1. Patterns of Chromosome Inheritance

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. © CNRI/SPL/Photo Researchers
18.1 Chromosomes and cell cycle
What is a karyotype?
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sister
chromatids
centromere
homologous
autosome pair
Sex chromosomes are not homologous in males.
The 46 chromosomes of a male
© CNRI/SPL/Photo Researchers

5. The cell cycle 2 parts: Interphase:
18.1 Chromosomes and cell cycle
The cell cycle
2 parts:
Interphase:
G1 stage – cell doubles its organelles; cell grows in size
S stage – DNA replication occurs
G2 stage – proteins needed for division are synthesized
Cell division (mitosis and cytokinesis):
Mitosis – nuclear division
Cytokinesis – cytoplasmic division

6. The cell cycle Interphase S growth and DNA replication G2 G1
18.1 Chromosomes and cell cycle
The cell cycle
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Interphase S
growth and DNA
replication G2 G1
growth and final
preparations for
division
growth M
Mitosis
Cytokinesis
elophase
Prophase
Anaphase
Metaphase T

7. 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
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duplication
duplicated
chromosome
chromosome
division

8. 18.2 Mitosis
The spindle in mitosis
Centrosome – the microtubule organizing center of the cell
Aster – an array of microtubules at the poles (ends of the cell)
Centrioles – short cylinders of microtubules that assist in the formation of spindle fibers
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centrioles
in centrosome
spindle
fibers
chromosomes
aster
pole

9. 18.2 Mitosis
Overview of mitosis
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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
centrosome
centriole
chromosome
2n = 4
DNA REPLICATION
DURING INTERPHASE
duplicated
chromosome
consisting of
two sister
chromatids
centromere
2n = 4
MITOSIS
2n = 4
2n = 4

10. 1. Mitosis: Prophase Chromosomes condense and become visible
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Chromosomes condense and become visible
Nuclear envelope fragments
Nucleolus disappears
Centrosomes move to opposite poles
Spindle fibers appear and attach to the centromere
aster
20 µm
duplicated
chromosomes
20 µm
nuclear
envelope
fragments
centromere
chromatin
condenses
nucleolus
disappears
spindle
fibers forming
Early Prophase
Prophase
Centrosomes have duplicated.
Chromatin is condensing into
chromosomes, and the nuclear
envelope is fragmenting.
Nucleolus has disappeared, and
duplicated chromosomes are visible.
Centrosomes begin moving apart,
and spindle is in process of forming.

11. 18.2 Mitosis
2. Mitosis: Metaphase
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Chromosomes line up at the middle of the cell (equator)
Fully formed spindle
spindle
pole
20 µm
chromosomes
at equator
20 µm
centromere
centromeric
spindle fiber
centromeric
spindle fiber
polar spindle fiber
Early Metaphase
Metaphase
Each chromatid is attached
to a spindle fiber. Some
spindle fibers stretch from each
spindle pole and overlap.
Centromeres of duplicated chromosomes
are aligned at the equator (center of
fully formed spindle). Spindle fibers
attached to the sister chromatids
come from opposite spindle poles.

12. 18.2 Mitosis
3. Mitosis: Anaphase
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Sister chromatids separate at the centromeres and move towards the poles
daughter chromosome
20 µm
Anaphase
Sister chromatids part and become daughter
chromosomes that move toward the spindle
poles. In this way , each pole receives the same
number and kinds of chromosomes as the parental cell.

13. 4. Mitosis:Telophase and cytokinesis
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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)
cleavage furrow
16µm
nucleolus
Telophase
Daughter cells are forming
as nuclear envelopes and
nucleoli reappear . Chromosomes will
become indistinct chromatin.

