DNA Replication

Packet #17

Chapter #16

Sunday, January 10,

2016

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HISTORICAL FACTS ABOUT DNA

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2016

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Historical DNA Discoveries

• 1928 • Frederick Griffith finds a substance in heat-killed

bacteria that “transforms” living bacteria • 1944

• Oswald Avery, Cloin MacLeod and Maclyn McCarty chemically identify Griffith’s transforming principle as DNA

• 1949 • Erwin Chargaff reports relationships among DNA bases

that provide a clue to the structure of DNA • 1953

• Alfred Hersey and Martha Chase demonstrate that DNA , not protein, is involved in viral reproduction.

• 1953 • Rosalind Franklin produces an x-ray diffraction image

of DNA

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Historical DNA Discoveries II

• 1953 • James Watson and Francis Crick propose a

model of the structure of DNA. • 1958

• Matthew Meselson and Franklin Stahl demonstrate that DNA replication is semi conservative replication

• 1962 • James Watson, Francis Crick and Maurice

Wilkins are awarded the Nobel Prize in Medicine for discoveries about the molecular structure of nucleic acids.

• 1969 • Alfred Hershey is awarded the Nobel Prize in

Medicine for discovering the replication mechanism and genetic structure of viruses

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Griffith Experiment

• The Griffith experiment, conducted in 1928, was one of the first experiments suggesting that bacteria are capable of transferring genetic information through a process known as transformation.

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Hershey Chase Experiment

• Hershey and Chase conduced an experiment using viral DNA to show that the DNA was the genetic material being inserted into the bacteria and used to replicate more viruses.

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STRUCTURE OF DNA

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Introduction I

• DNA is an organic macromolecule known as a nucleic acid.

• Nucleic Acids are composed of building blocks known as nucleotides.

• Nucleotides have three parts: -

• Phosphate

• Sugar

• Nitrogenous bases

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DNA Nucleotides

• Multiple DNA nucleotide subunits link together to form a single DNA strand.

• DNA nucleotides are composed of: - • Phosphate • Sugar

• Deoxyribose • Nitrogenous Bases

• Purines (Two Rings) • Adenine • Guanine

• Pyrimidines (One Ring) • Thymine • Cytosine

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DNA Nucleotides II

• Nucleotides are linked together by covalent phosphodiester bonds

• Each phosphate attaches to the 5’ end (carbon #5) of one deoxyribose and to the 3’ end (carbon #3) of the neighboring deoxyribose

• Makes up the sugar-phosphate backbone

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DNA Strands

• Each DNA strand, that is composed of multiple nucleotides, has a head and a tail.

• Head = 5’ end

• Phosphate group

• Tail = 3’ end

• Hydroxyl group

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DNA Molecule

• Each DNA molecule consists of two DNA strands (polynucleotide chains) that associate as a double helix

• The two strands/chains run antiparallel

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Base-Pairing Rules for DNA Chargaff Rules

• The two DNA strands are joined together at the nitrogenous bases.

• Holding the bases together, and allowing the formation of the double helix, are hydrogen bonds.

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Base-Pairing Rules for DNA Chargaff Rules II

• Adenine forms two hydrogen bonds with thymine

• Guanine forms three hydrogen bonds with cytosine • These pairings are

known as Chargaff’s rules

• A always pairs with T

• G always pairs with C • Complementary base

pairing

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Chargaff Rules III

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MODELS OF DNA REPLICATION

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Models of DNA Replication

• There were three models proposed about how DNA replicates.

• However, the one that stood the test was semi-conservative replication.

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Models of DNA Replication II

• In semi-conservative replication, each “old” strand of DNA is used to create a new complementary strand.

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INTRODUCTION TO DNA REPLICATION

The Players

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Introduction to the Strands

• Template Strands {The Parental Strands} • These are the strands

being copied • The original DNA

strands

• During DNA replication, both strands are copied

• This means that there are TWO template strands

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Introduction to the Strands II

• Complementary Strands {The Daughter Strands} • The NEW DNA strands

produced from the Template Strands

• During DNA replication, there are TWO complementary strands

• Always remember that the process started with TWO template strands

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Origin of Replication & Bi-directionality.

• DNA replication is bidirectional and starts at the origin of replication • The process proceeds in

both directions from that point.

• A eukaryotic chromosome may have multiple origins of replication

• Allows the process to occur faster and more efficient

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Introduction to the Making of the Complementary Strand

• DNA replication/synthesis, of the complementary strands, proceed in a 5’ to 3’ direction.

• Nucleotides can ONLY be added to the 3’ end.

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Introduction to the Making of the Complementary Strand

• Since DNA nucleotides can only be added to the 3’ end, it causes one of the complementary strands to be produced continuously and the other discontinuous • The continuous strand is

called the leading strand • The discontinuous strand is

called the lagging strand

• Is first synthesized as short Okazaki fragments before becoming one strand

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ENZYMES OF DNA REPLICATION & THE STEPS OF DNA REPLICATION Sunday, January 10,

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Enzymes of DNA Replication

• Helicase • Unzips DNA double-helix

• Topoisomerases • Prevents tangling and

knotting of DNA as the while the strands are unzipped.

• RNA primase • Initiates the formation of

“daughter” strands • Forms a segment known as

the RNA primer • The RNA primer contains

the nitrogenous base Uracil

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Enzymes of DNA Replication II

• DNA Polymerase III • Enzyme that catalyzes the

polymerization (making) of nucleotides

• Adds Deoxyribonucleotides (nucleotides only found in DNA, as opposed to RNA) to the 3’ end of a growing nucleotide chain

• Acts at the replication fork

• DNA Polymerase I • A type of DNA polymerase

will change the RNA primers into DNA

• Changing the base Uracil into Thymine

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Enzymes of DNA Replication III

• DNA Ligase

• Enzyme responsible for joining Okazaki fragments forming the Lagging Strand

• Gyrase

• Returns the DNA strands into a Double Helix

• Zips the DNA back together

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DNA Replication—The Big Picture

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DNA Replication—Lagging Strand

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POST DNA REPLICATION

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Check Points…

• Recall, from the cell cycle, that there are check points during the process.

• One of those check points is to check for incorrect base-pair matching.

DNA Excision Repair DNA Polymerase II

• On some occasions, errors in nucleotides may occur while making the new DNA strand. • Errors such as mismatches

& dimers may occur.

• To correct these errors, the enzymes nuclease, DNA polymerase II and DNA ligase are used during the process known as excision repair.

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Telomeres, Telomerase & DNA Shortening

• At the end of eukaryotic chromosomes are known as telomeres • Short, repetitive DNA

sequences that do not contain genes.

• Typically 100 to 1000 nucleotides

• TTAGGG (Humans)

• Telomeres help protect the organism's genes from being eroded through successive rounds of DNA replication.

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Telomeres, Telomerase & DNA Shortening

• Telomeres shorten each cell cycle (DNA replication sequence) but can be extended using the enzyme telomerase • Absence of telomerase in

certain cells may be the cause of “cell aging”

• Cells having a limited number of cell divisions

• Most cancer cells have telomerase to maintain the telomeres and possibly resist apoptosis.

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REVIEW