Chapter 17.1 & 17.2
Process from Gene to Protein
Flow of Genetic Info
• DNA inherited by an organism leads to specific
traits by dictating the synthesis of proteins.
• Gene expression – is a regulated process in
which DNA directs protein synthesis
• Proteins serve as the link between genotype
and phenotype.
Basic Principles
• Gene does not build a protein directly
• RNA is bridge between DNA and protein
synthesis
• RNA – ribose sugar, uracil replaces thymine,
single stranded
• 20 essential amino acids are the monomers
arranged in linear order that make up the
polypeptides in a protein
Basic Principles
• Transcription- is the process where RNA is
copied from one molecule to another from
DNA
*much like replication DNA provides
template for assembling sequence of RNA
- produces mRNA strand that will
carry the genetic message of DNA to
protein synthesizing machinery of
the cell
Basic Principles
• Translation – synthesis of a polypeptide from
mRNA.
-translates mRNA to amino acid
sequence of a polypeptide
- site of translation is ribosomes in
eukaryotes
Basic principles
• Evolutionary reasons for RNA intermediate:
1. provides protection for DNA and its
genetic information
2. allows more copies of a protein to be
made simultaneously
3. each RNA transcript can be translated
repeatedly.
Genetic code
• Triplets of nucleotide
bases are the smallest
units of uniform length
that can code for all 20
essential amino acids
• known as codons
• Transcribed and
translated in 5’ to 3’
direction
Deciphering code
• 61 of 64 triplets code for amino acids
• 3 designated as stop or termination signals
only to mark end to translation
• One codon (AUG) has dual function; codes for
methionine and serves as start signal
• More that one triplet may code the same
amino acid, but none of the same triplet
codes for a different amino acid
- ex: GAA and GAG code glutamic acid
17.2 Closer look at Transcription
• RNA polymerase is the enzyme responsible for
prying 2 DNA strands apart and pairing RNA
nucleotides to complementary base pair
• 5’ to 3’ direction
• Does not require primer to begin
• Promoter – sequence where RNA polymerase
attaches and begins transciption
Stages of Transcription
•
•
•
•
Initiation
Elongation
Termination
General to both
prokaryotes and
eukaryotes
• Promoter determines
which DNA strand will
serve as template
Prokaryotic Transcription
• Without nucleus, translation of mRNA can
begin while transcription is in progress
• One RNA polymerase initiates transcription
binding directly to promoter sequence
• Transcribed terminator sequence from DNA
functions to stop transcription.
Eukaryotic Transcription
• Nucleus separates transcription and mRNA
processing
*additional mRNA processing takes place
before translation
• Translation takes place in ribosome outside
nucleus
• 3 different RNA polymerases needed
– RNA polymerase II used in mRNA synthesis
Eukaryotic Transcription
• Transcription factors serve to mediate binding
of RNA polymerase and initiate transcription
• TATA box is a crucial promoter DNA sequence
that forms the initiation complex
• Elongation stage occurs as nucleotides are
added to RNA molecule; DNA double helix reforms
• Transcription terminated when polymerase falls
off DNA.