1. Cellular reproduction part 2

2. Chromosomes are matched in homologous pairs
MEIOSIS AND CROSSING OVER
Chromosomes are matched in homologous pairs
Somatic cells of each species contain a specific number of chromosomes
Human cells have 46, making up 23 pairs of homologous chromosomes
Chromosomes
Centromere
Sister chromatids
Figure 8.12

3. Homologous Chromosomes
Humans have 23 pairs of homologous chromosomes
22 pairs – autosomes – found in both males and females
1 pair – sex chromosomes, XX = female,
XY= male

4. Homologous Chromosomes
Matched pairs of chromosomes
Similar in size, shape, and banding pattern
Both carry genes controlling the same inherited characteristics (the version of the gene may be different)
The genes are located at the same locus
One chromosome of each pair is inherited from the mother, the other from the father

5. Multicellular diploid adults (2n = 46) Mitosis and development
The human life cycle
Haploid gametes (n = 23)
Egg cell
Sperm cell
MEIOSIS
FERTILIZATION
Diploid zygote (2n = 46)
Multicellular diploid adults (2n = 46)
Mitosis and development
Figure 8.13

6. Human Life Cycle
Diploid cells (2n) – cells that contain both homologous chromosomes. In humans the diploid number is 46.
Haploid cells (n) – cells with one copy of each homologous chromosome. The gametes (egg and sperm) are haploid. In humans the haploid number is 23.

7. Meiosis
Meiosis
The division that reduces the number of chromosomes by half.
In animals, meiosis results in the formation of haploid egg and sperm cells.

8. Figure 8.15

9. Figure

10. Two nuclear divisions occur: Meiosis I
During prophase I homologous chromosomes pair – synapsis
During prophase I the paired chromosomes exchange chromosome parts – crossing over
Homologous chromosomes are separated
2 cells produced each containing one copy of each homologous chromosome

11. Figure

12. Meiosis Meiosis II Not preceded by the replication of DNA
Sister chromatids of each chromosome are separated
Produces 4 haploid cells

13. Meiosis
Meiosis produces 4 cells that
Are haploid
Chromosome makeup of each is different from each other and the parent cell

14. Figure 8.17

16. Meiosis
Spermatogenesis
Formation of sperm by meiosis
Occurs in special cells (spermatogonia) in the testes
All 4 haploid cells become sperm

18. Meiosis
Oogenesis
Formation of an egg by meiosis
Occurs in special cells (oogonia) in the ovaries
Unequal divisions of the cytoplasm during meiosis I and meiosis II result in the formation of 1 haploid egg and 3 haploid polar bodies
Only the egg can be fertilized

20. Genetic Recombination
Genetic Recombination – the production of gene combinations different from those carried by the parent
There are 4 processes that contribute to genetic recombination.

21. Figure 8.18

22. Independent Assortment of Chromosomes
The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametes
This results in 2n possible combinations of gametes
For humans 2n = 223 = 8 million possible combinations
Random fertilization also increases variation in offspring

23. C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black
White
Pink
Tetrad in parent cell (homologous pair of duplicated chromosomes)
Chromosomes of the four gametes
Figure 8.17A, B

24. Homologous chromosomes carry different versions of genes
The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci

25. Tetrad
Chaisma
Centromere
Figure 8.18A

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

27. Crossing over a closer look Meiosis II
Animation
(must insert Miller and Levine lecture cd ch11)
Meiosis 1
Crossing over a closer look
Meiosis II
Figure 8.19

28. Genetic Recombination
Crossing over
The exchange of genetic information between 2 homologous chromosomes.
Occurs during prophase I.
Random fertilization
Depends on which sperm cell and its chromosome combinations fertilizes which egg

29. Preparation of a karyotype
Fixative
Packed red
And white
blood cells
Hypotonic solution
Blood culture
Stain
White
Blood cells
Centrifuge 3 2 1
Fluid
Centromere
Sister chromatids
Pair of homologous chromosomes 4 5
Figure 8.19

30. A karyotype is a photographic inventory of an individual’s chromosomes
ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE
A karyotype is a photographic inventory of an individual’s chromosomes
To study human chromosomes microscopically, researchers stain and display them as a karyotype
A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes

31. 8.21 Accidents during meiosis can alter chromosome number
Abnormal chromosome count is a result of nondisjunction
Either homologous pairs fail to separate during meiosis I
Nondisjunction in meiosis I
Normal meiosis II
Gametes
n + 1
n + 1
n – 1
n – 1
Number of chromosomes
Figure 8.21A

32. Or sister chromatids fail to separate during meiosis II
Normal meiosis I
Nondisjunction in meiosis II
Gametes
n + 1
n – 1 n n
Number of chromosomes
Figure 8.21B

33. Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome
Egg cell
n + 1
Zygote 2n + 1
Sperm cell
n (normal)
Figure 8.21C

34. Connection: An extra copy of chromosome 21 causes Down syndrome
This karyotype shows three number 21 chromosomes
An extra copy of chromosome 21 causes Down syndrome
Figure 8.20A, B

35. The chance of having a Down syndrome child goes up with maternal age
Figure 8.23

36. Alterations of Chromosomes
In most cases abnormal chromosome number results in spontaneous abortion long before birth.
Nondisjunction in the sex chromosomes has less of an affect on survival

37. Connection: Abnormal numbers of sex chromosomes do not usually affect survival
Nondisjunction can also produce gametes with extra or missing sex chromosomes
Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes

38. Table 8.1

40. Connection: Alterations of chromosome structure can cause birth defects and cancer
Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer
Four types of rearrangement are deletion, duplication, inversion, and translocation

41. Duplication – the fragment joins to a homologous chromosome.
Deletion
Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.
Duplication
Duplication – the fragment joins to a homologous chromosome.
Homologous chromosomes
Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.
Inversion
Reciprocal translocation
Translocation– attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.
Nonhomologous chromosomes
Figure 8.23A, B

42. Alterations of Chromosomes
Abnormalities in the structure of the chromosome may cause disorders (Figure 8.23A)
Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.
Duplication – the fragment joins to a homologous chromosome.

43. Alterations of Chromosomes
Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.
Translocation (Figure 8.23B) – attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.

44. Chromosomal changes in a somatic cell can cause cancer
A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia
Chromosome 9
Reciprocal translocation
Chromosome 22
“Philadelphia chromosome”
Activated cancer-causing gene
Figure 8.23C