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DNA and proteins The genetic code • Polypeptides are chains of amino acid residues joined by peptide bonds. • Proteins are large polypeptides. • There are 20 different aa and their sequence determines the structure and function of the protein. • The sequence of bases in a DNA molecule determines the sequence of aa. • A gene is a length of DNA that codes for one (or more) polypeptides. • A genome is the entire sequence of DNA of an organism (about 25000 genes in the human genome). • Each gene occupies a specific locus (position) on a chromosome and each chromosome consists of one molecule of DNA. • The DNA is wrapped around basic histone proteins (Chromatin) • In between genes is non coding DNA • Several genes that function together are called a cistron Genes code for polypeptides such as: Structural proteins (keratin, collagen, actin, myosin) Enzymes electron carriers Haemoglobin Antigens, immunoglobulins Cell surface receptors Channel proteins The genetic code • The sequence of bases in a gene determines that protein that it will code for • The bases are in groups of 3 called codons • Each codon codes for an amino acid The genetic Code translated according to the codes on mRNA Features of the Genetic Code • • • • • It is a triplet code It is degenerate It has punctuation It is widespread but not universal It is a non overlapping code Principles of Protein synthesis • • DNA stays in the nucleus Proteins are made on ribosomes in the cytoplasm SO • • The genetic code must be copied to messenger RNA (mRNA) mRNA leaves the nucleus via the nuclear pores taking the code to the ribosomes • So the process of using the genetic code held in DNA to make a protein occurs in 2 stages: 1. Transcription – the code on part of a DNA molecule is copied to a mRNA molecule. 2.Translation – the mRNA takes the code to a ribosome where it is used to make a polypeptide. Transcription • The process by which the base sequence of a gene is converted into a complimentary base sequence of mRNA • One strand of the DNA (template strand) is used to make mRNA the other stand is the coding strand. • Only part of a DNA molecule is transcribed at one time. • This part of the DNA molecule ‘unzips’ – DNA helicase breaks the hydrogen bonds between the bases. Transcription Transcription • The part of the DNA molecule to be transcribed enters the nucleolus to unwind and ‘unzip’ • DNA helicase breaks the hydrogen bonds between the bases • RNA polymerase catalyses the binding of activated free nucleotides to the template • Uracil binds to adenine NOT thymine • The nucleotides condense together forming phospho diester bonds • The mRNA is complimentary to the template strand and a copy of the coding strand mRNA and the Genetic Code Translation • The second stage of gene expression/protein synthesis • It is the assembly of polypeptides at the ribosome based on the sequence of codons • This means it is the translation of the sequence of bases into a sequence of amino acids Several ribosomes attach to mRNA forming a polysome Ribosomal groove mRNA attaches to small ribosomal subunit Made in the nucleolus from rRNA and protein Transfer RNA (tRNA) • Made in the nucleus and passes into the cytoplasm. • It is a length of RNA that folds into hairpin shapes to form a clover leaf • It can attach to a specific aa in the cytoplasm • It can recognise the specific mRNA codon for the aa it carries tRNA structure Smallest RNA Three exposed bases at one end where an amino acid can bind Over 20 different tRNA’s Recognises a specific activation enzyme Three unpaired nucleotide bases at the other end Translation mRNA used to make polypeptide chain (protein) 1. 1. mRNA attaches itself to the small subunit of a ribosome so 6 bases of the mRNA are exposed to the large subunit. 2. The first exposed mRNA codon is always AUG (start codon) 3. A tRNA molecule (with its aa -met) with an anticodon complimentary to the 1st codon lines up in position P 4. Complimentary base pairs form H bonds between the codon and anticodon (UAC with the AUG) 5. Another tRNA (Pro) complimentary base pairs with the next codon in the ribosome at position A. 6. The enzyme peptidyl transferase forms a peptide bond between the two aa (met and Pro) 7. The first tRNA is released and leaves the ribosome without its aa. Translation 2 8. The ribosome moves along the mRNA 3 bases to the next codon. 9. The second tRNA molecule (Pro) is now position P. 10.A new tRNA molecule pairs with the mRNA in position A bringing its aa (Tyr). 11.A growing polypeptide is formed in this way until a stop codon is reached. End of Translation 12.A stop codon (UAA, UAC or UAG) on the mRNA signals the ribosome to leave the mRNA. 13.A newly synthesised protein is now complete and can take up its tertiary structure. 14.Some proteins need to be activated by cAMP to take up a 3D shape that is the best fit to their complimentary molecule e.g by binding to an allosteric site on an enzyme Translation - outline A polysome Overview of Translation