1. Cell Reproduction Part II
Meisosis

2. MEIOSIS AND CROSSING OVER
8.12 Chromosomes are matched in homologous pairs
The somatic (body) cells of each species
Contain a specific number of chromosomes
For example human cells have 46
Making up 23 pairs of homologous chromosomes

3. The chromosomes of a homologous pair
Carry genes for the same characteristics at the same place, or locus
Chromosomes
Centromere
Sister chromatids
Figure 8.12

4. 8.13 Gametes have a single set of chromosomes
Cells with two sets of chromosomes
Are said to be diploid
Gametes, eggs and sperm, are haploid
With a single set of chromosomes

5. Sexual life cycles
Involve the alternation of haploid and diploid stages
Web activity
Mitosis and development
Multicellular diploid adults (2n = 46)
Diploid zygote (2n = 46) 2n
Meiosis
Fertilization
Egg cell
Sperm cell n
Haploid gametes (n = 23)
Figure 8.13

6. Quiz 8.11 – 8.13 Name 2 functions of mitosis(why do cells divide?).
Two chromosomes composing a pair are called?
What is a somatic Cell?
What is a gamete?
What are you thankful
for this Thanksgiving?
Chromosomes
Centromere
Sister chromatids

7. 8.14 Meiosis reduces the chromosome number from diploid to haploid
Meiosis, like mitosis
Is preceded by chromosome duplication
But in meiosis
The cell divides twice to form four daughter cells
Produces haploid gametes in diploid organism.

8. The first division, meiosis I
Starts with synapsis, the pairing of homologous chromosomes.
XX XX
Tetrad Tetrad
In crossing over
Homologous chromosomes exchange corresponding segments

9. Meiosis I separates each homologous pair
And produce two daughter cells, each with one set of chromosomes
Meiosis II is essentially the same as mitosis
The sister chromatids of each chromosome separate
The result is a total of four haploid cells
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10. MEIOSIS I: Homologous chromosomes separate
The stages of meiosis
MEIOSIS I: Homologous chromosomes separate
INTERPHASE PROPHASE I METAPHASE I ANAPHASE I
Centrosomes (with centriole pairs)
Sites of crossing over
Spindle
Microtubules attached to kinetochore
Metaphase plate
Sister chromatids remain attached
Nuclear envelope
Chromatin
Sister chromatids
Tetrad
Centromere (with kinetochore)
Homologous chromosomes separate
Figure 8.14 (Part 1)

11. Haploid daughter cells forming Sister chromatids separate
PROPHASE II METAPHASE II ANAPHASE II
TELOPHASE I AND CYTOKINESIS
TELOPHASE II AND CYTOKINESIS
Cleavage furrow
Haploid daughter cells forming
Sister chromatids separate
MEIOSIS II: Sister chromatids separate
Figure 8.14 (Part 2)

12. 8.15 Review: A comparison of mitosis and meiosis
Parent cell (before chromosome replication)
Chromosome replication
Chromosomes align at the metaphase plate
Tetrads align at the metaphase plate
Sister chromatids separate during anaphase
Homologous chromosomes separate during anaphase I; sister chromatids remain together
No further chromosomal replication; sister chromatids
separate during anaphase II
Prophase
Metaphase
Anaphase Telophase
Duplicated chromosome (two sister chromatids)
Daughter cells of mitosis 2n
Daughter cells of meiosis I n
2n = 4
Tetrad formed by synapsis of homologous chromosomes
Meiosis i
Meiosis ii
Prophase I
Metaphase I
Anaphase I Telophase I
Haploid n = 2
Daughter cells of meiosis II
Figure 8.15

13. 8.16 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring
Each chromosome of a homologous pair
Differs at many points from the other member of the pair
The arrangement of homologous pairs at metaphase I of meiosis affects the resulting gametes.

14. Two equally probable arrangements of chromosomes at metaphase I
Random arrangements of chromosome pairs at metaphase I of meiosis
Lead to many different combinations of chromosomes in eggs and sperm
Combination 1
Combination 2
Combination 3
Combination 4
Gametes
Metaphase II
Two equally probable arrangements of chromosomes at metaphase I
Possibility 1
Possibility 2
Figure 8.16

15. Random fertilization of eggs by sperm
Greatly increases this variation

16. 8.18 Crossing over further increases genetic variability
Genetic recombination
Which results from crossing over during prophase I of meiosis, increases variation still further
Web activity
Chiasma
Tetrad
Centromere
TEM 2,200
Figure 8.18A

