Molecular Biology

The study of DNA and how it serves as a chemical basis of heredity

DNA and RNA Nucleic acids Polymers of nucleotides called polynucleotides Nucleotides (monomers) are made up of – –Nitrogenous base – Adenine, Thymine (Uracil), Cytosine, Guanine –5-carbon sugar (pentose) –Phosphate group Nucleotides are joined together by covalent bonds between the sugar of one nucleotide and the phosphate of another nucleotide –This forms a sugar-phosphate backbone Nitrogenous bases extend from the sugar-phosphate backbone

Sugar-phosphate backbone DNA nucleotide Phosphate group Nitrogenous base Sugar DNA polynucleotide DNA nucleotide Sugar (deoxyribose) Thymine (T) Nitrogenous base (A, G, C, or T) Phosphate group

Sugars – ▫DNA – deoxyribose ▫RNA – ribose Nitrogen Bases ▫Can be divided into  Purines - larger; double rings  Adenine and Guanine  Pyrimidines – single structure rings  Thymine, cytosine, uracil

Sugar (deoxyribose) Thymine (T) Nitrogenous base (A, G, C, or T) Phosphate group

Sugar (ribose) Uracil (U) Nitrogenous base (A, G, C, or U) Phosphate group

Pyrimidines Guanine (G) Adenine (A) Cytosine (C)Thymine (T) Purines

Ribose Cytosine Uracil Phosphate Guanine Adenine

Structure of DNA James D. Watson and Francis Crick deduced the secondary structure of DNA, with X-ray crystallography data from Rosalind Franklin and Maurice Wilkins DNA is made up of two polynucleotide chains joined together by hydrogen bonding between bases, twisted into a helical shape This is known as a double helix! The sugar-phosphate backbone is on the outside The nitrogenous bases are perpendicular to the backbone in the interior They act like the rungs of the ladder

Twist

Hydrogen bond Base pair Partial chemical structureComputer model Ribbon model The two DNA strands are antiparallel to each other (facing opposite way)

Structure of DNA Chargaff’s rule  That the nitrogenous bases have complimentary bases Specific pairs of bases give the helix a uniform shape –A pairs with T, forming two hydrogen bonds –G pairs with C, forming three hydrogen bonds What would be complementary to CAGGATTCAGTACCG ?

Hydrogen bond Base pair Partial chemical structureComputer model Ribbon model

Process Both the strands of DNA separate from each other and act as templates for new strands of DNA to be made They use the base pairing rule and enzymes to do this. The daughter DNA is complementary to the parent DNA Semiconservative model – each strand of the DNA is made up of an old and new strand of DNA

Meselson and Stahl Did an experiment to prove that replication is semiconservativeexperiment

Parental molecule of DNA

Parental molecule of DNA Nucleotides Both parental strands serve as templates

Parental molecule of DNA Nucleotides Both parental strands serve as templates Two identical daughter molecules of DNA

DNA Replication Happens in both directions at many sites simultaneously on both strands DNA replication begins at the origins of replication –These sites have specific DNA sequences of nucleotides –Proteins attach here and help the DNA unwind at the origin –This produces a “bubble” –Replication proceeds in both directions from the origin –Replication ends when products from the bubbles merge with each other

Origin of replication Parental strand Daughter strand Bubble Two daughter DNA molecules

Each strand of DNA has a 5’ and a 3’ end  The 5’ and 3’ refer to the carbon number on the sugar  The 3’ end has an OH  The 5’ end has a Phosphate The strands are opposite of each other DNA Replication

3 end 5 end 3 end 5 end  P P P P P P P P

DNA replication occurs in the 5’ 3’ direction –This is because DNA polymerase can only add nucleotides on in the 3’ direction because of the OH –The daughter strand can only grow in the 5’ to 3’ direction –Replication is continuous on the parent 3’ 5’ template –Replication is discontinuous on the parent 5’ 3’ template, forming short segments (Okazaki fragments) DNA Replication

Proteins that help with Replication Helicase- an enzyme that helps to unwind the DNA strands. Creates replication fork. SSBs – Keep the two strands of DNA apart RNA Primase – Attaches an RNA primer to the template DNA polymerase – helps to add nucleotides and proofreads DNA as well as removes mismatched pairs RNAase H – Removes the RNA primer Ligase – the glue that connects Okazaki fragments

Parental DNA 3 5 DNA polymerase molecule DNA ligase 3 5 Overall direction of replication Daughter strand synthesized continuously Daughter strand synthesized in pieces DNA Replication Animation