CENTRAL DOGMA AND GENE REGULATION
Gene (DNA) first make mRNA the mRNA makes protein. One gene coding sequence produces one protein subunit.
This is the essence of the CENTRAL DOGMA
Genes contain an open reading frame that can be transcribed, as well as regulatory sequences such as
promoters and enhancers, which control the transcription of the open reading frame. Visit this website for more
details: http://en.wikipedia.org/wiki/DNA
Genes are DNA (mRNA) made of nucleotides. Traits are manifestations of proteins.
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.
1. 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.
2.
STUDY THE FIGURE TO MAKE CERTAIN YOU UNDERSTAND THE PROCESSES
Codon: the triplicate code found on mRNA that codes for each of the 20 amino acids, for start (methionine) and stop
Genetic Code is the set of rules by which information encoded in genetic material (DNA or RNA
sequences) is translated into proteins (amino acid sequences) by living cells
The code is universal: all cell in all organisms use the same
GENE REGULATION: Determines when a protein is expressed (produced) in a cell. Some proteins are always
expressed while others are expressed intermittently (inducible).
Plants and other Eukaryotes express specific gene sequences depending on the environmental or developmental
sequence. Each or Most genes are accompanied by a promoter sequence that binds to the mRNA polymerase when
the time is right. The binding of mRNA polymerase to the promoter sequence is affected by “control elements” on
the chromosome. When the correct transcription factor binds to the control element the RNA polymerase can bind
to the promoter. A transcription factor is anything that can bind to the control elements of the chromosome.
The Signal Transduction Pathway is the sequence of events that transmits the messenger into a response.
It has 3 steps:
1. Reception: this occurs at the plasma membrane when the hormone or other signaling molecule binds to a
membrane receptor
2. Transduction: activation of a cytoplasmic second messenger
3. Expression: the second messenger activates a transcription faction. The activated transcription factor binds
to a control element promoting transcription. Translation follows transcription.
Bacteria use operons; The lac operon is not important for the Botany Class
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.
Study this image as it shows lactose (purple) associating and dissociating with the repressor. Repressor binds to the
operator. RNA polymerase binds to the promoter.