Transcription Flashcards
1
Q
Transcription
A
- Process in which nucleotide sequence of gene is used as a template to direct synthesis of RNA made up of complementary base sequences
- DNA molecules too large to move through nuclear pores in nuclear envelope → info on DNA transcribed into smaller mRNA
2
Q
Gene
A
- Specific sequence of nucleotides in a DNA molecule which codes for a specific sequence of amino acids in one polypeptide chain
- Located in fixed position (locus) on chromosome
- Specifies a particular biological function → phenotype
3
Q
Components of a gene (3)
A
- Promoters → recognition site for binding of RNA polymerase and regulatory proteins (general transcription factors) → initiate transcription
- Termination sequence
- Transcription unit → template and non-template strand
4
Q
Key features of transcription (5)
A
- Formation of single-stranded RNA
- Synthesis in 5’ to 3’ direction
- Catalysed by RNA polymerase (multimeric complex) → assembly of ribonucleotides and formation of phosphodiester bonds
- Complementary base pairing
- Recognition sequences → begins at promoter, ends at termination sequence
5
Q
DNA strands (2)
A
- Template = non-coding = antisense
2. Non-template = coding = sense
6
Q
Transcription steps (3)
A
- Initiation
- Elongation
- Termination
7
Q
- Initiation (Eukaryotes) (3)
A
- General transcription factors first assemble along
the promoter at TATA box, before recruiting RNA polymerase & positioning it correctly on the promoter, forming transcription initiation complex. - RNA polymerase unzips the 2 strands of the DNA double helix by breaking H bonds between complementary base pairs
- Only one strand is used as the template to synthesise
mRNA.
8
Q
- Initiation (Prokaryotes) (4)
A
- Sigma factor associates with core RNA polymerase forming the RNA polymerase holoenzyme
- As the holoenzyme scans along the DNA, its sigma factor recognises and binds to the promoter.
- RNA polymerase unzips the 2 strands of the DNA double helix at promoter by breaking H bonds between complementary base pairs
- Only one strand is used as a template to synthesise mRNA.
9
Q
- Elongation (Eukaryotes and prokaryotes) (5)
A
- Free ribonucleotides bind to template DNA strand by complementary base pairing.
- A forms 2 H bonds with U, T forms 2 H bonds with A, C forms 3 H bonds with G*
- RNA polymerase catalyses formation of phosphodiester bonds between adjacent ribonucleotides to form sugar phosphate backbone
- mRNA strand synthesised and elongated in the 5’ to 3’ direction
- As the RNA polymerase continues to move down the template strand, the region of DNA that has just been transcribed reanneals.
10
Q
- Termination (Eukaryotes) (2)
A
- RNA polymerase transcribes a (termination) sequence on the DNA, which codes for a polyadenylation signal (AAUAAA) in the pre-mRNA.
- Proteins (endonucleases) bind at a point (10 to 35 nucleotides) downstream of the polyadenylation signal to cut and free the pre-mRNA from the polymerase.
11
Q
- Termination (Prokaryotes) (2)
A
- After transcribing through a termination sequence the transcribed terminator folds back to form a hairpin loop
- The loop acts as a termination signal that causes the mRNA and RNA polymerase to be released
12
Q
Post-transcriptional modification (Eukaryotes) (3)
A
- Addition of methylguanosine cap to 5’ end of pre-mRNA
- RNA Splicing
- Synthesis of poly A tail (polyadenylation)
13
Q
- Addition of methylguanosine cap to 5’ end of pre-mRNA (3)
A
- 5’ cap protects mRNA from degradation by ribonucleases that degrade RNA form the 5’end
- Serves as a recognition signal for the small ribosomal subunit to assemble & begin translation
- Facilitates the export of mature mRNA from the nucleus
14
Q
- RNA Splicing
A
- Requires ATP
- Spliceosomes excise introns (non-coding seq) & join exons (coding seq)
15
Q
- Synthesis of poly A tail (polyadenylation) (3)
A
- Enzyme poly A polymerase which adds adenine nucleotides downstream of polyadenylation sequence, AAUAAA.
- Protects mRNA from degradation by ribonucleases → more stable template for translation
- Directs the export of mRNA through nuclear pores into the cytoplasm
