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The names of the basal transcription factors begin with “TFII” (this is the transcription factor for RNA polymerase II) and are specified with the letters A–J. The transcription factors systematically fall into place on the DNA template, with each one further stabilizing the preinitiation complex and contributing to the recruitment of RNA polymerase II.

The processes of bringing RNA polymerases I and III to the DNA template involve slightly less complex collections of transcription factors, but the general theme is the same. Eukaryotic transcription is a tightly regulated process that requires a variety of proteins to interact with each other and with the DNA strand. Although the process of transcription in eukaryotes involves a greater metabolic investment than in prokaryotes, it ensures that the cell transcribes precisely the pre-mRNAs that it needs for protein synthesis.

Evolution connection

The evolution of promoters

The evolution of genes may be a familiar concept. Mutations can occur in genes during DNA replication, and the result may or may not be beneficial to the cell. By altering an enzyme, structural protein, or some other factor, the process of mutation can transform functions or physical features. However, eukaryotic promoters and other gene regulatory sequences may evolve as well. For instance, consider a gene that, over many generations, becomes more valuable to the cell. Maybe the gene encodes a structural protein that the cell needs to synthesize in abundance for a certain function. If this is the case, it would be beneficial to the cell for that gene’s promoter to recruit transcription factors more efficiently and increase gene expression.

Scientists examining the evolution of promoter sequences have reported varying results. In part, this is because it is difficult to infer exactly where a eukaryotic promoter begins and ends. Some promoters occur within genes; others are located very far upstream, or even downstream, of the genes they are regulating. However, when researchers limited their examination to human core promoter sequences that were defined experimentally as sequences that bind the preinitiation complex, they found that promoters evolve even faster than protein-coding genes.

It is still unclear how promoter evolution might correspond to the evolution of humans or other higher organisms. However, the evolution of a promoter to effectively make more or less of a given gene product is an intriguing alternative to the evolution of the genes themselves. H Liang et al., “Fast evolution of core promoters in primate genomes,” Molecular Biology and Evolution 25 (2008): 1239–44.

Promoter structures for rna polymerases i and iii

In eukaryotes, the conserved promoter elements differ for genes transcribed by RNA polymerases I, II, and III. RNA polymerase I transcribes genes that have two GC-rich promoter sequences in the -45 to +20 region. These sequences alone are sufficient for transcription initiation to occur, but promoters with additional sequences in the region from -180 to -105 upstream of the initiation site will further enhance initiation. Genes that are transcribed by RNA polymerase III have upstream promoters or promoters that occur within the genes themselves.

Eukaryotic elongation and termination

Following the formation of the preinitiation complex, the polymerase is released from the other transcription factors, and elongation is allowed to proceed as it does in prokaryotes with the polymerase synthesizing pre-mRNA in the 5' to 3' direction. As discussed previously, RNA polymerase II transcribes the major share of eukaryotic genes, so this section will focus on how this polymerase accomplishes elongation and termination.

Although the enzymatic process of elongation is essentially the same in eukaryotes and prokaryotes, the DNA template is more complex. When eukaryotic cells are not dividing, their genes exist as a diffuse mass of DNA and proteins called chromatin. The DNA is tightly packaged around charged histone proteins at repeated intervals. These DNA–histone complexes, collectively called nucleosomes, are regularly spaced and include 146 nucleotides of DNA wound around eight histones like thread around a spool.

For polynucleotide synthesis to occur, the transcription machinery needs to move histones out of the way every time it encounters a nucleosome. This is accomplished by a special protein complex called FACT    , which stands for “facilitates chromatin transcription.” This complex pulls histones away from the DNA template as the polymerase moves along it. Once the pre-mRNA is synthesized, the FACT complex replaces the histones to recreate the nucleosomes.

The termination of transcription is different for the different polymerases. Unlike in prokaryotes, elongation by RNA polymerase II in eukaryotes takes place 1,000–2,000 nucleotides beyond the end of the gene being transcribed. This pre-mRNA tail is subsequently removed by cleavage during mRNA processing. On the other hand, RNA polymerases I and III require termination signals. Genes transcribed by RNA polymerase I contain a specific 18-nucleotide sequence that is recognized by a termination protein. The process of termination in RNA polymerase III involves an mRNA hairpin similar to rho-independent termination of transcription in prokaryotes.

