In the next series of articles I will be covering transcription in prokaryotes. I suppose that this is a topic that you have been introduced to in previous biology classes that you have taken. We will get a more in-depth look into the subject. However, before we dive in, let’s review a few basic concepts.

What is the central dogma of biology?

The central dogma of biology is simply the idea that information flows from DNA to RNA to proteins. DNA contains the instructions for making proteins, which are the building blocks of cells. RNA is responsible for carrying the instructions from DNA to the protein-making machinery of the cell (ribosomes).

What is the difference between prokaryotes and eukaryotes

The main difference between eukaryotes and prokaryotes is that eukaryotes contain a clearly defined nucleus and membrane-bound organelles while these are absent in prokaryotes. Other than that, the differences are summarized in this article.

How is the DNA oriented?

In DNA, nucleotides are linked by phosphodiester bonds connecting carbon-5 on one ribose sugar to carbon-3 on the next. This produces a “polarity” whereby, on one strand, the nucleotides run from 5′ to 3′ and on the opposite strand, the nucleotides run from 3′ to 5′.

What is the difference between RNA and DNA?

DNA contains deoxyribose while RNA contains ribose sugar. The difference between ribose and deoxyribose is that ribose has an OH functional group at carbon-2 while DNA has a H at this position. DNA contains the nucleotides adenine, cytosine, guanine, and thymine while RNA contains adenine, cytosine, guanine, and uracil.

RNA is generally described as single stranded while DNA is double stranded. The description of RNA as single stranded may be over simplistic since RNA is capable of forming intermolecular bonds leading to the formation of 3-dimentaional tertiary structures.

How do we name positions on the DNA?

Positions on the DNA are given a number relative to the first nucleotide put down by the RNA polymerase, a position called +1 (or the initiation site). The +2 position is simply the second nucleotide put down by RNA polymerase, the +3 the third, and so on. If the position is -1, it means the nucleotide is one place before the +1 nucleotide. Likewise, -2 means 2 nucleotides before the +1 nucleotide, etc. All the + positions greater than +1, are referred to as being “downstream”. All the – positions are referred to as being “upstream”.

Image by Dr. Dan Krane, Wright State University

When and where does transcription begin?

Transcription begins when RNA polymerase binds to DNA of the gene at the promotor. The promotor in prokaryotes includes a TTGACG region at -35 upstream of the initiation site, a TATAAT region -10 upstream of the initiation site, and a +1 region where transcription starts. A protein required for transcription called the sigma factor sits between -35 and -10.

Source: Wikimedia Commons

What are the general steps in transcription?

Transcription is the first step in the process of gene expression. It is the process of converting DNA into RNA. In prokaryotes, transcription occurs in the cytoplasm. The first step is the binding of RNA polymerase to the DNA template. Next, RNA polymerase begins to unwind the DNA double helix and separate the two strands. Once the strands are separated, RNA polymerase can begin to synthesize RNA from nucleotides on one of the DNA strands. The RNA molecule will be complementary to the template strand. Finally, RNA polymerase will release the RNA molecule and bind to another part of the DNA template.

How much does it cost prokaryotes to transcribe a gene?

It can take anywhere between 12 to 100 ATP to make a single amino acid. Therefore, consider a prokaryote like E. coli that has about 4401 genes. Each of those 4401 genes can potentially make 10 proteins. Assuming that there are approximately 300 amino acids in a prokaryotic protein, the energy cost can be as high as 4401 x 10 x 300 x 100 = 1.32 billion ATPs.

Next, we will look at what happens at the initiation of transcription.

Reference: Krane, D. 2021. Bio 2110 Molecular Biology Video Lecture. Wright State University – Lake Campus.

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Courtney Simons
Courtney Simons
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Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.