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DNA Molecular Biology of the Gene Genes • biological blueprints • give attributes & traits • every nucleus, in every cell carries genetic blueprint • every cell has all information needed to make a complete you • located on chromosomes • humans have 46 • each contain thousands of genes Genes • share genes with all living organisms • 98% match chimpanzees • 99.9% match all other humans • differences exist at particular sites • causes each of us to be unique Genes & DNA • genes are made of DNA – deoxyribonucleic acid • macromolecule • made of 4 different nucleotides • paired in precise manner • order of nucleotidesgenetic code • DNA gives instructions to make proteins • each 3 combinations of nucleotides = one amino acid DNA • nucleic acid • macromolecule composed of smaller subunits –nucleotides • contains • carbon sugar-deoxyribose • nitrogenous base • 1-3 PO4 groups • contains 4 different nucleotides • each with different nitrogenous base • bases are found in 2 major groups • Purines – double ring structures – adenine (A) – guanine (G) • Pyrimidines – single ring structures – thymine (T) – cytosine (C) – uracil (U) DNA NUCLEOTIDES Sugar-Phosphate Backbone • bases are linked via dehydration synthesis into phosphodiester bonds • phosphate of one nucleotide covalently bonds to sugar of next • forms sugar-PO4 backbone • nitrogenous bases are arranged as appendages along backbone Sugar-Phosphate Backbone DNA • structure determined by Watson & Crick-1953 • discovered DNA is double stranded helix • composed of two strands • wrapped around each other in helical formation • core -bases of one DNA strand bonded to bases in other strand • if think of DNA molecule as ladder – sugar-phosphate backbone would be sides of ladder – paired bases would be rungs DNA • base pairing is specific • A-T • G-C • amount of A = amount of T • one strand is complementary to the other Replication • cells divide & reproduce daily • giving rise to 2 daughter cells • with same genetic makeup • before cell can divide, DNA must duplicate • called-replication • uses template mechanism Replication • to begin • strands of DNA must separate • double helix unwound by helicase – breaks H bonds between base pairs REPLICATION • unwinding takes place in a replication bubble • new strand of DNA is formed in both directions on both strands of DNA in bubble Replication • proceeds in both directions • DNA strand has 3’ end & 5’ end • at one end carbon 3 of sugar is attached to –OH group • at other end carbon 5 is attached to a phosphate group • DNA polymerase – enzyme – binds single nucleotides into new strand of DNA – works only in 3' to 5' direction • consequently DNA synthesis only occurs in 5' to 3' direction • means one daughter strand can be made as continuous strand – leading strand • other is made in short pieces • linked together with DNA ligase – lagging strand REPLICATION • each strand of DNA is used as template to make new, complementary strand • semi-conservative replication REPLICATION • at completion of process 2 DNA molecules have been formed each identical to original • one strand of each of new DNA molecules is strand of original DNA • other strand is complementary strand made during replication • semi conservative replication PHENOTYPIC EXPRESSION • small sections of chromosomes are genes • genetic makeup is genotype • expression of genes into specific traits is phenotype – result of proteins • one geneone protein Expression of Genotype • protein production is dictated by DNA • information about specific proteins is transferred to another nucleic acid-RNA • RNA is translated into a protein Genetic Code • DNAmRNAproteins • Proteins are long strands of amino acids held by peptide bonds • each has unique amino acid sequence • language of DNA is chemical • must be translated into different chemical languagethat of polypeptides • DNA language is written in linear sequence of nucleotide bases that comprise itAACCGTTGGACAC • specific sequence of bases makes up a gene glu lys ser ala met phe leu glu Expression of Genotype • transfer of information from DNA to RNA and then to proteins takes place in two processes • Transcription • Translation Transcription • DNA directs ribonucleic acid synthesis • transfers genetic information from DNA to RNA RNA • nucleotides – ribonucleotides • same basic components as DNA • single strand • 5 C sugar-ribose • phosphate groups • nitrogenous bases – same as DNA – one exception • RNA has Uracil (U) instead of T • base pairing rules are same • Uracil is substituted for thymine • U-A not T-A Types of RNA • Messenger – mRNA • Ribosomal – rRNA • Transfer – tRNA • all involved in translation Transcription • DNAmRNA • nucleic acid language of DNA is rewritten as sequence of RNA bases Transcription • process of transferring genetic information from DNA to RNA • similar to DNA replication • DNA is used as template to make RNA Transcription • stands of DNA must separate • only one serves as template • nucleotides take their places one at a time along template using same base pairing rules as replication except AU • 3 stages • Initiation • Elongation • Termination Initiation • RNA polymerase attaches to promoter – specific nucleotide sequence • RNA synthesis begins • RNA polymerase decides which strand to use as template • strand used- antisense strand • other stand-sense strand Elongation • RNA strand grows longer • RNA strand peels away from template allowing separated DNA strands to come back