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Download DNA & THE GENETIC CODE (protein synthesis)
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A sequence of bases on a DNA molecule which codes for a sequence of amino acids in a polypeptide chain A sequence of 3 DNA bases in a gene which code for a single amino acid 4 Bases = Examples ACGT AGC CTA A code that starts at one end of the gene and reads each codon independently of the next AUGAGCACCGAG amino acid 1 amino acid 2 amino acid 3 amino acid 4 More than one base triplet can code for a particular amino acid If the order of the bases is important there are 64 different ways of arranging four bases in a triplet code. HOWEVER only 20 different amino acids have to be coded for. • Some codes are used as initiation codons to indicate the beginning of a gene • Some are stop codons and indicate where a gene terminates • Others are used as alternative codons for some amino acids This involves 2 processes TRANSCRIPTION Copying a gene in the nucleus to make mRNA translation Using the mRNA to make proteins in the cytoplasm tRNA Ribosome mRNA transcription translation DNA POLYPEPTIDE IN NUCLEUS ENERGY (ATP) IN CYTOPLASM DNA cannot move into the cytoplasm where polypeptides are produced. This reduces the possibility of damage to the DNA. The DNA code is transcribed (copied) into a single stranded messenger RNA (mRNA) molecule in the nucleus. The process is controlled by the enzyme RNA polymerase. Only one of the DNA strands of the double helix carries the genetic code. This is called the coding or sense strand. This sense strand acts as a template for mRNA production. DNA helicase separates the 2 strands by causing hydrogen bonds to break between the complementary base pairs in the gene and the DNA to unwind. RNA polymerase attaches to the initiation code of the gene and moves along the template adding free RNA nucleotides to the exposed DNA nucleotides Free complementary RNA nucleotides in the nucleoplasm attach to the exposed DNA bases. adenine thymine pairs with on the RNA on the DNA uracil on the RNA adenine pairs with on the DNA whilst guanine always pairs with cytosine RNA polymerase joins the sugarphosphate backbone of the mRNA by condensation reactions. The completed mRNA molecule passes through the nuclear pores in the nuclear membrane into the cytoplasm. There are only 2 copies of each gene in the nucleus, but transcription allows many copies of mRNA to be available for translation and protein synthesis. During transcription the whole gene is copied into mRNA. However genes contain long sequences of non-coding DNA called introns. These are not required to make the protein therefore before the mRNA is used, these sequences are removed. Part of DNA / gene to be copied DNA template double stranded INTRON (noncoding) EXON transcription mRNA after Transcription single stranded modification Modified mRNA used by ribosome to make protein • interactive transcription • Carries amino acids to the ribosomes during translation • tRNA is single stranded • Folded back in itself to form a clover shape, held by hydrogen bonds • Specific amino acids are attached to one end • A 3 base anticodon at the other end is complementary to a specific mRNA codon Put the sentences in the correct order and then either rewrite or cut and paste • This is an enzyme which copies RNA into DNA • Transcription involves unpairing of bases in one region of the DNA helix (the gene being read), followed by the synthesis of a strand of mRNA. • This carries a triplet code sequence complementary to the template strand of DNA (i.e. DNA acts as a template) • mRNA acts as a messenger to carry information to the ribosome in the cytoplasm. • Translation involves decoding/reading the triplet message on mRNA. • Each codon, 3 bases, has a natural complementary sequence of 3 bases, called the anticodon. • This set of 3 bases is attached to a specific tRNA molecule that carries and transfers a specific amino acid. • The specific amino acid is determined by the sequence of the triplet code and anticodon. • The complementary triplets on mRNA and tRNA pair together in ribosomal sites (peptidyl & aminoacyl). • Ribosomes are situated in the cytoplasm, hence the need for mRNA & tRNA. • When 2 amino acids sit side by side, at the two sites on the ribosome, a condensation reaction occurs and a peptide bond will form between them.