Chromosomes, Mitosis, Meiosis: Understanding Cell Division

1. Chapter 8

2. Chromosomes, Mitosis, Meiosis • The Chromosome • A. How all these terms fit together: • Nucleotide – Basic unit of DNA • Gene – Sequence of DNA coding for a protein • Chromosome – Many genes • Genome – all the chromosomes in one cell 46 chromosomes in the human genome

3. B. Structure of a chromosome 1. DNA wrapped around specialized proteins called histones Centromere Chromatid a. this is a “single chromosome” (meaning DNA has not been replicated). b. when we see a chromosome under the microscope, the DNA has been replicated, so what we see are “doubled chromosomes”. We can’t see them otherwise. Sister Chromatids

5. C. Terms to help you understand Mitosis and Meiosis 1. Mitosis – division of the nucleus 2. Meiosis – making of gametes (egg and sperm) 3. Single chromosome – DNA has not been replicated 4. Doubled chromosome – DNA has been replicated 5. Homologous chromosomes a. all chromosomes occur in pairs (you get one from your mom {egg} and one from your dad {sperm}) so you can think of one in the pair being the “female chromosome” and the other in a pair being the “male chromosome” b. they are the same size and shape c. they are carrying the same genes on them (ie…each gene you have occurs as a pair)

6. 6. Diploid cell (abbreviated 2n) – a cell that contains both chromosomes of a homologous pair (in a human, this would be a cell with 46 chromosomes in it) 7. Haploid cell (abbreviated n) – a cell that contains only one chromosome of a homologous pair a. egg and sperm cell 8. Gametes – fancy term for reproductive cells a. haploid (egg and sperm) b. when gametes fuse, you get a zygote – one diploid cell that divides many times to make a whole organism.

7. Lifecycle of a cell A. Interphase 1. Most of the cell’s life is spent in this phase 2. This phase is divided into 3 parts a. G1 = Gap 1 The cell makes proteins The cell grows The cell makes more organelles b. S = Synthesis When DNA replication occurs (single chromosomes become doubled) c. G2 = Gap 2 More proteins are produced to prepare for cell division B. Mitosis 1. This is where the nucleus divides into two nuclei C. Cytokinesis 1. This is where the cytoplasm is divided and two cells are produced

8. Mitosis: What is it? • One nucleus dividing into two. • The two nuclei are identical • Occurs in all cells except egg and sperm. Occurs in “My--Toes”.

9. Phases of Mitosis • Prophase • Metaphase • Anaphase • Telophase P. MAT

10. Prophase • Chromosomes coil • Nuclear envelope breaks down (disappears) Early Prophase Late Prophase

11. Metaphase • Chromosomes line up in the Middle of the cell

12. Anaphase • Doubled chromosomes move Apart to opposite sides of the cell Early Anaphase Late Anaphase

13. Telophase • Chromosomes uncoil • Nucleus reappears • Cytokinesis occurs – cytoplasm splits resulting in 2 cells.

16. MEIOSIS: The process of making egg and sperm cells 1. 1 reproductive cell (ovarian cell or testicular cell) goes through this process and makes 4 haploid cells. 2. 1 diploid cell makes 4 haploid cells. HOW????

17. PHASES OF MEIOSIS Meiosis 1: Interphase 1 Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Meiosis 2: Interphase 2 Prophase 2 Metaphase 2 Anaphase 2 Telophase 2

18. Meiosis 1: Purpose is to separate the homologous pairs (diploid to haploid) • Interphase 1: DNA replicates • Prophase 1: Chromosomes coil Nucleus disappears Homologous chromosomes pair up to form a “tetrad” As tetrad forms, parts of the 4 chromatids can be exchanged. This is called “crossing over” The result is an exchange of genes which causes the variations among organisms.

19. Metaphase 1: The Tetrads move to the middle of the cell. • Anaphase 1: Tetrads move apart (the homologous pairs are separating) • Telophase 1: Chromosomes uncoil Nucleus reappears Cytokinesis occurs (resulting in 2 cells) Result is 2 haploid cells (the pairs separated) The chromosomes are still doubled, so they need to go through 1 more division.

20. Meiosis 2: Purpose is to separate the doubled chromosomes Process is like mitosis except DNA does not replicate (the chromosomes are already doubled). • Interphase 2: Nothing happens (DNA does NOT replicate) • Prophase 2: Chromosomes coil up Nucleus disappears • Metaphase 2: Doubled Chromosomes move to the middle of the cell. • Anaphase 2: Doubled chromosomes move apart • Telophase 2: Chromosomes uncoil Nucleus reappears Cytokinesis occurs Result is 4 haploid cells. These 4 cells are either egg or sperm.

24. Normal Female 46 XX

26. Chromosomal Mutations A. Deletion 1. The loss of a chromosome region. a. Causes include – radiation, viral attach, environmental factors b. Results in loss of at least one gene…probably more c. Example: “cri-du-chat” High-pitched cry sounds like a cat slow growth Small head Wide-set eyes Downward slant to the eyes Small jaw Low-set ears (may be malformed) Skin tags just in front of the ear Partial webbing or fusing of fingers or toes Single line in the palm of the hand Mental retardation Slow or incomplete development of motor skills

27. B. Duplication/Insertion 1. Repeated gene sequence on a chromosome. 2. Example: Fragile X syndrome a. twice as many males as females have this. b. there is a region on the X chromosome that has many replications which elongates the X chromosome. Family history of Fragile X syndrome, especially a male relative Mental retardation Large testicles after puberty Large body size Tendency to avoid eye contact Hyperactive behavior Large forehead or ears with a prominent jaw

28. C. Inversion 1. A segment that separated from a chromosome and then was inserted at the same place but in reverse. This reversal alters the position and order of the chromosome’s genes. a. may or may not have a major effect. b. may cause mental retardation and/or birth defects D. Translocation 1. A part of one chromosome that has exchanged places with a corresponding part of another, nonhomologous chromosome. (example—chromosome 14 and 21 exchange information) a. may cause a miscarriage b. may not effect carrier, but their offspring may be at risk of having Down syndrome or never be viable.

29. D. Nondisjunction – When chromosomes fail to separate during meiosis. 1. can cause abnormalities in autosome number. a. Down Syndrome (have an extra chromosome 21) 2. can cause abnormalities in sex chromosome number a. Turner Syndrome (XO) Female that has only one X chromosome (the egg did not have an X chromosome) The ovaries never become functional Breasts do not develop Stocky build A webbed neck Difficulty in spatial patterns Mental retardation

30. b. Triple X (XXX) An egg with two X’s is fertilized by a sperm with an X Tendency toward learning disabilities. Usually no physical abnormalities c. Klinefelter Syndrome (XXY) An egg with two X’s is fertilized by a sperm with a Y Male in appearance, but testes are underdeveloped, and breasts may be enlarged. Arms and legs tend to be longer than average Poor muscular development Sparse body hair Some learning disabilities

31. d. Jacob Syndrome (XYY) Taller than average Muscular Persistent acne Slightly below normal intelligence. At one time people used to think humans with Jacob syndrome were likely to be criminally aggressive, but this has been found not to be true.

32. Turner Syndrome Down Syndrome Triple X Klinefelter syndrome