Molecular Genetics - Transcription

Question 1

What is transcription?

Answer 1

• The process in which a strand of DNA is copied to produce a complementary RNA strand

Question 2

Where does transcription occur?

Answer 2

In the nucleus

Question 3

What are the 3 main steps of transcription?

Answer 3

1. Initiation
2. Elongation
3. Termination

• Same process for all genes, whether mRNA, tRNA, or rRNA produced

Question 4

What occurs in the initiation step?

Answer 4

• RNA polymerase binds to promoter sequence of the gene to be transcribed + “unzips” the DNA
• Unzipping the DNA exposes the template strand (must be 3’ to 5’)

Question 5

What is the promoter?

Answer 5

• Sequence recognized by RNA polymerase
• “Upstream” from gene to be transcribed
• Has a particular sequence of many A-T base pairs called the TATA box
• Since only 2 hydrogen bonds between A-T, less energy is required to unzip at this location

Question 6

What occurs in the elongation step?

Answer 6

• RNA polymerase “reads” nucleotide sequence of DNA template strand (3’ to 5’) and build complementary RNA strand (5’ to 3’)
• As RNA polymerase moves along DNA template, the DNA rewinds behind it and the DNA downstream unwinds and unzips

Question 7

What occurs in the termination step?

Answer 7

• RNA polymerase reaches termination sequence at end of gene
• This causes it to detach from DNA strand and release new RNA molecule, DNA molecule finishes rewinding
• Termination sequence is required so the RNA contains correct # of nucleotides

Question 8

What are the “coding”/”sense” strand and “antisense” strand?

Answer 8

• Coding/sense strand: non-template strand (same sequence as RNA, except for uracil)
• Antisense: template strand (complementary to RNA)

Question 9

What are post-transcriptional modifications?

Answer 9

• If transcript is mRNA, it must be processed before use
• Three events: 5’ cap added, poly-A-tail added, removal of introns

Question 10

5’ cap added

Answer 10

• A sequence of seven guanine nucleotides is added to start of pre-mRNA molecule
• Cap is recognized by ribosomes, initial attachment site during translation

Question 11

Poly-A-tail

Answer 11

• Approx. 50-250 adenine nucleotides added to 3’ end of transcript
• Poly-A-tail helps protect transcript from degradation by RNA digesting enzymes in cytosol

Question 12

What are introns and exons?

Answer 12

• Introns: non-coding sequences of gene, “interspersed regions”, must be removed from proper mRNA function
• Exons: coding sequences of gene, “expressed regions”

Question 13

Removal of introns

Answer 13

• Small nuclear ribonucleoproteins (snRNPs) bind to introns, signaling for removal
• Removal is done by spliceosomes (protein-rRNA combinations) cut out introns and stick exons together