
 Transcription
 Translation
 Intron, exon
 Triplet
 Codon, anticodon
 Start codon, stop codon
 RNA polymerase
 mRNA, tRNA, rRNA
 Mutation
Introduction
During the early part of the 20th century there was much discussion and debate over what genetic material was
made of. Initially proteins were thought to be the source, mainly due to the fact that they could be formed into
so many different shapes. By the 1950s experiments had shown that DNA was actually what carried the genes.
However, the exact structure and function eluded scientists. Building on research by earlier studies, Watson and
Crick finally described what we now know to be the double-helix structure of DNA, and found that it was
consistent among all living organisms. The arrangement of the nucleotides varied, but the basic concept was
sound. Understanding the shape and components of DNA allowed molecular biologists to begin unraveling the
secrets of genetic coding and replication.
DNA is comprised of four nucleotides: thymine, adenine, guanine, and cytosine. These bases match in
consistent and specific ways, with T and A always bonding to each other, and G and C to each other. These
inter-base bonds are through hydrogen bonding. The way these bases attach causes
a spiral to form, making the classic double-helix shape of DNA.
DNA Replication
Copying a strand of DNA is not complicated when you break it into steps.
1. An enzyme, helicase, breaks the hydrogen bonds between nucleotides in a
section of DNA.
2. DNA polymerase moves along the DNA strand in a specific direction, breaking
the bonds as it moves.
3. DNA polymerase also takes free nucleotides and matches them appropriately
(A to T and C to G).
4. DNA ligase then splices sections of the newly created strand together, sealing
them.
5. When the process is completed, two new DNA strands have been formed.
Each of the new strands is partially made up of the “parent” strand, a process
called semiconservative replication.
This process takes place in the nucleus of the cell during the S phase of Interphase.
Transcription
Simply put, transcription is the act of writing down or copying something so that it can be viewed in a different
location at a different time. Think about a transcript of a speech. You can read that transcript in your home and
know what the speaker said, even if you weren’t actually present at the time the speech was given.
The transcription of DNA is done for the same reason. The nuclear
pores are too small for DNA to move through (review mitosis and
cellular structure). However, proteins are made outside of the nucleus.
We need some process to take the code on the DNA and move it out of
the nucleus and to the endoplasmic reticulum. We also only need a
small section of the DNA to make a single protein, and don’t need the
entire strand. Once again, the process isn’t difficult if you break it down
in steps.
1. RNA polymerase opens a section of DNA
2. The same RNA polymerase then begins to match nucleotides
and bind them together. Important point: RNA does not contain
thymine! Instead, adenine bonds to uracil. One of the main
differences between DNA and RNA is that DNA contains thymine
and RNA contains uracil. Adenine will bond to either.
3. Once the gene has been copied, the newly created messenger
RNA (mRNA) breaks off.
4. Introns (unwanted sections of the mRNA) are removed and the
remaining RNA spliced together.
5. The mRNA leaves the nucleus.
Lasagna analogy, part 1: Let’s say that you really want to make home-made lasagna. This is not easy, and not
something that you could simply throw together without knowing how. You don’t have any cookbooks at home,
but this is a special meal so you have to make it work. You go to your local library and look through their
cookbooks, finding a great one of Italian recipes. Obviously, you can’t bring your oven and cooking supplies into
the library (nucleus). Unfortunately the book is in the reference section, so you can’t take it out of the library.
However, the library does have a photocopy machine you can use. Now, you don’t need the entire book (DNA
strand) to make your meal. You just need a single recipe (gene). So you quickly copy the recipe, not worrying
about the rest of the book (transcription). When you get the copies done, you notice that there are pictures of
other foods that you really don’t need. So you throw away these pages (introns), staple the remaining pages
together (splicing) and you smile as you leave the library and head home to begin cooking.
Translation
Let’s keep it simple again. When you translate something you take it from a language you don’t know and copy
it into a language that you do know. The same thing happens with cellular translation. The cell can’t read the
instructions from the DNA directly. Putting mRNA into the cytoplasm won’t cause the cell to perform its
functions right away. That code has to be translated into the “language” of proteins, which the cell can
understand. Let’s look at the steps.
1. mRNA is bound to the ribosome on the rough
endoplasmic reticulum, specifically at the start
codon (a codon is considered to be a grouping of
three nucleotide bases).
2. Transfer RNA (tRNA) is in the cytoplasm, bound to
specific amino acids. One end of the tRNA has an
“anticodon”, again three bases.
3. Within the ribosome the anticodon of the tRNA
binds to the codon on the mRNA. So A-G-G on the
mRNA would bind to an tRNA with U-C-C as the
anticodon.
4. The ribosome moves down one codon (triplet) and
another tRNA binds to the new codon.
5. When two tRNA molecules are next to each other in
the ribosome, the amino acids on each will form a
peptide bond.
6. The first tRNA then disconnects from the mRNA,
going into the cytoplasm to again bond to an amino acid.
7. The process continues, causing elongation of the polypeptide chain.
8. When the ribosome reaches a stop codon, the process halts, and the completed protein breaks away to
be further processed or to be used in the cell.
Lasagna analogy, part 2: You get home with your new recipe, and looking at it you notice that the Italian
cookbook was written in Italian! You don’t happen to speak Italian, and it’s too late to go back and find a new
book. Luckily your next door neighbor, Giuseppe, is from Italy. You call him over, and he agrees to translate the
recipe for you. Inside the kitchen (rough ER) you take the recipe (mRNA), and with the help of your friend
(ribosome) you start to cook. Giuseppe tells you what each line (codon) means, so you can then find the right
ingredients (tRNA). As he translates for you, you put the various ingredients together, eventually creating your
lasagna (polypeptide). Once the lasagna is completed, he can go back home, waiting for the next time you try to
cook Italian food.
Both transcription and translation involve three basic steps: initiation, elongation, and termination.
Mutations
Changes in the nucleotide sequence in the DNA are called mutations. This can happen through various methods,
and can cause different kinds of rearrangement. Nucleotides can be added, deleted, switched, and so on. When
the DNA pattern is changed, this alters the mRNA. When that abnormal mRNA bonds to the ribosome, tRNA
matches up as normal. However, the code may have changed. Because each tRNA binds only to a specific
amino acid, this can alter the pattern of amino acids. If you recall the basic biochemistry of proteins, you know
that a small change in the primary structure can lead to changes in the secondary, tertiary, and quaternary
structures. These changes can significantly alter the final shape and therefore function of the protein.
Concepts
 Understand who first described the structure of DNA.
 Understand the structures of DNA and RNA. How are they similar and how are they different?
 Know the different nucleotides found in DNA and RNA.
 Understand the process of DNA replication, and describe all of the steps.
 Understand the process of transcription and translation and how they lead to the creation of
polypeptides.
 Understand how mutations can affect the DNA and therefore lead to problems with proteins.
Review Material
MyBiology.com—Study guides and resource for this text. Specifically look at MP3 Tutor and all Web Activities