Transcription Flashcards
Transcription
DNA directed synthesis of RNA
Transcription is the first step of…
Gene Expression
RNA is transcribed from…
only ONE strand of DNA from a gene
Template Strand
The strand that serves as a template for complementary RNA transcript formation
Template strand is also known as:
1) Non-coding strand
2) Anti-sense strand
Why is the template strand also called the “non-coding strand”?
Because its sequence is not the same as the RNA produced from it (it has a complementary sequence)
Non-Template Strand
AKA –> Coding Strand / Sense Strand
–> The strand that is NOT transcribed
Why is the non-template strand also called the “coding strand”?
Because its sequence is “the same” as that of the RNA transcribed from the template strand
–> Both the non-template strand and RNA transcript are complementary to the template strand
Every time a gene is transcribed, the ________________ is always used as the template
SAME STRAND
Why is the same strand of a gene always used as the template during every round of transcription?
Particular DNA sequences associated with a gene determines how RNA polymerase is oriented when it binds to the DNA which then established which strand will be utilized as the template
–> This DOES NOT mean the same strand is used for ALL genes: it is gene specific which strand is the template
Genes are READ: ______________
So the RNA transcript is produced: ______________
Genes are read 3’ —-> 5’
RNA transcript is produced 5’ —–> 3’
Differences between DNA and RNA (3)
1) RNA uses Uracil instead of Thymine
2) RNA is usually single stranded
3) RNA contains ribose instead of deoxyribose
How does RNA being single stranded affect its stability?
Makes it less stable than DNA
–> It is more vulnerable to degradation/digestion by nucleases
How does ribose affect the stability of RNA?
Makes it less stable
–> Under basic conditions the hydroxyl group on 2’ C may be deprotonated which can act as a neucleophile and hydrolyze (break) the phosphodiester bonds between nucleotides
Importance of stability to RNA and DNA:
1) DNA is chemically stable: Important because DNA is PERMANENT –> The cell does NOT want the DNA to be changed
2) RNA is chemically unstable: Important because RNA needs to be short-lived/temporary so that the cell can control its quantity and therefore the amount of protein produced
–> cell needs to be able to degrade RNA to control protein levels
Major Cellular RNAs
1) mRNA –> Messenger RNA
2) tRNA –> Transfer RNA
3) rRNA –> Ribosomal RNA
4) snRNA –> Small Nuclear RNA
5)ncRNA –> Non-Coding RNA
mRNA
Messenger RNA
–> Encodes for proteins (what ends up getting translated)
tRNA
Transfer RNA
–> Functions during translation in which it serves as an “adaptor” molecule to bring the correct amino acid to the translation machinery (ribosome)
rRNA
Ribosomal RNA
–> Functions during translation in the structure of ribosomes
–> AND certain rRNAs are involved in the CATALYSIS of peptide bond formation
What is the most abundant RNA?
rRNA
snRNA
Small Nuclear RNA
–> Functions during RNA splicing
Structural RNAs
RNAs that are encoded by genes but DO NOT get translated into proteins:
1) tRNAs
2) rRNAs
3) snRNAs
–> Made of RNA but function kind of like proteins
ncRNA
Non-Coding RNA
–> We got no clue what they do: preliminary studies have found some evidence they may be involved in gene expression regulation
RNA Polymerase
An enzyme that uses a single stranded DNA template to synthesize a complementary strand of RNA
RNA polymerase builds RNA strand in the…
5’ to 3’ direction
–> Adds nucleotides to the free 3’ end of the synthesizing strand
(Like DNA polymerase in this way)
RNA polymerase also has ____________ activity
HELICASE ACTIVITY
–> Unwinds the DNA by itself to free the template strand (no other enzyme needed to separate the DNA strands)
RNA polymerase DOES NOT NEED…
a PRIMER –> Able to start a nucleotide chain from scratch
(unlike DNA polymerase)
Similarities between RNA and DNA Polymerases: (2)
1) Template-directed nucleic acid polymerization
2) Works in the 5’ to 3’ direction, adding nucleotides to the free 3’ end of the new strand
Differences between DNA and RNA production: (7)
1) DNA uses ATGC and RNA uses AUGC
2) DNA uses dNTPs and RNA uses NTP (nucleotide triphosphates)
3) DNA stays in nucleus and RNA can leave
4) DNA replication = double stranded molecule
RNA transcription = single stranded transcript
5) DNA rep. = ALL of genome is copied
RNA transcrip. = copies only a SEGMENT of DNA
6) DNA Polymerase needs primer and RNA polymerase does NOT
7) DNA rep. = CAREFUL PROOFREADING
RNA transcrip. = Very minimal to no proofreading
What is the reason for difference in proofreading between DNA and RNA?
DNA –> Needs extensive proofreading as any errors left in the code would be inherited causing incorrect genetic code to proliferate
RNA –> Doesn’t matter as much; so much RNA is produced that a few errors here or there wont have a huge impact as any faulty proteins produced would just get degraded (not a permanent problem if there is an error in the code)
RNA Polymerase in Prokaryotes vs Eukaryotes
Prokaryotes = Only have ONE RNA polymerase that produces ALL RNA molecules
Eukaryotes = Have THREE RNA polymerases
1) RNA Polymerase I
2) RNA Polymerase II
3) RNA Polymerase III
RNA Polymerase I
Synthesizes 3 rRNAs
RNA Polymerase III
Synthesizes tRNA + 1 rRNA
(Think Three = Transfer) –> The T’s match
RNA Polymerase II
The main one!
–> Synthesizes pre-mRNAs AND snRNAs
In-Vitro Transcription
Put into a test tube:
1) DNA
2) RNA Polymerase
3) NTPs (ATP, GTP, CTP, UTP)
4) Mg2+ (a cofactor of RNA polymerase)
–> RNA transcription will occur RANDOMLY –> Non-specific transcription
How does in-vivo transcription differ from in-vitro?
In-vivo is SPECIFIC –> Highly regulated
In-vitro is NON-SPECIFIC –> Not regulated and random
What regulates in-vivo transcription?
1) Recruitment of RNA polymerase
2) Transcription factors
3 main stages of transcription:
1) Initiation
2) Elongation
3) Termination
Transcription Initiation (General overview)
The recruitment of RNA polymerase and binding of it to the promoter leading to the unwinding of DNA and beginning of RNA synthesis
Promoter
Region of the gene that contains the start site of transcription and regulates efficiency of transcription
–> DNA sequences that define the start site of transcription