1. Chapter 18 Patterns of Chromosome Inheritance
Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.

2. Points to Ponder What is the structure of chromosomes?
What is the cell cycle and what occurs during each of its stages?
Explain what mitosis is used for, in what cells and the 4 stages.
Explain the 2 divisions of meiosis.
What is meiosis used for and in what cells?
Compare and contrast mitosis and meiosis.
Compare and contrast spermatogenesis and oogenesis.
What are trisomy and monosomy?
What most often causes these changes in chromosome number?
What are the syndromes associated with changes in sex chromosomes?
Explain the 4 changes in chromosome structure.

3. 18.1 Chromosomes and cell cycle
Chromosomes: A review
Humans have 46 chromosomes that are in 23 pairs within a cell’s nucleus
Pairs of chromosomes are called homologous chromosomes
Autosomes are the 22 pairs of chromosomes that control traits that do not relate to gender of an individual
Sex chromosomes are the 1 pair that contain the genes that do control gender
Cells (body cells) that have 46 (2N) chromosomes are called diploid
Cells (sex cells) that have only 23 (N) chromosomes not in pairs are called haploid

4. What is a karyotype? Computer arrangement of chromosomes in pairs

5. Karyotype In dividing cells Chromosomes are duplicated
Each chromosome is composed of two identical parts, called sister chromatids
Each sister chromatid contain the same genes
Genes of inheritance
Chromatids are held together at a region called centromere
Centromeres hold sister chromatids together until certain phase of mitosis

6. The cell cycle Interphase: 20 hr, 90% of cell cycle
G1 stage
Cell doubles its organelles and grows in size
Accumulate material needed for DNA synthesis
Proteins needed to change chromatin into chromosomes
S stage
DNA replication occurs; duplicated chromosomes
G2 stage
proteins needed for division are synthesized
Proteins found in microtubules
Cell division (mitosis and cytokinesis): 4 hrs
Mitosis – nuclear division
Each new nucleus contains the same number and kind of chromosomes as the former cell
Cytokinesis – cytoplasmic division

7. 18.1 Chromosomes and cell cycle
The cell cycle

8. Chromosome structure in mitosis
Chromosomes contain both DNA and proteins (collectively called chromatin)
Chromosomes that are dividing are made up of 2 identical parts called sister chromatids
The sister chromatids are held together at a region called the centromere

9. The spindle in mitosis Centrosome Aster Centrioles
microtubule organizing center of the cell
Aster
array of microtubules at the poles (ends of the cell)
Centrioles
short cylinders of microtubules that assist in the formation of spindle fibers

10. 18.2 Mitosis
Overview of Mitosis
A diploid cell makes and divides an exact copy of its nucleus
Used in cell growth and cell repair
Occurs in body cells
4 phases:
Prophase
Metaphase
Anaphase
Telophase

11. 1. Mitosis: Prophase Chromosomes condense and become visible
Nuclear envelope fragments
Nucleolus disappears
Centrosomes move to opposite poles
Spindle fibers appear and attach to the centromere

12. 18.2 Mitosis
2. Mitosis: Metaphase
Chromosomes line up at the middle of the cell (equator)
Fully formed spindle

13. 18.2 Mitosis
3. Mitosis: Anaphase
Sister chromatids separate at the centromeres and move towards the poles

14. 4. Mitosis:Telophase and cytokinesis
Chromosomes arrive at the poles
Chromosomes become indistinct chromatin again
Nucleoli reappear
Spindle disappears
Nuclear envelope reassembles
Two daughter cells are formed by a ring of actin filaments (cleavage furrow)

15. Importance of Cell Cycle and Mitosis
Cell cycle important for well-being and involved in
Growth and injury repair
Malfunctioning cell cycle result in
Benign tumor or a cancerous tumor
Cell Cycle Control System
Reception:
- External signal delivers message to receptor in plasma membrane of receiving cell
Transduction:
- receptor relays signal to protein inside cell cytoplasm
Activates Gene
Response:
- protein production that stimulate or inhibit the cell cycle
proto-oncogenes  stimulate cell cycle
Tumor suppressor genes  inhibit cell cycle

16. Overview of Meiosis
Two nuclear divisions occur to make 4 haploid cells
Used to make gametes (egg and sperm)
Occurs in sex cells
Has 8 phases (4 in each meiosis I & II)

17. 18.3 Meiosis
Meiosis I
Prophase I
Homologous chromosomes pair (synapsis) and crossing-over occurs in which there is exchange of genetic information
Homologous chromosomes
chromosomes look alike and carry genes for the same traits
Synapsis
line up of homologous chromosomes side by side

