1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles

2. Hereditary Similarity and Variation
Living organisms
Are distinguished by their ability to reproduce their own kind
Only oaks can produce oaks and elephants can make more elephants

3. Genetics Is the scientific study of heredity and hereditary variation
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings

4. Offspring acquire genes from parents by inheriting chromosomes
Parents endow their offspring with coded information in the genes
The tens of thousands of genes we inherit from parents
Genes are segments of DNA
Inherited information is passed on in the form of gene`s specific sequence (letters)
Similar with reading “apple” and imaginig it
Cells translate gentic “sentences” into visible characteristics

5. Transmission of hereditary traits has molecular basis in the replication of DNA
Produces the copies which are passed from parents to offspring
Gametes- in animals and plants reproductive cells; vehicles that transmit genes from one generation to the next

6. DNA of eukaryotic cells is in nucleus
Exception: small amounts in mitochondria and chloroplasts
All living species have characteristic number of chromosomes
46 in human somatic cells

7. Each gene in an organism’s DNA
Has a specific locus on a certain chromosome
We inherit
One set of chromosomes from our mother and one set from our father

8. Comparison of Asexual and Sexual Reproduction
In asexual reproduction
One parent produces genetically identical offspring by mitosis
Exact copy of themselves
Individual that reproduces asexually
gives rise to a clone (group of
genetically identical individuals)
Figure 13.2
Parent
Bud
0.5 mm

9. In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Not exact replicas, but variations

10. Fertilization and meiosis alternate in sexual life cycles
A life cycle
Is the generation-to-generation sequence of stages in the reproductive history of an organism

11. Sets of Chromosomes in Human Cells
In humans
Each somatic cell has 46 chromosomes, made up of two sets (23 each)
One set of chromosomes comes from each parent
Upon condensation they can be examinated by microscopy

12. 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

13. Homologous chromosomes
Are the two chromosomes composing a pair
Have the same characteristics
May also be called autosomes
Same length, centromere position and staining pattern

14. 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
Only small parts of X and Y are homologous
They determine an individual sex

15. We inherit one chromosome pair from each parent
Maternal set (from mother)-one set of 23 chromosomes
Parental set (from father)-one set of 23 chromosomes
We represent the number of chromosomes in a single set as n

16. A diploid cell Has two sets of each of its chromosomes
In a human has 46 chromosomes (2n = 46)
Number of chromosomes in somatic cells

17. Unlike somatic cells
Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes
For humans haploid number is 23
Number of chromosomes found in gamete
The set of 23 consists of 22 autosomes and one sex chromosome

18. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity
The ovaries and testes produce haploid gametes by meiosis
Human life cycle begins when those two haploid cells fuse
Fusion is called fertilization
Result- fertilized egg (zygote) is diploid
Contains two haploid sets of chromosomes

19. Meiosis Takes place in two sets of divisions, meiosis I and meiosis II
These division result in four daugther cells
Each with only half as many chromosomes as parent cell

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

21. Interphase Chromosomes replicate during S phase but remain uncondensed
Each replicated chromosome consists of 2 genetically identical sister chromatides

22. Prophase I Chromosomes begin to condense
Homologous chromosomes loosely pair along their lengths
Kinetochores attach to microtubules from one pole to the other (late prophase I)

23. Metaphase I
Pairs of homologous chromosomes are arranged on the metaphase plate
One chromosome of each pair facing each pole
Both chromatids of homologue are attached to kinetochore microtubules from one pole

24. Anaphase I
Chromosomes move toward the poles guided by spindle apparatus
Sister chromatides remain attached at the centromere
Homologous chromosomes each composed of two sister chromatids move toward opposite pole

25. Telophase I and cytokinesis
Each half of the cell has a complete haploid set of chromosomes
But each chromosome is still composed of two sister chromatids
Cytokinesis occurs simultaneously with telophase I forming two haploid daughter cells
Formation of cleavage furrow

26. No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II
Since chromosomes are already replicated

27. Prophase II Spindle apparatus forms
Chromosomes move toward the metaphase II plate (late prophase II, not shown on the figure)

28. Metaphase II
The chromosomes are positioned on the metaphase plate as in mitosis
Sister chromatides are not genetically identical
Kinetochores of sister chromatides are attached to microtubules extending from opposite poles

29. Anaphase II Centromers separate, sister chromatids come apart
Sister chromatids move as two individual chromosomes toward opposite poles

30. Telophase II and cytokinesis
Nuclei form, chromosomes begin to decondensing and cytokinesis occurs
Meiotic division of one parent cell produces four daughter cells
Each with haploid set of (unreplicated) chromosomes
Each daughter cell is genetically distinct from another one and from parent cell

31. A Comparison of Mitosis and Meiosis
Meiosis can be distinguished from mitosis by three events in Meiosis l:
Synapsis and crossing over- during prophase I (synapsis-homologous chromosomes become physically connected; crossing over- rearrangment between non-sister chromatids)
Tetrads on metaphase plate- at metaphase I paired homologous chromosomes (tetrads) are so positioned
Separation of homologues-at anaphase I

32. A Comparison of Mitosis and Meiosis

33. Evolutionary Significance of Genetic Variation Within Populations
Is the raw material for evolution by natural selection
Natural selection results in the accumulation of those genetic variations favored by environment

34. Mutations Sexual reproduction
Are the original source of genetic variation
Sexual reproduction
Produces new combinations of variant genes, adding more genetic diversity

35. Summary Chapter 13
Heredity/variation
Genes
Asexual and sexual reproduction
Life cycle
Meiosis
Difference between mitosis and meiosis