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DNA Structure and Function Nucleic Acids • Monomer = nucleotides • Structure = three parts: sugar, phosphate, and nitrogen-containing base Functions • Function #1: Nucleotide monomers can be used as “energy currency” • Examples = ATP / ADP Functions • Function #2: Stores genetic information (traits and inheritance) • Examples = DNA, RNA Nucleotide Structure • Nucleotides – the building blocks of nucleic acids • 1. 2. 3. Made of: Phosphate Sugar Nitrogenous Base Sugar-Phosphate Backbone • Nucleotides connected together with what type of bond? • Alternating sugars and phosphates Nitrogenous Bases • Four bases: – – – – Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Base Pairing • Bases form hydrogen bonds with each other – A with T – C with G PURINE with? PYRIMIDINE with? DNA C G T A Base pair C Hydrogen bond T A T G C G A T A C G C G T T C A G A A T A T A G A Ribbon model T C T Partial chemical structure Computer model 3-D DNA Structure • Based on the angle of the bonds (remember what a C with 4 bonds looks like), DNA forms a DOUBLE HELIX DNA Structure • Hydrogen Bonds occur between Nucleotide Bases • the bonds between which 2 bases are stronger? • thousands of bases, thousands of bonds, thousands of big twists Polarity • Is DNA polar or non-polar? Determining that DNA is the Genetic Material • GRIFFITH: 1928 • "Transforming factor” Determining that DNA is the Genetic Material Head DNA Tail Tail fiber 300,000 • HERSHEY – CHASE: 1952 • Determined that the heredity material was DNA not protein • Studied the bacteriophage T2 Determining that DNA is the Genetic Material Phage Bacterium Radioactive protein DNA Batch 1 Radioactive protein Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. Batch 2 Radioactive DNA Empty protein shell Phage DNA Radioactivity in liquid Centrifuge Agitate in a blender to separate phages outside the bacteria from the cells and their contents. Pellet Centrifuge the mixture Measure the so bacteria form a radioactivity in pellet at the bottom of the pellet and the test tube. the liquid. Radioactive DNA Centrifuge Pellet Radioactivity in pellet Determining the Structure of DNA • CHARGOFF: 1949 • Different species have different amounts of A, T, C, G • A always equals T • C always equals G Determining the Structure of DNA • FRANKLIN and WILKINS: 1950’s • X-ray crystalographic determination that DNA is a double helix Determining the Structure of DNA • WATSON and CRICK: 1953 • Double helix structure of DNA Determining the Structure of DNA • http://www.pbs.org/wgbh/nova/photo51/ DNA Structure in more detail Two strands run in opposite directions DNA is ANTIPARALLEL • 3’ and 5’ ends • 3’ has OH group • 5’ has phosphate group Nucleotide Assembly Organization of DNA • Prokaryotes – Several million base pairs one circular piece – Related genes grouped together – Mostly coding DNA – Nucleus? 23 Organization of DNA • Eukaryotes – Billions of base pairs – several linear chromosomes – Genes not grouped – Mostly non-coding DNA – Nucleus? 24 DNA in the cell • Humans have 3 billion nucleotide base pairs • Roughly 6 feet of DNA in the nucleus of each cell • How does it all fit? DNA is tightly packaged into chromosomes CHROMATIN - eukaryotes ONLY • Histone proteins: the spools around which DNA winds • 9 Histone proteins + DNA = Nucleosome – core of 8 histone proteins – DNA wrapped around – 1 “H1” histone Higher Chromatin Organization And even higher chromatin organization! And the highest organization! Meet the chromosome Chromosome Animation • http://www.biostudio.com/demo_freeman_ dna_coiling.htm Gene Expression • Every cell in your body came from 1 original egg and sperm • Every cell has the same DNA and the same genes 32 Gene Expression • Every cell in your body came from 1 original egg and sperm • Every cell has the same DNA and the same genes • Each cell is different, specialized • Differences due to gene expression – Which genes are turned on – When the genes are turned on – How much product they make 33 Gene Expression • DNA must be copied when cells divide so that DNA is the same in all cells DNA Replication • The Watson-Crick model of DNA structure suggested a mechanism for its replication – DNA strands separate – Enzymes use each strand as a template to assemble new nucleotides into complementary strands A T A T C G C G G C G A T A T A T Parental molecule of DNA A C C Nucleotides Both parental strands serve as templates T A T A T G C G C G C G C G C T A T A T A T A T A Two identical daughter molecules of DNA DNA Replication • http://www.fed.cuhk.e du.hk/~johnson/teachi ng/genetics/animation s/dna_replication.htm DNA Replication Summary • Helicase breaks hydrogen bonds to separate strands • DNA Polymerase pairs new bases and creates two new strands Semiconservative • Leading strand = continuous • Lagging strand = segments connected by ligase DNA Replication in Eukaryotes • Eukaryotic DNA is very long and linear • replication begins at many points DNA Replication in Prokaryotes • prokaryotic DNA is circular • replication begins at one point Repair of Mistakes • Error rate during replication: 1 in 100,000 nucleotides = about 3,000 mistakes per replication (human) • Error rate in completed daughter DNA molecules: 1 in 10,000,000,000 nucleotides Repair of Mistakes • Enzymes proofread DNA sequences during DNA replication and repair damaged DNA – Mismatch repair enzymes replace incorrectly paired nucleotides – Nucleotide excision repair enzymes chop out incorrect or damaged bases Repair of Mistakes • When proofreading and repair mechanisms fail, an error becomes a mutation – a permanent change in the DNA sequence When would the cell replicate DNA? • Any time it is going to divide so that new cells have enough DNA DNA Structure Review • Covalent bonds on sides connecting nucleotides • Hydrogen bonds between bases • A pairs with T • C pairs with G • 5’ end = phosphate • 3’ end = OH on sugar Cell Division Cell Division Questions to consider… Where? Why? How? Cell Division • Eukaryotic cells utilize two types of cell division: • mitosis • meiosis Mitosis in everything else Meiosis in sexual reproduction. Why Divide? • To replace dead or dying cell • To produce more cells to enlarge the organism • Reproduction Binary Fission • asexual reproduction used by most prokaryotes and protists • Results in the reproduction of a livingcellby division into two equal or near-equal parts. BINARY FISSION BUDDING BUDDING SUMMARY OF MITOSIS PARENT CELL GROWTH SUMMARY OF WHAT THE DNA DOES DURING MITOSIS DNA in Mitosis Chromosome duplication Centromere Sister chromatids Chromosome distribution to daughter cells DNA Replication during Mitosis • DNA genes CHROMOSOME • Count number of CENTROMERES • Sister chromatids: copied chromosomes attached at centromere DNA Replication during Mitosis • KARYOTYPE: picture of chromsomes arranged into pairs • Humans have 23 pairs (46 total) chromosomes • http://en.wikipedia.org/wiki/List_of _number_of_chromosomes_of_va rious_organisms DNA Replication • The Watson-Crick model of DNA structure suggested a mechanism for its replication – DNA strands separate – Enzymes use each strand as a template to assemble new nucleotides into complementary strands A T A T C G C G G C G A T A T A T Parental molecule of DNA A C C Nucleotides Both parental strands serve as templates T A T A T G C G C G C G C G C T A T A T A T A T A Two identical daughter molecules of DNA DNA Replication Summary • Helicase breaks hydrogen bonds to separate strands • DNA Polymerase pairs new bases and creates two new strands Semiconservative • Leading strand = continuous • Lagging strand = segments connected by ligase