DNA Structure & Replication - PDF

Nucleic Acid Structure Levels of DNA Structure: Nucleotides – the building blocks of DNA Strand – a linear polymer (polynucleotide) strand of DNA Double helix – the two complementary strands of DNA Chromosomes – DNA associated with an array of...

Nucleic Acid Structure Levels of DNA Structure: Nucleotides – the building blocks of DNA Strand – a linear polymer (polynucleotide) strand of DNA Double helix – the two complementary strands of DNA Chromosomes – DNA associated with an array of different proteins into a complex structure Genome – the complete complement of genetic material in an organism 0  DNA Nucleotides have three components – Phosphate group – Pentose sugar Deoxyribose Loading… DNA = Deoxyribonucleic Acid – Nitrogenous base Purines – Adenine (A), Guanine (G) Pyrimidines – Cytosine (C), Thymine (T)  Sugar–phosphate backbone Nitrogenous 5 end bases Thymine (T) DNA is a liner polymer of nucleotides (polynucleotide) Adenine (A) Cytosine (C) Phosphate DNA nucleotide Guanine (G) Sugar (deoxyribose) 3 end Fig. 16-UN1 Purine + purine: too wide Adenine and guanine Loading… Pyrimidine + pyrimidine: too narrow cytosine and thymine Purine + pyrimidine: width was consistent with X-ray data Fig. 16-7 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm 5 end (a) Key features of DNA structure (b) Partial chemical structure (c) Space-filling model  Semiconservative DNA replication – Specific base pairing suggested a mechanism for copying for genetic material – each parent strand serves as a template for building a new daughter strand  Replication begins at origin of replication – provides an opening called a replication bubble that forms two replication forks – DNA replication proceeds in both directions Eukaryotes have multiple origins of replication along a chromosome Fig. 16-12b Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter (new) strand 0.25 µm Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes  Many proteins work together in DNA replication and repair catalyzes separation of DNA at Helicase the fork Primase generates the primer (starting DNA Polymerases point) generates DNA polymers Topoisomerase relieves additional twisting DNA ligase ahead of the fork Single-strand binding catalyzes ‘pde’ bonds to link DNA nucleotides proteins keep H bonds from reforming in the DNA DNA Replication Step 1. Unwinding Step 2. Initiation Step 3. Elongation of leading and lagging strands Loading… Step 4. Completion of lagging strand Step 5. Proofreading 0  DNA polymerase – Covalently links nucleotides – Deoxynucleoside triphosphates 0  Deoxynucleoside triphosphates – Free nucleotides with three phosphate groups – Breaking covalent bond to release pyrophosphate (two phosphates) provides energy to connect nucleotides 0  Leading strand – DNA synthesized continusously – Primase makes a single RNA primer – DNA polymerase adds nucleotides in a 5’ to 3’ direction – RNA primers are removed by DNA polymerase and replaced with DNA – DNA ligase joins adjacent DNA fragments 0  Lagging strand – DNA synthesized from 5’ to 3’ direction – Discontinuous as Okazaki fragments – Okazaki fragments consist of RNA primers plus DNA – RNA primers removed by DNA polymerase and replaced with DNA – DNA ligase joins adjacent DNA fragments 0 Problems for linear DNA No way to complete the 5 end Repeated rounds of replication produce shorter DNA molecules  Eukaryotic chromosomes have nucleotide sequences called telomeres – prevent erosion of genes near ends – shortening of telomeres may be connected to aging