Lecture 9 Flashcards
How can splicing explain how the same gene can produce different proteins when translated
The mRNA transcript can be spliced in different ways which accounts for differences in the proteins produced
What is meant by SnRNPs and what do they consist of
Small nuclear ribonucleo proteins are structures that make up the spliceosome apparatus. They consist of small nuclear RNAs and proteins
Describe the role of rRNAs
Ribosomal RNAs are a major constituent of ribosomes. They are very large and very abundant and catalyse protein synthesis
Polymerase action causes an uncoiling of the DNA that provides a force on the upstream DNA, how is this tension relived
Topoisomerases release the tension by either making single or double stranded breaks in the DNA upstream hence removing some of the coils from the superhelix
What is significant about viruses when it comes to protein synthesis
Viruses break the central dogma of DNA–>RNA–>Protein whereby reverse transcriptase enzymes can make DNA from RNA
Which RNA polymerase transcribes all protein coding genes
RNA Polymerase II
mRNA accounts for the majority of RNA, T or F
F – RNA only accounts for between 3-5% of all RNA
What is meant by the secondary structure of RNAs
Secondary RNA structure refers to the base pairing that occurs within a single RNA strand.
Which end of the mRNA strand is polyadenylated
3’ end
Describe the synthesis of the 5’ cap in eukaryotes
The 5’ cap is present in all eukaryotic mature mRNAs. Its consists of a methyl guanosine trisphosphate cap added to the first nucleotide. The 5’-5’ linkage is unusual and provides stability to the mRNA. The 5’ cap is also required for binding of the eukaryotic initiation factors. The 5’ cap is added when the mRNA is around 20-40 nucleotides long and begins to emerge from RNA polymerase
Specific transcription factors bind close to the promoter region, T or F
F – they bind far away
What is the name given to the machinery that carries our RNA splicing and what does it consist of
Spliceosome – a nuclear complex made up of about 150 proteins and 5 RNAs
What is the role of RNA polymerase I
Transcribes rRNA genes (28S, 18S and 5.8S)
PolyA tails can be up to 200 residues in length, T or F
T
Explain RNA polymerases role in synthesising the polyA tail
RNA polymerase contains a tail that is highly phosphorylated. The negative charge of these phosphates resembles the RNA backbone negative charge. This serves as a docking point for RNA binding proteins which are held there prior to binding to particular RNA sequences known as polyA signals.
What sequence represents the PolyA signal sequence?
AAUAAA
Recall the structure of a mature mRNA from start to finish
Methyl guanosine cap –> 5’ UTR –> START –> Coding Sequence –> 3’ UTR –> PolyA Tail
How many families of tRNAs are there
49
The tertiary structure of RNA refers to its interactions with other RNAs, T or F
F – tertiary structure is the RNA strands 3D conformation
Give an example of a promoter sequence to which a general transcription factor binds to
TATA box – consisting of a TATAA/TAA/T sequence that lies 30 base pairs upstream of the coding sequence
Which RNA polymerase transcribes all tRNAs, 5S rRNAs and other snRNAs
RNA Polymerase III
What is meant by the primary sequence of RNA
The polyribonucleotide sequence
Splicing is specific to eukaryotic transcription, T or F
T
What is the role of cleavage stimulating factor and cleavage/polyadenylation specific factor in the synthesis of the PolyA tail?
Cleavage stimulating factor (Cstf) and cleavage/polyadenylation specific factor (CPSF) bind to the negatively charged tail of RNA polymerase II but then jump off when the PolyA signal sequence is detected. They then bind to the polyA signal and cleave the RNA which then allows PolyA polymerase to synthesise a polyA tail
General transcription factors are required for all gene transcription, how do they act
They act to guide RNA polymerase and bind to the promoter sequence
What is significant about non-coding RNAs
Non-coding RNAs serve structural and enzymatic functions, acting more like proteins
Genes can be on either the sense or antisense stands of the DNA but not both, T or F
T
What is the purpose of the polyA tail
The polyA tail is added as part of the termination process. It acts to maintain the RNA stability and help in mRNA nuclear export and translocation.
What are the key differences between RNA and DNA
RNA replaces thymine with uracil. RNA is also synthesised as a single strand and thus is unstable and rapidly degraded. Finally RNA contains a ribose sugar backbone instead of a deoxyribose sugar. The difference is an -OH group replaces the H bonded to the 2’ carbon in the sugar ring
Which regions of pre-RNA are spliced out but only in eukaryotes
Introns
Explain how splicing occurs
The 2’ OH of an adenine branch site attacks the phosphodiester bond on guanine donor site. Cleavage at the donor site results in the formation of lariat. Next a 3’ OH in the guanine donor site attacks phosphodiester bond on a guanine acceptor site freeing the lariat which is then degraded
In RNA A pairs with U and C pairs with G however it is possible for unusual base pairing such as G with U, how is this described
Non-Watson-Crick pairing
What are the three functions of snRNAs
Recognise 5’ donor and branch sites, bring sites together and catalyse cleavage
The RNA molecule produced in transcription will have the same sequence as the DNA sequence that codes for that gene, T or F
F – it will have the opposite sequence to the template strand
What is the role of tRNA
tRNA participates in translation where its bound amino acid is added to the growing polypeptide chain when the tRNAs anticodon loop binds to the complimentary codon on the mature mRNA strand
How is transcription different from DNA replication
Multiple RNA polymerases bind to the same gene, no primers are needed, only one strand of DNA is used as the template and polymerase only moves in one direction. Finally the transcript doesn’t remain bound to the template as in semi-conservative replication
What can be said about the error rate in transcription
Its much higher than DNA replication
Due to internal folding within the molecule and complimentary base pairing, an RNA molecule appears as a 10th of the length of the corresponding DNA sequence, T or F
T
