RNA Structure & Transcriptase 😄

Question 0

Types of RNA

Answer 0

Ribosomal RNA or rRNA

• ” rampant”
• most common
• associated with several proteins as a component of the ribosomes
• prokaryotes have 3 distinct size species
• eukaryotes have 4 distinct size species

Transfer RNA or tRNA

• “tiny” because it is the smallest RNA
• adaptor molecule that carries a specific amino acid to the site of protein synthesis
• contains many unusual bases and extensive intra-chain base pairing
• each of the 20 amino acid has at least 1 tRNA

Messenger mRNA or mRNA

• carries genetic information from the nuclear DNA to the cytosol, where it is used as the template for protein synthesis
• in eukaryotic cells, modification include: a long sequence of adenine nucleotides on the 3’end (poly-A tail), a “cap” on the 5’-end consisting of a molecule of 7-methylguanosine attached backward (5’-5’) through a triphosphate linkage

Small nuclear RNA or snRNA
- a subset of RNAs significant involved in mRNA processing and gene regulation

Question 1

Polymers of nucleotides, but differ from DNA by containing:

• ribose instead of deoxyribose
• Uracol instead of thymine

Answer 1

RNA

Question 2

4-subunit enzyme that synthesizes RNA

Possesses 5’ –> 3’ polymerase activity

Requires the ff:

• Sigma factor
• Rho factor

Answer 2

RNA polymerase

Question 3

Recognizes the nucleotide sequence (promoter region) at the beginning of the length of the DNA to be transcribed

Answer 3

Sigma factor

Question 4

Required for termination of transcription of some genes

Answer 4

Rho factor

Question 5

Steps in Prokaryotic DNA transcription

Answer 5

Step 1: INITIATION

• RNA polymerase holoenzyme binds to the promoter region
• this sequence contains characteristic consensus nucleotide sequences that are highly conserved: Pribnow box, -35 sequence

Step 2: ELONGATION

• RNA polymerase copying one strand of the DNA double helix, pairing Cs with Gs and As with Us
• substrates are ribonucleoside triphosphates
• RNA polymerase does not require a primer and has no known exonuclease or endonuclease activity

Step 3: TERMINATION
- may be accomplished by RNA polymerase alone or may require p factor:
( p factor binds to a C-rich region near the 3’aend of the newly synthesized RNA and migrates along the 5’–>3’ direction until the termination site is reached; p independently termination required a stable hairpin loop turn and a palindrome sequence)

Question 6

Stretch of six nucleotides (5’-TATAAT-3’) centered about 8 to 10 nucleotides to the left of the transcription start site

Answer 6

Pribnow box

Question 7

Second consensus nucleotide sequence (5’-TTGACA-3’) about 35 bases to the left of the transcription start site

Answer 7

-35 Sequence

Question 8

There are 3 distinct classes of RNA polymerase in the nucleus of eukaryotic cells

Answer 8

RNA polymerase I for large rRNAs in the nucleolus

RNA polymerase II for mRNAs

RNA polymerase III for tRNAs and some other small rRNAs in the nucleoplasm

Question 9

TATA or Hogness box, CAAT box and GC box

Serve as binding sites for proteins called general transcription factors, which in turn interact with each other and with RNA polymerase II

Answer 9

Promoter sequences

Question 10

DNA sequences that increase the rate of initiation of transcription by binding to specific transcription factors called activators

Answer 10

Enhancers

Question 11

Linear copy of the transcriptional unit, the segment of DNA between specific initiation and termination sequences

Answer 11

Primary transcript

Question 12

In both prokaryotic and eukaryotic cells, they are synthesized from long precursor molecules called PREERIBOSOMAL RNAs

These precursors are cleaved and trimmed by ribonucleases, producing the 3 largest rRNAs

Answer 12

rRNAs

Question 13

Also made from longer precursor molecules

These must have an intervening sequence (intron) removed, and the 5’-and 3’-ends of the molecule are trimmed by ribonuclease

A 3’-CCA sequence is added and bases at specific positions are modified, producing unusual bases

Answer 13

tRNAs

Question 14

Prokaryotic mRNA is generally identical to its primary transcript, whereas eukaryotic mRNA is extensively modified

A 7-methyl-guanosine “cap” is attached to the 5’-terminal end of the mRNA

A long poly-A tail (not transcribed from DNA) is attached to the 3’aend of most mRNAs

Answer 14

mRNAs

Question 15

Regulation of gene expression in prokaryotes usually involves either initiation or termination of transcription

Answer 15

Genetic regulation

Question 16

A set of structural genes coding for a group of proteins required for a particular metabolic function along with the regulatory region that controls the expression of the structural genes

Answer 16

Operon

Question 17

Portion of the bacterial chromosome that controls the synthesis of the enzymes involved in lactose metabolism:

Z gene: encodes a B-galactosidase
Y gene: encodes a galactoside permease, the transport protein required for the entry of lactose into the cell
A gene: encodes a thiogalactoside transacetylase enzyme, whose function is unknown
i gene: encodes a lac repressor protein that is constitutively expressed and locate at a distant site in the DNA

Answer 17

Lactose Operon