The Structure of DNA All life on earth uses a chemical called DNA to carry its genetic code or blueprint. In this lesson we be examining the structure of this unique molecule. {Point out the alligator’s eyes in the first picture.} By the way, can you make out what this is? *************************************************************** [The goal of this presentation is to introduce high school biology students to the chemical structure of DNA. It is meant to be presented in the classroom while accompanying the teacher’s lecture, under the control of the teacher.]

DNA by the Numbers Each cell has about 2 m of DNA. The average human has 75 trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only 0.000000002 m. The earth is 150 billion m or 93 million miles from the sun. If you unravel all the DNA in the chromosomes of one of your cells, it would stretch out 2 meters. If you did this to the DNA in all your cells, it would stretch from here to sun more than 400 hundred times!

DNA – deoxyribonucleic acid is the nucleic acid that stores and transmits genetic info. from one generation to the next. present in all organisms, but different (unique) in each individual, except for identical twins.

DNA DNA is often called the blueprint of life. In simple terms, DNA contains the instructions for making proteins within the cell. Why is DNA called the blueprint of life?

History of DNA

History of DNA Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA Proteins were composed of 20 different amino acids in long polypeptide chains

Transformation Griffith 1928-worked with virulent S and nonvirulent R strain Pneumoccocus bacteria found that R strain could become virulent (transform) when it took in DNA from heat-killed S strain Avery 1944- suggested that DNA was probably the genetic material that was “transformed”

Griffith Experiment

History of DNA Chromosomes are made of both DNA and protein Hershey & Chase 1952- experiments on bacteriophage viruses proved that DNA was the cell’s genetic material Radioactive 32P was injected into bacteria!

Chargaff’s Rule Adenine pairs with Thymine Guanine pairs with Cytosine The bases form weak hydrogen bonds T A G C

Discovery of DNA Structure Erwin Chargaff 1950 showed the amounts of the four bases on DNA (A,T,C,G) In a body or somatic cell: If… A = 15% then… T = __% so… G = 35% C = __%

DNA Structure Rosalind Franklin & Maurice Wilkins - 1952 took diffraction x-ray photographs of DNA crystals (2 sides, twisted) Watson & Crick -1953 built the first model of DNA using Franklin’s x-rays (double helix)

Rosalind Franklin

Watson and Crick

Helix Most DNA has a right-hand twist with 10 base pairs in a complete turn Left twisted DNA is called Z-DNA or southpaw DNA Hot spots occur where right and left twisted DNA meet producing mutations

Antiparallel Strands One strand of DNA goes from 5’ to 3’ (sugars) The other strand is opposite in direction going 3’ to 5’ (sugars)

DNA Structure

The Shape of the Molecule DNA is a very long double stranded polymer of nucleotides. The basic shape is like a twisted ladder or zipper. This is called a double helix. {Show students a model of the double helix. Explain what a spiral is and a helix is.}

One Strand of DNA Each nucleotide monomer contains 1 phosphate group 1 sugar (deoxyribose) *1 nitrogenous base One strand of DNA has many millions of nucleotides. nucleotide S B P S B P S B {Point to the 3-D mode, if you have one, to show the parts as you discuss them.} P S B P S B

DNA Nucleotide O=P-O O N CH2 O C1 C4 C3 C2 Phosphate Group O Nitrogenous base (A, G, C, or T) O CH2 O C1 C4 C3 C2 5 Sugar (deoxyribose)

DNA P O 1 2 3 4 5 P O 1 2 3 4 5 G C T A

One Strand of DNA The backbone of the molecule is alternating phosphates & deoxyribose sugar (phosphodiester bond) The “rungs/teeth” are nitrogenous bases (4 possible) C, T, A, or G phosphate deoxyribose {Point to the 3-D mode, if you have one, to show the parts as you discuss them.} bases

4 possible Nitrogenous Bases Double ring PURINES Adenine (A) Guanine (G) Single ring PYRIMIDINES Thymine (T) Cytosine (C) A or G T or C

Base-Pairings Purines only pair with Pyrimidines Hydrogen bonds required to bond Guanine & Cytosine C G 3 H-bonds

Hydrogen bonds are required to bond Adenine & Thymine 2 H - bonds

DNA Double Helix “Rungs of ladder” Nitrogenous Base (A,T,G or C) “Legs of ladder” Phosphate & Sugar Backbone

Two Stranded DNA DNA has two strands that fit together something like a ladder or zipper. The rungs or teeth are the nitrogenous bases but why do they stick together? {Point to the 3-D model to show the parts as you discuss them.}

DNA Two strands coiled called a double helix Sides made of a pentose sugar Deoxyribose bonded to phosphate (PO4) groups Middle made of nitrogen bases bonded together by weak hydrogen bonds

Hydrogen Bonds The bases attract each other because of hydrogen bonds. Hydrogen bonds are weak but there are millions of them in a single molecule of DNA. Cytosine always pairs with Guanine C N H G

Hydrogen Bonds, cont. When making hydrogen bonds, Cytosine always pairs with Guanine Adenine with Thymine A C N T H O

Remember the Strands are Antiparallel O 1 2 3 4 5 P O 1 2 3 4 5 G C T A

The nitrogen bases of each strand pair with the bases on the complementary strand The order of the bases makes up the genetic code. A T C G

Question: DNA –C G T A T G- What would be the complementary DNA strand for the following DNA sequence? DNA –C G T A T G-

Answer: DNA –C G T A T G- comp DNA –G C A T A C-

Question: If there is 30% Adenine, how much Cytosine is present?

