dna structure & replication chapter 15 continued bedford county public schools – jami n. key
TRANSCRIPT
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DNA Structure & Replication
Chapter 15 continued
Bedford County Public Schools – Jami N. Key
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The Structure of DNADouble helix
Each nucleotide is made up of: Deoxyribose (sugar) A phosphate group A nitrogenous base
Adenine, guanine, cytosine, thymine A, G = purines
2 carbon rings C, T = pyrimidines
1 carbon ring
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The Structure of DNABase-Pairing Rules:
Chargaff’s Rules Guanine pairs with cytosine Thymine pairs with adenine
DNA strands are anti-parallel They run in opposite directions 5’ and 3’ ends
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DNA ReplicationBig Picture:
A new and identical molecule of DNA is made, using the old one as a template
Occurs in the nucleus
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DNA ReplicationDNA replication begins at the origin of
replication, a special sequence of DNA
2 strands are separated by helicase, forming a replication bubble
Replication fork is formed at each end of the replication bubble
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DNA ReplicationAt replication fork, nucleotides “line up” with
their complementary mates, according to the base-pairing rules
DNA polymerase III attaches the nucleotides to the exposed bases of the DNA strand
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DNA Replication: A Summary
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Leading Strand
DNA replication is different on the 2 strands
DNA polymerase can only add nucleotides to the 3’ end of a DNA strand
Along one template strand, the leading strand, DNA polymerase III just follows the replication fork (replicates continuously in one strand)
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Lagging Strand
On the other strand of DNA, the lagging strand – DNA polymerase must work in the opposite direction of the replication fork
Short segments of DNA– Okazaki fragments – are made
Okazaki fragments are joined by DNA ligase
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DNA ProofreadingDNA polymerase I proofreads each nucleotide
as it is added to the DNA strand
If there’s a mistake… it backs up removes the wrong nucleotide adds the right nucleotide
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Enzymes & Their Job in ReplicationHelicases- unwind the DNA strand
Single strand binding protein- holds the single strands apart for replication.
Primase- inserts RNA primer to begin replication process.
DNA Polymerase III- adds complementary bases to 3’ end of primer or new DNA strand.
DNA Polymerase I- removes RNA primer & inserts DNA nucleotides. (also proofreads)
DNA Ligase- “sews” Okasaki fragments of lagging strand together with covalent bonds.
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Semi-Conservative ReplicationDNA replication is semi-conservative
Each new DNA molecule has one old strand and one new strand