ch.16 - DNA Replication.docx

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Biological Sciences
BIOSC 0150
Christine Dahlin- Universityof Pittsburgh Johnstown

1 General Biology 1 Chapter 16 – DNA Replication Overview - How DNA structure discovered? - What has the key info on DNA structure? - How is DNA replicated in bacteria and eukaryotes? - Steps of replication o All the enzymes o Leading and lagging strands Nucleic acids - Information storage and transfer - DNA (deoxyribonucleic acid) gives instructions to make proteins o Make up genes: discrete unit of inheritance - RNA (ribonucleic acid) o Helps execute instructions o Protein get make What do nucleic acids & DNA have to do with chromosomes? - DNA & RNA – types of nucleic acid - Chromosomes – long strands of DNA coiled around proteins o 1 chromosomes = 1 molecules - Genes – particular sequences of DNA Chromosomes made of DNA From DNA to chromosomes - To prep for cell division (after replication), DNA condenses - After chromosomes replicated, condensed chromosomes are pair of sister chromatids (dyads) 2 General Biology 1 Chapter 16 – DNA Replication  DNA & RNA are different because:  the nucleotides have a different sugar group  RNA has a different base group than DNA  RNA typically exists as a single strand while DNA is double stranded  RNA is more flexible in shape than DNA Bases mostly carbon and nitrogen  Pyrimidines – 1 ring, smaller  Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)  Purines – 2 rings, larger  Adenine (A), Guanine (G) - Covalent bond (strong bond) hold the components of a single nucleotide together - Hydrogen bonds (weak bonds) hold bases of nucleotides on parallel strands together  Chargaff’s rule: in cells the amount of A = amount of T; amount of G = amount of C  Strands were symmetrical  DNA double helix was discovered by Watson, Crick and Franklin Who were Drs. Watson & Crick? - Scientists that developed the model for DNA structure received Nobel Prize - Assisted by Rosalind Franklin who photographed DNA using X-ray crystallography - Double helix – has a uniform diameter - Purine + purine = base pairing is too wide - Pyrimidine + pyrimidine = base pairing is too narrow - Purine + pyrimidine = perfect fit! Structure of bases dictates pairing - Adenine (A) can form 2 bonds with thymidine (T) - Guanine (G) can form 3 bonds with cytosine (C) DNA - Strands also run antiparallel subunits face opposite directions - 5’ end: phosphate group; 3’ end: hydroxyl group Noted: Review of 5C sugars from chapter 5! 1C – where base attached to 2C – tell whether you have hydrogen group (DNA) or hydroxide group (RNA) 3C – connected to the next nucleotide 5C – where phosphate attached to Why is base pairing important for replication??? - Strands get split - Each complementary strand has the info. to make the other - Each strand serves as a template: o Nucleotides line up along template to form a new strand & follow base-pairing rules 3 General Biology 1 Chapter 16 – DNA Replication Conservative model – one DNA double helix molecule consists of the original DNA double helix template while the other DNA molecule is composed entirely out of new nucleotides Dispersive model – each DNA double helix molecule contain a mixture of old and new DNA strand. Semi-conservative Model – each DNA double helix molecule consists of one parental strand and one new strand DNA replication - Bacteria – 1 long circular chromosome o 5.6 million nucleotide pairs o Take less than 1 hour to copy and divide into 2 cells - Humans – 46 chromosomes o Approximately 6 billion pairs of nucleotide o Take a few hours to copy! o Very few mistakes - Replication in bacteria is simpler - Eukaryotes’ DNA replication has similar process with bacteria’s DNA replication in bacteria: - One single circular DNA chromosome replicate into 2 daughter DNA molecules - The replication bubble expand on both side until they split into 2 daughter DNA molecules DNA replication in a eukaryote: - Double stranded DNA on the origin - Multiple replication bubble due to expansion of the 2 strands - They further expand until bubbles fuse into 2 daughter DNA molecules - Replication has completed  In both cell types, replication proceeds in both directions away from origin Origins of replication - Short stretch of DNA with specific nucleotide sequence - Where replication starts - Bacteria have one origin of replication - Eukaryotes have many origins of replication Replication fork – have a Y fork shape - Y region is when parental DNA double helix strands unwind - DNA with one strand going from 5’ to 3’ direction with another strand going from 3’ to 5’ direction. 4 General Biology 1 Chapter 16 – DNA Replication - Several proteins participate in unwinding/separation of DNA - Topoisomerase – relieves pain, breaks, swivels and rejoins strands as needed - Helicase – enzyme that separates parent strands - Single strand binding proteins (at each side of the fork) – stabilized unwound parental strands Primase (enzyme) - Untwisting of DNA causes strain! Topoisomerase RNA Now, we can’t still add DNA because: primer - Enzymes are specific: enzymes that make DNA can’t initiate synthesis (Ex – DNA polymerase III) Helicase - Problems occur: no exisiting end for new strand, Single strand nothing to bind the DNA double strands, no –OH group to make DNA binding proteins Solution: RNA primer Primer – short RNA stretch that added by primase - Provides a 3’ hydroxyl group where more nucleotides can attach! -
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