The Human Genome and Chromosomal Basis of Heredity and Chromosomal Disorders

1. The Human Genome and Chromosomal Basis of Heredity and Chromosomal Disorders Chromosomes were found to be the bearer of genetic factor Ömer Faruk Bayrak

2. WHAT IS GENE? 2005 2003 DNA Double Helix, Watson & Crick Nature, 1953 Human genome Project Inactivation of different X genes

3. The physical and functional unit of heredity that carries information from one generation to the next • DNA sequence necessary for the synthesis of a functional protein or RNA molecule.

4. GENE • Gene were first detected and analyzed by Mendel and subsequently by many other scientist (Mendel stated that physical traits are inherited as “particles”) • Mendel did not know that the “particles” were actually Chromosomes & DNA • Subsequent studies shows the correlation betweentransmission of genes from one generation to generation (Segregation and independent assortment) and the behavior of chromosomes during sexual reproduction, specifically the reduction division of meiosis and fertilization. • These and related expt. provided a strong early evidence that genes are usually located on chromosomes.

5. What are the requirements to fulfill as a genetic material? • 1. The genotype function or replication: • The genetic material must be capable of storing genetic information and transmitting this information faithfully from parents to progeny, generation after generation. • 2. The phenotype function or gene expression • The genetic material must control the development of phenotype of the organism, be it a virus, a bacterium, a plant or animal. • That is, the genetic material must dictate the growth and differentiation of the organism from single celled zygote to the mature adult.

7. DNA STRUCTURE Nucleic acids first called “nuclein” because they were isolated from cell nuclei by F. Miescher in 1869 • Each nucleotide is composed of (1) a Phosphate group (2) a five – carbon sugar (or Pentose), and (3) a cyclic nitrogen containing compound called a base.

8. In DNA, the sugar is 2-deoxyribose (thus the name deoxyribonucleic acid) In RNA, the sugar is ribose (thus ribonucleic acid).

9. Adenine and Guanine are double ring base called Purines 6-aminopurine 2-amino-6-oxypurine Cytosine, thymine, and uracil are single-ring base called Pyrimidines. 4-amino-2-oxypyrimidine 2,4-oxypyrimidine 2,4-oxy-5-pyrimidine

10. 16.6 Base pairing in DNA

12. Chargaff’s rule • The composition of DNA from many different organisms was analyzed by E.Chargaffand his colleagues. • It was observed that concentration of thyminewas always equal to the concentration of adenine (A = T) • And the concentration of cytosine was equalto the concentration of guanine(G = C). • This strongly suggest that thymine and adenine as well as cytosine and guanine were present in DNA with fixed interrelationship. • Also the total concentration of purines(A +G) always equal to the total concentration ofpyrimidine(T +C). However, the (T+ A)/ (G+C) ratio was found to vary widely in DNAs of different species.

13. The earth is 150 billion m or 93 million miles from the sun. Did you know? • Each cell has about 2 m of DNA. • The average human has 75 trillion cells. • The average human has enough DNA to go from the earth to the sun more than 400 times. • DNA has a diameter of only 0.000000002 m.

14. DNA replication After publishing their model, W&C made a hypothesis for the replication of DNA.  a.  Hydrogen bonds break, and the two strands separate. b.  Each strand now serves as a template for a new complimentary strand. c.  Nucleotides are connected and the daughter DNA molecules are formed.

19. 16.8 Three alternative models of DNA replication

20. 16.13 Synthesis of leading and lagging strands during DNA replication

21. - Once hydrogen bonds begin to break, replication bubbles begin to form at points along the DNA strand. - Bubbles form at sites called origins of replication.  - DNA replication proceeds in both directions from the origin of replication.

23. Human Chromosomes • Humans have 46 chromosomes organized as 23 pairs that are homologous because each pair contains the homologous genes • Humans are genetically diploid= 2 copies of each chromosome, except for the sex chromosomes (X+Y) that are non-identical • Each species has a characteristic set of chromosomes.

