Chapter 14 DNA Early scientists unsure if DNA or protein was genetic material Hershey Chase experiment determined that DNA was genetic material DNA is polymer of nucleotides each made up of 1 a ribose 2 phosphate group and a 3 nitrogenous base A T C G DNA synthesized from nucleotide triphosphates dATP dGTP dCTP dTTP Complementary base pairing A and T C and G Ribose and phosphate group form a phosphodiester backbone 5 to 3 DNA follows semiconservative replication each new DNA strand has one newly have a single origin of replication because they have only one Bacteria chromosomes synthesized strand and one old strand place where replication begins multiple sites along each chromosome where DNA synthesis begins Leading strand strand synthesized TOWARD the replication fork Lagging strand Synthesized in the direction AWAY from the replication fork Eukaryotic chromosomes have multiple origins of replications because they have DNA replication DNA Polymerase attached nucleotides to existing DNA requires primer DNA helicase opens double helix to allow enzymes to attach to each strand Single strand DNA binding proteins SSBPs bind to the single stranded DNa to prevent refolding The enzyme primase makes RNA primers on which DNA nucleotides can be added to during DNA polymerization the RNA of the primer is later replaced with DNA Topoisomerase enzymes relieves supercoiling tension that occurs at the replication Leading strand vs lagging strand DNA ligase catalyzes the joining of Okazaki fragments into a continuous strand Synthesis of the Lagging Strand DNA fragments must be added in short groups called Okazaki fragments RNA at 5 end from RNA primer and DNA at 3 end All the enzymes used for DNA replication are in a huge complex called the replisome DNA polymerase III extends Okazaki fragments Replicating the Ends of Linear Chromosomes The ends of linear chromosomes are called telomeres which contain many repeats of a Telomere length serves as a record of cell division and limits the number of times a six base sequence somatic cell can divide Telomerase is an enzyme that solves this problem by adding additional six base repeats to the end of the leading strand Telomerase is only active in reproductive organs somatic cells those not active in gamete formation lack telomerase so the chromosomes of somatic cells gradually shorten with each mitotic division getting small as an individual ages Mistakes in Replication DNA polymerase may pair the wrong nucleotide to a chain Proofreading is when the cell checks for mistakes and remedies them Exonuclease is the enzyme that removes incorrect nucleotides at the end of a nucleotide Endonuclease is the enzyme that removes incorrect nucleotides within not at the end chain and acts in the 3 to 5 direction of a nucleotide chain Nucleotide excision repair is where a repair complex identifies a mismatch in a nucleotide sequence cuts the mutant DNA free and allows DNA polymerase and ligase to fill in the gap DNA polymerase I removes RNA primers and replaces it with DNA Chapter 16 Transcription RNA processing Translation Flow of information from DNA to gene products DNA RNA protein products Gene a DNA sequence that is expressed to form a functional product either RNA or polypeptide Transcription Enzymes called RNA polymerase help synthesize does not need primer Ribonucleotide triphosphate NTPs is the nucleotide monomer that attaches to the existing building RNA RNA polymerase is what catalyzes the formation of a phosphodiester bond between the growing mRNA chain and the new ribonucleotide The strand that is read by the enzyme is the template strand The strand that is not being read is the non template strand or coding strand It matches what the RNA being transcribed should look like except that the U s are T s RNA polymerase synthesizes in the 5 to 3 direction following along the DNA strand s 3 to 5 direction RNA polymerase cannot initiate transcription on its own A protein subunit called a sigma must bind to the RNA polymerase to start transcription Together the RNA polymerase and sigma form a holoenzyme Promoters are the sections of DNA where transcription begins they are 40 50 base pairs long and have a six base pair sequence known as the 10 box because it is centered about 10 bases from the point of initiation for RNA polymerase to start transcription Sigma guides RNA polymerase to the promoters DNA that is located in the direction RNa polymerase moves during transcription is said to be downstream from the point of reference DNA located in the opposite direction is said to be upstream The place where transcription starts is called the 1 site There is another base pair sequence that occurs in promoters known as the 35 box because it is centered about 35 bases upstream from the 1 box Transcription begins when sigma binds to the 35 and 10 boxes which supports the hypothesis that sigma is a regulatory protein The type of sigma protein in the RNA polymerase holoenzyme determines what type of gene will be transcribed RNA processing in Eukaryotes Operons are rare in eukaryotes usually always found in prokaryotes After transcription the products is an immature primary transcript or pre mRNA RNA splicing is process where introns not expressed are removed from RNA and the remaining exons are spliced together snRNPs bind near the introns forming a complex called the spliceosome No introns and exons in prokaryotes There are no introns and exons in prokaryotes because operons do not need processing everything is together already and introns and exons are only found in eukaryotes 5 cap is adding consisting of methylguanylate and 3 phosphate groups 3 poly A tail is added to 3 end consisting of a long row of adenine nucleotides the cap and tail protect mRNAs from degradation and enhance efficiency of translaion Translation In prokaryotes Ribosomes attach to mRNAs and begin synthesizing proteins BEFORE transcription is complete in fact multiple ribosomes attach to each mRNA forming a polyribosome this way many copies of a protein can be produced from a single mRNA In eukaryotes transcription and translated are separated in time and space so they occur separately like bacteria polyribosomes form Aminoacyl Synthetase enzyme that attaches amino acid to tRNA Ribosomal RNA rRNA what allows ribosomes to do their job very important Translation has 3 steps 1 2 Initiation mRNA binds to ribosome small subunit initiator tRNA binds to start codon at P site middle one large subunit of
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