From gene to protein

Question 1

transcription initiation

Answer 1

1) transcription factors protein bind to promoter (TATA box sequence)

2) RNA polymerase protein binds in the correct place and orientation

Question 2

transcription elongation

Answer 2

1) separates into 2 DNA strands exposing 10-20 nucleotides at a time

2) It then starts moving in the 3’ → 5’ direction on the template strand of DNA, adding the complementary RNA nucleotides together. This forms a growing strand of RNA in the 5’ → 3’ direction, which then trails off from the polymerase

Question 3

transcription termination

Answer 3

1) controlled by DNA sequence called the polyadenylation signal sequence, which is transcribed as AAUAAA in the pre-mRNA

2) Proteins in the nucleus bind to this AAUAAA sequence

3) About 10-35 nucleotides later, these proteins cut the RNA transcript off from the polymerase

4) The polymerase keeps transcribing but the RNA produced is degraded by enzymes, which eventually catch up to the polymerase and dissociate it from the DNA

Question 4

transcription initiation complex

Answer 4

complex made of transcription factor and polymerase

Question 5

mRNA

Answer 5

messenger RNA, carries protein information from the DNA in a cell’s nucleus to the cell’s cytoplasm

Question 6

tRNA

Answer 6

transfer RNA, serves as an adapter between genetic instructions in mRNA and the amino acids

Question 7

rRNA

Answer 7

ribosomal RNA, forms ribosomes

Question 8

5’ cap

Answer 8

a modified form of a guanine nucleotide, and occurs after the first 20-30 nucleotides have been transcribed

Question 9

poly-A-tail

Answer 9

After the pre-mRNA is released, an enzyme adds 50-250 additional adenine nucleotides to the 3’ end. This is called a Poly-A tail

Function:
- facilitate export of mRNA from the nucleus
- help protect mRNA from degradation by hydrolytic enzymes
- help ribosomes attach to the 5’ end of the mRNA

Question 10

introns

Answer 10

non-coding sequences

Question 11

exons

Answer 11

coding sequences

Question 12

splicesome

Answer 12

large complex consisting of proteins and small RNAs

Question 13

RNA splicing (steps)

Answer 13

1) splicesome binds to several short nucleotide sequences along an intron on the pre-mRNA

2) The intron is then released and rapidly degraded, and the spliceosome joins together the two neighbouring exons

3) RNA catalyzes the splicing reaction

Question 14

ribozymes

Answer 14

RNA molecules that function as enzymes

Question 15

alternative RNA splicing

Answer 15

the same genes can give rise to several different polypeptides, depending on which segments are treated as exons during RNA processing

Question 16

anticodon

Answer 16

tRNA end to bind to a specific mRNA codon (3 nucleotides)

Question 17

Aminoacyl-tRNA synthetase

Answer 17

protein that joins the amino acids to the corresponding tRNA (uses ATP)

Question 18

wobble (or degeneracy)

Answer 18

position of the last nucleotide on the anti-codon who is flexible and binds with multiple codons

Question 19

ribosomes

Answer 19

translation occurs in it,
made of a small and large subunit made of rRNA, joined together only when mRNA is there

Question 20

binding site for tRNA (steps)

Answer 20

1) tRNAs in the cytoplasm randomly enter the A site (right) until one with the matching anticodon is found

2) The P site (middle) contains a tRNA that has already entered the ribosome through the A site. The created amino acid chain leaves the ribosome through the exit tunnel

3) Discharged tRNAs exit the ribosome in the E site (left)

Question 21

Translation initiation

Answer 21

1) AUG initiate the start of translation

2) The small ribosomal subunit binds to a specific initiator tRNA, which carries the amino acid methionine. It then binds to the 5’ cap of the mRNA and then scans downstream until it reaches the start codon, which the initiator tRNA binds to

3) the large ribosomal subunit attaches, forming a complete translation initiation complex

Question 22

Translation elongation

Answer 22

1) Amino acids are added one by one to the C-terminus (a.k.a. carboxyl end) of the growing chain as tRNAs move in and out of the ribosome

2) Codon by codon, the mRNA moves out of the ribosome (5’ end first)

3) Empty tRNAs return to the cytoplasm, where they are reloaded with the correct amino acid

Question 23

Translation termination

Answer 23

1) Elongation continues until a stop codon is reached (UAG, UAA or UGA)

2) A protein shaped like a tRNA (called the release factor) binds to the stop codon in site A of the ribosome

3) A water molecule binds to the end of the polypeptide chain instead of an amino acid. This releases the polypeptide out through the exit tunnel