12-2 DNA Replication Author stream

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Slide1:

The Central Dogma of Molecular Biology What is the Central Dogma of Molecular Biology? Describes the flow of genetic information from DNA to RNA to Proteins DNA Replication Transcription Translation Today’s Target is to describe the semi-conservative replication of DNA

Recall…:

Recall… DNA & RNA are nucleic acids, an important macromolecule in organisms that stores and carries genetic information

DNA Bonding:

DNA Bonding Purines (small word, big base) Adenine Guanine P y rimidines (big word, small base) C y tosine Th y mine Chargaff’s rules A=T, C=G Hydrogen Bonds attractions between the stacked pairs; WEAK bonds

A Purine Always Binds with A Pyrimidine:

A Purine Always Binds with A Pyrimidine

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What is DNA Replication? DNA Replication is when: Parent DNA makes 2 exact copies of DNA Why?? Occurs in production of new cells so each new cell can have its own FULL copy of DNA for growth and repair

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DNA Replication                                                                                                     DNA Replication is “semi-conservative” Meaning each newly synthesized molecule contains 1 “ parent template” strand and 1 new “ daughter ” strand

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DNA Synthesis The DNA bases on each strand act as a template to synthesize a complementary strand Recall that Adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C) The process is semiconservative because each new double-stranded DNA contains one old strand (template) and one newly-synthesized complementary strand DNA Replication A G C T G T C G A C A G C T G T C G A C A G C T G T C G A C A G C T G T C G A C T C G A C A G C T G

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DNA Synthesis The DNA bases on each strand act as a template to synthesize a complementary strand Recall that Adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C) The process is semiconservative because each new double-stranded DNA contains one old strand (template) and one newly-synthesized complementary strand DNA Replication A G C T G T C G A C A G C T G T C G A C A G C T G T C G A C A G C T G T C G A C T C G A C A G C T G

DNA Replication Steps::

DNA Replication Steps: 1. Initiation involves assembly of replication fork (bubble) at origin of replication 2. Elongation Parental strands unwind and daughter strands are synthesized. the addition of bases by proteins 3. Termination : the duplicated chromosomes separate from each other. Now, there are 2 IDENTICAL copies of DNA.

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I. BEGINNING OF DNA REPLICATION (INITIATION) (- ase means enzyme) 1. DNA helic ase (think “ heli ”) binds to the DNA at the replication fork untwist (“unzips”) DNA using energy from ATP Breaks hydrogen bonds between base pairs Single-stranded DNA-binding proteins (SSBP) stabilize the single-stranded template DNA during the process so they don’t bond back together.

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DNA Replication Step 1: Initiation Helicase unwinds DNA forming a “ replication fork ”

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DNA Replication Step 2: Elongation- -- Adding New Nucleotides 1. RNA PRIMASE adds a RNA primer to each template strand as a starting point for replication 2. DNA POLYMERASE reads the template strand (3’ to 5’) and adds new complimentary nucleotides (5’ to 3’) 3. DNA being made in the direction of the replication fork is called the leading strand

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DNA Replication 4. R eplication occurs in two directions. DNA polymerase can only add new nucleotides in the 5’ to 3’ direction (leading strand) 5. An RNA primer is laid down on the other strand, and new nucleotides are added 5’ to 3’ moving away from the replication fork. This is the lagging strand and the segment of DNA produced is called an OKAZAKI FRAGMENT

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DNA Replication 6. Helicase unwinds some more and the leading strand is extended by DNA polymerase adding more DNA nucleotides. Thus, the leading strand is is made continuously.

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DNA Replication The top template strand is called the lagging strand . 7. DNA polymerase adds new to the lagging strand. This produces the second Okazaki fragment. Thus, the lagging strand is made in pieces (discontinuously)

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DNA Replication Step 3: Termination 1. A different type of DNA POLYMERASE removes the RNA primer and replaces it with DNA 2. DNA LIGASE joins the two Okazaki fragments to produce a continuous chain 3. Each new DNA molecule is rewound by HELICASE . Each molecule is identical

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DNA Replication Summary and Other Facts: Leading Strand : 1 primer, 5’ to 3’ continuous Lagging Strand : multiple primers, 5’ to 3’ discontinuous In humans, DNA polymerase adds 50 nucleotides every second DNA polymerase can proofread its own work and repair 1 in 10,000 bases are in error, after proofreading, rate of mutation is 1 in 10,000,000

Replication enzymes you need to know…:

Replication enzymes you need to know… GYRASE HELICASE DNA polymerase PRIMASE LIGASE single-stranded binding proteins

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Gyrase - unwinds DNA coil Helicase – enzyme that breaks H-bonds DNA Polymerase – enzyme that catalyzes connection of nucleotides to form complementary DNA strand in 5’ to 3’ direction (reads template in 3’ to 5’ direction) Leading Strand – transcribed continuously in 5’ to 3’ direction Lagging Strand – transcribed in segments in 5’ to 3’ direction ( Okazaki fragments ) DNA Primase – enzyme that catalyzes formation of RNA starting segment ( RNA primer ) DNA Ligase – enzyme that catalyzes connection of two Okazaki fragments DNA Replication

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DNA Replication DNA Polymerase Enzyme that catalyzes the covalent bond between the phosphate of one nucleotide and the deoxyribose (sugar) of the next nucleotide DNA Polymerization

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DNA Replication Check out these animations and reviews: BioCoach Biosynthesis of DNA practice BioCoach adding new DNA practice BioCoach enzymes and molecules of replication practice DNA structure and replication self-quiz

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Summary: DNA Replication

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In order for cells to grow and divide, they must be able to make new copies of DNA that they can pass on to their daughter cells. This process is called DNA replication . 3.4.1

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3.4.1 In the first step of replication, the enzyme DNA Helicase unwinds the double helix . It then separates the DNA strands by breaking the hydrogen bonds between the bases. Unzipping the two strands allows access to the individual nucleotides that are needed for the replication process to continue.

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3.4.1 Next, DNA polymerase enzymes attach to the newly separated strands of DNA. The enzyme then uses free nucleotides and synthesizes complementary strands by reading the original strands. When the process is finished, two identical DNA strands have been produced.

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3.4.1

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3.4.2 3’ TGCCAAGGCTTGAG 5’ 3’ TGCCAAGGCTTGAG 5’ During the process of synthesizing DNA, DNA polymerase reads the original strands and adds complementary base pairs. See below for an example. 5’ ACGGTTCCGAACTC 3’ 3’ TGCCAAGGCTTGAG 5’ 5’ ACGGTTCCGAACTC 3’ 5’ ACGGTTCCGAACTC 3’ This complementary base pairing is important for conserving the base sequence of DNA. It ensures that each new strand is identical to the original.

Slide32:

3.4.3 DNA replication is considered to be semiconservative because each of the parent strands serves as a template and make up half of the resulting copies. New DNA strands are 50% new and 50% old

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