基因检测 专业讲课稿.ppt

* To turn a gene’s DNA sequence of bases into a protein’s sequence of amino acids requires an intermediary molecule called mRNA. Base pairing is used to create mRNA from one of the DNA strands. Every T in the DNA strand pairs with an A in the mRNA strand, and Cs and Gs in the DNA pair with Gs and Cs in the mRNA. However, if there is an A in the DNA sequence, it pairs with a new base called Uracil (U) in the mRNA strand. Thus, if the DNA sequence is TACGCAATATGCATT, the mRNA sequence becomes AUGCGUUAUACGUAA. * The sequence of the bases of an mRNA (A, U, G, C) directly spells out the sequence of amino acids in the protein. Every three bases in the mRNA sequence codes for a single amino acid and is called a “codon.” There are 64 different ways to arrange the four bases in mRNA--e.g., UCA, UCG, AAA, and AGC--and yet there are only 20 different amino acids. This means that one amino acid can have more than one codon. For example, valine has four codons (GUU, GUC, GUA, and GUG), while histidine only has two (CAU and CAC). The mRNA code also has one “start” and three “stop” signals. The codon AUG codes for an amino acid called methionine that always signals the start of protein building. Three other codons--UAG, UGA, and UAA--do not code for any amino acid. Instead, they always signal the mRNA to stop protein building. * Proteins are built within a cell structure called a ribosome--a protein factory. First the mRNA attaches itself to the ribosome. Then specific adapter molecules, called transfer RNAs (tRNAs), match themselves to their appropriate codons. Each codon has a corresponding tRNA that carries a specific amino acid at one end. Here is the beginning of a new protein. The first tRNA carrying methionine matched itself to the first codon. Then a second tRNA carrying arginine matched itself with the second codon. A chemical reaction joined one free end (the hook) of the methionine molecule to a free end (the eye) of the arginine molecule, and methionine’s tRNA was rele