Section summary

Transcription in eukaryotes involves one of three types of polymerases, depending on the gene being transcribed. RNA polymerase II transcribes all of the protein-coding genes, whereas RNA polymerase I transcribes rRNA genes, and RNA polymerase III transcribes rRNA, tRNA, and small nuclear RNA genes. The initiation of transcription in eukaryotes involves the binding of several transcription factors to complex promoter sequences that are usually located upstream of the gene being copied. The mRNA is synthesized in the 5' to 3' direction, and the FACT complex moves and reassembles nucleosomes as the polymerase passes by. Whereas RNA polymerases I and III terminate transcription by protein- or RNA hairpin-dependent methods, RNA polymerase II transcribes for 1,000 or more nucleotides beyond the gene template and cleaves the excess during pre-mRNA processing.

Art connections

[link] A scientist splices a eukaryotic promoter in front of a bacterial gene and inserts the gene in a bacterial chromosome. Would you expect the bacteria to transcribe the gene?

[link] No. Prokaryotes use different promoters than eukaryotes.

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Questions & Answers

how to pronounce the word cyanobacteria
siva Reply
explain the term transpiration pull
Tank Reply
what Is corona
INGIEBE Reply
Describe the process of protein sythesis?
Kizito Reply
What is a ploidy level
Francis Reply
Ploidy refers to the number of chromosomes. We have 23 pairs of chromosomes in somatic cells. Sex cells are haploid thus 23 chromosomes vs. 46 Chromosomes.
Eric
Evolution is evolvement according to one's environment. Let's use humidity as an example. A person from a very cold environment would not be used to hot humid weather. But over time their body would slowly, slowly adapt.
Eric
Give me (3) types of biodiversity
Jay
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Namutebi
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INGIEBE
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Kiana Reply
it's a process or analayzing some program
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Fahamia
is a study of
Sarah
unfolding or unrolling hence the process of development of growth
Sarah
is the periodic change of the structural features of an organism
Rahim
I think
Rahim
process of developing
Nikita
hi guiz
Mohammed
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Kaniki
why are diagrams not available
Evans Reply
How can nucleotide molecules of DNA be constructed?construct a six molecules of DNA?
Julia Reply
You take a Deoxyribose sugar. The Deoxyribose sugar has a hydrogen on the second carbon vs. an OH group. The nitrogenous base binds to the #1 Carbon and a phosphate binds to the 5'. When the 2nd NBase bonds, it does so on the 3' Carbon. Thus you have 5' to 3' directional growth.
Eric
what is the explanation of local biomes in Nigeria
Kabir
It would be savanna, tropical grassland.
Eric
describe the breathing mechanism in the body fish.
Jackson Reply
what is genetics.
Jackson
the study of hereditary
Julia
state the importance of biodiversity of organisms in an area?
Chris Reply
Let's compare The Brazilian Rain Forest to Antarctica. There many, many types of flora and fona in The Rain Forest. So many that we are still discovering them vs. the variety in Antarctica.
Eric
it boosts the ecosystem where each species has an important tole
Kizito
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Adam Reply
Biology is the study of living things.
Patrick
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Jackson
anatomy,genetics,parasitology
Evans
Botany
Kizito
what is meosis
Mabatho Reply
did you mean meiosis? meiosis this is the type of cell division where one diploid parent cell produces two daughter cells that are haploid and genetically different from the parent cell.
Patrick
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Mabatho
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Mabatho
Am also struggling as well
Patrick
nothing comes on a silver platter, you need to put effort in everything you are doing not forgetting God. study as if there is no tomorrow. "Suffer now and enjoy the future"
Patrick
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Odhiambo
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Patrick
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Mabatho
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Patrick
what's a nervous system
tessy Reply
what's the human brain
tessy
The human brain consist of the cerebrum, cerebellum and brain stem. The cerebral cortex is composed of neuronal cell bodies, which is the gray matter. The white matter is composed of neuronal axons. The brain is the motherboard of our system.
Eric
the structure of paramecium
Charity Reply
yes
Sarah
Ho
Nevers
It is a unicellular, eukaryotic, autotrophic, organism whose movement is done by cilia.
Eric
what is codominant
Eyuel
what is endocytosis
Stephen Reply
Endocytosis is when the cell membrane surrounds and engulfs an antigen, viral particles, etc. This forms a vesicle which moves through the cytoplasm. And merges with a designated organelle.
Eric

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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