DNA: The Double Helix

1. DNA: The Double Helix Deoxyribonucleic Acid

2. DNA Structure

3. DNA Structure • DNA has a basic structure called a “double helix” • DNA has two strands that form the spiral shape • These strands are made of repeating subunits called nucleotides

4. Nucleotides

5. Nucleotides • Composed of three parts: • Sugar (Deoxyribose) • Phosphate Group • Nitrogenous Base (A, T, C, G)

6. Bonds. Many Bonds.

7. Bonds. Many Bonds. • DNA’s double helix is held together with bonds – think spiral staircase! • Covalent Bonds • Found between the sugar of one nucleotide and the phosphate of another • Form the railings of the staircase • Hydrogen bonds • Found between the bases • Form the steps of the staircase • Each full turn has 10 nucleotide pairs

8. Nitrogenous Bases

9. Nitrogenous Bases • There are four types of bases. • Adenine (A) • Thymine (T) • Guanine (G) • Cytosine (C) • Grouped by rings • Pyrimidines: One ring • C and T • Purines: Two rings • A and G

10. Base-Pairing Rules • American biochemist Erwin Chargaff discovered that the bases always pair off together! • A and T • C and G • This is called “complementary base pairing”

11. DNA and Chromosomes Nucleotides  DNA  Genes  Chromosomes

12. DNA and Chromosomes • The number of chromosomes varies by species from one to many • Eukaryotes have two sets and are diploid • One from mother, one from father • Prokaryotes and sex cells have one set and are are haploid • Matching pairs of maternal and paternal chromosomes are called homologous chromosomes

13. Human Chromosomes

14. Human Chromosomes • Humans have 46 chromosomes • Humans have 23 sets of chromosomes • 22 are homologous pairs • 1 is a set of sex chromosomes • Other living things? • Fruit flies have 4 pairs of chromosomes • Rice plants have 12 pairs of chromosomes • Dogs have 39 pairs of chromosomes

15. DNA Replication • Before a cell divides, the DNA inside must copy itself • This is done through DNA Replication DNA Replication Animation

16. DNA Replication • Step 1: Unwinding • DNA helicase binds to the DNA • Helicase is an enzyme • Helicase begins to move along the DNA and breaks the hydrogen bonds • The best area for this is one where there are a lot of A-T bonds because there are only 2 bonds to break

17. DNA Replication • Step 2: Replication Fork • Helicase causes the replication fork to form where the strands separate • RNA Primase attaches a starter of RNA nucleotides at the initiation point • DNA Polymerase attach to strands to start adding nucleotides • Polymerase is an enzyme

18. DNA Replication • Step 3: Strand Growth Begins • Polymerase adds complementary nucleotides, found floating in the nucleus • Leading Strand (Blue) adds bases continuously • Lagging Strand (Red) adds bases in fragments • Called Okazaki fragments • RNA Primase inserts RNA primer into the gaps to hold everything in place

19. DNA Replication • Step 4: Strand Growth Continues • Strands continue to add more bases • Notice that the leading strand follows the motion of replication fork • The lagging strand adds in the opposite direction

20. DNA Replication • Step 5: Ligase Joins Pieces • Exonuclease strips away the RNA primer and DNA polymerase fills in the gaps • Enzyme called DNA Ligase joins the fragments of DNA together

21. DNA Replication • Step 6: Strands Are Completed • Four single strands of DNA are created, resulting in two new double helixes • Each helix has one new strand and one old strand • Semi-conservative replication