Unit 3 Chapter 16: Transcription and Translation

Question 1

Generally, what happens during transcription and where does it occur

Answer 1

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

Question 2

What is the template strand (antisense)

Answer 2

the strand that is read by the enzyme

Question 3

What is the coding strand (sense)

Answer 3

• 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)

Question 4

What are the roles of RNA polymerases 1,2,3

Answer 4

*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

Question 5

What are promoters

Answer 5

-binding sites in DNA where transcription begins

Question 6

What is the transcription complex

Answer 6

TATA box a promoter sequence , centred 30 bases upstream from transcription start site (upstream = in opp direction that RNA polymerase moves during transcription)

Question 7

What molecule makes the initial contact with DNA that starts transcription in eukaryotes

Answer 7

basal transcription factors (work same as sigma) bind to appropriate promoter region in DNA, then RNA polymerase follows

Question 8

What direction does RNA polymerase move during transcription

Answer 8

3’ to 5’ synthesizing RNA in 5’ to 3’ direction

Question 9

What is the elongation phase

Answer 9

when RNA polymerase moves down DNA synthesizing RNA

Question 10

What is the termination phase

Answer 10

when transcription stops

-usually happens when RNA polymerase reaches a termination sequence on the DNA

Question 11

What does the coding region always begin with

Answer 11

start codon

-methionine

Question 12

What must happen to the RNA produced from transcription (primary RNA transcript) before it can be translated

Answer 12

it must be processed

Question 13

How is RNA processed after transcription

Answer 13

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

Question 14

What are exons

Answer 14

final, usable part of the mRNA after RNA splicing

Question 15

What are introns

Answer 15

the non-coding sequences in the mRNA that interfere and must be removed

Question 16

How does RNA splicing work

Answer 16

1. snRNPs (special RNAs) bind to 5’ exon-intron boundary and to a key adenine ribonucleotide near the end of the intron
2. then other snRNPs come forming a spliceosome
3. intron forms a loop when 5’end is covalently bonded to a key adenine at its connecting point
4. loop is cut out , phosphodiester bond links remaining exons together

Question 17

How are caps and tails added

Answer 17

1. as soon as 5’ end of RNA leaves RNA polymerase, enzymes add a 5’ cap
2. 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

Question 18

What is the role of untranslated regions

Answer 18

to stabilize mature RNA and regulate translation

Question 19

What is the role of caps/tails

Answer 19

protect mRNAs from degradation by ribonucleases and enhance efficiency of translation

Question 20

Where does translation occur

Answer 20

in ribosomes in cytosol

Question 21

What is tRNA and how does it work

Answer 21

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

Question 22

How are codons writtion

Answer 22

5’ to 3’

Question 23

How are anticodons written

Answer 23

3’ to 5’

Question 24

What is the wobble hypothesis

Answer 24

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

Question 25

What are the wobble pairing rules

Answer 25

• 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

Question 26

In bacteria, why can translation start even before transcription is done

Answer 26

there is no nuclear envelope to separate the process so they are connected

Question 27

What is a polysome

Answer 27

when many codons bind to the same transcript starting at the start codon one after another

Question 28

What is the point of the wobble

Answer 28

allows 40 tRNAs to bind to all 61 mRNA codons

Question 29

What is the first step of translation

Answer 29

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

Question 30

What is the large subunit of a ribosome

Answer 30

where peptide formation takes place

Question 31

What is the small subunit of a ribosome

Answer 31

holds mRNA in place during translation

Question 32

What are the A, P and E sites

Answer 32

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)

Question 33

What are the 3 steps for translation

Answer 33

1. protein synthesis
2. elongation
3. termination

Question 34

Where does protein synthesis begin/end

Answer 34

begins at N terminus and proceeds to C terminus

Question 35

What are the 3 steps for protein synthesis

Answer 35

1. tRNA moves into A site and anticodon binds to codon
2. peptide bond forms between amino acid held by tRNA in A and the growing peptide chain which is held by a tRNA in P
3. 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)

Question 36

How does translation begin

Answer 36

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

Question 37

What is elongation in translation

Answer 37

• 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)

Question 38

Is a ribosome an enzyme or a ribozyme

Answer 38

ribozyme, made entirely of RNA therefore protein synthesis is catalyzed by RNA

Question 39

What happens in termination of translation

Answer 39

• 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

Question 40

What happens to the newly made protein chain

Answer 40

• folding

- chemical modification such as additions of sugar or lipids or carb-based sorting signal, phosphorylation, etc.

Question 41

Can proteins begin to function before translation is done

Answer 41

yes in both euks/proks