Title: Unit 4 Learning Module
Course: SAT 170 Introductory Biotechnology > Section: SAT 170 Introductory Biotechnology

Unit 4 Learning Module

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Learning Module 4:
Genes and Gene Expression
We learned in our last unit of material that DNA is the molecule that is passed from one generation to the next carrying the information for all of our characteristics. This information is coded within the sequence of adenines, thymines, guanines and cytosines of a DNA molecule. The process by which that code if copied (transcription) and interpreted (translation) is known as gene expression.

Figure 1

Figure 2

Central Dogma of Molecular Biology

The central dogma of molecular biology states that information flows from DNA → RNA → protein. Transcription is the process by which DNA is copied into RNA. That RNA can then be translated into a protein. These processes will be explained in more detail below.  Each gene dictates the amino acid sequence of a protein; RNA acts as the intermediary between the gene and the ribosome (recall from Unit 2 that the ribosome if the site of protein synthesis).

See Figure 1: The central dogma for the flow of genetic information in living organisms is from DNA, to RNA, to protein. Some viruses are able to also use RNA to create DNA (hence the double arrow between DNA and RNA).

Gene Expression

Transcription is the process by which one strand of a DNA molecule serves as a template for the synthesis of RNA. There are several key players in the process of transcription including: mRNA, RNA polymerase and the transcription unit.

- mRNA is the complement to the DNA molecules that is synthesized during transcription. mRNA synthesis follows the same rules of complementary base pairing as we saw in DNA replication, with the exception that RNA contains uracil in place of thymine. Thus, if the DNA sequence reads ATCCGTA, the complementary mRNA would read UAGGCAU. Basically, anytime you would think to include a thymine in the complement you would instead include a uracil for the mRNA.
- RNA polymerase is similar to DNA polymerase. The role of RNA polymerase is to synthesize the complementary mRNA from the DNA template.
- The transcription unit is the segment of DNA that is involved in transcription. It includes the gene that is to be transcribed as well as a promoter, a start site and a termination site. The role of the promoter is to serve as the site of binding for RNA polymerase. As their name indicates, the start site and termination site serve as the point where transcription begins and ends, respectively.

See Figure 2: Notice how the RNA polymerase uses the DNA strand as a template for the synthesis of the complementary mRNA molecule.

Figure 3

Transcription occurs in three different steps: initiation, elongation and termination.

- During translation initiation, RNA polymerase binds to the promoter of the transcription unit.
- After binding to the transcription unit RNA polymerase synthesizes the complementary mRNA using one strand of DNA as a template during translation elongation. Like DNA replication, this process occurs in the 5’ to 3’ direction.
- The stop sequence at the termination site signals RNA polymerase to release the DNA template and the newly synthesized mRNA. The DNA rewinds back into its original shape and structure, while the mRNA proceeds to the next step of gene expression.

See Figure 3: This image illustrates the binding of RNA polymerase to the DNA molecule which serves as transcription initiation. The elongation step requires that new RNA nucleotides are added to extend the mRNA chain. Notice how the mRNA, DNA and RNA polymerase disassemble once transcription termination occurs.