Understanding Chromosomes: Structure and Function of DNA Molecules

1. Chapter 6 DNA Structure and Function

2. 6.1 A Hero Dog’s Golden Clones • Trakr, the search dog that located the final survivor of the 9/11 attacks, later died of toxic exposure • Trakr’s DNA lives on in his genetic copies (clones) • To make a clone from an adult cell, researchers must reprogram its DNA to function like the DNA of an egg • Cloning mammals is unpredictable; few implanted embryos result in a live birth, and many have serious health problems

3. James Symington and Trakr at Ground Zero, September 2001

4. 6.2 Chromosomes • Each DNA molecule consists of two strands twisted into a double helix • In cells, DNA molecules and their associated proteins are organized into chromosomes • DNA wraps around “spools” of proteins called histones that allow chromosomes to pack tightly

5. Chromosome Structure • When the cell prepares to divide, it duplicates all of its chromosomes • Sister chromatid • One of two attached members of a duplicated eukaryotic chromosome • Centromere • Constricted region in a eukaryotic chromosome where sister chromatids are attached

6. A Duplicated Chromosome centromere one chromatid its sister chromatid a chromosome (unduplicated) a chromosome (duplicated)

7. Chromosome Structure A) Proteins structurally organize DNA into chromosomes.

8. Chromosome and Uncondensed DNA C) DNA, the substance, extracted from human cells. B) A human chromosome.

9. 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 46 chromosomes • Diploid • Cells having two of each type of chromosome characteristic of the species (2n)

10. Two Types of Eukaryotic 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 • In humans, XY and XX

11. 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

12. Human Female Karyotype A) Karyotype of a female human, with identical sex chromosomes (XX).

13. Bird Female Karyotype B) Karyotype of a female chicken, with nonidentical sex chromosomes (ZW).

14. ANIMATED FIGURE: Chromosome structural organization To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

15. ANIMATION: Karyotype preparation

16. 6.3 Fame, Glory, and DNA Structure • A DNA molecule consists of two strands of nucleotide monomers running in opposite directions and coiled into a double helix • DNA nucleotide • One five-carbon sugar (deoxyribose) • Three phosphate groups • One nitrogen-containing base (adenine, thymine, guanine, or cytosine)

17. Four nucleotides in DNA base three phosphate groups adenine (A) deoxyadenosine triphosphate

18. Four nucleotides in DNA thymine (T) deoxythymidine triphosphate

19. Four nucleotides in DNA cytosine (C) deoxycytidine triphosphate

20. Four nucleotides in DNA guanine (G) deoxyguanosine triphosphate

21. Discovering DNA Structure • Erwin Chargaff • Discovered the relationships between DNA bases • Rosalind Franklin • Discovered structure of DNA by x-ray crystallography • Maurice Wilkins • Experimental evidence of DNA structure • James Watson and Francis Crick • Built the first accurate model of a DNA molecule

22. Chargaff’s Rules • 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

23. Rosalind Franklin’s x-ray diffraction image of DNA

24. Watson, Crick, and Wilkins B) Watson (left) and Crick (right) with their model of DNA. C) Maurice Wilkins.

25. The Double Helix • Watson and Crick proposed that DNA consists of two strands of nucleotides, running in opposite directions, coiled into a double helix • Hydrogen bonds between bases hold the two strands together • Only two kinds of base pairings form: A to T, and G to C (Chargaff’s rule)

26. DNA Structure • Covalent bonds between sugar and phosphate form the backbone of each chain sugar–phosphate backbone hydrogen bonds link internally positioned nucleotide bases A) Structure of DNA, as illustrated by a composite of three different models. The two sugar–phosphate backbones coil in a helix around internally positioned bases.

27. DNA’s Base-Pair Sequence • The order of nucleotide bases in a DNA strand (DNA sequence) is genetic information • The order of bases varies among species and among individuals • The two strands of a DNA molecule are complementary

28. Complementary Base Pairs

29. ANIMATED FIGURE: DNA close up To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

30. ANIMATED FIGURE: Subunits of DNA To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

31. 6.4 DNA Replication and Repair • DNA replication • Process by which a cell copies its DNA before it divides • Each strand of the double helix serves as a template for synthesis of a new, complementary strand of DNA • Results in two double-stranded DNA molecules identical to the parent • Semiconservative replication • One strand of each molecule is parental (old), and the other is new

32. 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 • Each type of DNA polymerase requires a primer in order to initiate DNA synthesis • DNA ligase seals any gaps to form a continuous strand

33. DNA Replication and Repair • DNA polymerase • DNA replication enzyme; assembles a new strand of DNA based on sequence of a DNA template • Primer • Short, single strand of DNA that base-pairs with a targeted DNA sequence • DNA ligase • Enzyme that seals breaks in double-stranded DNA

34. DNA Replication enzymes primer DNA polymerase DNA ligase

35. How Mutations Arise • Proofreading by DNA polymerase corrects most base-pairing errors during DNA replication • Uncorrected errors in DNA replication may become mutations • Mutation • A permanent change in DNA sequence

36. How Mutations Arise • Ionizing radiation (x-rays, most UV light, and gamma rays) can knock electrons out of atoms, breaking DNA • Non ionizing radiation (UV light 320–380 nm) forms nucleotide dimers that kink DNA and increase mutation rate • Chemicals in tobacco smoke transfer methyl groups (CH3) to nucleotide bases in DNA, causing mispairs during replication • Breakdown products of many environmental pollutants bind irreversibly to DNA, causing replication errors

37. Effects of Ionizing Radiation A) Major breaks (red arrows) in chromosomes of a human white blood cell after exposure to ionizing radiation. Pieces of broken chromosomes often become lost during DNA replication.

38. Effects of Thymine Dimers thymine dimer B) Thymine dimer. This type of DNA damage is caused by exposure to UV light between 230 and 380 nanometers in wavelength.

39. ANIMATED FIGURE: DNA replication details To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

40. ANIMATED FIGURE: Duplication To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

41. ANIMATED FIGURE: Translocation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

42. 6.5 Cloning Adult Animals • Reproductive cloning • Technology that produces an exact genetic copy of an individual (clone) • Somatic cell nuclear transfer (SCNT) • Nuclear DNA from an adult somatic cell is transferred into an unfertilized, enucleated egg • Common in livestock breeding

43. Somatic Cell Nuclear Transfer A) A cow egg is held in place by suction through a hollow glass tube called a micropipette. DNA is identified by a purple stain.

44. Somatic Cell Nuclear Transfer B) Another micropipette punctures the egg and sucks out the DNA. All that remains inside the egg’s plasma membrane is cytoplasm.

45. Somatic Cell Nuclear Transfer C) A new micropipette prepares to enter the egg at the puncture site. The pipette contains a cell grown from the skin of a donor animal.

46. Somatic Cell Nuclear Transfer D) The micropipette enters the egg and delivers the skin cell to a region between the cytoplasm and the plasma membrane.

47. Somatic Cell Nuclear Transfer E) After the pipette is withdrawn, the donor’s skin cell is visible next to the cytoplasm of the egg. The transfer is now complete.

48. Somatic Cell Nuclear Transfer F) An electric current causes the foreign cell to fuse with and empty its nucleus into the cytoplasm of the egg. The egg begins to divide, and an embryo forms.

49. Clone produced by SCNT A) Liz the championship Holstein cow (right) with her clone.

50. ANIMATED FIGURE: How Dolly was created To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE