L5-Transcription and Translation

Question 1

What is the function of a sigma protein?

Answer 1

The sigma factor recognizes the appropriate site on the DNA for RNA synthesis to begin. It easily dissociates from the core enzyme when transcription begins.

Question 2

Explain the structure and purpose of the promoter region.

Answer 2

Promoters are specific DNA sequences that bind RNA polymerase. They are initiation sites that RNA polymerase must recognize before RNA synthesis. They have two fairly conserved sequences: TATAAT (Pribnow box at -10) and TTGACA (-35 bases).

Question 3

How is termination for RNA synthesis controlled?

Answer 3

• • Specific base sequence-rich in GC containing an inverted repeat with a central nonrepeating section and a stretch of U:A base pairs (weak).
• • Rho (in E. coli)-Rho tightly binds to RNA and moves down chain to RNA polymerase-DNA complex. When RNA polymerase pauses at Rho-dependent ermination site (on DNA template), RHO causes both RNA and RNA polymerase to be released fromDNA.

Question 4

Explain the importance of being polycistronic.

Answer 4

mRNA encoding a group of cotranscribed genes is called polycistronic mRNA. When this is translated, several polypeptides are sytnthesized one after another by the same ribosome. This enables expression for these genes/protein that are often in the same biochemical pathway or needed under the same conditions to be coordinate.

Question 5

Describe the structure and function of an rRNA transcription unit.

Answer 5

An rRNA transcription unit contains three rRNAs in prokaryotes: 16S rRNA, 23S rRNA, and 5S rRNA. These are contranscribed and are quite stable. They are responsible for creating the enzymatic portions of the ribosome.

Question 6

What is the purpose of a start and stop codon?

Answer 6

The start codon ensures that translation begins at the proper codon/with the proper reading frame. The stop codon ensures that translation stops.

Question 7

Define open reading frame.

Answer 7

A sequence of DNA or RNA that can be translated into a polypeptide.

Question 8

Why is there codon bias?

Answer 8

Codon bias is correlated with a corresponding bias in the concentration of different tRNA molecules. (A tRNA corresponding to a rarely used codon will be in relatively short supply) Exact origin of codon bias is unclear.

Question 9

Describe the structure and function of a tRNA.

Answer 9

• • 73-93 nucleotides, contain purine and pyrimidine bases that differ somewhat from standard bases found in RNA because they are chemically modified
• • 3’ end/acceptor stem has unpaired CCA sequence where adenosine connects by ester linkage to amino acid
• • Carries the anticodon that base pairs with codon on mRNA and transfers the amino acid onto growing polypeptide.

Question 10

Describe the steps involved in protein synthesis.

Answer 10

• • Initation-begins with a free 30S ribosomal subunit. Initiation complex forms of 30S subunit, mRNA, formylmethionine tRNA, and several initiation proteins called IF1, IF2, and IF3. (GTP required). 50S ribosomal subunit adds. 16S rRNA binds to ribosome binding site.
• • Initation-prokaryotic mRNA can use start codon other than AUG (most common alternative is GUG).
• • Elongation-Incoming charged tRNA first attaches at acceptor site (A site). P site (peptide site) is the site where the growing polypeptide chain is held. Polypeptide bond is formed between amino acids of the tRNA in A and P site.
• • Translocation-GTP moves ribosome three nucleotides exposing a new codon at A site. tRNA that was in P site is released at E/exit site.
• • Ribosome moves along mRNA (mRNA doesn’t move through ribosome).
• • Termination-Specific proteins called release factors recognize the stop codon and cleave the attached polypeptide from the final tRNA and release the finished product. Ribosome subunits dissociate.

Question 11

What are the components of a ribosome?

Answer 11

30S and 50S ribosomal subunits. Each subunit contains specific ribosomal RNAs and ribosomal proteins (referred to as “translation factors”)

Question 12

What functional role does rRNA play in protein synthesis?

Answer 12

• 16S rRNA is involved in initiation through base pairing with the Shine-Dalgarno sequence
• Helps hold mRNA in ribosome
• Help position tRNA in the A and P sites.
• Actual formation of peptide bonds is catalyzed by rRNA (23S rRNA)

Question 13

Do genes for tRNAs have promoters and start codons?

Answer 13

The gene for a tRNA needs a promoter in order for it to be transcribed. It lacks a start codon because it does not get translated.