Chapter 17: From Gene to Protein Flashcards
What do tRNAs contain?
An anticodon
Overview of elongation of translation
Ribosome moves along mRNA adding amino acids to the growing polypeptide chain
3 steps of translation initiation
- rRNA in small subunit binds to the complementary sequence of mRNA (ribosomal binding site) helped by initiation factors
- Initiator tRNA binds to start codon
- Large subunit attaches to form translation initiation complex
Where does translation occur?
The cytoplasm
What do snRNPs do?
Recognize specific sequence at the end of introns and cuts out introns and joins exons back together
What does translation do?
Turns mRNA into protein
What makes up a snRNP (small nuclear ribonucleoprotein)?
snRNAs + proteins
What must happen to pre-mRNA before it can be transported to the cytoplasm for translation?
It must be modified in the nucleus with a 5’ cap and a 3’ Poly-A tail
How is transcription terminated in eukaryotes?
Poly(A) signal is the termination signal
Where are transcription and translation separated?
Eukaryotes
What helps RNA polymerase to recognize promoters in eukaryotic transcription? In bacteria?
In eukaryotes = basal transcription factors
In bacteria = sigma proteins
Enzymes which attach the amino acid to the tRNA
Aminoacyl-tRNA synthetases
What strand is the same as the RNA strand (except T and U) and is not read by RNA?
DNA coding strand
Overview of termination of transcription (bacteria)
Transcription proceeds until the polymerase reaches the terminator sequence which causes the enzyme to dissociate from the DNA
How is tRNA transcribed?
Transcribed from DNA template and folds to form a 3D structure
What does adding the 5’ cap and 3’ Poly-A tail do?
- Facilitate export from nucleus
- Protect from degradation in the cytoplasm by nucleases
- Help ribosomes attach to the 5’ end for translation
Overview of initiation of translation
Small unit of the ribosome with the initiator tRNA (methionine) already attached binds to the leader sequence of mRNA and scans for AUG and the large subunit binds and completes translation initiation complex
Enzyme adds 50-250 adenines to 3’ end
3’ Poly-A tail
What does transcription do?
Makes a single stranded copy of mRNA that can leave the nucleus. DNA —> mRNA
What do synthetases require to bring specific amino acids and tRNAs together?
ATP. Energy
What could happen if the 5’ cap and 3’ poly-a tail weren’t around to assist ribosomes?
Ribosomes could attach to any random AUG in the strand and not the start one on the 5’ end
What strand does the RNA read?
The DNA template strand
What does RNA polymerase do in elongation of transcription (bacteria)?
- Unwinds the helix and exposes 10-20 nucleotides at a time
- Synthesizes mRNA from 5’-3’ without the need of a primer
- Adds 50 nucleotides per second
What is the E ribosome site?
Where the empty tRNA exits
What are the 3 sites of a ribosome?
E, P, and A
How do synthetases work?
Their active site can only fit specific combinations of tRNA and amino acids (ensures anticodon brings in the correct amino acid)
Triplet which is complementary to codon on mRNA
Anticodon
What can holoenzymes do (bacteria)?
Recognize and bind to promoters in initiation of transcription
Overview of elongation of transcription (bacteria)
DNA is unwound and RNA polymerase creates a strand of mRNA which is complementary to the template strand of DNA. RNA polymerase synthesizes the mRNA in the 5’ to 3’ direction. Occurs at a rate of 50 bases per second
Where does transcription occur in eukaryotes?
The nucleus
What can silent mutations create?
A splice site that isn’t supposed to be there (happens with Progeria)
What typically happens to a single mRNA?
It is simultaneously translated by multiple ribosomes (polyribosomes)
Overview of termination of translation
Ribosome hits a stop codon and the subunits separate and the polypeptide is released
What must happen to a polypeptide for it to become a protein?
Correct secondary and tertiary folding
What catalyzes the formation of peptide bonds?
rRNA
What does polypeptide folding into a protein sometimes require?
Assistance of molecular chaperones
What are the 3 steps of transcription?
- Initiation
- Elongation
- Termination
Initial transcript in eukaryotes
pre-mRNA
What are polyribosomes?
Multiple ribosomes on mRNA
What does tRNA do?
Translates message from nucleic acids to amino acids
What is tRNA with an amino acid attached called?
Charged
What do tRNAs carry?
A specific amino acid
What are some key players in translation?
tRNAs and ribosomes
What does sigma do (bacteria)?
Guides RNA polymerase to start of a gene in initiation of transcription
What is the A ribosome site?
Incoming (active) tRNA
What 3 chemical modifications can occur after translation?
- Attachment of sugars, lipids, phosphate groups, etc
- Cleavage of polypeptide chain (like insulin)
- Interaction of multiple subunits (quaternary structure)
Flexible base pairing in the 3rd position (not always A and U or C and G pairing in the third position which allows for less tRNAs)
Wobbles
How long does it take to make a ribosome to make a polypeptide?
Less than a minute
The part of eukaryotic genes that are coding segments
Exons
Consist of a small and large subunit composed of protein and ribosomal RNA (rRNA)
Ribosomes
Where can untranslated regions be found?
After the 5’ cap before the start codon and after the stop codon before the poly-a tail
What doesn’t RNA polymerase need that DNA polymerase does?
A primer
How is mRNA read?
3 bases at a time (codons) to build a polypeptide
What are the 3 steps of translation termination?
- Elongation continues until a stop codon is reached
- A release factor (protein shaped like a tRNA) binds to the stop codon in the A site that causes the polypeptide to be released
- The translation assembly breaks apart
What types of polymerases do eukaryotes have involved in transcription as opposed to bacteria?
3 main polymerases that each produce a certain type of RNA instead of just RNA polymerase
What are the 3 steps of translation?
- Initiation
- Elongation
- Termination
How many synthetases are there?
- 1 for each amino acid
What is an untranslated region with base pairs AAUAAA found after the stop codon before the 3’ poly-a tail called?
Polyadenylation signal
How does termination of transcription occur in bacteria?
RNA forms a hairpin loop and RNA polymerase releasees transcript
What is the P ribosome site?
tRNA with growing polypeptide
What differences are seen in the promoters of eukaryotic transcription as opposed to bacterial?
Promoters are more divers in eukaryotes (TATA box)
What does translation elongation require?
Proteins (elongation factors) and energy
What allows there to be less tRNAs than total codons?
Wobbles which are flexible base pairing in the 3rd position (not always A and U or C and G pairing in the third position which allows for less tRNAs)
What two purposes do introns serve?
- Some introns contain sequences which play a role in regulating gene expression
- They allow for a single gene to code for more than one polypeptide (alternative RNA splicing)
What is alternative RNA splicing?
Some exons can be left out of final transcript leading to more protein possibilities. More proteins can be made from the much smaller number of genes
How do prokaryotic and eukaryotic ribosomes differ?
They are very similar in structure but subtle differences are exploited in medicine. Ex: tetracycline can inactivate bacterial ribosomes without harming human ribosomes
Overview of initiation of transcription (bacteria)
Transcription factors and RNA polymerase bind to the promoter of the gene (which genes are transcribed is tightly controlled)
What is RNA polymerase + sigma called (bacteria)?
Holoenzyme
What do different sigma proteins do (bacteria)?
Bind promoters with different DNA sequences
The part of eukaryotic genes that are noncoding DNA
Introns
What is a spliceosome?
snRNPs that cut out introns and join exons back together
How does translation elongation occur?
Amino acids are added one by one to growing polypeptide chain
Modified guanine added to 5’ end
5’ cap
