D1.2 Transcription and Translation
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What is gene expression?
The process by which information in DNA is used to synthesize a functional product (usually a protein), via transcription + translation.
What are the 3 stages of gene expression in eukaryotes?
• Transcription → DNA → pre-mRNA (in nucleus) • RNA processing (editing) → introns removed, exons spliced • Translation → mRNA → polypeptide (in cytoplasm)
What is the central dogma?
DNA → RNA → Protein (flow of genetic information)
What are introns and exons?
Introns: non-coding regions (removed) Exons: coding regions (expressed)
Where do transcription and translation occur?
Transcription: nucleus Translation: cytoplasm (ribosomes)
Why can gene expression be regulated?
Because not all genes are needed at all times → cells control which genes are “on” or “off”.
What is the genetic code?
A set of rules that determines how mRNA codons are translated into amino acids.
What is a codon?
A triplet of bases on mRNA that codes for one amino acid.
How many codons exist?
64 total 61 code for amino acids 3 are STOP codons
What is the start codon?
AUG → codes for methionine (Met)
What are stop codons?
UAA, UAG, UGA → terminate translation
What does it mean that the genetic code is universal?
Almost all organisms use the same genetic code → evidence for common ancestry (LUCA).
What does “non-ambiguous” mean?
Each codon codes for only one amino acid.
What does it mean that the genetic code is degenerate?
Multiple codons can code for the same amino acid.
How does degeneracy provide “safety”?
Reduces impact of mutations Some base substitutions do not change amino acid → silent mutations
What is transcription?
Synthesis of RNA from a DNA template using RNA polymerase.
Which strand is used in transcription?
Template (antisense) strand
What is the coding strand?
DNA strand identical to mRNA (except T → U)
Direction of transcription?
RNA is synthesized 5’ → 3’
What enzyme is responsible for transcription?
RNA polymerase (specifically RNA polymerase II in eukaryotes)
What are promoter and terminator regions?
Promoter: start site (RNA polymerase binds) Terminator: stop signal
What happens during transcription (step-by-step)?
RNA polymerase binds promoter DNA unwinds RNA nucleotides added (complementary base pairing) RNA strand elongates Polymerase detaches at terminator
Why can multiple RNA polymerases transcribe one gene?
Increases rate of protein production.
What happens to pre-mRNA after transcription?
It is processed into mature mRNA.