Dna Makes Rna Makes Protein

The blueprint for any organism is contained within its genome in the form of chromosomes and is written in the universal four "base" language of adenine (A), guanine (G), cytosine (C), and thymine (T). Chromosomes are built of chromatin, double stranded DNA wrapped around a multi-protein complex core comprised of histone proteins. This DNA contains the language (DNA or nucleotide sequence) that can be read and translated into proteins, and these areas of DNA are called genes.12 In higher organisms, ranging from yeast to plants and man, practically all genes are interrupted, with sequences coding for protein (coding exons) separated by regions of non-coding DNA called introns. The beginning and ends of genes are usually marked by exons that do not code for parts of the protein, the so called non-coding exons. Within coding exons, contiguous groups of three bases (codons), form the genetic code. The 64 (43) individual codons specify for the 20 amino acids from which proteins are made, or signal the start (initiator methionine codon) or end (stop codon) of translation. It is therefore the contiguous order of the codons within a gene that delineate the linear amino acid sequence of the protein produced.

In general, gene expression describes the production of RNA and, subsequently, protein from a gene. This process can be split into three major parts: transcription of the gene in the nucleus to make primary RNA, splicing of the primary transcript to form the mature messenger RNA (mRNA) and translation of mRNA in the cytoplasm to produce the protein for which the gene codes (fig 28.1). Transcription is carried out by an enzyme called RNA polymerase II (RNA pol II) under the direction of specialised basal transcription factors that form a multi-protein complex with RNA pol II on the gene promoter.1 The promoter contains the start site of transcription, usually designated +1, which marks the beginning of the first exon of the gene and hence corresponds to the first nucleotide of the mRNA. Binding sites for various transcription factors, which are DNA binding proteins with highly specific affinities for particular DNA sequences, sequester transcription factors to the gene where they participate in boosting (or, in some cases, repressing) the level of transcription. Transcription factors may bind within the promoter or lie within areas called enhancers that are located distally—usually upstream, but occasionally downstream, of the promoter. Considerable effort has focused on identifying promoter and enhancer sequences responsible for directing gene transcription and the identification of the factors that act on them.3 Once bound to their cognate DNA sequences, transcription factors help drive the rate at which RNA pol II initiates fresh rounds of transcription. The polymerase moves along the gene making a primary RNA copy of one strand of the DNA duplex, copying both exonic and intronic sequences. This primary RNA transcript is subsequently processed to remove the intron derived sequences and the

Figure 28.1 The process of gene expression. Chromosomes are scaffolds of DNA organised around protein (histones) in units called nucleosomes. DNA is unwound from histones before transcription of a gene by RNA polymerase II and transcription factors (coloured). The primary RNA transcript, which is a copy of both exonic (red) and intronic (blue) DNA sequences, is processed subsequently to remove intronic sequences (mRNA splicing). The resulting mRNA is then exported to the cytoplasm for translation and subsequent post-translational modification such as methylation, glycosylation or phosphorylation. Detection of mRNA by northern blot is illustrated: the blot shows that mRNA for the slow skeletal isoform of troponin T (TnTs) is expressed only in adult skeletal muscle (Sk) and not in fetal (F) or adult (H) heart or liver (L). Rehybridisation of the blot with a probe to 18S rRNA (18S) shows presence of RNA in each lane. Protein expression is analysed by western blotting: in the example shown, a universal antibody recognising all three troponin I (Tnl) isoforms shows distribution of fast skeletal (f), slow skeletal (s) and cardiac (c) isoforms in adult skeletal muscle (Sk) and fetal heart (F). Figure courtesy of KA Dellow.

Figure 28.1 The process of gene expression. Chromosomes are scaffolds of DNA organised around protein (histones) in units called nucleosomes. DNA is unwound from histones before transcription of a gene by RNA polymerase II and transcription factors (coloured). The primary RNA transcript, which is a copy of both exonic (red) and intronic (blue) DNA sequences, is processed subsequently to remove intronic sequences (mRNA splicing). The resulting mRNA is then exported to the cytoplasm for translation and subsequent post-translational modification such as methylation, glycosylation or phosphorylation. Detection of mRNA by northern blot is illustrated: the blot shows that mRNA for the slow skeletal isoform of troponin T (TnTs) is expressed only in adult skeletal muscle (Sk) and not in fetal (F) or adult (H) heart or liver (L). Rehybridisation of the blot with a probe to 18S rRNA (18S) shows presence of RNA in each lane. Protein expression is analysed by western blotting: in the example shown, a universal antibody recognising all three troponin I (Tnl) isoforms shows distribution of fast skeletal (f), slow skeletal (s) and cardiac (c) isoforms in adult skeletal muscle (Sk) and fetal heart (F). Figure courtesy of KA Dellow.

exons joined together (RNA splicing). Following some 5' and 3' modifications, such as the addition of a 3' poly-adenylic acid tract (polyA tail), the final mRNA product is exported from the nucleus to the cytoplasm, where it serves as a template for the production of protein by the ribosomes. Subsequent post-translational modifications such as cleaving off any propeptide or leader sequences which direct the protein to its ultimate destination in the cell or the attachment of phosphate or acetyl groups to specific amino acid residues may be necessary to produce the final functional form of the protein.

Was this article helpful?

0 0
Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Post a comment