BIOLOGY 11 Mrs. Garrison NUCLEIC ACIDS AND PROTEIN SYNTHESIS Nucleic Acids Deoxyribonucleic acid (DNA) Ribonucleic acid (RNA) Both Composed of nucleotides Sugar + Phosphate + bases Deoxyribonucleic acid (DNA) Deoxyribose (sugar) Phosphate Bases Adenine Guanine Cytosine Thymine Double-stranded alpha-helix Sugar and phosphate alternate to form "rails" of ladder Bases pair between the "rails" to form "rungs" Adenine pairs with thymine Guanine pairs with cytosine Carries hereditary information Chromosomes made of chromatin (=DNA + histone proteins) DNA replicates itself prior to cell division Need exact copy of each double-stranded DNA molecule Enzymes required at each step Double strands of DNA separate Each strand acts as template on which a new complementary strand is made by pairing the bases of new nucleotides with the bases of the template DNA strand New complementary strand is identical to old strand paired with template Have 2 identical double-stranded DNA molecules Each is half old DNA and half new DNA Synthesis of complementary strand occurs from 5' to 3' direction Mistakes corrected for by enzymes Ribonucleic Acid (RNA) Single stranded molecule from which protein is built Composed of nucleotides Ribose (sugar) Phosphate Bases Adenine Guanine Cytosine Uracil (no thymine) RNA synthesis called transcription RNA is built from a segment of DNA called a gene Requires enzymes and energy Occurs in nucleus on uncoiled DNA Double DNA strands separate One strand acts as template New nucleotides are added by base pairing to build a complementary RNA strand from the DNA template, starting at promoter site on DNA Guanine pairs with cytosine of DNA Cytosine pairs with guanine of DNA Adenine pairs with thymine of DNA Uracil pairs with adenine of DNA RNA strand pulls away from DNA and DNA strands again pair up RNA strand is modified Non-coding portions (introns) cut out Coding portions (exons) spliced together RNA strand leaves nucleus through pores in nuclear membrane Three types of RNA Messenger RNA (mRNA) - codes for amino acids of protein Ribosomal RNA (rRNA) - combines with enzymes to form 2 ribosomal subunits, which are location of protein synthesis Transfer RNA (tRNA) - brings amino acids to ribosome Proteins Protein synthesis is called translation Genetic code Information coded for by the base sequence of mRNA (which was coded for by the base sequence of DNA) is used to build a sequence of amino acids, forming a protein mRNA bases arranged in codons of 3 bases each tRNA has a triplet anticodon which pairs with mRNA codon Each tRNA carries specific amino acid, depending on the anticodon Each amino acid has 2 to 4 different codons which code for it Initiation of translation Small ribosomal unit binds to mRNA at start codon (AUG) Initiator tRNA brings in first amino acid Large ribosomal subunit binds to complex Chain elongation As each succeeding mRNA codon is moves into the binding site on ribosome, the tRNA with the proper anticodon comes in with its amino acid Each successive amino acid is bound to the previous amino acid by a peptide bond and the previous tRNA releases its amino acid and leaves Chain termination Occurs when a stop codon (UAG, UAA, UGA) is reached Polypeptide chain released to cytoplasm or ER (if modification required) Mistakes Insertions or deletions of DNA nucleotides result in incorrect mRNA codons mRNA codons following the mistake will be misread Three insertions or deletions will bring the codons back into the correct reading frame Replacement of correct nucleotide with incorrect nucleotide will result in just one mRNA codon being wrong The first two nucleotides of codon most important,a mistake causing the third nucleotide to be wrong will often cause no problem Mistakes in DNA will therefore cause mistakes in mRNA codons, which will cause mistakes in amino acid sequence (primary structure) of protein Mistakes in primary structure may result in mistakes in tertiary structure and a non-functioning protein