Lec 4: Transcription and Translation

Question 1

What is hnRNA?

Answer 1

The primary RNA transcript that has not undergone processing to remove introns

Question 2

Is the template strand sense or antisense? Is the non-template strand sense or antisense?

Answer 2

The template strand is antisense

Non-template strand is sense

Question 3

What are the 5 classes of RNA in human cells?

Answer 3

rRNA: ribosomal RNA
tRNAs: transfer RNA, adapter in protein synthesis between mRNA and amino acids
mRNAs: encode proteins
snRNAs: components of spliceosome RNA splicing
negative regulator RNA (miRNAs, siRNA, piRNA, lncRNA)

Question 4

What do the 3 RNA polymerases synthesize?

Answer 4

RNA Pol 1 = 28s, 18s, 5.8s rRNAs
RNA Pol 2 = mRNAs, some miRNAs and snRNAs
RNA Pol 3 = tRNAs, 5s rRNAs, some snRNAs and miRNAs
Mitochondria have another RNA polymerase

Question 5

How is the location and direction of transcription on the DNA template set?

Answer 5

Set by the promoter sequence

Question 6

What are the 2 important proximal promoter element, and generally what types of genes are they found within?

Answer 6

TATA box
CAAT box
typically found in tissue-specific genes (i.e. housekeeping genes do not have them)

Question 7

What controls transcription of many housekeeping genes? What else can generally control the transcript?

Answer 7

Housekeeping genes typically have sequences rich in CG found in CpG islands, modified by methylation of C residues
-many genes have enhancers or silencers that are located further away on the genome

Question 8

Describe the steps of transcription.

Answer 8

1. Begins at TATA box
2. TATA box is recognized by transcription factor TFIID, which enables TFIIB
3. Remaining transcription factors bind along with RNA polymerase
4. TFIIH uses ATP to unwind DNA and phosphorylates RNA pol II so that it can begin the elongation phase of transcription

Question 9

What is another name for the primary transcript, and what happens to it before it is processed?

Answer 9

hnRNA

• capped by addition of 7-methyl G residue to 5’ end by guanylyl transferase through 5’ - 5’ phosphodiester bon
• poly-A tail added by poly A polymerase

Question 10

What protein catalyzes RNA processing, and where does it occur? How does the removal of introns occur?

Answer 10

• catalyzed by small nuclear ribonucleoproteins (snRNPs)
• occurs in splceosome
• introns start with a GU and end with AG, so removal of intron occurs through attack of the conserved A of the branch site and the G of the GU at the beginning of the intron

Question 11

What 2 diseases are caused by mutations in splice sites?

Answer 11

beta-thalassemia

Duchenne’s muscular dystrophy

Question 12

What 3 codons correspond to a stop codon?

Answer 12

UAA, UAG, UGA

Question 13

What is the link between mRNA and the amino acid sequence of the new protein? What is added to their 3’ end and by what?

Answer 13

tRNAs

-all tRNAs have a CCA added to the 3’ end by nucleotidyl transferase

Question 14

What would be the anticodon on a tRNA for the mRNA sequence of AUG?

Answer 14

anticodon would be UAC

Question 15

What is there a greater number of, tRNAs or mRNA possible codons?

Answer 15

There are more codons than tRNAs, so many tRNAs can recognize more than 1 codon due to wobble base pairing between the third position of codon in mRNAs (which corresponds to the 1st position of anticodon in tRNAs)

Question 16

What is the site of protein synthesis? What 2 subunits make it up, and what RNA molecules are within each subunit?

Answer 16

Ribosomes

• Large (60S) and small (40S) subunits
• 60S subunit contains 28S, 5,8S, and 5S rRNAs
• 40S subunit contains 18S rRNA

Question 17

What catalyzes protein synthesis?

Answer 17

In the ribosome, ribosomes act as ribozymes

-RNA component catalyzes peptidyl transferase reaction

Question 18

What attaches amino acids to tRNAs?

Answer 18

attached through carboxyl groups to CCA sequence at 3’ end of tRNAs by aminoacyl-tRNA synthetases

Question 19

What are the 2 adapters needed for translation?

Answer 19

1. aminoacyl-tRNA synthetase (couples tRNA to amino acid, called charging)
2. tRNA molecule (anticodon forms base pairs with mRNA)

Question 20

What molecules are in the complex that initiates protein synthesis?

Answer 20

Met-tRNA (since AUG met is the start codon)
Ten initiation factors
GTP
40S ribosome

Question 21

Once the start codon AUG is found by the protein synthesis initiation complex, what happens?

Answer 21

brings in 60S ribosomal subunit generation 80S initiation complex

• second charged tRNA binds next codon)
• peptidyl transferase catalyzes formation of peptide bond between carboxyl group of one amino acid and amino group of the next amino acid

Question 22

How is protein synthesis terminated?

Answer 22

once reaching a stop codon, there is a binding of a release factor to the A-site of a stop codon