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• • • • • • • • • • • • • • • • • •• What is central dogma? From DNA to Protein All proteins consist of polypeptide chains – A linear sequence of amino acids Each chain corresponds to the nucleotide base sequence of a gene What is the first step?1. Transcription: Enzymes uses base sequence of a gene as template to make strand of RNA Two DNA strands unwind in a specific region RNA polymerase assembles strand of RNA – – Covalently bonds RNA nucleotides (adenine, guanine, cytosine, uracil) according to nucleotide sequence of exposed gene What is the second step? 2. Translation – – Information in the RNA strand is decoded (translated) into a sequence of amino acids Prokaryotes and eukaryotes In prokaryotic cells (no nucleus) – Transcription and translation occur in cytoplasm In eukaryotic cells – – Genes are transcribed in the nucleus Resulting mRNA is translated in the cytoplasm Three types of RNA Messenger RNA (mRNA) – Carries protein-building codes from DNA to ribosomes Ribosomal RNA (rRNA) – Forms ribosomes (where polypeptide chains are assembled) Transfer RNA (tRNA) – Delivers amino acids to ribosomes RNA and DNA compared DNA – – – – exists as double-stranded molecules hereditary information double helix contains deoxoyribose sugar • RNA • • RNA Modification: Alternative Splicing • • • • • • • – – – – Disposable copies of hereditary information and some are catalytic exists as a single stand. contains ribose instead of deoxyribose contains uracil in place of thymine Before mRNA leaves the nucleus: – – – – – Introns are removed Some exons are removed along with introns; remaining exons are spliced together in different combinations Poly-A tail is added to 3’ end of new mRNA What is the genetic code? Messenger RNA (mRNA) carries DNA’s protein-building information to ribosomes for translation mRNA’s genetic message is written in codons – Sets of three nucleotides along mRNA strand Codons Codons specify different amino acids – A few codon signals stop translation Sixty-four codons constitute a highly conserved genetic code