Transcription and Translation

Question 1

What are the 3 ways RNA differs from DNA?

Answer 1

• RNA is single-stranded
• RNA contains ribose instead of deoxyribose
• And has uracil instead of thymine

Question 2

What are the 3 types of RNA involved in protein synthesis?

Answer 2

1) mRNA-the encoded DNA sequence that is translated into a polypeptide or protein by tRNA
2) rRNA-structural component of ribosome
3) tRNA-RNA which is able to transfer amino acids to the ribosome according to the mRNA transcript

Question 3

What are the 3 forms in RNA structure?

Answer 3

• Primary structure-sequence of nucleotides
• Secondary structure -hairpin loop forms as the RNA molecule is covered to double stranded
• Tertiary structure- The RNA molecule folds upon itself and binds to the unpaired region of the RNA molecule.

Question 4

What are the key differences between eukaryotic and prokaryotic mRNA processing?

Answer 4

1) Prokaryotic mRNA is not further processed and can be directly translated, when the 5’phosphate is removed. RNA modifications in prokaryotes only occur in tRNA and in rRNA. (e.g. pseudouricil and thiouracil) and when CCA transferase is added to the 3’ end.
2) in eukaryotes mRNA is processed through capping and the addition of the poly-A tail which is needed to stabilize the molecule, and protect the transcript from being degraded ny RNases before being transported out of the nucleus.

Question 5

What are the 3 phases involved during transcription?

Answer 5

1) Initiation
2) Elongagtion
3) Termination

Question 6

In transcription, what are the events that occur during the initiation phase.

Answer 6

During the initiation phase RNA polymerase recognizes and binds to promoter sequence on the DNA template strand in which it then unwinds DNA and begins by inserting the first 3 nucleotides. Keep in mind that during RNA synthesis the three phosphates at the 5’ end are retained.

Question 7

During the elongation phase of transcription what happens?

Answer 7

RNA polymerase continues to add complementary nucleotides in the 5’-3’ direction antiparallel to the template strand. Be sure to remember that T is replaced by U.

Question 8

How is RNA polymerase able to recognize a promoter sequence?

Answer 8

through the use of sigma factors the RNA polymerase is able to recognize and bind to the promoter sequence.

Question 9

How is Transcription terminated by Rho-dependent termination?

Answer 9

Rho-dependet transcription termination occurs when the Rho protein recognizes a particular DNA sequence known as the rut sequence; which causes the Rho protein to race after RNA polymerase in order to create a hairpin loop essentially slowing down and dislodging RNA polymerase and thus ending transcription.

Question 10

What are the 2 factors required for Rho-independent transcription termination occur?

Answer 10

• Does not require Rho protein
• Hairpin formation occurs causing RNA polymerase to slow down
• Immediate long stretch of A or T which further disrupts the RNA polymerase

Question 11

What is a gene?

Answer 11

A region of DNA encoding for a particular polypeptide chain or functional RNA such as rRNA or tRNA.

Question 12

What is a codon?

Answer 12

A sequence of three nucleotide bases that form the mRNA code for an amino acid.

Question 13

What is an anticodon?

Answer 13

An anticodon is a set of three ribonucleotide bases in a molecule of tRNA complementary to mRNA codon.

Question 14

What ensures the correct amino acid is placed during translation?

Answer 14

During translation the anticodon formed by tRNA forms hydrogen bonds with the codons of the mRNA this is what ensures the proper amino acid is placed in the proper sequence during translation.

Question 15

What is the start codon?

Answer 15

The start codon is a sequence of three nucleotides in mRNA that specifies the first amino acid in the synthesis of the polypeptide chain.

• Represented as AUG or GUG in prokaryotes
• Just AUG in eukaryotes

Question 16

What is a nonsense codon or a Stop codon?

Answer 16

A sequence of three nucleotides that stop mRNA transcription which can be represented as UAA, AUG, and UGA.

Question 17

What are the 4 key features that describes the genetic code?

Answer 17

• 3 nucleotides code for an amino acid
• the code is expressed by mRNA so U instead of T is used.
• Redundancy allows for more than one triplicate code to be used for an amino acid.
• Codon usage is the particular triplet code favored by a particular species and is referred to as codon bias or the preferential codon.

Question 18

What is a promoter sequence?

Answer 18

A nucleotide sequence that is upstream of the gene to which RNA polymerase binds at he initiation of transcription.

Question 19

What is am operator?

Answer 19

A nucleotide sequence through which binding of a repressor protein can control transcription of a gene or operon.

Question 20

What is an Operon?

Answer 20

A series of genes whose transcription controlled by a single operator.
- Remember, genes can also be carried out on different strands.

Question 21

What are the 4 steps taken in order to initiate the translation process?

Answer 21

1) 30S binds to the ribosome binding site located on mRNA
2) 30s reaches the start codon AUG or GUG
3) Formation of the initiation complex. (Includes 30S, tRNA w/ UAC anti codon carrying f-MET+initiation factor)
4) 50S attaches to the initiation complex; releasing the initiation factors and thus creating the 70S ribosome.

Question 22

How many sites are on the 70S ribosome and what do they do?

Answer 22

The 70S ribosome contains an A site, a P site and an E site. The A site (acceptor site) is where the first ammoniacal-tRNA is added. Where the P site (peptide site) is where the growing amino acid chain is temporarily held as the next codon is attached. The E site or (exit site) is where the peptide is released from the ribosome forming the polypeptide chain .

Question 23

What are the 4 steps involved during Translation termination?

Answer 23

1) the ribosome locates the stop codon
2) the release factors break the covalent bond that holds the finished protein to the terminal tRNA
3) The 30S and 50S ribosomal subunits fall apart to be recycled
4) During the elongation process the protein has folded into its functional 3-D shape

Question 24

What the 3 steps involved during translation elongation?

Answer 24

1) Peptide bond formation occurs between the polypeptides of the A and P site; as a result of the ribozyme peptidyl transferase.
2) polpetides shifts from the P site to the E site of the ribosome
3) 70S ribosome moves 1 codon down the mRNA to allow for the decoding of the next codon in the message, a process known translocation.

Question 25

What are chaperones and what does their presence in the cell allow for?

Answer 25

Chaperones are proteins capable of assisting in the proper folding and assembly of proteins but not while performing their biological function.

Question 26

What is the Sec system and how does it work?

Answer 26

The Sec system allows for the passage of proteins through the inner membranes of the cell. The protein is able to do so by binding to both Sec A&B which in turn attach to SecYEG which ultimately aids in the transfer across the membranes.

Question 27

What is a targeting factor?

Answer 27

Factors that guide proteins to membranes and help them pass in and out of the membranes.

Question 28

How many types of secretion system are gram-negative bacteria?

Answer 28

4 secretion systems:

• type I (secretes only one protein)
• type II (represented by the cholera toxin secretion system)
• type III (like the sec system in that it aids in the passing through of proteins inside Eukaryotic cells)
• type IV (related to plasmid conjugation systems).