MUTATIONS

Mutation is heritable changes in DNA. Somatic mutations are passed on to daughter cells, but only germ line mutations are passed on to sexually produced offspring. In 1901, Hugo de Vries studied the inheritance in evening primrose (Oenothera glazioviana) and observed change in one of the alleles of a gene as a result of mutation and the organism carrying it was called a mutant. In gene mutation also called point mutation, an allele of a gene changes. Silent mutations result in no change in amino acids, when altered mRNA is translated into a polypeptide. Missense, nonsense and frame-shift mutations do cause changes in the amino acids produced. Chromosomal mutations (deletions, duplications, inversions or translocations) involve large regions of a chromosome. Spontaneous mutations occur because of instabilities in DNA or chromosomes. Induced mutations, although often detrimental to an individual organism, are the raw material of evolution.

     

Transposable Genetic Elements

These are DNA sequences which can move from one position to another in the genome. Barbara Mc Clintock (1948) discovered transposons in corn and received the noble prize in 1983. They are present in both prokaryotes and eukaryotes. When transposons move from one location to another, they break and mutate genes. They have played a role in evolution and contributed to genetic diversity.

Gene Expression

In 1908 A. Garrod, who studied the inheritance of human disease alkaptonuria, linked the biochemical phenotype to missing enzyme, and thus to an abnormal gene. He wrote a book entitle “In born errors of metabolism” and gave the evidence that gene expresses itself by making protein. Later on Beadle and Tatum’s experiments on metabolic enzymes on the bread mold Neurospora led to one-gene-one-enzyme hypothesis, which is modified into one-gene-one-polypeptide; the function of gene is to code for a specific polypeptide.

 The expression of gene to form a polypeptide occurs in two major steps:

Transcription copies the information of a DNA sequence (a gene) into corresponding information in an RNA sequence.

Translation converts this RNA sequence into the amino acid sequence of a polypeptide.

Transcription   The genetic information in the DNA is not expressed directly but is transferred via messenger RNA (mRNA).  The long molecules of mRNA are assembled by complementary base pairing along one strand of the DNA helix. This process, called transcription, is catalysed by the enzyme RNA polymerase. RNA transcription proceeds in three steps: initiation, elongation and termination. Initiation requires a promoter, to which RNA polymerase binds. Elongation of the RNA molecule proceeds from the 5’ and 3’ end.

   Each sequence of three nucleotides in the coding region of the mRNA molecule is the codon for a specific amino acid. Eukaryotes have three different RNA polymerases. RNA polymerase II transcribes protein coding genes (fig B.3.3). Most eukaryotic genes contain noncoding sequences called introns, which are removed from the pre-mRNA transcript. The primary transcript of a eukaryotic gene is modified while still in the nucleus. First, its 5’ and 3’ ends are modified; by the addition of a G cap at 5’ end and a poly A tail at 3’ end. The introns are removed from pre-mRNA by RNA spilcing. A complex of mRNPs and enzymes, called a spliceosome, forms at the consensus sequences that lie between introns and exons. The spliceosome cuts out the introns and joins the exons together, results in the final mRNA strand which leaves the nucleus and travels to ribosome in the cytoplasm in the cell.

 In 1961, while studying the DNA structure Francis Crick observed that the DNA code is made up of called a triplet, the DNA sequences is transcribed into complementary sequence of mRNA bases. The genetic code is a ‘’language’’ of triplets of mRNA nucleotide basis (codons) corresponding to 20 specific amino acids; there are start and stop codons as well. The code is redundant (an amino acid may be represented by more that one codon), but not ambiguous (no single codon represents more than one amino acid.

Page No B.24 figure B.3.3 Transcription: The three step process involving one of the strands of DNA as a template on which free RNA nucleotides assemble into strand of mRNA.(Diagram)

Translation Protein synthesis takes place at the ribosome. A ribosome is formed of two subunits, one large and one small, each consisting or rRNA complexed with specific proteins. Another group of RNA molecule, known as transfer RNA (tRNA), is also required for protein synthesis. These molecules can carry an amino acid on one end, and they have can triplet of the bases, the anticodon. The tRNA molecule is the adapter that pairs the correct amino acid with each mRNA codon during protein synthesis. There is at least one kind of tRNA molecule for each kind of amino acid found in the protein. ((Fig. B.3.4).