Nucleic Acid, Replication, Transcription and mRNA processing Part 4 - (Week 5)

Question 1

What are steps in transcription?

Answer 1

1. Initiation of RNA synthesis - not requiring a primer
2. New nucleotides = added to the 3’ end of the RNA
3. DNA unwinds at the front of the transcription bubble
4. Unwinding occurs

Question 2

What happens as a result of transcription?

Answer 2

• DNA = exposed = RNA polymerase binds the appropriate ribonucleotides
• RNA synthesis follows the same base-pairing rules as DNA, except uracil, instead of thymine
• RNA polymerase reaches the end of the gene it releases the mRNA = DNA rewinds

Question 3

What is the transcription unit?

Answer 3

• sequence of nucleotides in DNA that codes for a single RNA molecule, along with the sequences necessary for its transcription
• contains a promoter, RNA-coding sequence + a terminator

Question 4

What is a promoter?

Answer 4

• DNA sequence where RNA polymerase attaches

Question 5

What is the terminator?

Answer 5

the sequence signaling the end of transcription in bacteria

Question 6

Review: What are the stages of transcription?

Answer 6

• Initiation
• Elongation
• Termination

Question 7

What happens in initiation?

Answer 7

• RNA polymerase binds to the promoter region of DNA
• initial RNA synthesis, no primer required
• location of the consensus sequence determines position of the start site
• promoter element = TATA box, = crucial in forming initiation complex in eukaryotes
• transcription factors mediate the binding of RNA polymerase + initiation of transcription
• completed assembly of transcription factors + RNA polymerase bound to a promoter

Question 8

What is the transcription initiation complex?

Answer 8

• transcription factors + RNA polymerase bound to a promoter

Question 9

What happens in elongation?

Answer 9

• carried out by the action of RNA polymerase
• as RNA polymerase moves along the DNA - untwists the double helix
• transcription progresses at a rate of 40 nucleotides per second in eukaryotes
• gene transcribed simultaneously by several RNA polymerases
• nucleotides = added to the 3’ end of the growing RNA molecule

Question 10

RNA polymerase can synthesise from nothing. True or False?

Answer 10

True - in elongation

Question 11

What happens in termination?

Answer 11

• mechanisms are different in bacteria & eukaryotes
• bacteria, polymerase stops transcription at the end of the terminator sequence + the mRNA can be translated without further modification
• eukaryotes, RNA polymerase transcribes the polyadenylation signal sequence, RNA transcript = released 10-35 nucleotides past this polyadenylation sequence

Question 12

What happens in RNA synthesis?

Answer 12

• is complementary + antiparallel to the template strand
• new nucleotides = added to the 3’ -OH group of the growing RNA; so transcription proceeds in a 5’ to 3’ direction
• nontemplate strand = not usually transcribed

Question 13

How do both strands of DNA encode genes?

Answer 13

• RNA = transcribed from one DNA strand
• both DNA strands can act as the template
• DNA strand that is transcribed for a given mRNA = temed the template strand
• DNA strand = nontemplate strand
• not always top + bottom strands used - interchangeable

Question 14

When RNA = transcribed from a gene, which strand of DNA = typically used?

Answer 14

• the template strand

Question 15

During transcription, the RNA molecule = synthesised in the 3’ to 5’ direction. True or False?

Answer 15

True

Question 16

What do enzymes in the eukaryotic nucleus do?

Answer 16

• modify mRNA (RNA processing)

- before the genetic messages are dispatched to the cytoplasm

Question 17

How do eukaryotic cells modify RNA after transcription?

Answer 17

• RNA splicing
• modifying the ends of the mRNA
• addition of the 5’ cap
• addition of the poly(A) tails (3’)
• cannot be used for synthesis of proteins

Question 18

What are the concepts of split genes?

Answer 18

• most eukaryotic genes + their RNA transcripts have long noncoding stretches of nucleotides that lie between coding region
• noncoding regions = intervening sequences/introns
• other regions = exons - eventually expressed + translated into amino acid sequences

Question 19

Within the concept of split genes, what do the genes include?

Answer 19

• DNA sequences that code for all exons + introns
• sequences at the beginning + end of the RNA = not translated into a protein, including the entire transcription unit (promoter, RNA coding sequence)

Question 20

What does RNA splicing do?

Answer 20

• removes introns
• joins exons, creating an mRNA molecule with a continuous coding sequence
• intronic regions = more common

Question 21

What is a spliceosome?

Answer 21

• nuclear complex of proteins + RNA (one of the largest + most complex of all molecular structures)

Question 22

What are functional + evolutionary importance of introns?

Answer 22

• some introns contain sequences that may regulate gene expression
• some genes can encode more than 1 kind of polypeptide, depending on which segments = treated as exons during splicing
• alternative RNA splicing
• the number of different proteins an organism can produce = much greater than its number of genes

Question 23

How does the alternation of mRNA ends work?

Answer 23

• each end of a pre-mRNA molecule = modified in a particular way
• 5’ end - addition of the 5’ cap
• 3’ end - addition of the poly(A) tail

Question 24

What modifications are shared from the alteration of mRNA ends?

Answer 24

• facilitate the export of mRNA to the cytoplasm
• protects mRNA from hydrolytic enzymes
• help ribosomes attach to the 5’ end

Question 25

Which mRNA processing event adds stability to the mRNA?

Answer 25

• both a & b add stability

Question 26

Pre-mRNA is not ready, premature and cannot even carry out transcription. True or False?

Answer 26

True - its transcripts = processed prior to translation

Question 27

What is the central dogma of molecular biology?

Answer 27

• explanation of flow of genetic info within a biological system
• “DNA makes RNA, and RNA makes protein”