Genetics: DNA & Protein Synthesis - Transcription

Question 1

What is transcription?

Answer 1

• The production of an mRNA copy of a gene which is made from DNA

Question 2

Where does transcription occur in eukaryotes and prokaryotes?

Answer 2

• Eukaryotes
- Nucleus

• Prokaryotes
- Cytoplasm

Question 3

What are the main steps of transcription?

Answer 3

• RNA polymerase attaches to DNA
• Complementary mRNA is formed
• RNA polymerase reaches stop signal
• Splicing in eukaryotes

Question 4

What is the promoter and terminator region of a gene?

Answer 4

• Promoter
- Start of gene

• Terminator
- End of gene

Question 5

Describe the process of transcription

Answer 5

• RNA polymerase attaches to DNA

• at promoter region
• Enzyme (DNA helicase in euk.) breaks hydrogen bonds between two DNA strands
• Strands separated and one is used as template for mRNA copy

• Complementary mRNA is formed

• Free RNA nucleotides lined up along bases on template strands
• Complementary base pairing
• RNA polymerase joins nucleotides, forms phosphodiester bonds
• Forms mRNA strand (pre-mRNA)

• RNA polymerase reaches stop signal

• Stops making mRNA when stop codon is reached
• RNA polymerase detaches from DNA

• Splicing in eukaryotes

Question 6

Describe the additional step in transcription in eukaryotes

Answer 6

• Splicing of pre-mRNA

• Pre-mRNA contain introns and exons, been copied into mRNA during transcription
• Splicing removes introns and joins exons together, forming mRNA strands

• Transport of mRNA

• Transcription and splicing occurs in nucleus
• mRNA leaves nucleus through a pore and attach to a ribosome in cytoplasm ready for translation

Question 7

Why do prokaryotic cells not undergo splicing?

Answer 7

• No introns in prokaryotic DNA

Question 8

Why are stop codons needed for DNA to stop production of pre-mRNA?

Answer 8

• Allows the DNA to rejoin

- With only 12 bases being exposed at a time to reduce the chance of damage to the DNA