Chapter 16: Transcription and Translation Flashcards
What is transcription?
Using DNA to code for RNA molecules
What main molecules are required for transcription?
- DNA template
- RNA polmerase
- rNTP (ribonucleotide triphosphate)
How many RNA polymerases are in bacteria? Eukaryotes?
1 in bacteria, 3 in eukaryotes (RNA pol. I, II, III)
The structure of rNTP
a base, ribose sugar and 3 phosphates
What is the strand that is read by RNA polymerase called?
template strand
The sequence of mRNA matches with what strand?
coding (non-template) strand
What protein does RNA polymerase need in order initiate transcription?
a sigma protein
What does a sigma protein do?
- creates a holoenzyme by binding to RNA polymerase
- guide RNA pol. to specific locations (promoters) where transcription should begin
- a regulatory protein
What binds to DNA first?
sigma protein (not RNA pol.)
Where on the DNA does sigma bind to?
promoter region
What are promoters?
initiates transcription (NOT actual start)
What the 2 promoter regions of bacterial DNA?
- 35 box (TTGACA)
- 10 box (TATAAT)–10 bases from where RNA pol. starts transcription
What place does bacterial transcription actually begin?
the +1 region
Are sigma proteins present in eukaryotes?
No.
In eukaryotes, what acts analogous to a sigma in bacteria?
basal transcription factors (need many to begin transcription)
What does RNA pol. I do?
synthesis of large ribosomal units
What does RNA pol. II do?
synthesis or mRNA
What does RNA pol. III do?
synthesis of small ribosomal units and tRNA
In eukaryotes, what is the promoter?
TATA box located 30 bases upstream to transcription site
What happens once sigma is released from promoter?
initiation ends, synthesis (elongation) begins
Once DNA helix unwinds to form what (in transcription of Protein synthesis)?
Transcription bubble
What is elongation?
when RNA pol. begins synthesizing mRNA
What do zipper amino acids do?
help separate the mRNA from the template DNA strand
What do rudder amino acids do?
help maneuver the template and non-template strand through the channels of RNA pol.
How does transcription ends?
when RNA pol. reaches a termination signal, RNA loops on itself to form a hairpin, therefore disrupting the interaction between RNA pol. and the mRNA
What is the difference between a nucleotide and a nucleoside?
nucleosides are similar to nucleotides except that they don’t have a phosphate group
How many phosphates can a nucleotide have?
up to 3
What are ribonucleosides?
RNA base + ribose sugar
What are deoxyribonucleosides?
DNA base + deoxyribose sugar
What is nucleic acid?
a polymer of nucleotide monomers linked together
Why does transcription and translation happen simultaneously in bacteria?
because bacterial (prokaryotic) cells lack a nucleus
What is an exon?
coding region of pre mRNA that is kept after splicing
What is an intron?
non-coding region of pre mRNA that is removed by spliceosomes
In eukaryotes, what types of RNA processing are needed before mRNA can be translated?
1) Capping with 5’ cap
2) Poly A tail at the 3’ end of RNA strand
3) Splicing out the introns
What is the purpose of capping the 5’ end of mRNA?
- to protect the 5’ end from degradation by exonucleases
- to stabilize mRNA before translation
- recognition site for translation?
What is the purpose of adding a row of adenine to the 3’ end of mRNA (poly A tail)?
-to protect the end from degradation
snRNPs are known as:
small nuclear ribonucleoproteins
In vitro hybridization of mRNA and their DNA templates produce loops of mRNA-DNA hybrids. These loops are
single-stranded DNA and they represent introns
What happens to the mature mRNA after post-transcriptional processing (in eukaryotes)?
exported for translation
What is the advantage of alternative splicing?
- a variety of mRNA transcripts can be produced
- this leads to the synthesis of many different proteins
What is translation?
peptides and proteins are sythesized from mRNA
What is a peptide?
a short chain of amino acids linked together by a peptide bond (ie. dipeptides, tri-peptides, polypeptides, etc.)
What molecules are required for translation?
- mRNA
- tRNA
- ribosome
- amino acids
What is an aminoacyl tRNA?
tRNA + amino acid
What are the main components of tRNA?
-anticodon, attached amino acid to CCA region and nucleotides (loops and secondary structured stem)
What are ribosomes made up of?
large and small subunits
What does the large subunit of ribosomes do?
-where peptides are formed
What does the small subunit of ribosomes do?
holds the mRNA in place during translation
What are anticodons?
set of 3 ribonucletides that form base pairs with the mRNA codon
What the 3 sites of ribosomes involved in translation?
- A (acceptor or aminoacyl) site
- P (peptide formation)site
- E (exit) site
The A site is responsible for:
bringing in the tRNA with AA attached
What happens in the P site?
tRNA holds the growing polypeptide chain
What happens in the E site?
the tRNA has no AA attached
What is the ribosomal binding site?
the mRNA sequence that forms complimentary base pairs with the rRNA from the small subunit
-the start of initiation of translation
When is initiation complete?
when the large sununit joins the ribosomal complex
The process of initiation (translation)
1) mRNA binds to small subunit via complimentary base pairing
2) Aminoacyl tRNA carrying f-met binds to AUG codon of mRNA
3) Large subunit binds, completing complex and leaving the initiator aminoacyl tRNA in the P site
The process of elongation (translation)
1) arrival of aminoacyl tRNA to A site
2) peptide bond forms (peptide chain is transferred from tRNA in P site to tRNA in A site)
3) translocation-repeat down the length of mRNA (ribosome moves down length of mRNA in 5’ to 3’ direction)
Do tRNAs have a stop anticodon?
No.
How does protein synthesis end?
when release factor fills the empty A site
What do release factors do?
- fills A site because similar in shape, size as tRNA
- hydrolyzes the bond linking tRNA in P site to polypeptide chain
- releases polypeptide chain
What sequences in the tRNA should be conserved? variable?
Conserved: the nucleotide part
Variable: the anticodon and amino acid part
