1. The Cellular Basis of Inheritance
Chapter 7

2. Sexual and Asexual Reproduction
A single parent
No genetic variation
All offspring are genetically identical to each other and the parent
Mutation is the only form of variation
Mechanisms
Mitosis
Binary fission
Vegetative propagation
Parthenogenesis
Pros and Cons
No energy expended to find a mate (pro)
No mate needed, a single individual can reproduce (pro)
No genetic variation (con)
Vegetative Propagation

3. Vegetative Propagation
Mitosis
Parthenogenesis

4. Sexual and Asexual Reproduction
Two parents required
most of the time—self fertilizing flowers
Genetic variation
Variation is important for survival and adaptation (and ultimately evolution)
Mechanism
Meiosis gamete formation  fertilization
Pros and Cons
Genetic variation (pro)
A mate is needed, no reproduction occurs for one individual(con)
Must expend energy to attract and find a mate (con)

5. Meiosis
A specialized type of cell reproduction specifically designed to create gametes (ova and sperm or spores) for sexual reproduction
For sexual reproduction the gametes must have 1/2 the number of chromosomes as the normal cells of an organism
For example human cells have 46 chromosomes
(23 homologous pairs)
If the sperm had 46 and the ovum had 46, then the resulting zygote would have 92 chromosomes!
Over time cells would have more and more chromosomes
Meiosis reduces the number of chromosomes in the parent’s cells from diploid (2n) to haploid (n)
Occurs only in sexually reproducing eukaryotic species

6. 6

7. Vocabulary Diploid Haploid Chromosomes
Two copies of each type of chromosome
Haploid
One copy of each type of chromosome
Chromosomes
Sister chromatids, centromere, duplicated and unduplicated
Homologous pair of chromosomes (homologues)
Two of the same type of chromosome
One paternal and one maternal

8. Vocabulary Somatic cells Germ cells Gamete Fertilization Zygote
Typical diploid cells of an animal
Germ cells
Cells committed to go through meiosis to produce gametes
Gamete
Reproductive cells (Egg/ova, sperm, spores)
Fertilization
A union of two cells (gametes) from two individual organisms
Zygote
The first cell of a new individual (result of fertilization)

9. Meiosis During meiosis there are two rounds of cell division Meiosis I
Separates homologous pairs
Meiosis II
Separates sister chromatids

10. Meiosis
The process of meiosis is very similar to mitosis, however there are some key differences that account for producing genetically variable, haploid gametes
The following slides will give an overview of meiosis especially pointing out the key differences from mitosis
However, the actual processes of each phase will not be presented as they are very similar to mitosis

11. Meiosis Meiosis I: Prophase I Synapsis Crossing over
Pairing of homologous chromosomes
The structure of the two chromosomes is called a tetrad
There are four strands of DNA, two sets of sister chromatids
Crossing over
While the homologs are closely associated they can swap segments of DNA
This creates novel combinations of gene traits in both strands

13. Meiosis Meiosis I: Prometaphase I
Spindle fibers attach to the kinetochore at the centromeres
Each of the pair of chromosomes is attached to opposite poles

14. Meiosis Meiosis I: Metaphase I
Homologous pairs line up at the metaphase plate (equator)
They are pushed and pulled by the mitotic spindle
Random alignment or independent assortment
Homologous pairs can be attached to either spindle pole
The pair can align in either orientation regardless of how other pairs are lining up
Increases the number of potential combinations of maternal and paternal traits (alleles) in resulting gametes
In humans there are 223 possible combinations (over 8 million)
Creates variability in offspring

16. Meiosis Meiosis I: Anaphase I
One of each duplicated chromosome is pulled towards a spindle pole by the microtubules
The other homologous chromosome is pulled to the opposite pole
This creates haploid cells by separating the homologous chromosomes

17. Meiosis Meiosis I: Telophase I
One of each type of chromosome has arrived at the poles
The homologous pairs are no longer together
For most species cytokinesis will occur
For a few, cytokinesis will occur after both rounds of meiosis
Cells often proceed directly into meiosis II without completely finishing telophase I

19. Meiosis Meiosis II: Prophase II
Both cells created by meiosis I will follow through the steps of meiosis II to divide a second time
The steps resemble mitosis of a haploid cell
Prophase II
Chromosomes are
Still duplicated (two chromatids each), but only one of each type of chromosome (haploid)
Generally still condensed following meiosis I
There is no DNA replication between meiosis I and II

20. Meiosis Meiosis II Metaphase II Anaphase II
Chromosomes line up at the equator
Anaphase II
Sister chromatids separate and are pulled towards opposite poles

