Reading Guide
Chapter 7: From DNA to protein
This chapter is about the characteristics and structure of DNA, the process of DNA replication,
the process of protein synthesis, and a look at ways to regulate gene expression. Let’s start with a quick
review of DNA. Think of some of the key ideas you already know about the structure and function of
DNA and make a list of them…..here are the keys ideas you should include.
All organisms (whether it be a bacterium or human) have a genome which consists of all the
genetic material of the cell. This genome is composed of DNA, and in all cells is in the form of doublestranded DNA. Bacterial cells have just one chromosome which is double-stranded and in a circular
form.
So what exactly is the chemical composition of DNA? It is composed of a phosphate-sugar
backbone, the sugar and phosphate are covalently bonded in alternating subunits. The sugar found in
the structure of DNA is deoxyribose and it is here that the nitrogenous bases bind. In DNA, these
nitrogenous bases are adenine, guanine, cytosine, and thymine. Can you draw a model of DNA?
A few more characteristics of DNA…it has two strands that are antiparallel, meaning that one
strand has the orientation 5’-3’ and the other is in the opposite orientation running 3’5’. What do
the terms 5’ (5-prime) and 3’ (3-prime) refer to? These are references back to the structure of the
sugar, deoxyribose, specifically the number of the carbon in the sugar. At the 3’ position there is a
hydroxyl group in deoxyribose and at the 5’ position there is a phosphate group. What kinds of bonds
are holding the sugar and phosphate groups together in the backbone of DNA? What kinds of bonds are
holding the bases together? Which reminds me….we need to mention that there are also base pairing
rules. If you know the sequence of bases on one strand of DNA you can predict the sequence in the
complementary strand. The rules for base pairing are adenine and thymine pair together and cytosine
and guanine pair together.
The DNA is essential for two key processes in the cell, cell replication and protein synthesis.
Every time a new cell is made the genetic information needs to be replicated. Let’s look this process
briefly first. In DNA replication, the two strands of DNA open and an enzyme reads one of the strands
and synthesizes a complementary strand of DNA. In bacterial cells this process begins at a location in
the chromosome called the origin of replication, and replication proceeds bidirectionally with the
enzyme DNA Polymerase synthesizing the new strand of DNA. DNA polymerase is an amazing enzyme
that can build a complementary strand of DNA in the direction 5’3’ and proofread itself to make sure
that the correct base is inserted. This enzyme will only start the process of replication when a primer is
placed on the DNA strand, essentially the restriction with DNA polymerase is that it can only build a new
strand with a starting fragment….or primer in place. This primer is made by another enzyme called
primase and consists of a short sequence of RNA. So….if you are following me I just stated that DNA
replication involves the synthesis of a new strand of DNA from an RNA primer. This means that the DNA
strand will have sections which consist of RNA, which is only temporary as the DNA Polymerase enzyme
will remove these sections of RNA and replace them with DNA. A few other terms…the replication fork
is the place where replication is occurring. There are other enzymes involved in the process of DNA
replication, helicases and DNA gyrase are important in unwinding the strand so that replication can
proceed.
One more thing about DNA replication. Remember the enzyme DNA Polymerase can only
synthesize a new strand in the direction 5’3’, if the DNA is double stranded with the strands in
antiparallel orientation, how does this happen? What is meant by the terms leading and lagging strands
of DNA? Can you describe the process of replication of these two strands? (Use Fig 7.6 as a guide).
DNA is also essential for gene expression in cells. Gene expression is used to describe the
process of protein synthesis, which includes transcription and translation. Transcription is the first step
towards synthesis if a protein and requires the enzyme RNA polymerase and the DNA (as a template).
Since we are synthesizing only one strand of RNA in the direction 5’-3’, the enzyme will bind to the
appropriate strand of DNA to generate a single strand of RNA. This process begins at a sequence in the
DNA called the promoter region and continues until the terminator sequence in the DNA is reached. It is
the promoters that regulate the direction of transcription, or rather indicates which of the DNA strands
is to be read by RNA Polymerase. Once the process of transcription is complete…what do you have? A
strand of RNA!
This strand of RNA is then processed through translation, where we have synthesis of a protein
as the final product. The steps in this process are not as important as understanding what components
are necessary. For translation you need your mRNA strand, amino acids, ribosomes, and tRNAs. The
mRNA associates with the ribosomes