DNA Structure and Function

1. DNA Structure and Function Chapter 6

2. 1.1 Impacts/IssuesHere Kitty, Kitty, Kitty, Kitty, Kitty • Making clones (exact genetic copies) of adult animals is now a common practice in research and animal husbandry

3. 6.2 Chromosomes • A eukaryotic chromosome is a molecule of DNA together with associated proteins • Chromosome • Structure made of DNA and associated proteins • Carries part or all of a cell’s genetic information

4. Chromosome Structure • Sister chromatid • One of two attached members of a duplicated eukaryotic chromosome • Centromere • Constricted region in a eukaryotic chromosome where sister chromatids are attached

5. Chromosome Structure • Proteins organize DNA structurally • Allow chromosomes to pack tightly • Histone • Type of protein that structurally organizes eukaryotic chromosomes • Nucleosome • A length of DNA wound around a spool of histone proteins

6. Chromosome Structure

7. Chromosome Number • A eukaryotic cell’s DNA is divided into a characteristic number of chromosomes • Chromosome number • Sum of all chromosomes in a cell of a given type • A human body cell has 23 pairs of chromosomes • Diploid • Cells having two of each type of chromosome characteristic of the species (2n)

8. Examples of Chromosome Number

9. Types of Chromosomes • There are two types of eukaryotic chromosomes: autosomes and sex chromosomes • Autosomes • Paired chromosomes with the same length, shape, centromere location, and genes • Any chromosome other than a sex chromosome • Sex chromosomes • Members of a pair of chromosomes that differ between males and females

10. Sex Chromosomes: Sex Determination in Humans

11. diploid reproductive cell in female diploid reproductive cell in male X Y X X eggs sperm X X X Y X Y X XX XY XX XY X union of sperm and egg at fertilization Stepped Art Fig. 6-3a, p. 104

12. Karyotype • Karyotyping reveals characteristics of an individual’s chromosomes • Karyotype • Image of an individual’s complement of chromosomes arranged by size, length, shape, and centromere location

13. Constructing a Karyotype

14. 6.3 Fame and Glory • Erwin Chargaff • Discovered the relationships between DNA bases • Rosalind Franklin • Discovered the basic structure of DNA by x-ray crystallography • James Watson and Francis Crick • Built the first accurate model of a DNA molecule

15. Key Players • Rosalind Franklin, Maurice Wilkins, James Watson, and Francis Crick

16. The Double Helix • A DNA molecule consists of two strands of nucleotide monomers running in opposite directions and coiled into a double helix • DNA nucleotide • A five-carbon sugar (deoxyribose) • Three phosphate groups • One nitrogen-containing base (adenine, thymine, guanine, or cytosine)

17. The Double Helix • Two double-helix strands are held together by hydrogen bonds between nucleotide bases • Chargaff’s rules • Bases of the two DNA strands in a double helix pair in a consistent way: A = T and C = G • Proportions of A and G vary among species

18. The Four DNA Nucleotides

19. DNA Structure

20. Patterns of Base Pairing • The order of bases (DNA sequence) varies among species and among individuals • Each species has characteristic DNA sequences • DNA sequence • The order of nucleotide bases in a strand of DNA

21. 6.4 DNA Replication and Repair • A cell replicates its DNA before it divides • Each strand of the double helix serves as a template for synthesis of a new, complementary strand of DNA • DNA replication results in two double-stranded DNA molecules identical to the parent

22. DNA Replication and Repair • During DNA replication, the double-helix unwinds • DNA polymerase uses each strand as a template to assemble new, complementary strands of DNA from free nucleotides • DNA ligase seals any gaps to form a continuous strand

23. DNA Replication and Repair • DNA replication • Duplication of a cell’s DNA before cell division • DNA polymerase • DNA replication enzyme; assembles a new strand of DNA based on sequence of a DNA template • DNA ligase • Enzyme that seals breaks in double-stranded DNA

24. DNA Replication

25. 1) The two strands of a DNA molecule are complementary: their nucleotides match up according to base-pairing rules (G to C, T to A). 2) As replication starts, the two strands of DNA unwind at many sites along the length of the molecule. 3) Each parent strand serves as a template for assembly of a new DNA strand from nucleotides, according to base-pairing rules. 4) DNA ligase seals any gaps that remain between bases of the “new” DNA, so a continuous strand forms. The base sequence of each half-old, half-new DNA molecule is identical to that of the parent. Stepped Art Fig. 6-8, p. 108

26. Animation: DNA replication

27. DNA Replication: The Double Helix

28. Checking for Mistakes • DNA repair mechanisms fix damaged DNA • Proofreading by DNA polymerase corrects most base-pairing errors • DNA repair mechanisms • Any of several processes by which enzymes repair DNA damage

29. Mutations • Uncorrected errors in DNA replication may become mutations • Mutation • A permanent change in DNA sequence

30. 6.5 Cloning Adult Animals • Reproductive cloning technologies produce an exact genetic copy of an individual (clone) • Reproductive cloning • Technology that produces genetically identical individuals

31. Somatic Cell Nuclear Transfer • Somatic cell nuclear transfer (SCNT) • Method of reproductive cloning in which nuclear DNA from an adult somatic cell is transferred into an unfertilized, enucleated egg • Therapeutic cloning • Using SCNT to produce human embryos for research

32. Somatic Cell Nuclear Transfer

33. Clones • Clone produced by somatic cell nuclear transfer

34. Digging into Data:The Hershey Chase Experiments

35. Fig. 6-12a, p. 113

36. 35S remains outside cells Virus proteins labeled with 35S DNA being injected into bacterium Virus DNA labeled with 32P 32P remains inside cells Labeled DNA being injected into bacterium Fig. 6-12b-c, p. 113