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Chromosomes and Cell Cycle

Chromosomes and Cell Cycle. All genetic material of a cell is called the genome. Genome is composed of DNA Long molecules of DNA organized for cell division are called chromosomes Each chromosome contains thousands of genes. Chromosomes contain genes. Chromosomes stained orange.

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Chromosomes and Cell Cycle

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  1. Chromosomes and Cell Cycle

  2. All genetic material of a cell is called the genome • Genome is composed of DNA • Long molecules of DNA organized for cell division are called chromosomes • Each chromosome contains thousands of genes

  3. Chromosomes contain genes

  4. Chromosomes stained orange

  5. DNA material in a cell • Found to be loosely coiled in the cell and is called chromatin • For cellular division the DNA is condensed to form chromosomes

  6. DNA condensation generates chromosomes

  7. DNA is packaged into chromosomes DNA wraps around a histone core forming nucleosomes. Multiple nucleosomes are organized into solenoids solenoid

  8. Condensed further to form chromosomes

  9. What happens to a chromosome if you remove the scaffolding proteins? • Histones have been removed from this metaphase chromosome • Scaffolding proteins retain shape of the chromosome

  10. Do all organisms have DNA? • Yes!

  11. Do all organisms have • The same amount of DNA? • The same number of chromosomes?

  12. The number of chromosomes varies with each organism

  13. Humans have 23 pairs of chromosomes • 22 pairs of autosomes • 1 pair of sex chromosomes

  14. Chromosome analysis - Giemsa staining • Giemsa stain is used to visualize chromosomes • There are 22 pairs of autosomes and 1 pair of sex chromosomes

  15. Chromosome analysis - Fluorescence in-situ hybridization (FISH) A technique to visualize specific regions of chromosomes using fluorescent DNA probes.

  16. Chromosome analysis - Chromosome Paints • DNA is labeled with different fluorescent dyes • Dyes “paint’ specific regions

  17. Remember that chromosomes represent duplicated DNA • These structures are formed to facilitate the proper partitioning of DNA during cell division • The structure represents twice as much DNA, since DNA replication has occurred

  18. q (long arm) centromere p (short arm) Chromosome Structure

  19. Chromosome split the chromatids during cell division

  20. All cells need to divide to pass on their genetic information by cell division

  21. Cell Cycle reflects the life of a cell

  22. Cell Cycle • G1 phase • First growth phase, cells are making proteins • S phase • DNA replication occurs • G2 phase • Growth phase, checking of DNA • M phase • Cell division

  23. Mitosis

  24. Mitosis

  25. Mitosis

  26. How does cancer relate to the cell cycle? • There are checkpoints in the cell cycle that regulate each stage • G1 • G2 • M • Cancer cells have lost the control of cell division

  27. Checkpoints in the cell cycle

  28. Do all human cells divide by mitosis? • Remember that mitosis provides a new cells with a copy of the EXACT same chromosomes, 23 pairs • What about the sex cells? How many chromosomes are present in them?

  29. Meiosis is a special type of cellular division • Sex cells have ONE set of 23 chromosomes • Sex cells are considered haploid • When two haploid cells fuse they form a diploid cell (one that has 23 PAIRS of chromosomes)

  30. How do we end up with half the number of chromosomes in meiosis? • Need TWO rounds of division • Meiosis I • Meiosis II • Final result is 4 daughter cells with haploid chromosomes

  31. Meiosis Overview

  32. Meiosis I

  33. Meiosis II

  34. Crossing Over in Meiosis

  35. Summary

  36. Comparison of mitosis and meiosis

  37. Key points to notice • Only in meiosis I does synapsis occur which allows for recombination or crossing over ofchromosomes • Does this process occur in mitosis?

  38. Nondisjunction occurs when chromosomes fail to separate • Can occur either during mitosis or meiosis • What happens if it occurs in Meiosis I? • What happens if it occurs in Meiosis II?

  39. Trisomy 21 • Down syndrome • Most common chromosome number abnormality • Occurs at a rate of 1/700 births

  40. Incidence of Down syndrome increases with age of mother

  41. Human Sex Determination Textbook Story: XX = Female XY = Male With a few exceptions….

  42. Aneuploidy can also occur with Sex Chromosomes XXY= Klinefelter Syndrome- phenotypically male with normal intelligence, sterile XO = Turner syndrome- phenotypically female, sterile, usually normal intelligence XYY = no general traits (tall) XXX = healthy, “normal”

  43. The SRY Gene

  44. Down Syndrome may be inherited • Robertsonian translocation accounts for 5% of down syndrome cases • Occurs between chromosomes 21 and 14

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