A2 GENESStructure of genesGene familiesGene expressionGene promoterIntrons and exonsPseudogenes

1. Structure of genesIn physical terms, a gene is a discrete segment of DNA with a base sequence that encodes the amino acid sequence of a polypeptide, or a RNA nucleotide sequence.Genes vary in size from less than 100 base pairs to several million base pairs.The genes are very dispersed and are separated by intergenic DNAalso called extragenic region (Fig. 1).

Figure 1. Gene in DNA molecule

The two strands of a gene are called template strand (also called antisense/ noncoding strand ) and nontemplate strand (also called sense/ coding strand).The capacity of DNA molecules to store information is enormous: the number of different combinations of the four bases is 4n.

2. Gene familiesSome genes are organized into groups or clusters, which are called operons and multigene families.Operons are gene clusters found in bacteria. In an operon, genes are regulated in a coordinated way and encode proteins with closely related functions. e.g. Lac operon (Fig. 2)

Figure 2. The lac operon. Three genes (Lac Z, Y and A) are arranged and regulated together

In higher organisms, clustered genes exist as multigene families. Unlike operons, the genes in a multigene family are identical or very similar and are not regulated coordinately.Simple and complex multigene families: the genes are identical (e.g. 5S rDNA) or not (e.g. globin gene family) respectively (Fig. 3).

Ribosomal RNA genesIntergenic DNA(a) Simple multigene familyFigure 3. Multigene family

3. Gene expressionGene expression is the process by which information stored in the base sequence of DNA molecule is made available to the cell.Central dogma, originally proposed by Crick, states that information is transferred from DNA to RNA to protein, which are named transcription and translation respectively.

Reverse transcription: in retrovirus, an enzyme called reverse transcriptase copy RNA into DNA.The biological information contained within the genes acts as a set of instructions for synthesizing proteins at the correct time and in the correct place.

The flow of genetic information

4. Gene promotersGene expression is highly regulated. Different genes are active in different cells.Expression of genes is regulated by a segment of DNA sequence present upstream of the coding sequence known as the promoter.

RNA polymerase and transcription factors recognize and bind sequences conserved in the promoter and initiates transcription.The expression of a gene in a cell is determined by the promoter sequence and its ability to bind RNA polymerase and transcription factors.

5. Introns and ExonsIn higher organisms, the coding information is usually split into a series of segments of DNA sequence called exons.The sequences that separate exons are introns which do not code. Introns are usually much longer than exons.

Splicing: the removal of introns from primary transcript and leaves the exons and the coding information continuous.

6. PseudogenesErrors in base sequences make it impossible for genes to contain useful information, these genes are called pseudogenes. Therefore, pseudogenes are evolutionary relics.Examples: globin pseudogenes

QuestionsExplain: Gene family, Exon, Intron, Splicing, Promoter, PseudogeneDescribe the central dogma.