Introduction to Gene expression:

In multicellular organism, each and every cell in the body has similar genetic information; individual cells encompass different structural and functional features. Gene expression is the most basic level at which genotype gives increase to the phenotype. The genetic code stored in the DNA in form of nucleotide sequence is interpreted through gene expression and the properties of the expression products give increase to the organism's phenotype. 

Control of Gene Expression:

Gene expression is a procedure by which genes coded information is transformed to the structures operating in a cell. The procedure is employed by all known life - eukaryotes, prokaryotes and viruses - to produce the macromolecular machinery for life. Gene expression provides the cell control over structure and function and is the foundation for cellular differentiation, morphogenesis and the versatility and adaptability of any organism.

Gene Expression in Bacteria: 

Bacterial cells are genetically much simpler than eukaryotic cells, having just one chromosome and only around 3,000 genes. Bacteria can be grown fast in large numbers under controlled conditions in the lab; they have been particularly helpful for studying the regulation of gene expression. Francois Jacob and Jacques Monod in the year 1960, formulated a powerful model of the control of gene expression in bacterial cells, based on their investigation of enzyme synthesis in E. coli. The Jacob-Monod model suggests that three portions of the chromosome are comprised in controlling transcription of the structural genes, the regulator gene, the operator region and the promoter region.

The regulator gene controls indirectly the action of the structural genes. The regulator gene is positioned near the structural genes and encodes the information for the synthesis of the repressor protein. If the repressor binds to the operator, it blocks the promoter's binding sites for RNA polymerase and therefore prevents transcription of the structural genes. In this system, the genes identifying specific enzymes are inactive till turned on by an inducer substance. In a negative control system like the lac operon, a repressor protein binds to the operator and turns off the transcription. If the repressor protein is inactive, the operator is turned on and transcription and translation automatically take place.

In a positive control system, proteins termed transcription factors bind to the promoter and activate transcription.

Hormonal Control of Gene Expression:

In higher animals and plants signals in different glands and/ or secretory cells in some way stimulate target tissue or target cells to experience dramatic modifications in their metabolic patterns. These changes often include modified pattern of differentiation which are generally dependent on altered patterns of gene expression.

Peptide hormones like insulin and steroid hormones like estrogen, progestrone, testosterone (in animals such as mammals) and ecdysone (in insects). In higher animals, hormones are synthesized in the specialized secretory cells termed as endocrine cells and are discharged into the blood stream. The peptide hormones don't generally enter cells due to their relative large size.

Their consequences are mediated by receptor proteins positioned in target-cell membranes and through the intracellular levels of secondary messenger termed as cyclic AMP (cAMP). The cAMP activates a protein kinase which activates numerous specific enzymes. The steroid hormones on the other hand are small molecules which readily enter cells via the plasma membrane. Once inside the suitable target cells, the steroid hormones become joined to specific receptor proteins that are present only in the cytoplasm of the target cells. The hormone-receptor protein complexes activate the transcription of specific and accurate genes by binding to specific DNA sequences present in the cis-acting regulatory areas of the genes.

Introduction to Genetic Code:

The vital question of gene expression is how the order of nucleotides in a DNA molecule encodes the information which specifies the order of amino acids in a protein, that is, the correspondence among nucleotide triplets and amino acids in proteins. The letters A, G, T, and C correspond to the nucleotides found in the DNA. They are organized into three-letter code words termed as codons and the collection of such codons makes up the genetic code.

The Nature of the Genetic Code:

The genetic code is the set of rules through which information encoded in the genetic material (that is, DNA or mRNA sequences) is translated to proteins (amino acids) by living cells. In the year 1961 Francis Crick and his colleagues reasoned that the genetic should likely comprise of a series of blocks of information, each and every block corresponding to an amino acid in the encoded protein.

They further hypothesized that the information in one block was probably a series of three nucleotides specifying a specific amino acid; they arrived at the number three as a two nucleotide block will not yield sufficient different combinations to code for the 20 different types of amino acids which generally occur in proteins.

Within genes that encode proteins the nucleotide sequences of DNA is generally in increment of three consecutive nucleotide with no penetration between the increments, each and every block of three nucleotide code of one amino acid. These three nucleotide blocks are termed as codons. Translation takes place on the ribosome; first the initial part of mRNA transcribed in a gene binds to an rNRA molecule interwoven in the ribosome, the mRNA lays on the ribosomes in such a manner that only three nucleotide part of the mRNA molecule - the codon is exposed at the polypeptide making site as each and every bit of the mRNA message is exposed in turn. A molecule of tRNA in the complementary three nucleotide sequences or anticodon binds to the mRNA, as the tRNA molecule carries a specific amino acid, that amino acid and no other is added to the polypeptide chain in that position.

Protein synthesis takes place as a series of tRNA molecules bond one after the other to the exposed part of mRNA molecule as it moves via the ribosomes, each of these tRNA molecules has joined to it an amino acid and the amino acid it brings to the ribosome is added one after the other to the end of a growing polypeptide chain. The anticodon of a tRNA is three nucleotide long; the base sequences of the tRNA anticodons are complementary to the related sequences of mRNA. As there are four different types of nucleotides in mRNA (C, G, A, U) there are 43 or 64 distinct 3 letter code words or codons possible. The list of different mRNA codons specific for each of the 20 amino acids is termed as the Genetic code. The genetic code is similar in all organisms with just a few exceptions. A specific codon like AGA corresponds to the similar amino acid (Arginine) in bacteria as in humans.

Note: That 3 out of the 64 codons - UAA, UAG and UGA don't correspond to triplets which are recognized by any activating enzyme. Such 3 codons, termed as nonsense codons they serve up as chain terminators or as stop signals in the mRNA message, marking the end of a polypeptide that is, they state where the polymerization of amino acids to a protein is to stop.

The codon AUG both codes for methionine and serves up as an initiation site. The first AUG in an mRNA's coding area is where translation into protein starts, marking the starting of a polypeptide amino acid sequence. The ribosome employs the first AUG which it encounters in the mRNA message to signal the beginning of its translation.

Features of the Genetic Code:

1) The code is a triplet codon.

2) The code is non-overlapping that is, in translating mRNA molecule the codons don't overlap however are sequentially ordered.

3) The code is commaless that is, no punctuation and once the reading is commenced at a specific codon, there is no punctuation among codons and the message is read in a continuing series of nucleotide triplets till  a translation stop codon is reached.

4) The genetic code is unambiguous, having a specific codon for all time coding for the similar amino acid.

5) The code is global ranging from bacteria to man.

6) Some of the codes act as start codons (AUG).

7) Some act as stop codons (UAA, UAG and UGA).

8) The code has polarity that is; it is always read in a fixed direction the 5′ 3′ direction.

9) Degenerate: The code is degenerate that is, more than one Condon might specify the similar amino acid.

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