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 information2. allows more copies of a protein to be made simultaneously3. 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 re-forms

• Transcription terminated when polymerase falls off DNA.