1. Chapter 13: Meiosis and Sexual Life Cycles

2. Cell Theory
Cell Theory: Living things are made of cells and cells reproduce from other cells
Asexual Reproduction: Cells divide creating identical cells
A clone is a group of genetically identical individuals from the same parent
Sexual Reproduction: Gametes fuse to create zygote
Unique combination of chromosomes gives rise to offspring with unique set of characteristics

3. Asexual Reproduction All prokaryotes
Some protists, invertebrates, plants
Some multicellular organisms reproduce both sexually and asexually
Budding: creation of new individuals from growths of existing individuals
Fission: splitting of one organism into 2
Fragmentation and Regeneration: Breaking of body part, followed by growth and development

4. Asexual Reproduction Advantages: Disadvantages:
Sessile animals can reproduce without mate
Produce many offspring quickly, without energy invested in sperm and eggs or in mating
Produce offspring with optimal genetics for environment
Disadvantages:
Genetically uniform populations
Prevents adaptation of population in changing environment

5. Sexual Reproduction
Most animals and plants reproduce by sexual reproduction
Offspring created by fertilization= fusion of 2 haploid gametes to produce a diploid zygote
Egg, Sperm
Offspring has unique genetic characteristics

6. Sexual Reproduction Advantages Disadvantages
High genetic variability among offspring
High variability= greater chance for adaption for changing environments
Evolution by natural selection
Disadvantages
Must find mate
High energy requirements for courtship, gamete production

7. Living Cells
Genetics is the scientific study of heredity and variation
Heredity is the transmission of traits from one generation to the next
Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

8. Living Cells Sexual Reproduction: Gametes fuse to create zygote
Gametes= egg and sperm cells (Haploid)
Genes are the units of heredity, and are made up of segments of DNA
Genes are passed from parent to offspring through gametes
Fertilization= Fusion of egg and sperm cell
Zygote= Fertilized egg (Diploid)
After fertilization, zygote goes from a single cell to a multicellular organism through mitosis

9. Gametes Gametes contain haploid number of chromosomes
Single set of autosomes + one sex chromosome
Haploid gametes conserve original number of chromosomes in offspring
If two diploid cells (2n) fused, resulting offspring would be 4n
If two haploid cells (n) fused, resulting offspring would be 2n

10. Chromosomes Sex chromosomes Autosomes Determine sex of offspring
Genes can have other functions too
Mammals= Males have XY chromosomes, Females have XX chromosomes
Female gametes= all contain X chromosome
Male gametes= 50% contain X and 50% contain Y
Autosomes
Non-sex chromosomes
In humans, 22 pairs of autosomes

11. Human Karyotype
Haploid (n)=23
Diploid (2n)=46

12. Chromosome Terminology
Diploid Cell: 2n = 6
Haploid= n= 3
Following DNA synthesis, each of the six chromosomes consists of 2 sister chromatids, connected at the centromere
Sister Chromatids
Homologous
Chromosome
Centromere
Blue: Paternal Chromosomes
Pink: Maternal Chromosomes

13. Chromosome Terminology
Locus: location of gene on chromosome
Maternal and paternal chromosomes can have different versions of the same gene
Allele: alternative version of a gene

15. Plants and some algae exhibit an alternation of generations
Life cycle has a diploid and haploid multicellular stage
The diploid organism, called the sporophyte, makes haploid spores by meiosis
Each spore grows by mitosis into a haploid organism called a gametophyte
A gametophyte makes haploid gametes by mitosis
Fertilization results in a diploid sporophyte

16. In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage
The zygote produces haploid cells by meiosis
Each haploid cell grows by mitosis into a haploid multicellular organism
The haploid adult produces gametes by mitosis

17. Life Cycles
Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis
However, only diploid cells can undergo meiosis
In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring

18. Meiosis
Like mitosis, meiosis is preceded by the replication of chromosomes
Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II
The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis
Each daughter cell has only half as many chromosomes as the parent cell

19. Meiosis Cell division Two consecutive nuclear divisions + cytokinesis
Cells go through G1, S, and G2 phases prior to meiosis
All chromosomes duplicated during S phase
Two consecutive nuclear divisions + cytokinesis
Meiosis 1: separates homologous chromosomes
Cells are haploid after end of Meiosis 1
Meiosis 2: separates sister chromatids
Similar to Mitosis
End Result: 4 haploid gametes

