Nucleic Acid, Replication, Transcription and mRNA processing Part 4 - (Week 5) Flashcards
What are steps in transcription?
- Initiation of RNA synthesis - not requiring a primer
- New nucleotides = added to the 3’ end of the RNA
- DNA unwinds at the front of the transcription bubble
- Unwinding occurs
What happens as a result of transcription?
- 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
What is the transcription unit?
- 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
What is a promoter?
- DNA sequence where RNA polymerase attaches
What is the terminator?
the sequence signaling the end of transcription in bacteria
Review: What are the stages of transcription?
- Initiation
- Elongation
- Termination
What happens in initiation?
- 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
What is the transcription initiation complex?
- transcription factors + RNA polymerase bound to a promoter
What happens in elongation?
- 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
RNA polymerase can synthesise from nothing. True or False?
True - in elongation
What happens in termination?
- 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
What happens in RNA synthesis?
- 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
How do both strands of DNA encode genes?
- 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
When RNA = transcribed from a gene, which strand of DNA = typically used?
- the template strand
During transcription, the RNA molecule = synthesised in the 3’ to 5’ direction. True or False?
True
What do enzymes in the eukaryotic nucleus do?
- modify mRNA (RNA processing)
- before the genetic messages are dispatched to the cytoplasm
How do eukaryotic cells modify RNA after transcription?
- 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
What are the concepts of split genes?
- 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
Within the concept of split genes, what do the genes include?
- 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)
What does RNA splicing do?
- removes introns
- joins exons, creating an mRNA molecule with a continuous coding sequence
- intronic regions = more common
What is a spliceosome?
- nuclear complex of proteins + RNA (one of the largest + most complex of all molecular structures)
What are functional + evolutionary importance of introns?
- 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
How does the alternation of mRNA ends work?
- 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
What modifications are shared from the alteration of mRNA ends?
- facilitate the export of mRNA to the cytoplasm
- protects mRNA from hydrolytic enzymes
- help ribosomes attach to the 5’ end
Which mRNA processing event adds stability to the mRNA?
- both a & b add stability
Pre-mRNA is not ready, premature and cannot even carry out transcription. True or False?
True - its transcripts = processed prior to translation
What is the central dogma of molecular biology?
- explanation of flow of genetic info within a biological system
- “DNA makes RNA, and RNA makes protein”
