Unit 7 Flashcards
rRNA
A class of RNA molecules serving as the components of ribosomes, the complexes that carry out the synthesis of proteins
mRNA
A class of RNA molecules that are translated by the ribosomes to make proteins. They are intermediaries, carrying information for the synthesis of a protein from one or a few genes to the ribosome
tRNA
Adaptor molecules that faithfully translate the information in mRNA into a specific sequence of amino acids
Identify the pentose ring (ribose vs deoxyribose)
RNA has a -OH group at the 2’ carbon making it ribose. DNA has a -H at the 2’ carbon making it deoxyribose
Which nitrogen bases are found in DNA and RNA?
DNA: ACTG
RNA: ACUG
Distinguish between monocistronic and polycistronic mRNA?
If a single mRNA carries the code for only one polypeptide, the mRNA is monocistronic.
If a single mRNA carries the code for two or more different polypeptides the mRNA is polycistronic
Is eukaryotic mRNA monocistronic or polycistronic? What about prokaryotic?
- Eukaryotic: monocistronic
- Prokaryotic: polycistronic
Is DNA usually double or single stranded?
Double stranded
Is RNA usually double or single stranded?
Single stranded
Does single stranded RNA have any double stranded regions? Point out a hiarpin loop.
Yes, complementary strands in two single strands of RNA (or within a single strand of RNA that folds back on itself to align the residues can pair with each other)
- Hairpin loops form between nearby self-complementary sequences (palindromic sequences)
Within double stranded regions of RNA, define two types of base pairs
AU
GC
What are features of a double stranded RNA?
- Bulge when there are mismatched pairs
- Internal loops (when there are also mismatched pairs)
- Hairpin when there are palindromic sequences
Give an overview of transcription by RNA polymerase
- The reaction involves two Mg2+ ions coordinated to the phosphate groups of the incoming nucleoside triphosphates (NTPs) and the 3 Asp residues, which are highly conserved in the RNA Polymerases of all species. One Mg2+ facilitates the attack of the 3’-hydroxyl group on the alpha phosphate of the NTP; the other Mg2+ facilitates displacement of the pyrophosphate
- RNA polymerase and the transcription bubble move from left to right along the DNA. The DNA is unwound ahead and rewound behind as RNA is transcribed. As the DNA is rewound, the RNA-DNA hybrid is displaces and the RNA strand is extruded.
- Movement of an RNA Polymerase along DNA tends to create positive supercoils (overwound DNA causes the DNA strands to melt apart from the tension) ahead of the transcription bubble and negative supercoils (underwound
Compare and contrast RNA polymerases and DNA polymerase I considering:
substrates
- RNA Polymerase: NTP
- DNA Polymerase: dNTP
Compare and contrast RNA polymerases and DNA polymerase I considering:
Primer needed
- RNA Polymerase: does not need primer
- DNA Polymerase: needs a primer
Compare and contrast RNA polymerases and DNA polymerase I considering:
Direction of Synthesis
Both are synthesized in the 5’ to 3’ direction
Compare and contrast RNA polymerases and DNA polymerase I considering:
Nucleophilic attack in bond formation
Both have a 3’-OH group act as a nucleophile, attacking the alpha phosphate of the incoming NTP or dNTP and releasing PPi
Compare and contrast RNA polymerases and DNA polymerase I considering:
Role of pyrophosphate and pyrophosphate in the overall reaction
For both, the hydrolysis of PPi drives the overall reaction towards the products (production of lengthened RNA/DNA)
Compare and contrast RNA polymerases and DNA polymerase I considering:
Requirements for a template
- RNA Polymerase: Yes (DNA template)
- DNA Polymerase: Yes (DNA Template)
What is meant by the terms “template” and “non-template” strands
The DNA strand that serves as the template for RNA synthesis is called the template strand. The DNA strand complementary to the template, the nontemplate strand, is identical to the RNA transcribed (with T replaced with U)
Does the same strand of chromosome always serve as the template strand?
The bottom strand acts as template for these transcripts; however it is possible to have them transcribed with the top strand (only some are transcribed with the top strand). However, it’s usually the bottom strand because that is in the 3’ to 5’ direction
Discuss the quaternary structure of RNA polymerase
- 6 subunits.
- The RNA polymerase looks similar to a claw, with the pinceres formed by the Beta and Beta’ subunits
- The sigma subunit (sigma70) rests on top of the RNA polymerase and threads through the RNA exit channel
Does RNA polymerase have 3’ to 5’ exonuclease activity? Why is it unnecessary?
No, RNA polymerase doesn’t. This is because nearly all RNAs are eventually degraded and replaced, a mistake in an RNA molecule is less of a consequence to the cell than a mistake in the permanent information stored in DNA
What is meant by the term consensus sequence?
