Central Dogma and Gene Regulation

CENTRAL DOGMA AND GENE REGULATION

Proteins are made of amino acids; a string of amino acids is a polypeptide and when the chain folds into its 3-D shape it is a protein.

Protein synthesis requires, transcription and translation.

1. Transcription :: the information on DNA is copied onto a length of mRNA (DNA mRNA)
2. Translation ; the polypeptide chain is formed (RNA protein)

Transcription occurs in the nucleus. The transfer of information from DNA to RNA is through complimentary base pairing. Transcription uses the enzyme RNA polymerase to deliver the correct complimentary base. Steps in transcription:

1. An unedited version of mRNA is made known as “pre-mRNA”. The pre-mRNAinclude exon (expression sequences) and introns (insertion sequences)
2. The pre-mRNA is edited; the intron are removed.
3. A cap is added at the start site and a poly A++ tail is added at to the termination site.
4. The resulting mRNA is called “transcript”.

Translation occurs in the cytoplasm on the ribosomes (rER) and requires mRNA, tRNA and rRNA. Each plays a different role.

mRNA has the codon (a string of three bases that determines the amino acid);

tRNA joins the correct amino acid to the correct codon sequence by codon-anticodon base pairing; rRNA forms into the two subunits of the ribosome and acts as a workbench.

During translation:

a. mRNA binds to the small ribosome and the first tRNA binds to the start (AUG) codon on the mRNA

b. the large subunit now attaches to the small subunit

c. the polypeptide chain elongates. The second tRNA molecule binds to an mRNA codon at the A site. This

tRNA brings another amino acid . The ribosome moves down the mRNA chain, allowing the tRNA to read each codon in the mRNA. The tRNA brings an amino acid for each codon it reads.

d. Termination of the Growing Chain. The tRNA reaches one of the three stop codons in the mRNA. This signals the completion of translation. Ribosomes fall all and polypeptide goes to the golgi for packaging and delivery.

Codon: the triplicate code found on mRNA that codes for each of the 20 amino acids, for start (methionine) and stop

Genetic Code: the inventory of linkages between nucleotide triplets and the amino acids they code for:

GENE REGULATION: Determines when a protein is expressed (produced) in a cell. Some proteins are always expressed while others are expressed intermittently (inducible).

The Lac Operon: This is an example of a inducible expression. For E. coli to metabolize lactose several proteins must be produced by a cell.

a. b- galactosidase which hydrolyses lactose into glucose and galactose

b. permease enzyme which transports lactose into cells

In E.coli, these proteins are controlled by a cluster of genes and promoter sequences known as the Lac operon.

The operon includes:

i-gene- a regulator gene codes for a repressor protein

promoter- the binding site for mRNA polymerase

operator – binds to the repressor

z-gene- codes for b-galactosidase

y-gene- codes for permease enzyme

a- gene codes for a transaminase protein

When no lactose is around the repressor is bound to the operator preventing transcription of DNA by RNA polymerase.

When lactose is around, repressor binds to lactose, freeing the operator and allowing RNA polymerase to transcribe DNA and produce the two enzymes needed to digest lactose.