Chapter 15 - Gene expression

One gene - One enzyme hypothesis

One gene codes for One enzyme; has been proven false

Central Dogma of Genetic

DNA → transcription → RNA → translation → protein

Codons

The “code” of DNA, a three nucleotide code for an amino acid; nonoverlapping; Can be represented as RNA or DNA code

Transcription stages

Happens on the DNA; Initiation → elongation → termination

Initiation in transcription

Promoters signal the start point several dozen nucleotides up stream; Transcription factors mediate the binds of RNA polymerase;

Transcription initiation complex

The completed Assembly of transcription factors and RNA polymerase

TATA box

Crucial in forming intitation complex in eukaryotes

Elongation in transcription

untwists the DNA and prints 40 nucelodtides/sec

Termination in prokaryotes

polymerase stops transcription at the end of the terminator and the mRNA can be ranslated without further modification

Termination in Eukaryotes

RNA polymerase transcribes the polyadenylation signal sequence, and transcription stops 10-35 nucleotides past this signal

RNA processing

5’ cap and a poly-A tail 3’ are added for stability; and introns are cut out and exons are spliced together

Introns

A non-coding region of a mRNA that is spliced out

Exon

A coding region of mRNA that is spliced together

Alternate splicing

mRNA can be spliced together in different ways to code for different proteins

Translation

After transciption; initiation → Elongation → termination; all three stages require proteins called factors.

tRNA

holds amino acid for ribosomes added by amino acyl-tRNA synthase

Anticodon

The opposite matching set of nucleotides for a codon

Wobble

The third codon is flexible and allows for some tRNAs to bind to more than 1 codon

Sites of Ribosome

A site → P site → E site

A site

Holds an amino acid that will be added to the polypeptide chain

P site

Holds the polypeptide chain

E site

exit site, where tRNA leaves

Initiation of transcription

mRNA binds to the small unit of RNA and a matching tRNA binds to the mRNA, making an initiation complex, then the large unit attaches and finishes the ribosome; Takes energy

Elongation of transcription

1. Codon recongination (takes energy)

2. Peptide bond formation

3. Translocation (takes energy)

   Goes in the 5’→3’ Direction

Peptide bond formation

the polypeptide chain is added to the A site amino acid

Termination of translation

When a stop codon reaches the A site, it addes a protein called a release factor, this adds water to the polypeptide chain and releases it.

Polyribosomes / coupled transciption

Multiple ribosomes can print on the same RNA

Nucleotide-pair substitution

replaces one nucleotide pair with the other

Silent mutation

Has no effect on the amino acid produced

Missense mutation

code for the wrong amino acid

Nonsense Mutation

Changes the codon for an amino acid into a stop codon

Insertions and deletions

The addition or loss of a nucleotide pairs in a gene; producing a frameship mutation

Promoter

a region of DNA upstream of a gene where relevant proteins (such as RNA polymerase and transcription factors) bind to initiate transcription of that gene