together • RNA strand formed is directly complementary to its DNA template • each time C is found in antisense strand of DNA template a G is paired with it Termination • RNA polymerase reaches special sequence of bases in templateterminator • ends transcription • RNA polymerase detaches Post-transcriptional Modifications • in prokaryotic cells RNA can function immediately • in eukaryotic cells RNA is processed before moving to cytoplasm for translation • post-transcriptional modifications • capping-tailing • splicing-ligating • ligation Capping-Tailing • nucleotides are added to either end of RNA • “G” nucleotide(s) might be added to one end; “A” nucleotides might be added to other • additions make RNA more stable • protects molecule from attack by enzymes • helps ribosomes recognize mRNAA Splicing & Ligation • precursor mRNA contains exons & introns • exons – segments containing information for formation of proteins • Introns – internal non-coding regions • before mRNA can leave nucleusintrons must be removed from strand • Introns are spliced out • exons are ligated (or attached) together • RNA can now move to cytoplasm through nuclear membrane pores Translation • conversion of nucleic acid language into protein language • proteins are macromolecules-polymers of amino acids • 20-common to all organisms • sequence of nucleotides in mRNA dictates sequence of amino acids in polypeptide • sequence of bases in molecule of DNA is genetic code GENETIC CODE • DNA & RNA are made of 4 different nucleotides • there are 20 amino acids • if each nucleotide coded for one amino acidcould only be 4 amino acids • if each 2 coded for onecould be 16 amino acids • smallest number of bases that can code for 20 amino acids is 3 • particular triplet of nucleotides in mRNA is a codon – specific for a particular amino acid • 64 possible triplet codes • code is redundant – more than one codon for each amino acid Codons • 61 code for amino acids • some have regulatory purposes – start & stop translation • AUG-start codon – codes for METmethionine • UAA, UAG, UGA- stop codons – tell ribosomes to end polypeptide synthesis Genetic Code • highly conserved • same in all organisms • genes can be transcribed & translated even if transferred from one species into another • opened door for genetic recombinant technology & genetic engineering Translation • amino acids are not able to recognize codons of mRNA • requires an interpreter – intermediate that can understand language of one form & translate message into another • tRNA (transfer RNA) is interpreter • picks appropriate amino acid & recognizes appropriate codon in mRNA • converts 3 letter code of nucleic acids into amino acidsproteins tRNA • composed of one strand of RNA • chain twists & folds making some double stranded areas • one end-special triplet of basesanticodon • contains complementary sequence of bases to sequence of bases in mRNA • recognizes bases in mRNA by applying standard base pairing rules • other end-site where amino acid can attach • enzyme recognizes both tRNA & its amino acid partner • there are at least 32 different tRNA in eukaryotic cells • anticodons are redundant • there is at least one anticodon for each amino acid Translation • ribosomes coordinate process of translation • ribosomes are formed from 2 subunits each made of proteins & rRNA (ribosomal RNA) • completely assembled ribosome has binding site for mRNA on its small subunit & two binding sites for tRNA on its large subunit Translation Stages • Initiation • Elongation • Termination Initiation • mRNA molecule binds to small ribosomal subunit • special initiator tRNA binds to specific codon-AUG – start codon • Anticodon-UAC • start codon also carries amino acid methionine • Next – large ribosomal subunit binds to small one creating functional ribosome • initiator tRNA fits into one of two tRNA binding sites on ribosome-P site • other tRNA binding site-A site is vacant • P site holds tRNA containing growing peptide chain • A site holds tRNA carrying amino acidsext amino acid to be added to chain Elongation • amino acids are added one by one to first amino acid • ribosome moves along mRNA in the 5'-to3'direction • tRNA (anticodon) corresponding to second codon binds to A site, carries amino acid Elongation • peptide bond forms between carboxyl group of one amino acid & amino group of next • after peptide bond forms-ribosome shifts, or translocates, causing tRNA to occupy the P site Elongation • movement brings next mRNA codon to be translated into A site • process begins again • elongation continues until stop codon is reached Termination • UAA, UAG & UGA are stop codons • when one of these sequences is detetectedpeptide released from last tRNA • ribosome splits back into its separate subunits Mutations • any change in nucleotide sequence of DNA • production of mutationsmutagenesis • some-spontaneous • some due to mutagens • radiation, chemicals & viruses • two categories – base substitutions – insertions & deletions Base Substitutions • Point mutation – replacement of one nucleotide for another • may go unnoticed • may cause significant issues • hemophilia • sickle cell anemia • Huntingtons Chorea • Tay Sachs disease Insertion & Deletion • mRNA is read as a series of triplet codons during translation • adding or deleting one base changes reading frame for tRNA • Frame-shift mutations – dramatic effects – all nucleotides downstream from insertion or deletion will be regrouped into different codons – result is usually nonfunctional protein