17. Tetrad (homologous pair of chromosomes in synapsis)
How crossing over leads to genetic variation
Coat-color genes
Eye-color genes C E
Tetrad (homologous pair of chromosomes in synapsis) c e
Breakage of homologous chromatids 1 C E c e
Joining of homologous chromatids 2 C E
Chiasma c e 3
Separation of homologous chromosomes at anaphase I C E C e c E c e 4
Separation of chromatids at anaphase II and completion of meiosis C E
Parental type of chromosome C e
Recombinant chromosome c E
Recombinant chromosome c e
Parental type of chromosome
Figure 8.18B
Gametes of four genetic types

18. 8.21 Accidents during meiosis can alter chromosome number
Abnormal chromosome count is a result of nondisjunction
The failure of homologous pairs to separate during meiosis I
The failure of sister chromatids to separate during meiosis II

19. Nondisjunction in meiosis I
Normal meiosis II
Gametes
n + 1
n 1
Number of chromosomes
Nondisjunction in meiosis II
Normal meiosis I
n -1 n
Number of chromosomes
Figure 8.21B
Figure 8.21A

20. Fertilization after nondisjunction in the mother
Sperm cell
Egg cell
n (normal)
n + 1
Zygote
2n + 1
Figure 8.21C

21. CONNECTION
8.22 Abnormal numbers of sex chromosomes do not usually affect survival
Nondisjunction can also produce gametes with extra or missing sex chromosomes
Leading to varying degrees of malfunction in humans but not usually affecting survival
Figure 8.22A
Poor beard
growth
Breast
Development
Under-developed
testes
Figure 8.22B
Characteristic facial
features
Web of
skin
Constriction
of aorta
Poor breast
development
Under developed
ovaries

22. Human sex chromosome abnormalities

23. CONNECTION
8.23 Alterations of chromosome structure can cause birth defects and cancer
Chromosome breakage can lead to rearrangements
That can produce genetic disorders or, if the changes occur in somatic cells, cancer

24. Chromosomal Abnormalities Deletions, duplications, inversions, and translocations
Reciprocal translocation
Nonhomologous chromosomes
Deletion
Duplication
Inversion
Homologous chromosomes
Figure 8.23B
“Philadelphia chromosome”
Chromosome 9
Chromosome 22
Reciprocal translocation
Activated cancer-causing gene
Figure 8.23C
Figure 8.23A

25. 8.17 Homologous chromosomes carry different versions of genes
The differences between homologous chromosomes
Are based on the fact that they can bear different versions of a gene at corresponding loci
Tetrad in parent cell (homologous pair of duplicated chromosomes) e c E C
White
Pink
Meiosis
Black
Brown
Chromosomes of
the four gametes
Eye-color genes
Coat-color genes
Brown coat (C); black eyes (E)
White coat (C); pink eyes (e)
Figure 8.17B
Figure 8.17A

26. ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE
8.19 A karyotype is a photographic inventory of an individual’s chromosomes
A karyotype
Is an ordered arrangement of a cell’s chromosomes

27. Preparation of a karyotype from a blood sample
Blood culture
Fluid
Centrifuge
Packed red and white blood cells
Hypotonic solution
Fixative
White blood cells
Stain
Centromere
Pair of homologous chromosomes
Sister chromosomes
2,600X
A blood culture is centrifuged to separate the blood cells from the culture fluid. 1
The fluid is discarded, and a hypotonic solution is mixed with the cells. This makes the red blood cells burst. The white blood cells swell but do not burst, and their chromosomes spread out. 2
Another centrifugation step separates the swollen white blood cells. The fluid containing the remnants of the red blood cells is poured off. A fixative (preservative) is mixed with the white blood cells. A drop of the cell suspension is spread on a microscope slide, dried, and stained. 3
The slide is viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is entered into a computer, which electronically arranges them by size and shape. 4
The resulting display is the karyotype. The 46 chromosomes here include 22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to discern in the karyotype, each of the chromosomes consists of two sister chromatids lying very close together (see diagram). 5
Figure 8.19

28. CONNECTION 8.20 An extra copy of chromosome 21 causes Down syndrome
A person may have an abnormal number of chromosomes
Which causes problems

29. Down syndrome is caused by trisomy 21
An extra copy of chromosome 21
5,000
Figure 8.20A
Figure 8.20B

30. Infants with Down syndrome (per 1,000 births)
The chance of having a Down syndrome child
Goes up with maternal age
Age of mother 45 50 35 30 25 40 20 90 10 60 70 80
Infants with Down syndrome (per 1,000 births)
Figure 8.20C