18. 18.3 Meiosis
What is crossing over?
Crossing-over is the exchange of genetic information between nonhomologous sister chromatids during synapsis
This occurs during prophase I of meiosis and increases genetic variation

19. 18.3 Meiosis
Meiosis I

20. Meiosis I Metaphase I Anaphase I Telophase I
Homologous pairs lined at the equator
Number of possible chromosomal combinations in the gametes is 8,388,608 without accounting for cross-over
Anaphase I
Homologous chromosomes separate and move towards opposite poles
Telophase I
2 daughter cells result each with 23 duplicated chromosomes

21. Meiosis II Prophase II Metaphase II Anaphase II Telophase II
Chromosomes condense again
Metaphase II
Chromosomes align at the equator
Anaphase II
Sister chromatids separate to opposite poles
Telophase II
4 daughter cells result each with 23 unduplicated chromosomes

22. 18.3 Meiosis
Meiosis II

23. Significance of Meiosis
Keep the chromosome number constant from generation to generation
Asexual reproduction by binary fussion
Bacteria, protozoans, and yeasts
Bacteria can increase to over one million cells in about 7 hours
Sexual reproduction, requires more energy
Humans and other animals
Results in genetic recombination
Ensures offspring are genetically different compared to each other and their parents
Recombination is due to crossing-over and independent alignment of chromosomes
Benefit  generates diversity needed so that a few will be suited to new and different environmental circumstances

24. Mitosis vs. Meiosis Growth and repair of cells Occurs in body cells
18.4 Comparison of meiosis and mitosis
Mitosis vs Meiosis
Growth and repair of cells
Occurs in body cells
1 division
Results in 2 diploid genetically identical cells
Formation of gametes
Occurs in sex cells
2 divisions
Results in 4 haploid genetically different cells

25. Comparing mitosis and meiosis I

26. Comparing mitosis and meiosis II

27. The production of sperm and eggs
18.4 Comparison of meiosis and mitosis
The production of sperm and eggs
Oogenesis
Process of making eggs in females
During meiosis 1 egg and 3 polar bodies are formed
Polar bodies act to hold discarded chromosomes and thus disintegrate
Normally 1 egg per month is produced and ~400 during the entire reproductive cycle
Spermatogenesis
Process of making sperm in males
A continual process after puberty
About 400 million sperm are produced per day

29. Changes in chromosome number
Nondisjunction
when both members of a homologous pair go into the same daughter cell during meiosis I
when sister chromatids fails to separate in meiosis II.
Results of nondisjunction:
Monosomy: cell has only 1 copy of a chromosome
e.g. Turner syndrome (only one X chromosome)
Trisomy: cell has 3 copies of a chromosome
e.g. Down syndrome (3 copies of chromosome 21)
** Too many chromosomes is tolerated better than a deficiency of chromosomes
Survival is greater when monosomy and trisomy involves sex chromosomes
Because in female XX, one X chromosomes is a Barr body is inactive

30. Down Syndrome Most common autosomal trisomy Characteristics:
Three copies of chromosome 21
Characteristics:
Short stature, an eyelid fold, flat face, fissured tongue, round head, mental retardation
Genes that carry Down Syndrome
Located on bottom third of chromosome 21
Responsible for increased tendency toward leukemia, cataracts, accelerated rate of aging and mental retardation (Gart gene)

31. Changes in chromosome number
18.5 Chromosome inheritance
Changes in chromosome number

32. Changes in sex chromosome number
18.5 Chromosome inheritance
Changes in sex chromosome number
Turner syndrome (X)
short stature, broad shouldered with folds of skin on the neck, underdeveloped sex organs, no breasts
Klinefelter syndrome (XXY)
underdeveloped sex organs, breast development, large hands and long arms and legs
Poly-X female (XXX, XXXX)
XXX tends to be tall and thin but not usually retarded
XXXX are severely retarded
Jacobs syndrome (XYY)
tall, persistent acne, speech and reading problems

33. Changes in sex chromosome number

34. Changes in chromosome structure
Deletions: causes abnormalities
loss of a piece of the chromosome
Translocations
movement of chromosome segments from one chromosome to another nonhomologous chromosome
Duplications:
presence of a chromosome segment more than once in the same chromosome
Inversions: lead to altered gene activities, deletions and duplications
a segment of a chromosome is inverted 180 degrees resulting in a reversed chromosome

35. Changes in chromosome structure