1. Chapter 13
Overview: Hereditary Similarity and Variation
Living organisms
Are distinguished by their ability to reproduce their own kind
WARNING: This chapter relies heavily on a knowledge of definitions. Terminology is crucial to understanding meiosis.

2. Heredity
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings

3. Genetics
Is the scientific study of heredity and hereditary variation

4. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

5. Inheritance of Genes Genes Are the units of heredity
Are segments of DNA
Segments of nucleotides (A, T, C, G) arranged in sequence

6. Each gene in an organism’s DNA
Has a specific locus on a certain chromosome
Each chromosome is DNA and proteins
May have 100’s or 1000’s of genes
We inherit
One set of chromosomes from our mother and one set from our father

7. Comparison of Asexual and Sexual Reproduction
In asexual reproduction
One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm
This gives rise to clones – a group of genetically identical individuals

8. In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

9. Fertilization and Meiosis
Concept 13.2:
A life cycle
Is the generation-to-generation sequence of stages in the reproductive history of an organism

10. Sets of Chromosomes in Human Cells
In humans
Each somatic cell has 46 chromosomes, made up of two sets
One set of chromosomes comes from each parent
Somatic cells – all the cells other than sperm or egg

11. A karyotype
Is an ordered, visual representation of the chromosomes in a cell
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Figure 13.3

12. Homologous chromosomes
Are the two chromosomes composing a pair
Have the same characteristics (length, staining pattern, position of centromere)
May also be called autosomes

13. Sex chromosomes Are distinct from each other in their characteristics
Are represented as X and Y
Determine the sex of the individual, XX being female, XY being male

14. A diploid cell Has two sets of each of its chromosomes
One set came from mom, one from dad
Humans have 46 chromosomes (2n = 46)
n = the number of chromosomes in a set

15. In a cell in which DNA synthesis has occurred
Remember . . .
In a cell in which DNA synthesis has occurred
All the chromosomes are duplicated and thus each consists of two identical sister chromatids
Figure 13.4
Key
Maternal set of
chromosomes (n = 3)
Cell is in G2 phase
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

16. Unlike Somatic Cells,
Gametes - sperm and egg cells, are haploid cells, containing only one set of chromosomes
The haploid number is n = 23 in humans
22 autosomes
1 single sex chromosome

17. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity
The ovaries and testes produce haploid gametes by meiosis

18. During fertilization The zygote
These gametes fuse, forming a diploid zygote
The zygote
Develops into an adult organism

19. The human life cycle Key Figure 13.5 Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

20. Meiosis – Reduces the Number of Chromosomes
Concept 13.3:
Meiosis
Takes place in two sets of divisions, meiosis I and meiosis II
Results in 4 daughter cells, each genetically unique

21. The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes 1 2
Homologous
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II

22. Meiosis I
Reduces the number of chromosomes from diploid to haploid
Meiosis II
Produces four haploid daughter cells

23. Details of Meiosis Stages
CAUTION!
Stages of meiosis I and II use the same names as stages in mitosis
There are differences in these stages though!

24. Stages of Meiosis
Chromosomes replicated in S phase

25. Stages of Meiosis I Chromosomes begin to condense
Homologous chromosomes loosely pair along their lengths
In crossing over, DNA of nonsister chromatids break and rejoin to the other’s DNA
In synapsis homologues are held tightly along length
Tetrads – group of 4 chromosomes are visible
Also, all the happenings of Prophase seen in mitosis occur too

26. Stages of Meiosis I
Pairs of homologous chromosomes (in tetrads) move to the middle of the metaphase plate
Both chromatids of a homologous chromosome are attached to one kinetochore microtubule

27. Stages of Meiosis I Chromosomes move to opposite poles
Sister chromatids remain attached at centromere

28. Stages of Meiosis I
Each cell has a haploid set of chromosomes, but each chromosome still has sister chromatids
No chromosome replication takes place before meiosis II

29. Stages of Meiosis II Spindles form
By end of Prophase II, they attach at centromere

30. Stages of Meiosis II
Chromosomes moved to the middle of metaphase plate
Sister chromatids are not genetically identical

31. Stages of Meiosis II
Sister chromatids come apart, move to opposite poles

32. Stages of Meiosis II
Meiotic division of 1 cell produces 4 genetically different daughter cells
Each daughter cell has a haploid number of chromosomes

33. A Comparison of Mitosis and Meiosis
Meiosis I and II can be distinguished from mitosis because:
Meiosis reduces the number of chromosome sets
Mitosis conserves the number of chromosome sets
And also by:
By three events in Meiosis l

34. 1) Synapsis and crossing over
Homologous chromosomes physically connect and exchange genetic information

35. 2) Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

36. 3) Separation of homologues
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
In anaphase II of meiosis, the sister chromatids separate

37. (before chromosome replication)
A comparison of mitosis and meiosis
Figure 13.9
MITOSIS
MEIOSIS
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrad formed by
synapsis of homologous
chromosomes
Metaphase
Chromosomes
positioned at the
metaphase plate
Tetrads
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
MEIOSIS II
Daughter
cells of
meiosis I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Daughter cells of meiosis II n
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6
Text box of Figure 13.9 is excellent!

38. Genetic Variation and Evolution
Concept 13.4:
Reshuffling of genetic material in meiosis
Produces genetic variation

39. Origins of Genetic Variation Among Offspring
In species that reproduce sexually
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

40. 1) Independent Assortment of Chromosomes
Homologous pairs of chromosomes
Orient randomly at metaphase I of meiosis

41. In independent assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Possibility 2
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Figure 13.10

42. 2) Crossing Over Crossing over
Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

43. 3) Random Fertilization
The fusion of gametes
Will produce a zygote with any of about 64 trillion diploid combinations

44. Evolutionary Significance of Genetic Variation Within Populations
Is the raw material for evolution by natural selection