Answer: If 30% Adenine then 30% Thymine If 60% A-T; then 40% C-G Therefore,40% C-G would be 20% Guanine = __% Cytosine

DNA Replication

DNA REPLICATION

DNA Replication When a cell divides, DNA preserves individuality by passing exact copies of itself to the new cell

Replication Facts DNA has to be copied before a cell divides DNA is copied during the S or synthesis phase of interphase New cells will need identical DNA strands

Synthesis Phase (S phase) S phase during interphase of the cell cycle Nucleus of eukaryotes Mitosis -prophase -metaphase -anaphase -telophase G1 G2 S phase interphase DNA replication takes place in the S phase.

5’ to 3’ Sugars . When the DNA double helix unwinds, it resembles a ladder The sides of the ladder are the sugar-phosphate backbones The rungs of the ladder are the complementary paired bases The two DNA strands are anti-parallel (they run in opposite directions)

Steps in DNA Replication Occurs when chromosomes duplicate (make copies) Enzyme DNA Helicase unwinds & separates the 2 DNA strands by breaking the weak hydrogen bonds as enzymes “unzip” the molecule Each old strand of nucleotides serves as a template for each new strand New nucleotides move into complementary positions are joined by DNA polymerase DNA polymerase is an enzyme.

Anti-Parallel Strands of DNA On the left is the DNA double helix. When the helix is unwound, a ladder configuration shows that the uprights are composed of sugar and phosphate molecules and the rungs are complementary bases. Notice that the bases in DNA pair in such a way that the phosphate-sugar groups are oriented in different directions. This means that the strands of DNA end up running antiparallel to one another, with the 3’ end of one strand opposite the 5’ end of the other strand.

DNA Replication Begins at Origins of Replication One strand serves as a mold for another strand to be copied Two strands open forming Replication Forks (Y-shaped region) New strands grow at the forks Replication Fork Parental DNA Molecule 3’ 5’

Two New, Identical DNA Strands Result from Replication Replication is called semiconservative because each new double helix is composed of an old (parental) strand and a new (daughter) strand.

Another View of Replication Use of the ladder configuration better illustrates how complementary nucleotides available in the cell pair with those of each old strand before they are joined together to form a daughter strand.

DNA Replication Begins at Origins of Replication One strand serves as a mold for another strand to be copied Two strands open forming Replication Forks (Y-shaped region) New strands grow at the forks Replication Fork Parental DNA Molecule 3’ 5’

DNA Replication 1.Enzyme DNA Helicase unwinds & separates the 2 DNA strands by breaking the weak hydrogen bonds to unzip the chain Single-Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted 2. Free nucleotides match up & form H bonds to complete complementary base strand

3. Base pairs bond, DNA polymerase links phosphate of one nucleotide to the sugar of another. 4. Pairing continues until each “original” DNA strand has a complete matching strand (result 2 identical DNA strands) T A G C A T A T C G T A IF… T A G C A T and A T C G T A T A G C A T A T C G T A THEN…

Replication of Strands Replication Fork Point of Origin

Semiconservative Model of Replication Idea presented by Watson & Crick The two strands of the parental molecule separate, and each acts as a template for a new complementary strand New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA DNA Template New DNA Parental DNA

DNA Replication DNA polymerase can then add the new nucleotides Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides Primase is the enzyme that synthesizes the RNA Primer DNA polymerase can then add the new nucleotides

Direction of Replication DNA Replication DNA polymerase can only add nucleotides to the 3’ end of the DNA This causes the NEW strand to be built in a 5’ to 3’ direction RNA Primer DNA Polymerase Nucleotide 5’ 3’ Direction of Replication

Synthesis of the New DNA Strands The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork RNA Primer DNA Polymerase Nucleotides 3’ 5’

Synthesis of the New DNA Strands The Lagging Strand is synthesized discontinuously against overall direction of replication This strand is made in MANY short segments It is replicated from the replication fork toward the origin RNA Primer Leading Strand DNA Polymerase 5’ 3’ Lagging Strand 5’ 3’

Remember HOW the Carbons Are Numbered! O=P-O Phosphate Group N Nitrogenous base (A, G, C, or T) CH2 O C1 C4 C3 C2 5 Sugar (deoxyribose)

Lagging Strand Segments Okazaki Fragments - series of short segments on the lagging strand Must be joined together by an enzyme Lagging Strand RNA Primer DNA Polymerase 3’ 5’ Okazaki Fragment

Joining of Okazaki Fragments The enzyme Ligase joins the Okazaki fragments together to make one strand Lagging Strand Okazaki Fragment 2 DNA ligase Okazaki Fragment 1 5’ 3’

Proofreading New DNA DNA polymerase initially makes about 1 in 10,000 base pairing errors Enzymes (helicase) proofread and correct these mistakes The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors

DNA Damage & Repair Chemicals & ultraviolet radiation damage the DNA in our body cells Cells must continuously repair DAMAGED DNA Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA DNA polymerase and DNA ligase replace and bond the new nucleotides together

DNA makes proteins that are needed for growth, repair and all life functions Ex: collagen - cartilage and tendons hemoglobin – blood carries oxygen through the body keratin - hair and fingernails insulin – metabolizes blood sugars …muscles, skin, etc…

DNA  RNA  Protein Prokaryotic Cell DNA mRNA Ribosome Protein Transcription Translation DNA mRNA Ribosome Protein Prokaryotic Cell

Remember the Strands are Antiparallel O 1 2 3 4 5 P O 1 2 3 4 5 G C T A

DNA Stands for Deoxyribonucleic acid Made up of subunits called nucleotides Nucleotide made of: 1. Phosphate group 2. 5-carbon sugar 3. Nitrogenous base

Question: DNA 5’-CGTATG-3’ What would be the complementary DNA strand for the following DNA sequence? DNA 5’-CGTATG-3’

Answer: DNA 5’-CGTATG-3’ DNA 3’-GCATAC-5’