Initiation of mRNA Transcription in Eukaryotes Flashcards
RNA polymerase
The enzymes that perform transcription. This enzyme catalyzes the formation of the phosphodiester bonds between the nucleotides. It moves step-wise along the DNA, unwinding the DNA helix just ahead of the active site, in order to expose a new region of the template strand for complementary base-pairing. RNA polymerases do not require a primer
Differences between transcription and DNA replication (2)
- Unlike the new DNA strand, the RNA strand doesn’t remain hydrogen bonded to the DNA template strand. Behind the region where nucleotides are being added, the RNA chain is displaced and the DNA helix re-forms
- RNA molecules are much shorter than DNA molecules, because they are copied from a limited region of DNA
Sigma factor
Transcription initiation factor. It is a subunit that associates with the core RNA polymerase (in bacteria). It helps RNA polymerase to read the signals in DNA and know where to begin transcribing. Binds at -35 and -10, with transcription starting at +1
RNA polymerase holoenzyme
A bacterial enzyme that works with sigma factor. They form a complex that adheres weakly to DNA, and the holoenzyme generally rapidly slides along the DNA molecule and dissociates. When the holoenzyme encounters a promoter, the complex binds more tightly
Promoter
A special sequence of nucleotides that indicates the starting point for RNA synthesis. Called consensus sequences in prokaryotes. RNA polymerase binds tightly here because the sigma factor makes specific contacts with the bases on the outside of the DNA double helix
Initiation of transcription in prokaryotes (6)
- The RNA polymerase holoenzyme tightly binds at the promoter
- Opens the double helix to expose a short stretch of nucleotides on each strand called the transcription bubble
- The transcription bubble is stabilized by sigma factor binding to the unpaired bases on one of the exposed strands
- The other exposed DNA strand acts as a template strand for base-pairing with ribonucleotides
- RNA polymerase synthesizes RNA in a step-wise fashion- the polymerase moves forward one base pair for every nucleotide added
- Transcription ends at the terminator
Terminator
The DNA sequence where transcription ends (in prokaryotes). RNA polymerase halts and releases the newly made RNA molecule and the DNA template.
How do terminating signals stop RNA polymerase in prokaryotes?
For most genes, a termination signal consists of a string of A-T nucleotide pairs, which is preceded by a DNA sequence that becomes a hairpin structure when it base-pairs. The hairpin helps to disengage the RNA transcript from the active site
Consensus nucleotide sequence
All bacterial promoters contain related nucleotide sequences, which allows them to be recognized by the sigma factor. The consensus nucleotide sequence is derived by comparing many sequences with the same basic function and tallying up the most common nucleotides found at each position. Acts as an “average” for a large number of nucleotide sequences.
Eukaryotic polymerases
RNA polymerase 1, 2, and 3.
RNA polymerase 1 and 3
Transcribe the genes encoding transfer RNA (tRNA), rRNA, and various small RNAs
RNA polymerase 2
Transcribes most genes, including all of those that encode proteins
Differences in bacterial and eukaryotic RNA polymerase function (2)
- Bacterial RNA polymerase only needs one transcription initiation factor (sigma). Eukaryotic require many of these factors, called general transcription factors
- Eukaryotic transcription initiation must take place on DNA packaged into nucleosomes and higher-order forms of chromatin structure. These are absent in bacterial chromosomes
General transcription factors
The many factors required for transcription initiation in eukaryotes. They help to correctly position RNA polymerase at the promoter, pull apart the DNA strands so transcription can begin, and release RNA polymerase from the promoter to start elongation
General transcription factors for RNA polymerase 2 (5)
- TFIID
- TFIIB
- TFIIF
- TFIIE
- TFIIH