When a gene is to be expressed, the base sequence of DNA is copied or Peptide bonds form between the adjacent amino acids to finalise the structure. How the base sequences in DNA and RNA code for particular amino acids. amino acid, it would pay you to start from the beginning with the structure of DNA. The DNA is ultimately the code for the sequence of amino acids in a polypeptide chain, which forms a protein. Each grouping of three DNA nucleotide bases is a.
Reproduction, the genome and gene expression
The combination TTT, for example, codes for the amino acid phenylalanine. Regulatory regions of the gene also contribute to protein synthesis by determining when the gene will be switched on or off. Sciencing Video Vault Proteins In active genes, genetic information determines which proteins are synthesized and when synthesis is turned on or off. These proteins fold into complicated three-dimensional structures, somewhat like molecular origami.
Because each amino acid has specific chemical characteristics, the sequence of amino acids determine the structure and shape of a protein.
For example, some amino acids attract water, and others are repelled by it. Some amino acids can form weak bonds to each other, but others cannot. Proteins that catalyze accelerate chemical reactions, for example, have "pockets," which can bind specific chemicals and make it easier for a particular reaction to occur. Variations in the DNA code of a gene can change either the structure of a protein or when and where it is produced. If these variations change the protein structure, they could also change its function.
Degeneracy of the code may also be significant in permitting DNA base composition to vary over a wide range without altering the amino acid sequence of the proteins encoded by the DNA.
How is mRNA interpreted by the translation apparatus? These codons are read not by tRNA molecules but rather by specific proteins called release factors Section Binding of the release factors to the ribosomes releases the newly synthesized protein. The start signal for protein synthesis is more complex.
Polypeptide chains in bacteria start with a modified amino acid—namely, formylmethionine fMet. However, AUG is also the codon for an internal methio-nine residue, and GUG is the codon for an internal valine residue. Hence, the signal for the first amino acid in a prokaryotic polypeptide chain must be more complex than that for all subsequent ones. In bacteria, the initiating AUG or GUG codon is preceded several nucleotides away by a purine-rich sequence that base-pairs with a complementary sequence in a ribosomal RNA molecule Section Once the initiator AUG is located, the reading frame is established—groups of three nonoverlapping nucleotides are defined, beginning with the initiator AUG codon.
Initiation of Protein Synthesis. Start signals are required for the initiation of protein synthesis in A prokaryotes and B eukaryotes.
The base sequences of many wild-type and mutant genes are known, as are the amino acid sequences of their encoded proteins. In each case, the nucleotide change in the gene and the amino acid change in the protein are as predicted by the genetic code. Furthermore, mRNAs can be correctly translated by the proteinsynthesizing machinery of very different species. For example, human hemoglobin mRNA is correctly translated by a wheat germ extract, and bacteria efficiently express recombinant DNA molecules encoding human proteins such as insulin.
These experimental findings strongly suggested that the genetic code is universal. A surprise was encountered when the sequence of human mitochondrial DNA became known. Human mitochondria read UGA as a codon for tryptophan rather than as a stop signal Table 5.
Relationship Between DNA Bases Genes, Proteins and Traits | Sciencing
Mitochondria of other species, such as those of yeast, also have genetic codes that differ slightly from the standard one. The genetic code of mitochondria can differ from that of the rest of the cell because mitochondrial DNA encodes a distinct set of tRNAs.
Do any cellular protein-synthesizing systems deviate from the standard genetic code? Thus, the genetic code is nearly but not absolutely universal. Variations clearly exist in mitochondria and in species, such as ciliates, that branched off very early in eukaryotic evolution.
It is interesting to note that two of the codon reassignments in human mitochondria diminish the information content of the third base of the triplet e. Most variations from the standard genetic code are in the direction of a simpler code. Why has the code remained nearly invariant through billions of years of evolution, from bacteria to human beings? A mutation that altered the reading of mRNA would change the amino acid sequence of most, if not all, proteins synthesized by that particular organism.
Many of these changes would undoubtedly be deleterious, and so there would be strong selection against a mutation with such pervasive consequences. By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.