A sequence of nucleotides or amino acids that represent the most commonly observed sequence at a specific position in a set of related DNA, RNA, or protein sequences
ex. the -10 and -35 regions
Define the numbering system which is used to identify regions such as the -10 and -35 regions
The DNA base pairs that correspond to the beginning of an RNA molecule are given positive numbers, and those preceding the RNA start site are given negative numbers
*this is about DNA in relation to RNA
What is the significance of the -35 and -10 regions
They are both consensus sequences that serve as recognition sites for RNA polymerase and help recruit the enzyme to the promoter region
- the sigma factor initially binds to the -35 and -10 region with the RNA polymerase. This positions it correctly so the RNA polymerase itself can bind to these areas
What is the relationship between the “strength” of a promoter and the degree of match to a consensus sequence?
The closer the match to the consensus sequence, the stronger your promoter
Remember the consensus sequence is an average or general guess of what the nucleotide sequence in that area should look like. The closer the actual promoter sequence matches the consensus sequence, the stronger the promoter tends to be
What is the function of the sigma subunit?
It associates with the core enzyme of RNA Polymerase and also is responsible for recognizing and binding to specific DNA sequences within the promoter region, such as the -35 and -10 regions, introducing a large bend in DNA through the process.
Discuss the events in transcription initiation and elongation.
- RNA polymerase and core sigma factor subunit bind to the DNA promoter. At this point the complex is closed (meaning the DNA strands are together)
- Transcription bubble forms opening the complex
- Transcription is initiated (technically, at this stage the RNA polymerase is synthesized short RNA transcripts known as abortive transcripts. Once RNA polymerase has synthesized a sufficient length of RNA to stabilize its interaction with the DNA template, it can transition into elongation)
- Promoter clearance is followed by elongation.
- Elongation continues, sigma 70 dissociates and is replaced by NusA
- Transcription is terminated NusA dissociates, and the RNA polymerase is recycled
Is transcription usually regulated at the level of initiation, elongation, or termination? Is it ever regulated at the other levels?
Regulation can occur at any step in transcription, including elongation dn termination.
However, much of the regulation is directed at the polymerase binding and transcription initiation steps. Differences in promoter sequences are just one of several levels of control. The binding of protein sequences near or distant from the promoter can also affect levels of gene expression.
What does a strong promoter mean?
RNA polymerase will bind tighter so the frequency and rate at which transcription is initiated at a particular promoter is increased
Discuss two distinguishing features of the termination signal in the mRNA in rho independent termination of transcription in prokaryotes
- The palindromic sequence can fold back on itself, forming a hairpin structure. This disrupts the interaction between RNA and DNA template with the POLYMERASE
*this would occur if your RNA transcribed two sequences that are complementary to each other - After the hairpin structure, the DNA has an AAAAA sequence meaning the RNA sequence will be transcribed to be UUUUU. A and U have weak interactions, so the RNA and DNA would separate from each other, leading to termination
Describe the sequences typically found at a eukaryotic promoter
1) The TATA box is the major assembly point for the proteins of preinitiation complexes of Pol II
2) The Initiator Sequence (Inr): the DNA is unwound here and the transcription start site is usually within or very near this sequence
3) Additional sequences around the TATA box and downstream (to the right) of the Inr may be recognized one or more transcription factors
How do these sequences compare to those found at a prokaryotic promoter*
- Eukaryotic transcription machinery is in the nucleus (prokaryotes don’t have a nucleus)
- There are 3 types of RNA Polymerase
-Some of the promoters have a TATA box - Requires an array of other proteins at the promoter and elsewhere to begin transcription
What is a transcription factor?
protein factors required for transcription by RNA polymerase II in eukaryotes
Discuss the formation of a primary transcript and its processing to an mRNA
*newly synthesized RNA is called primary or precursor transcript
- Transcription occurs from a 5’ to 3’ direction and produces a primary transcript containing both exons and introns. The introns are spliced out, producing mature mRNA. The poly A tail is added to the mature mRNA after transcription and protects the mature RNA
During the processing of eukaryotic mRNA a “5’ cap” is added. Describe and list two functions of the cap
- Protects mRNA from ribonucleases
- Binds to specific cap-binding complexes of protein and participates in binding of the mRNA to that ribosome to initiate translation
- The 5’ cap is a residue of 7-methylguanosine linked to the 5’ terminal residue of the mRNA through an unusual 5’,5’ triphosphate linkage
Distinguish between “exons” and “introns”
Noncoding tracts that break up the coding region of the transcript are called introns and the coding segments are exons.
Do all eukaryotic genes contain introns? Do most?
No. The vast majority of genes in vertebrates contain introns. The occurrence of introns in other eukaryotes varies.
Does splicing take place at the level of DNA or RNA
Level of RNA. The introns are removed from the pre-mRNA