24. Eukaryotic Chromosome Structure • Genetic material in eukaryotes is organized to form linear chromosomes (one chromosome = one molecule) • Pulsed-field gel electrophoresis is used to separate individual chromosomes that migrate as distinct bands on a gel (visible evidence for chromosomes)

25. Chromatin Fiber Organization • Dark field electron microscopy shows fiber structure of chromosomes as beads on a string • Nucleosome= the fundamental unit of organization of the chromatin fiber • Each nucleosome contains a core particleof basic proteins = histones surrounded by 1.75 turns of DNA helix = 145 bp of DNA

26. Chromatin Fiber Structure • Core particle histone octamer contains two molecules each of: - histone H2A - histone H2B - histone H3 - histone H4 • Linker region connecting nucleosomes contains histone H1

27. Chromatin Fiber Structure • Primary Structure of DNA = double helix = 2nm duplex DNA • Duplex DNA winds around histone octamers to form nucleosomes= 11nmhistone fiber • Nucleosome fibers form left-handed helix with 6 nucleosomes per turn = 30 nm chromatin fiber (solenoid structure)

28. Organization of Nucleosomes The DNA molecule is wound one and three fourths turns around a histone octamer.

29. Various Stages of Chromosome Condensation Solenoidal Model of Chromatin

30. Chromosome Structure • 30 nm chromatin fiber condenses to metaphase chromatid = 1400 nm • Nonhistone protein complexes =scaffold: Required for the attachment of loops of chromatin fibers (confirmed by DNase digestion)

31. Chromosome Structure • Euchromatin= comprises most of the genome, transcriptionally active parts • Heterochromatin= highly condensed inactive chromatin located at centromeres and telomeres • Centromere= attachment point for sister chromatids and spindle fibers • Telomere= end of chromosome

32. Schematic Drawing of Metaphase Chromosome

33. Centromeres(Essential for chromosome segregation) • Centromeres = chromosome regions that contain the site of attachment for microtubules = kinetochore • Centromeres contain heterochromatin, condensed chromatin • In situ hybridization of metaphase chromosomes shows satellite DNA at centromeres

34. Telomeres(Essential for the stability of the chromosomal tip) • Telomeres are specialized regions of DNA at the ends of chromosomes • Telomeres contain short tandem DNA repeats that are added to ends by the enzyme = telomerase • Telomerase contains RNA primer complementary to telomere repeat

35. Function of Telomere Repeat and Telomerase

36. Sex Chromosomes • X and Y chromosomes = sex chromsomes which are non-identical but share some genes for pairing • Males are genetically haploid for most genes on the X chromosome which results in unique patterns of X-linked inheritance • Autosomes = non-sex chromosomes

37. Cell Division – Chromosome Division: Cell Cycle (Mammalian) • Cell division cycles occur in stages: - G1 = pre-DNA synthesis - S = DNA synthesis - G2 = post-DNA synthesis - M = mitosis: cell division occurs by precise steps which distribute one set of chromosomes to each of two daughter cells • Cell cycle takes about 18-24 hours in higher eukaryotes. • Mitosis takes about 1-2 hours. Mitosis: Meiosis: Interphase

38. The Cell Cycle of a Typical Mammalian Cell

39. Mitosis • Chromosome replication: exact duplicates are made during the S period = sister chromatids formed (interphase). -Stages of Mitosis- • Prophase- individual chromosomes become visible, spindle fibers organize and attach to centromeres of chromosomes • Metaphase- chromosomes line up in center of cell: alignment of chromosomes along the metaphase plate is a checkpoint to proceed to the next phase.

40. Anaphase- sister chromatids separate after centromere division: one member of each pair is pulled to either pole of the cell • Telophase- nuclei of two new cells reorganize; the cells are diploid = each contains both members of every pair of chromosomes *Chromosomes decondense until they are no longer visible. *Cytokinesis follows.

41. Mitosis Diploidity is maintained after mitosis.

42. Meiosis • Meiosisis a specialized type of cell division that occurs only in reproductive cells (e.g. eggs or sperms) • Two rounds of cell division result in the formation of gametes that are genetically haploid = contain only one copy of each pair of homologous chromosomes

43. Meiosis <Simplified overview of meiosis> *The behavior of a single pair of homologous chromosomes. *Each chromosomes already consists of two chromatids, joined at a single centromere.

44. Meiosis • Meiosis occurs in stages and requires two cell division events • Meiosis I: - Chromosomes duplicate in S phase - Homologous chromosomes pair: 4 strands of chromatids align - Homologous chromosomes are pulled to either pole of the cell at anaphase • Meiosis II: - Cell division occurs in the absence of chromosome duplication - Sister chromatids separate at anaphase as in mitotic division

45. Major Stages of Meiosis with Two Pairs of Homologous Chromosomes

46. Crossing-over (Chiasmasis) between Homologous Chromosomes * No cross-over between sister chromatids. * Random genotype formation in a gamete

47. Meiotic vs. Mitotic Division • Meiosisproduces four cells, each of which contains one copy of each pair of homologous chromosomes = genetically haploid (n) • Mitosis produces two cells that contain both members of each pair of homologous chromosomes = genetically diploid (2n)

48. Fig. 3.5

50. Human Karyotype