14. 18.3 Meiosis
Overview of meiosis
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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)
centromere
homologous
chromosome pair
nucleolus
centrioles
homologous
chromosome
pair
2n = 4
CHROMOSOME DUPLICATION
synapsis
2n = 4
sister
chromatids
MEIOSIS I
Duplicated
homologous pairs
synapse and
then separate.
MEIOSIS II
Sister chromatids
separate, becoming
daughter chromosomes.
n = 2
n = 2

15. Meiosis I Prophase I Metaphase I Anaphase I Telophase I
Homologous chromosomes pair (synapsis) and crossing-over occurs in which there is exchange of genetic information
Metaphase I
Homologous pairs lined at the equator
Anaphase I
Homologous chromosomes separate and move towards opposite poles
Telophase I
2 daughter cells result each with 23 duplicated chromosomes

16. 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
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sister chromatids
crossing-over
between nonsister
chromatids
chromatids
after
exchange
recombinant
daughter
chromosomes
synapsis
of homologues

17. Meiosis I 18.3 Meiosis 2n = 4 Animal Cell at Interphase Prophase I
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Animal Cell
at Interphase
2n = 4
Prophase I
Chromosomes have duplicated. Homologous
chromosomes pair during synapsis and
crossing-over occurs.

18. 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

19. Meiosis II 18.3 Meiosis n = 2 Prophase II Cells have one chromosome
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n = 2
Prophase II
Cells have one chromosome
from each homologous pair .

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

21. Comparing mitosis and meiosis
18.4 Comparison of meiosis and mitosis
Comparing mitosis and meiosis
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2n = 4
Prophase I
Synapsis and
crossing-over occur.
Metaphase I
Homologous pairs align
independently at the equator.
Anaphase I
Homologous chromosomes
separate and move towards the poles.
MEIOSIS I
2n = 4
Prophase
Metaphase
Chromosomes align
at the equator.
Anaphase
Sister chromatids separate and
become daughter chromosomes.
MITOSIS

22. Comparing mitosis and meiosis
18.4 Comparison of meiosis and mitosis
Comparing mitosis and meiosis
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Telophase I
Daughter cells are forming
and will go on to divide again.
n = 2
Daughter cells
Sister chromatids
separate and become
daughter chromosomes.
Four haploid daughter cells.
Their nuclei are genetically
different from the parent cell.
n = 2
n = 2
MEIOSIS I cont'd
MEIOSIS II
Daughter cells
Telophase
Daughter cells
are forming.
Two diploid daughter cells.
Their nuclei are genetically
identical to the parent cell.
MITOSIS cont'd

23. 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 500 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

24. The production of sperm and eggs
18.4 Comparison of meiosis and mitosis
The production of sperm and eggs
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SPERMATOGENESIS
primary
sper matocyte 2n
Meiosis I
secondary
spermatocytes
Meiosis II n
spermatids n
Metamorphosis
and maturation
sperm n
OOGENESIS
primary
oocyte 2n
Meiosis I
first
polar body n
secondary
oocyte n
Meiosis II
Meiosis II is completed
after entry of sperm
second
polar body n
Fertilization
cont'd
egg
sperm nucleus n n
fusion of sperm
nucleus and
egg nucleus
zygote 2n

25. Changes in chromosome number
18.5 Chromosome inheritance
Changes in chromosome number
Nondisjunction occurs when both members of a homologous pair go into the same daughter cell during meiosis I or 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)

26. Changes in chromosome number
18.5 Chromosome inheritance
Changes in chromosome number
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extra chromosome 21 21
Gart
gene a. b.
a: Courtesy Chris Burke; b: © CNRI/SPL/Photo Researchers, Inc.

27. 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

28. Changes in chromosome structure
18.5 Chromosome inheritance
Changes in chromosome structure
Deletions – loss of a piece of the chromosome (e.g. Williams syndrome)
Translocations – movement of chromosome segments from one chromosome to another nonhomologous chromosome (Alagille syndrome)
Duplications – presence of a chromosome segment more than once in the same chromosome
Inversions – a segment of a chromosome is inverted 180 degrees

29. Changes in chromosome structure
18.5 Chromosome inheritance
Changes in chromosome structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a a a a a a a b b b b b b b b l l c c c c c m c m + a c d d n d n d d d d c e e d e o e e e o e e f d f p f p f f e f f g g q q g g g f g g h r r h g
a. Deletion
b. Duplication
c. Inversion
d. Translocation

30. Changes in chromosome structure
18.5 Chromosome inheritance
Changes in chromosome structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a s s a a a b b t b t b h + c c
deletion
lost u u c c
translocation d v d v d d e e w e w e f x x f f f g g y g y g h z h z h