20. Meiosis in Humans N=23 N=46 N=46 N=23 N=23 N=23 N=23 N=23 Meiosis 2G1
S phase/G2
N=46
N=46
N=23
N=23
N=23

21. Prophase I= ~90% of the time required for meiosis
Chromosomes begin to condense
In synapsis, homologous chromosomes loosely pair up, aligned gene by gene
Each pair of chromosomes forms a tetrad, a group of four chromatids
Crossing over can occur
Nonsister chromatids exchange DNA segments
Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

22. Metaphase I
Tetrads line up at the metaphase plate, with one chromosome facing each pole
Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad
Microtubules from the other pole are attached to the kinetochore of the other chromosome

23. Anaphase I
Pairs of homologous chromosomes separate
One chromosome moves toward each pole, guided by the spindle apparatus
Sister chromatids remain attached at the centromere and move as one unit toward the pole

24. Telophase and Cytokinesis I
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells
Animal cells= Cleavage furrow
Plant cells= Cell plate

25. Meiosis II
Two haploid daughter cells undergo another round of cell division in Meiosis II
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated
Meiosis II is very similar to mitosis
Separation of sister chromatids

26. Meiosis II

27. Meiosis II
At the end of meiosis, there are four daughter cells, each with a haploid set of individual chromosomes
Each daughter cell is genetically distinct from the others and from the parent cell
Crossing over causes differences in sister chromatids

28. Mitosis vs Meiosis
Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell
Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

29. Mitosis
Meiosis
End Result:
2 Identical
Cells
End Result:
4 Unique Gametes

30. Mitosis vs Meiosis

31. Mitosis vs Meiosis
Three events are unique to meiosis, and all three occur in meiosis l
Synapsis and crossing over in prophase I: Homologous chromosomes physically connect and exchange genetic information
At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

32. Meiosis and Genetic Diversity
Natural selection results in the accumulation of genetic variations favored by the environment
Sexual reproduction contributes to the genetic variation in a population, which originates from mutations, through multiple processes:
Independent orientation of chromosomes
Random Fertilization
Homologous chromosomes can have different versions of genes
Crossing Over

33. 1. Independent orientation of chromosomes
The number of combinations possible when chromosomes sort independently into gametes is 2n, where n is the haploid number

34. 2. Random Fertilization
Potential for any chromosome combination to be fused with any other chromosome combination
Fusion of 2 human gametes: 70 trillion possible combinations
Each zygote has a unique genetic identity

35. 3. Homologous chromosomes can have different versions of a gene
Mutations (changes in an organism’s DNA) are the original source of genetic diversity
Mutations create different versions of genes called alleles
Reshuffling of alleles during sexual reproduction produces genetic variation

36. 3. Homologous chromosomes can have different versions of a gene
Meiosis 1
Meiosis 2 G1
S phase

37. 4. Crossing Over: Exchange of genetic information
Occurs during Prophase 1 in meiosis
Homologous chromosomes pair up and form tetrads
Sister chromatids from 2 chromosomes “cross-over” at chiasmata
Exchange of DNA
Genetic Recombination= production of gene combinations different from parent cell
Homologous Chromosomes
Sister Chromatids
Genetic Recombination

38. What happens when meiosis goes wrong?

39. Failure to Separate
Nondisjunction= chromosome pair does not separate in meiosis 1 or 2
Gamete created with extra chromosome
Majority of gametes with wrong number of chromosomes will not produce viable offspring
Trisomy 21= Downs Syndrome
1 out of 700 children born
Chance increases with age of mother
Produces physical effects
Round face, flattened nose, small teeth, short stature, failure to sexually develop
Susceptible to heart defects, respiratory infections, leukemia, Alzheimer’s disease

40. What about sex cells?
Abnormal numbers of sex cells do not usually have an effect on survival
XXY= Klinefelter Syndrome
Small testes, sterile, develops female body characteristics
XYY= Normal male
Taller than normal
XXX= Normal female
No obvious effects
XO= Turner Syndrome
Short stature, web of skin between shoulders and neck, sterile and lack of sexual development
Only case where 45 chromosomes is not fatal in humans

41. Errors can give rise to new species
Plants
Polyploidy= errors in meiosis resulted in more than 2 sets of chromosomes
Plants can self-fertilize, preventing the altered chromosome number from disrupting reproduction
Creates a new species
~50% of plants are polyploid
Wheat, potatoes, apples, cotton
Animals
Less common in animals
Fishes and amphibians
Rat= recent discovery in Chile
Rainbow Trout 2n=60