Cell Biology- Hejmadi4- DNA transcription

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What is transcription?
The mechanism by which a template strand of DNA is utilized by specific RNA polymerases to generate one of the three different types of RNA
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What is its biological significance?
allows selective expression of genes -can be regulated to control time, place and level of protein expression
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Where does a) transcription b)translation occur in (i) prokaryotes (ii) Eukaryotes?
a)(i)cytosol (ii)nucleus b)(i) cytosol (ii) cytosol
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What is the initiation codon almost always in eukaryotes? What does this code for?
ATG, Methionine
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In eukaryotes, what is the default position of a gene? Hence?
That the gene is switched off.. hence eukaryotes require activators.
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In eukaryotes, the polymerase enzyme cannot bind to the promoter on its own. What does it require?
transcriptional factors- to bind first, together forming a transcription initiation complex.
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What determines whether a gene is expressed in a cell?
Whether specific regulatory proteins are available in the cell
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what is an operon?
Continuous exons- found in bacteria which almost never possess introns
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What is the default position of a prokaryotic gene?
gene is switched on, so prokaryotes require repressors
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What does the rate of transcription of a prokaryotic gene depend on?
whether the promoter of the gene contains sequences that RNA polymerase can bind to tightly. (If so, the promoter will compete well for the RNA polymerase enzymes available in the cell, and this gene will be preferentially expressed.)
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What are the 3 main steps in the mechanism of transcription?
1)Initiation 2)Polymerisation 3)Termination
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Describe initiation step
Occurs at a promoter- a region on the DNA where RNA polymerase binds and begins to unwind the DNA.
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Describe Polymerisation step. What else is this called?
The polymerase begins reading the DNA template strand and building a complementary RNA strand from free nucleoside triphosphates, which are added to the 3’ hydroxyl group of the last nucleotide. This is called elongation.
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What releases the energy that fuels transcription?
During the reaction, a pyrophosphate ion is released and broken down
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What happens as RNA polymerase moves through?
The DNA double helix rewinds- like unwinding this is an energy-requiring process
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What happens during the termination step?
RNA polymerase reaches termination site and the RNA transcript is released from the template. DNA rewinds completely and RNA polymerase dissociates from it.
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What 3 steps are involved in transcriptional processing?
1)Capping 2)Polyadenylation (tailing) 3)Splicing
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Describe the capping process...
When the RNA chain is about 30 nucleotides long, the 5’ ends are modified by the addition of a guanine group: - mediated by guanylyltransferase – involves a 5’-5’ triphosphate linkage. – Happens before transcription is finished = co-transcriptionally
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capping process continued...
Methyl transferases then add methyl groups in the 7 position to form 7-methylguanosine caps. These are recognized by the translation machinery
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Give three functions of the caps
 Protect against degradation by nucleases  Serves as recognition site for ribosomes  Transports hnRNA from nucleus to cytoplasm
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What is hnRNA?
Heterogenous nuclear RNA
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What is poly-adenylation?
the addition of poly(A) residues at 3’ end.
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Describe how these poly(A) residues are added...
RNA is cleaved by ribonuclease 15-20 nucleotides downstream from the AAUAAA conserved sequence. The enzyme poly(A) polymerase and cleavage & polyadenylation specificity factor adds a series of adenines (generated from ATP) to 3’ end
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Give 2 functions of the poly(A) tail
 enhances the stability of eukaryotic mRNA  regulates its transport to the cytoplasmic compartment
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What is splicing?
the highly precise removal of intron sequences
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What does splicing begin with?
assembly of helper proteins and snRNPs at the intron/exon borders. These splicing factors act as beacons to guide small nuclear riboproteins to form the spliceosome.
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What are snRNPs?
Small nuclear ribonucleoproteins
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What does the spliceosome do? (4 steps- first 3 on here)
1)brings the exons on either side of the intron very close together, ready to be cut. 2)One end of the intron is cut and folded back on itself to join and form a loop 3)spliceosome then cuts the RNA to release the loop and join the two exons together
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4th step
The edited RNA and intron are released and the spliceosome disassembles.
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What type of reactions are all of the ones described above?
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What is alternative splicing?
a single gene can code for multiple proteins. In this process, particular exons of a gene may be included or excluded from the final processed mRNA produced. I,e. the cell can control which exons are spliced together.
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How does this explain why we have less genes than we originally thought?
splicing allows control of different exonic sequences that dictate different proteins being expressed in different tissues.
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How are mRNA's transported to the cytosol?
1)CAP nuclear transport receptor- recognises nuclear pore 2) 7-methylguanine directs it towards nuclear pore 3) polyA binding protein keeps structure in tact
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What are promotors?
The start site of transcription and also the point on the DNA template strand in which RNA polymerase binds to
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Where are promotors found in prokaryotes? Give an exmaple of one
near the 5' end of operons. Pribnow box- TATAAT
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Where are promotors found in eukaryotes? give the consensus promotor sequence for a)constitutive structural genes b)selective structural genes. What are they recognised by?
Near the 5' of genes. a)GGGCGG b)TATA. RNA polymerase II
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What is a consensus sequence?
sequence of DNA that have a similar structure and function in different organisms
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What is a constitutive gene?
A gene that is transcribed continually
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What do repressors do?
bind to non-coding inhibitory sequences (silencers) on the DNA strand that are close to or overlapping the promoter region, impeding RNA polymerase's progress along the strand, thus impeding the expression of the gene
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What is a silencer?
: DNA sequence capable of binding repressors. Again, not necessarily found close to the genes they supress
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What do activators do?
bind to enhancer sequences in DNA, resulting in encouraged expression for a gene through increasing the attraction of RNA polymerase to the promoter, through interactions with subunits of RNA polymerase or indirectly by changing the structure of DNA
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What is an enhancer?
where a transcriptional activator binds to to speed up the rate of transcription. These are not necessarily found close to the genes they enhance.
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What is its biological significance?


allows selective expression of genes -can be regulated to control time, place and level of protein expression

Card 3


Where does a) transcription b)translation occur in (i) prokaryotes (ii) Eukaryotes?


Preview of the front of card 3

Card 4


What is the initiation codon almost always in eukaryotes? What does this code for?


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Card 5


In eukaryotes, what is the default position of a gene? Hence?


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