Unit 3 Chapter 16: Transcription and Translation Flashcards
Generally, what happens during transcription and where does it occur
occurs in nucleus
NTP, (same as dNTP but with ribose) matches a base on the DNA template, once this happens RNA polymerase catalyses the formation of a phosphodiester linkage between the 3’ end of the mRNA that’s forming and the NTP; this process continues 5’ to 3’ forming the RNA that is complimentary to the gene
What is the template strand (antisense)
the strand that is read by the enzyme
What is the coding strand (sense)
- not the template strand
- the sequence of this strand matches the sequence of the RNA and codes for a peptide (except this has Ts and RNA has Us)
What are the roles of RNA polymerases 1,2,3
*note bacteria have only one RNA polymerase
1: transcribes genes that code for most of the rRNA found in ribosomes
2: transcribe protein-coding genes that make mRNA, and genes that code for RNA that function in ribosome assembly and in processing/regulation of mRNA
3: transcribe genes that code for tRNAs, one of the small rRNAs in ribosomes, for non coding RNAs, and for RNAs that function in ribosome assembly and regulation.processing of mRNA
What are promoters
-binding sites in DNA where transcription begins
What is the transcription complex
TATA box a promoter sequence , centred 30 bases upstream from transcription start site (upstream = in opp direction that RNA polymerase moves during transcription)
What molecule makes the initial contact with DNA that starts transcription in eukaryotes
basal transcription factors (work same as sigma) bind to appropriate promoter region in DNA, then RNA polymerase follows
What direction does RNA polymerase move during transcription
3’ to 5’ synthesizing RNA in 5’ to 3’ direction
What is the elongation phase
when RNA polymerase moves down DNA synthesizing RNA
What is the termination phase
when transcription stops
-usually happens when RNA polymerase reaches a termination sequence on the DNA
What does the coding region always begin with
start codon
-methionine
What must happen to the RNA produced from transcription (primary RNA transcript) before it can be translated
it must be processed
How is RNA processed after transcription
RNA splicing to remove sequences of non-coding base pairs which are in the DNA but not in the corresponding mRNA, addition of Caps and Tails
What are exons
final, usable part of the mRNA after RNA splicing
What are introns
the non-coding sequences in the mRNA that interfere and must be removed
How does RNA splicing work
- snRNPs (special RNAs) bind to 5’ exon-intron boundary and to a key adenine ribonucleotide near the end of the intron
- then other snRNPs come forming a spliceosome
- intron forms a loop when 5’end is covalently bonded to a key adenine at its connecting point
- loop is cut out , phosphodiester bond links remaining exons together
How are caps and tails added
- as soon as 5’ end of RNA leaves RNA polymerase, enzymes add a 5’ cap
- another enzyme cleaves the 3’ end after transcription and another one adds a long row of adenine nucleotides (not encoded on DNA) called the polyAtail
now RNA is considered mature
What is the role of untranslated regions
to stabilize mature RNA and regulate translation
What is the role of caps/tails
protect mRNAs from degradation by ribonucleases and enhance efficiency of translation
Where does translation occur
in ribosomes in cytosol
What is tRNA and how does it work
transfer RNA, an adaptor between mRNA and proteins
- needs ATP to attach to amino acid
- enzymes called aminoacyl tRNA synthetases catalyze the addition of the amino acids to tRNA (charging of tRNA)
- one or more tRNA per amino acid
- act as interpreter in translation
- all shaped like upside down L
- have anticodons that bind to mRNA
How are codons writtion
5’ to 3’
How are anticodons written
3’ to 5’
What is the wobble hypothesis
basically says that if the first 2 bases in a codon match then some tRNAs can still bond to some codons even if their 3rd base pair doesn’t match as long as it doesn’t change the protein that will be formed
What are the wobble pairing rules
- G (Anti) can bind with C or U on codon
- C(Anti) binds with G
- A(Anti) binds with U
- U(Anti) binds with A or G
- I (Anti) binds with A, U or C
In bacteria, why can translation start even before transcription is done
there is no nuclear envelope to separate the process so they are connected
What is a polysome
when many codons bind to the same transcript starting at the start codon one after another
What is the point of the wobble
allows 40 tRNAs to bind to all 61 mRNA codons
What is the first step of translation
conversion of each mRNA codon which begins when anticodon of tRNA binds to codon and ends when a peptide bond forms between the tRNA’s amino acid and the growing polypeptide chain
What is the large subunit of a ribosome
where peptide formation takes place
What is the small subunit of a ribosome
holds mRNA in place during translation
What are the A, P and E sites
so during translation 3 distinct tRNAs are lined up in the ribosome
@ A: the tRNA here carries an amino acid (acceptor)
@ P: the tRNA here holds the growing polypeptide chain (peptide bond formation)
@ E: the tRNA here no longer has an amino acid and is getting ready to leave (exit)
What are the 3 steps for translation
- protein synthesis
- elongation
- termination
Where does protein synthesis begin/end
begins at N terminus and proceeds to C terminus
What are the 3 steps for protein synthesis
- tRNA moves into A site and anticodon binds to codon
- peptide bond forms between amino acid held by tRNA in A and the growing peptide chain which is held by a tRNA in P
- ribosome moves ahead and all tRNAs shift one spot down the line (the one at E exits, P moves to E and A moves to P,etc)
How does translation begin
when a section of rRNA in small ribosomal subunit binds to mRNA (ribosome binding site), then tRNA carrying a methionine binds to starts codon, then large subunit joins complex and tRNA carrying methionine goes to P site
What is elongation in translation
- E and A sites are empty at start
- proceeds when tRNA binds to codon in A site
- when both P and A are occupied, tRNAs are in ribosomes active site and peptide bond formation/protein synthesis can occur
- translocation is the third step where the tRNAs shift their positions or get ejected into cytosol (if in E)
Is a ribosome an enzyme or a ribozyme
ribozyme, made entirely of RNA therefore protein synthesis is catalyzed by RNA
What happens in termination of translation
- when translocation opens the A site and exposes a stop codon on the mRNA, a release factor fills the A site because no tRNA has an anticodon that binds to stop codons
- polypeptide is freed
What happens to the newly made protein chain
- folding
- chemical modification such as additions of sugar or lipids or carb-based sorting signal, phosphorylation, etc.
Can proteins begin to function before translation is done
yes in both euks/proks
