Transcription

Question 1

Transcription

Answer 1

• 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

Question 2

Gene

Answer 2

• 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

Question 3

Components of a gene (3)

Answer 3

1. Promoters → recognition site for binding of RNA polymerase and regulatory proteins (general transcription factors) → initiate transcription
2. Termination sequence
3. Transcription unit → template and non-template strand

Question 4

Key features of transcription (5)

Answer 4

1. Formation of single-stranded RNA
2. Synthesis in 5’ to 3’ direction
3. Catalysed by RNA polymerase (multimeric complex) → assembly of ribonucleotides and formation of phosphodiester bonds
4. Complementary base pairing
5. Recognition sequences → begins at promoter, ends at termination sequence

Question 5

DNA strands (2)

Answer 5

1. Template = non-coding = antisense

2. Non-template = coding = sense

Question 6

Transcription steps (3)

Answer 6

1. Initiation
2. Elongation
3. Termination

Question 7

1. Initiation (Eukaryotes) (3)

Answer 7

1. 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.
2. RNA polymerase unzips the 2 strands of the DNA double helix by breaking H bonds between complementary base pairs
3. Only one strand is used as the template to synthesise
   mRNA.

Question 8

1. Initiation (Prokaryotes) (4)

Answer 8

1. Sigma factor associates with core RNA polymerase forming the RNA polymerase holoenzyme
2. As the holoenzyme scans along the DNA, its sigma factor recognises and binds to the promoter.
3. RNA polymerase unzips the 2 strands of the DNA double helix at promoter by breaking H bonds between complementary base pairs
4. Only one strand is used as a template to synthesise mRNA.

Question 9

2. Elongation (Eukaryotes and prokaryotes) (5)

Answer 9

1. Free ribonucleotides bind to template DNA strand by complementary base pairing.
2. A forms 2 H bonds with U, T forms 2 H bonds with A, C forms 3 H bonds with G*
3. RNA polymerase catalyses formation of phosphodiester bonds between adjacent ribonucleotides to form sugar phosphate backbone
4. mRNA strand synthesised and elongated in the 5’ to 3’ direction
5. As the RNA polymerase continues to move down the template strand, the region of DNA that has just been transcribed reanneals.

Question 10

3. Termination (Eukaryotes) (2)

Answer 10

1. RNA polymerase transcribes a (termination) sequence on the DNA, which codes for a polyadenylation signal (AAUAAA) in the pre-mRNA.
2. 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.

Question 11

3. Termination (Prokaryotes) (2)

Answer 11

1. After transcribing through a termination sequence the transcribed terminator folds back to form a hairpin loop
2. The loop acts as a termination signal that causes the mRNA and RNA polymerase to be released

Question 12

Post-transcriptional modification (Eukaryotes) (3)

Answer 12

1. Addition of methylguanosine cap to 5’ end of pre-mRNA
2. RNA Splicing
3. Synthesis of poly A tail (polyadenylation)

Question 13

1. Addition of methylguanosine cap to 5’ end of pre-mRNA (3)

Answer 13

1. 5’ cap protects mRNA from degradation by ribonucleases that degrade RNA form the 5’end
2. Serves as a recognition signal for the small ribosomal subunit to assemble & begin translation
3. Facilitates the export of mature mRNA from the nucleus

Question 14

2. RNA Splicing

Answer 14

• Requires ATP

- Spliceosomes excise introns (non-coding seq) & join exons (coding seq)

Question 15

3. Synthesis of poly A tail (polyadenylation) (3)

Answer 15

1. Enzyme poly A polymerase which adds adenine nucleotides downstream of polyadenylation sequence, AAUAAA.
2. Protects mRNA from degradation by ribonucleases → more stable template for translation
3. Directs the export of mRNA through nuclear pores into the cytoplasm

Question 16

Types of RNA (3)

Answer 16

1. Messenger RNA (mRNA)
2. Ribosomal RNA (rRNA)
3. Transfer RNA (tRNA)

Question 17

Messenger RNA

Answer 17

• Single stranded
• Protein-coding region + untranslated regions
• UTR includes 5’ cap and poly-A tail

Question 18

Role of mRNA (3)

Answer 18

1. Serves as a messenger that takes info out of nucleus via nuclear pore to cytoplasm where translation takes place
2. Acts as template for translation
3. Each codon within coding region of mRNA represents an amino acid in a polypeptide → sequence of codons determines polypeptide sequence