DNA Structure and Replication

DNA RNA Double stranded Deoxyribose sugar Adenine, guanine, cytosine, thymine Holds the information to make proteins Single stranded Ribose sugar Adenine, guanine, cytosine, uracil Messenger, transfer, ribosomal

DNA Structure Double strands of DNA are considered anti-parallel (arranged in opposite directions) DNA strands are in different orientations—one side 5’ carbon is “up”, other side 3’ carbon is up

DNA Structure DNA double helix is very similar to a twisted ladder The sides/backbone of the ladder are considered to made of a sugar phosphate complex The “rungs” of the ladder are the nitrogenous bases Backbone held together by covalent bonds Rungs held together by hydrogen bonds

DNA Structure A, T, G, C can be separated into 2 groups PYRIMIDINES Thymine Cytosine (Uracil in RNA) Single ring PURINES Adenine Guanine Double ring, 2x size of the others Thymine Adenine

DNA Packaging Eukaryotic DNA condensed (prokaryotic DNA is “naked”) Chromosomes are supercoiled, very tightly packed Histones (proteins) help with this packaging Amount of condensation is unbelievable: over 10,000x smaller when in the chromosome form

Amount of condensation is unbelievable: over 10,000x smaller when in the chromosome form If you were to have a fully extended molecule of DNA that stretched to be 100 yds long… A molecule that was 10,000x smaller than that would only measure .36 inches!

Beads on a string… Nucleosomes: strands of DNA coiled around 8 histone proteins DNA (--), histones (+) = attraction Coiled DNA cannot be transcribed Enzymes must uncoil DNA This helps eukaryotes regulate transcription

Theories of DNA replication Conservative Original DNA double helix acts as a template for producing a new template The original double helix is conserved Geoff Brown, IB OCC Website 11

The original double helix is destroyed Destructive The original double helix is destroyed Nucleotides from the original strands end up in all daughter strands Geoff Brown, IB OCC Website 12

The original DNA strands are conserved, but... Semi Conservative The original DNA strands are conserved, but... ... the strands are separated and each daughter double helix inherits one original strand Geoff Brown, IB OCC Website 13

DNA replication is semi conservative The two original strands act as templates for the synthesis of two new strands Free nucleotides pair with complementary bases A C T G Template strand Pairing of complementary bases preserves the sequence of bases when DNA replicates Geoff Brown, IB OCC Website 14

- unwinds the double helix The Details: A C T G The enzyme Helicase: - unwinds the double helix - breaks the hydrogen bonds between complementary bases - separates the two strands 15

A C T Helicase A C T A C T T A G T A G T A G Free nucleotides pair up with complementary bases, and are held in place, by hydrogen bonding 16

A C T T A G A A C T A C T T G T A G T A G T A G A C T T A G DNA Polymerase creates two new strands of DNA by creating covalent bond between adjacent nucleotides 17

Leading strand A C T A C T T A G A C T T A G T A G T A G 1) An RNA primer allows for the addition of RNA nucleotides that match exposed point of DNA after helicase splits the two strands A C T A C T 2) DNA polymerase III adds DNA nucleotides 5’ >> 3” T A G A C T T A G T A G T A G 3) DNA polymerase I removes RNA primer, replaces it with DNA nucleotides 18

Lagging strand Not continuous, slower, hence lagging strand New nucleotides added in fragments, then later chemical bonded to form a full strand Additional enzyme needed to complete this process DNA ligase

Lagging strand A C T T A G A C T A C T T G A T A G T A G 1) An RNA primer allows for the addition of RNA nucleotides that match exposed point of DNA after helicase splits the two stands 2) DNA polymerase III adds DNA nucleotides 5’ >> 3”, but in small chunks called Okazaki fragments 3) DNA polymerase I removes RNA primer, replaces it with DNA nucleotides 4) DNA ligase bonds fragments together Modified from Geoff Brown, IB OCC Website 20

DNA Replication-enzymes PROTEINS ROLE Helicase Unwinds the double helix at replication forks Primase Synthesizes RNA primer DNA polymerase III Synthesizes new strand by adding nucleotides onto the primer (5’ to 3’) DNA Polymerase I Removes the primer and replaces it with DNA DNA ligase Joins the ends of DNA segments and Okazaki fragments

DNA Replication Replication is insanely fast—up to 4,000 bp per second VERY few mistakes, but they do happen-all repaired by specific enzymes The same enzymes are responsible for repairing damaged DNA as a result of chemical damage or exposure to high energy waves (radiation)