Lesson Overview

1. Lesson Overview 12.2 The Structure of DNA http://www.brainpop.com/health/geneticsgrowthanddevelopment/dna/ http://www.youtube.com/watch?v=zwibgNGe4aY

2. What is DNA? Chromosomes and their genes are made of a molecule called DNA. DNA stands for deoxyribonucleic acid. Each chromosome is a very long molecule of tightly coiled DNA. DNA molecules carry the code that controls what cells are made of and what they do. Which part of a DNA molecule holds this information?

3. Nucleic Acids and Nucleotides • DNA is made up NUCLEOTIDES • 3 basic components: • 1. 5-carbon sugar called deoxyribose • 2. a phosphate group • 3. a nitrogenous base

4. Nitrogenous Bases and Covalent Bonds • Covalent bonds between their sugar and phosphate groups • Hydrogen bonds between base pairs (weaker)

5. A C G T DNA and base pairs The double helix ‘ladder’ of a DNA molecule is held together by ‘rungs’ made from pairs of chemicals called bases. There are four types of bases, and they are usually identified by their initials. adenine cytosine guanine thymine How do you think the four bases are paired?

6. A T C G How do bases pair together? Base pairs hold the two strands of the DNA helix together. The rules for base pairing are… ‘A’always pairs with ‘T’ ‘C’alwayspairs with ‘G’ There are millions of base pairs in a DNA molecule, and they always follow these rules. It is the sequence of these bases along a DNA molecule that forms the genetic code – it’s that simple!

7. Nitrogenous Bases and Covalent Bonds • DNA has four kinds of nitrogenous bases: • 1. adenine (A) – 2 rings, PAIRS with T • 2. guanine (G) – 1 ring, PAIRS with C • 3. cytosine (C) – 1 ring, PAIRS with G • 4. thymine (T) – 2 rings, PAIRS with A

8. Chargaff’s Rules • Chargaff discovered that the % of adenine [A] and thymine [T] bases are almost equal in any sample of DNA • The same thing is true for the other two nucleotides, guanine [G] and cytosine [C] • The observation that [A] = [T] and [G] = [C] became known as one of “Chargaff’s rules, ” or base-pairing rules

9. The Work of Watson and Crick (and Franklin) • In the 1950s, British scientist Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule. • Watson and Crick’s breakthrough model of DNA was a double helix, in which two strands were wound around each other

10. The Work of Watson and Crick • Watson and Crick’s breakthrough model of DNA was a double helix, in which two strands were wound around each other

11. Base Pairing

12. Lesson Overview 12.3 DNA Replication

13. The Replication Process • During replication, the DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing. • Each strand of the double helix of DNA serves as a template, or model, for the new strand.

14. The Replication Process • As each new strand forms, new bases are added • If the base on the old strand is A, then T is added to the new strand • Also, G is always paired to C

15. The Replication Process • Each DNA molecule resulting from replication has one original strand and one new strand

16. The Role of Enzymes • HELICASE – an enzyme that “unzips” a molecule of DNA by breaking the hydrogen bonds between base pairs and unwinding the two strands

17. The Role of Enzymes • DNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNA. • DNA polymerase also “proofreads” each new DNA strand, ensuring that each molecule is a perfect copy of the original

18. Replication in Living Cells • How does DNA replication differ in prokaryotic cells and eukaryotic cells? • Replication in most prokaryotic cells starts from a single point and proceeds in two directions until the entire chromosome is copied. (DNA is circular) • In eukaryotic cells, replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied