Chapter 8.2: Transcription: From DNA to RNA Flashcards
Define hairpin loops
Terminators often form hairpin loops (also called stem loops) in which nucleotides within the mRNA pair with nearby complementary nucleotides in the same molecule
What is a primary transcript?
o The result of transcription is a single strand of RNA known as a primary transcript
What are the 3 mechanisms of RNA splicing?
• 3 types of short sequences within the primary transcript—splice donors; splice acceptors, and branch sites—help ensure the specificity of slicing o These sites make it possible to server the connections between an intron and the exons that precede and follow it, and then join the formerly distant exons
Describe initiation of transcription
RNA polymerase binds to double-stranded DNA at the beginning of the gene to be copied. RNA polymerase recognizes and binds to promoters, specialized DNA sequences near the beginning of a gene where transcription will start. Although specific promoters vary substantially, all promoters in E. coli contain two characteristic short sequences of 6-10 nucleotide pairs that help bind RNA polymerase. In bacteria, the complete RNA polymerase (the holoenzyme) consists of a core enzyme, plus a σ (sigma) subunit involved only in initiation. The σ subunit reduces RNA polymerase’s affinity for the promoter. As a result, the RNA polymerase holoenzyme can hone in on a promoter and bind tightly to it, forming a so-called promoter complex. After binding to the promoter, RNA polymerase unwinds part of the double helix, exposing unpaired bases in the template strand. The complex formed between the RNA polymerase holoenzyme and an unwound promoter is called an open promoter complex. The enzyme identifies the template strand and chooses the two nucleotides with which to initiate copying. Guided by base pairing with these two nucleotides, RNA polymerase aligns the first two ribonucleotides of the new RNA, RNA polymerase aligns the first two ribonucleotides of the new RNA, which will be at the 5’ end of the final RNA product. The DNA transcribed as the 5’ end of the mRNA is often called the 5’ end of the gene. RNA polymerase then catalyzes the formation of a phosphodiester bond between the first two ribonucleotides. Soon thereafter, the RNA polymerase releases the σ subunit. This release marks the end of initiation.
What are introns?
The sequences found in the DNA of the gene but not in the mature mRNA are known as introns (for “intervening regions”)
What is the function of spliceosome?
• Splicing normally required a complicated intranuclease machine called the spliceosome, which ensures that all of the splicing reactions take place in concert
Describe termination of transcription
RNA sequences that signal the end of transcription are known as terminators. There are two types of terminators: intrinsic terminators, which cause the RNA polymerase core enzyme to terminate transcription on its own, and extrinsic terminators, which require proteins other than RNA polymerase to bring about termination. All terminators, whether intrinsic or extrinsic, are specific sequences in the mRNA that are transcribed from specific DNA regions. Terminators often form hairpin loops (also called stem loops) in which nucleotides within the mRNA pair with nearby complementary nucleotides in the same molecule. Upon termination, RNA polymerase and a completed RNA chain are both released from the DNA
What is the main difference in transcription initiation between prokaryotes and eukaryotes?
o A key difference with prokaryotes is that sequences called enhancers that can be thousands of base pairs away from the promoter are often also required for efficient transcription of eukaryotic genes o The result of transcription is a single strand of RNA known as a primary transcript o In eukaryotic organisms, most primary transcripts undergo RNA processing in the nucleus before they migrate to the cytoplasm to direct protein synthesis
Define terminators
RNA sequences that signal the end of transcription are known as terminators
Define transcription
• Transcription is the process by which the polymerization of ribonucleotides guided by complementary base pairing produces an RNA transcript of a gene
Describe elongation of transcription
When the σ subunit separates from the RNA polymerase, the enzyme loses its enhanced affinity for the promoter sequence and regains its strong generalized affinity for any DNA. These changes enable the core enzyme to leave the promoter yet remain bound to the gene. The core enzyme now moves along the chromosome, unwinding the double helix to expose the next single-stranded region of the template. The enzyme extends the RNA by linking a ribonucleotide positioned by its complementary with the template strand to the 3’ end of the growing chain. As the enzyme extends the mRNA in the 5’-to-3’ direction it moves in the antiparallel 3’-to’5’ direction along the DNA template strand Once an RNA polymerase has moved off the promoter, other RNA polymerase molecules can move in to initiate transcription
What is the function of the ribozymes?
RNA molecules that can acts as enzymes and catalyze a specific biochemical reaction
What are exons?
Sequences found in both a gene’s DNA and the mature messenger RBA are called exons (for “expressed regions”)
What happens during RNA processing?
o Adding a 5’ methylated cap and a 3’ poly-A tail o Removing introns from the primary transcript by RNA splicing
What is the transcription bubble?
• The region of DNA unwound by RNA polymerase is called the transcription bubble
