RNA & Genetic Code

Question 1

Which type of RNA carries the info specifying the amino acid sequence of a protein to the ribosome ?

Answer 1

mRNA (in nucleus)

• transcribed from DNA template strand in nucleus via RNA polymerases’
• read as codons
• eukaryotes=monocistronic; prokaryotes=polycistronic

Question 2

Which type of RNA is responsible for translating the language if nucleic acids into that of amino acids & peptides ?

Answer 2

tRNA (in ribosome)

• anticodon pairs w/ mRNA in ribosome
• charge & activated w/ an amino acid (attached @ 3’ OH end)—–> done by aminoacyl-tRNA synthetase

Question 3

Which type of RNA is synthesized in the nucleolus ?

Answer 3

rRNA

-many function as ribozymes: enzymes composed of RNA molecules

Question 4

What are the start and stop codons ?

Answer 4

AUG (start)

UAA, UGA, UAG (stop)—> cause for release of protein from ribosome

Question 5

The variable position within a codon is known as ?

Answer 5

Wobble position (3rd base)
-Tends to be silent or degenerate meaning that is mutated it will have no adverse effects on polypeptide sequence

Question 6

What is the difference between missense & nonsense mutations ?

Answer 6

Missense is when one amino acid substitutes for another, & nonsense is when a codon now encodes for a stop codon (truncation)

Question 7

The creation of mRNA from DNA is known as ?

Answer 7

Transcription

Question 8

Where does RNA polymerase II bind to start transcription ?

Answer 8

TATA box (promoter region)

Question 9

The primary transcript formed after the termination of the RNA polymerase III enzyme is called ?

Answer 9

Heterogeneous nuclear RNA (hnRNA)

Question 10

What steps consist of post-transcriptional processing and lead to the functional release of hnRNA from the nucleus ?

Answer 10

1. Splicing of introns (via spliceosomes) and ligation of exons
   - In spliceosomes, snRNP/snRNA complex identifies 5’&3’ spplice sites of introns
2. @5’ end of hnRNA, a 7-methylguanylate triphosphate cap is added as a site for ribosome binding and protects against degradation
3. A poly-A tail is added to the 3’ end & protects the message against rapid degradation

Question 11

What structure is composed of proteins and rRNA ?

Answer 11

Ribosomes

Question 12

In prokaryotes, what sequence does the small subunit of the ribosome bind to ?

Answer 12

Shine-Dalgarno sequence in 5’ untranslated region

Question 13

What is the process of initiation for translation ?

Answer 13

1. small ribosomal subunit binds to 5’ cap
2. tRNA binds to AUG
3. large ribosomal subunit binds and forms complex
   * all this is assisted by initiation factors

Question 14

What is the process of elongation of translation ?

Answer 14

1. A site holds incoming aminoacyl tRNA complex
2. P site holds tRNA w/ growing polypeptide chain & where MET binds due to start of polypeptide. Peptidyl transferase is required for tRNA moving from A to P due to peptide bond formations (requires GTP)
3. E site is where now uncharged tRNA unbinds from mRNA & is ready to be recharged
   * elongation factors aid in all this

Question 15

In prokaryotic cells, what is known as a cluster of genes transcribed as a single mRNA ?

Answer 15

An operon

-Jacob-Monod model: describes structure and function

Question 16

What does the Jacob-Monod model consist of ?

Answer 16

Structural gene: codes for protein of interest
Operator site: non-transcribable region of DNA capable of binding a repressor protein
Promoter site: site for RNA polymerase to bind
Regulator gene: codes for repressor

Question 17

What system consists of a repressor protein being tightly bound to the operon and acting as a roadblock for transcription ?

Answer 17

Inducible system

• Inducer must bind repressor protein
• Analogous to competitive inhibition

Question 18

A transcriptional activator in prokaryotes that signal when glucose levels are low is known as ?

Answer 18

CAP catabolite activator protein

-Signals that alternative carbon sources should be used

Question 19

The system that allows constant production of protein is known as ?

Answer 19

Repressible system

-Repressor needs to bind with corepressor to form complex, then bind to operator to stop transcription

Question 20

Transcription activating proteins that search the DNA for specific binding motifs are known as ?

Answer 20

Transcription factors

• DNA binding domain: binds to specific nucleotide sequence in promoter region or to DNA response element (sequence of DNA that binds to only specific transcriptional factors)
• Activation domain: allows for binding of several transcription factors

Question 21

How is gene amplification accomplished in eukaryotes ?

Answer 21

By enhancers (control of one gene’s expression by multiple signals) & gene duplication (copies of relevant gene)

Question 22

Coactivators involved in chromatin remodeling are known as ?

Answer 22

Histone acetylases: decreases + charge on lysine and weakens interaction of histone w/ DNA

*Histone deacetylases: do the opposite!