DNA – structure and component (12-1) How does DNA control the function of genes? Genes: carry info from one generation to the next Influence the heritable characteristics Easily copied DNA = polymer made of monomers = nucleotides Shaped as a double alpha helix Nucleotide = 5 carbon sugar(deoxyribose), phosphate group(PO4-)and nitrogenous base (adenine and guanine = purines – double ring or thymine and cytosine are pyrimidines – single ring) Backbone of DNA chain = phosphate – sugar alternating - phosphate of one nucleotide is attached to the sugar of the next nucleotide etc Polynucleotide strand has DIRECTIONALITY 5’ end (end that starts with phosphate group ) to the 3’ end (with the OH group on the sugar) The “rungs” of the DNA ladder = paired nitrogenous base Always one purine with one pyrimidine: AT and CG However, the sequence of nitrogenous bases can be joined tog in any order on a single side of the double helix CHROMOSOMES AND DNA REPLICATION Vocabulary: Chromatin: in eukaryotic organisms the tightly packed DNA and protein(histones) Nucleosomes: beadlike structure comprised of a handful of histones wrapped with the tightly coiled DNA to form a thick fiber 1 DNA REPLICATION Structure of DNA helps explain how DNA can be copied/replicated Each strand of the double helix has the info to construct the other half because of the specific base pairing DNA strands = complimentary DNA replication occurs at HUNDREDS of places along the strand, occurring in BOTH DIRECTIONS until the strand is completely replicated REPLICATION FORKS: sites where separation and replication take place (www.scilinks.org webcode cbn 4122) also: PHSchool.com webcode cbp 4122 DUPLICATING DNA REPLICATION – before cell division – DNA is copied DNA molecule separates into two strands Each strand acts as a template for ordering nucleotides into a NEW complimentary strand (base pairing rules) Where there was one double stranded DNA molecule now there are 2 double stranded DNA molecules, each an exact Replica If original strand has bases TACGTT new strand becomes ATGCAA – each strand of the “unzipped” DNA molecule is replicating itself Our cells have 46 DNA molecules – one double helical molecule/chromosome 6 billion base pairs – our cells can take a few hours to copy all of this DNA! How Replication Occurs 2 ENZYMES! And regulatory molecules(more proteins) 1. replication begins at sites called “origins of replication” – short stretches of DNA with a specific sequence of nucleotides 2. Proteins that initiate DNA replication recognize the sequence and attach 3. strands separate once protein attaches creating a replication “bubble” 4. Replication proceeds in BOTH directions 5. eukaryotic chromosome has hundreds – thousands replication origins 6. replication “bubbles” fuse – speeding up replication process 7. replication fork at each end of “bubble” 8. Helicases – enzymes that unwind the double helix at replication forks 9. single strand binding proteins bind to unpaired DNA 10. Primer – a short chain of RNA is the initial nucleotide chain produced – enzymes that synthesize DNA can’t initiate the synthesis of a polynucleotide – PRIMASE synthesizes PRIMER by using the DNA template 11. the new DNA strand will start from 3’ end of RNA primer 12.DNA polymerase(11) – joins individual nucleotides to RNA primer to produce a DNA molecule complimentary to the “parent” strand AND “proofreads” for mistakes in replication How does the antiparallel arrangement of the double helix affect replication? DNA polymerases can add nucleotides ONLY to the 3’ end of primer A new DNA strand can elongate only in the 5’3’ direction (remember each double helix has a strand that begins with 5’ and its complimentary strand that begins with 3’ – so if the polymerase can add 3 nucleotides to the 3’ strand only - then the new complimentary strand is starting with 5’ and leading to the 3’ of its other end SEE DIAGRAM DNA strand moving from 5’3’ = leading strand DNA strand moving away from replication fork also moves in 5’ 3’ direction but on the other side of the “bubble” from the replication fork = lagging strand Lagging strand is synthesized in fragments = OKAZAKI FRAGMENTS DNA ligase joins the Okazaki fragments into a continuous strand QUALITY CONTROL 1. DNA polymerases proofread each nucleotide – removes incorrect nucleotides – synthesis resumes 2. Mismatch repair – enzymes remove and replace errors in replication 3. nucleotide excision repair – damaged DNA – an enzyme – nuclease cuts out the damaged portion – new nucleotides fill in the “gap” Ends of chromosomes = telomeres = special nucleotide sequences Hundreds of 6 nucleotide sequences comprise a telomere Telomeres protect the genes of a chromosome During continual replication the telomeres shorten ultimately leading to death of a cell Germ cells(egg/sperm) have telomerase which prevents the shortening of the telomere - cancer cells also have telomerase ONE GENE ONE PROTEIN – Protein synthesis 4 5