Overview of Transcription

*These notes were taken from LG 2.3

LG:

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Overview of Transcription

Transcription

• This is the process by which a segment of DNA directs the synthesis of an RNA.

• Here, DNA does not produce proteins directly:

  • One strand of DNA must be copied first into a complementary RNA sequence.

Process of Transcription

• In the 5’ to 3’ direction, an enzyme called RNA polymerase unwinds the DNA (responsible for polymerization of RNA).

• Like DNA replication, transcription starts with the opening and unwinding of a small portion of the DNA double helix, followed by complementary base pairing between incoming nucleotides and the DNA template.

• Unlike DNA replication, the RNA strand does not remain hydrogen bonded to the DNA template strand in transcription.

• Also, RNA molecules synthesized during transcription are much shorter than DNA molecules.

  • A comparison of RNA polymerase and DNA polymerase is presented below.

Table 3.2a. RNA Polymerase vs. DNA Polymerase

    In eukaryotes, the binding of several transcription factors that make the binding of RNA polymerase possible is activated by unique regions in the DNA called promoters (upstream). The major types of RNA are presented below.

Table 3.2b. The Different Types of RNAs

Stages of Transcription

I. Initiation

    Initiation of RNA transcription from one of the two strands of the DNA molecule proceeds as follows:

Prokaryote	Eukaryote
RNA polymerase (RNAP) can recognize and bind itself to the promoter (ρ factor)	Transcription factors (TFs) mediate RNA polymerase binding and transcription initiation
No initiation complex	TFs + RNAP II = Transcription Initiation Complex (TIC)
Consensus sequence of the promoter (AT-rich regions): Pribnow box – TATAAT	Consensus sequence of the promoter (AT-rich regions): TATA box – TATAAA

II. Elongation

• A single gene can be transcribed simultaneously (i.e. more than one mRNA molecule are being transcribed at the same time).

• DNA is double stranded: sense strand (coding strand or non-template strand; and antisense strand (non-coding strand or template strand).

    RNA nucleotides complementary to the template strand of the DNA are added to the 3’ end of the growing strand. DNA unwinds 10-20 bases at a time for pairing with RNA nucleotides, then the double helix reforms.

    Before the transcription of a gene is completed, a protein called Rho (ρ) factor binds to the termination site and brings about the termination of transcription.

Prokaryote	Eukaryote
Rho-Dependent Termination
Rho (ρ) factor recognizes the transcribed RNA rut (C-rich), upstream of the real terminator sequence	RNAP II continues along the DNA strand until it reaches the terminator sequence
Terminator (RNA sequence) serves as termination signal	Polyadenylation signal sequence transcribes the polyadenylation signal (AAUAAA)
The mRNA can be translated without further modification	After 10-35 nucleotides downstream from AAUAAA, a pre-mRNA is released
Rho catches up with RNAP (paused at termination sequence) and allows release	RNAP II continues to transcribe and is eventually dissociated

Post-Transcriptional Processing in Eukaryotes

• The RNAP II does not only transcribes DNA into RNA, but also bears pre-mRNA-processing proteins on its tails, which are then transferred to the nascent RNA at the appropriate time.

• The likely order of events in producing a mature mRNA from a pre-mRNA is shown below.

• Some regions of the mRNA will not be translated.

• Some non-coding sequences called introns are interspersed between coding (expressed) sequences called exons.

  • The intron loops out as snRNPs (small nuclear ribonucleoprotein particles; complexes of snRNAs and proteins) which bind to signals at the end of each intron.

  • snRNPs join with other proteins to form a spliceosome. The intron is excised and the exons are then spliced together.

• The alteration of mRNA ends includes the following processes:

  • (1) 5’ capping or the addition of a modified G cap;

  • (2) 3’ polyadenylation or the addition of a poly-A tail;

  • (3) UTRs (untranslated regions) for ribosome binding signals.

• These modifications share several functions:

  • (1) to facilitate the export of mRNA from the nucleus to the cytoplasm

  • (2) to prevent the degradation of mRNA by hydrolytic enzymes; and

  • (3) serve as signals to the rRNA for attachment to the 5’ end.

• The structure of a typical eukaryotic protein coding mRNA including the UTRs is shown on the left in Figure 3.2j.