21. Meiosis Meiosis II Telophase II
New nuclear envelopes form around all four new haploid nuclei
Cytoplasmic division results in four cells each containing a haploid number of unduplicated chromosomes

22. Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II

23. Meiosis Meiosis introduces genetic variations in traits
Two parents both contribute genes to their offspring
One of each autosomal chromosome and one sex chromosome are passed to offspring
Thus offspring get two of each type of chromosome
Each pair of chromosomes carries the same genes
Genes may not be identical
The differences or traits of the same gene are called alleles

24. Meiosis
Meiosis introduces genetic variations (mixes of different alleles) in traits
Crossing over
Prophase I
Random alignment
Metaphase I

25. Meiosis Results of meiosis
1 diploid parent cell  4 genetically variable haploid daughter cells

26. From gametes to offspring
Sexual reproduction
Meiosis  four haploid cells
Gamete formation
Sperm formation
Ova/Egg formation
Fertilization
Fusion of the haploid nuclei of the ovum and sperm
Creates a diploid zygote
Adds to variation
Which sperm fertilizes which egg is a matter of chance (see next chapter!)

27. Questions When homologs swap DNA this is called?
T or F: Homologs have a predictable pattern they use to line up during metaphase I.
What is separated during Anaphase I?
What is separated during Anaphase II?
What processes of meiosis contribute to genetic variation?
How many cells are produced by meiosis?

28. Disorders in Chromosome Number
During meiosis homologs may fail to separate
Referred to as nondisjunction
Affects the chromosome number at fertilization
Normal gamete (n) + nondisjunction gamete (n + 1) = zygote (2n +1)
Trisomic zygote will have three of one type of chromsome and two of every other type
Normal gamete (n) + nondisjunction gamete (n 1 1) = zygote (2n -1)
monosomic zygote will have one of one type of chromsome and two of every other type

29. Metaphase I Anaphase I Telophase I Metaphase II Anaphase II
Telophase II
Figure An example of nondisjunction during meiosis. Of the two pairs of homologous chromosomes shown here, one fails to separate during anaphase I. The chromosome number is altered in the resulting gametes.
Stepped Art
Fig , p. 212

30. Disorders in Chromosome Number
Trisomy and monosomy
Trisomy (polyploid) is common in some plants, insects, and fish
Usually fatal in humans
Trisomy 21 individuals will survive infancy
Down’s syndrome

31. Disorders in Chromosome Number
Changes in Sex Chromosome Number
Turner syndrome (XO)
Inherit an unstable Y from the dad
Female, short, non sexually mature
XXX syndrome
Usually does not result in physical or medical problems
Klinefelter syndrome (XXY)
Overweight, tall, normal intelligence, estrogen > testosterone
XYY syndrome
Taller than average, mild mental impairment
NOT predisposed to a life of crime

32. Chromosome Structural Rearrangements
Changes in chromosome structure
Are rare
Usually cause drastic health effects
Responsible for ~1/2 of miscarriages
Genetic disorders
Sometimes evolutionarily important
Occur spontaneously or induced by exposure to certain chemicals or radiation

33. Chromosome Structural Rearrangements
Changes in chromosome structure
Duplication
Happens during prophase I of meiosis
Crossing over occurs unequally between homologs
One chromosome will have a deleted segment
The other will have the duplication
Huntington’s

34. Chromosome Structural Rearrangements
Changes in chromosome structure
Deletion
The loss of some portion of a chromosome
Cause serious disorders and are often lethal in mammals
Cri-du-chat
Deletion in chromosome 5
Causes mental impairment and an abnormally shaped larynx

35. Chromosome Structural Rearrangements
Changes in chromosome structure
Inversion
A segment of DNA is flips in the reverse direction
Usually no genes are lost
Causes infertility
Inverted segments cause homologs to mispair during meiosis

36. Chromosome Structural Rearrangements
Changes in chromosome structure
Translocation
A piece of one chromosome may break and attach
to a different chromosome or
to a different part of the same chromosome
Burkett's lymphoma
reciprocal translocation between chromosomes 8 and 14
Can cause infertility (mis-pairing during meiosis)

37. Chromosome Structural Rearrangements
Chromosome changes in Evolution
Large-scale changes in chromosomes can lead to speciation
Most are usually lethal or cause genetic disorders and infertility
In a very few instances chromosome changes can be adaptive
Multiple globin chain genes possibly arose due to duplications
An individual homozygous for an inversion could become the founder of a new species

38. Summary Sexual and Asexual Reproduction Meiosis Errors in Meiosis
Meiosis I
Meiosis II
Variation
Errors in Meiosis
Chromosome number
Chromosome rearrangements