Transcription: Start and Stop

The Central Dogma

What is the “central dogma”? The central dogma of molecular biology is basically the idea explaining how proteins are made, from transcription of the DNA to the translation of mRNA.

Transcription (RNA synthesis)

Transcription, also known as RNA synthesis, is the process of making the mRNA from the DNA.

There are two strands of our DNA: the coding strand and the template strand.
The coding strand runs from 5′ to 3′. The template strand is its opposite, its antiparallel; it runs from 3′ to 5′.

At the promoter region, the enzyme RNA polymerase unwinds the DNA at its promoter region. The RNA polymerase then runs down the template strand and copies the DNA by matching the DNA’s nucleotides with its complementary.

Remember these nucelotide complementaries:

  • A ——- T (DNA)

  • A ——-U (RNA)

  • C——–G (DNA)

The RNA polymerase makes the mRNA strand that runs from 5′ to 3′.

  • Coding Strand: 5′ to 3′
  • Template Strand: 3′ to 5′
  • mRNA Strand (RNA polymerase runs in this direction): 5′ to 3′

Here is an example of what each strand would look like during the transcription process.

DNA Template: [[ 3‘  ATTCCGGAA 5‘ ]]

mRNA Strand:            [[ 5‘  UAAGGCCUU3‘]]

DNA Coding:      [[ 5‘ TAAGGCCTT 3‘ ]]

Test Tip: If you’re taking a test and it asks you to find the codes for the mRNA strand, a fast way to figure out the mRNA strand’s letters is to just look at the coding strand and switch all of the T’s for U’s. Don’t forget to label its head 5′ and bottom end 3′!

As the RNA polymerase goes down the template strand, the unwounded DNA rewinds into its original configuration. Transcription stops at the termination site.

Start and Stop of Transcription:

  • Start: Promoter Region

  • Stop: Termination Region (usually a row of T’s)

    • Why is the termination region of transcription usually a row of T’s? Great question! The complementary nucleotide of T is A, and there are always two hydrogen bonds between the T and A. Unlike the nucleotides T and A, complementary C and G are bonded to each other with three hydrogen bonds. Logically, two hydrogen bonds are weaker than three hydrogen bonds. Thus, it is easier for the RNA polymerase to fall off of the termination region and be released from the DNA.
    • Eukaryotes vs. Prokaryotes? For eukaryotes, termination factors are required in order for the mRNA to be released, but for prokaryotes, termination factors are usually not required. The termination factors are molecules that bind to the termination signal region and causes the RNA polymerase to fall off. Again, the RNA polymerase begins to fall off along a string of approximately 300 base pairs, which are usually nucleotides of T.
    • As for prokaryotes, the mRNA strand has an inverted repeat along with a string of Us . Like T’s, U’s form only double bonds with A’s, and it is likely that the RNA polymerase can fall off more easily along this row of weaker bonds. Additionally, the inverted repeat in the mRNA folds into a loop structure, further disrupting the binding of the RNA polymerase to the DNA at hand. This then induces the Rho-independent transcription termination.

Transcription could also synthesize other types of RNAs. Besides mRNA, transcription is also the production of miRNA, tRNA, and rRNA.

Transcription and Direction

Exons and Introns

Many eukaryotes have exons and introns within their mRNA strands right after transcription.

Exons are also known as coding regions, and introns are known as noncoding regions. Thus, introns are the ones removed from the mRNA because they are parts that do not code for anything.

  • Exons: Coding regions —-> not removed
  • Introns: Noncoding regions —-> removed

Start and Finish of Transcription

Where does transcription start and end? How does a RNA polymerase know where to attach to the DNA and start transcribing?

RNA polymerases recognize transcriptions sites slightly differently between bacteria and eukaryotes.

Source:

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