Concepts of Biology: The Cellular Basis of Inheritance

1. Concepts of Biology: The Cellular Basis of Inheritance

2. Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (Credit a: modification of work by Frank Wouters; credit b: modification of work by Ken Cole, USGS; credit c: modification of work by Martin Pettitt)

3. Meiosis and Sexual Reproduction • Every eukaryote, from yeast to you, uses meiosis to create gametes. Meiosis is at the core of sexual reproduction. • Why does this process predominate? • Why don’t organisms just clone themselves? Or reproduce asexually?

4. Meiosis and Sexual Reproduction • Cloned organisms are uniformly susceptible. • Evolutionarily successful organisms need variation to buffer against the changing nature of the environment. • The mixing of parental genes is what meiosis accomplishes for each new generation.

5. In animals, sexually reproducing adults form haploid gametes from diploid germ cells.

6. Fungi, such as black bread mold (Rhizopusnigricans), have haploid-dominant life cycles.

7. Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. (Credit c: “fern”: modification of work by Cory Zanker; credit c: “gametophyte”: modification of work by “Vlmastra”/Wikimedia Commons)

8. Meiosis • Meiosis occurs in germ tissue (e.g., testes and ovaries). • Meiosis is reductional – DNA is copied once and divided twice so that the products of the second division are haploid gametes (e.g., sperm and eggs). • The process ensures mixing and new combinations will be produced.

10. Crossing Over • In the previous illustration of the effects of crossing over, the blue chromosome came from the individual’s father and the red chromosome came from the individual’s mother. • Crossover occurs between non-sister chromatids of homologous chromosomes. • The result is an exchange of genetic material between homologous chromosomes. • The chromosomes that have a mixture of maternal and paternal sequences are called recombinant and the chromosomes that are completely paternal or maternal are called non-recombinant.

11. Variation • Crucial to this mixing is crossing over in prophase I. • After homologues find each other and pair perfectly, random portions are exchanged. • It is impossible to create gametes identical to the gametes that created you.

13. Independent Assortment • Synapsed homologous pairs orient randomly on the metaphase plate after prophase I. • This behavior results in independent assortment. • Along with crossing over, it is the source of new genetic combinations.

14. CONCEPT IN ACTION • Review the process of meiosis, observing how chromosomes align and migrate, at the website link. • http://openstaxcollege.org/l/animal_meiosis2

15. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes. In anaphase I, the homologous chromosomes are separated. In prometaphaseII, microtubules attach to individual kinetochores of sister chromatids. In anaphase II, the sister chromatids are separated.

17. CONCEPT IN ACTION • For an animation comparing mitosis and meiosis, go to the website link. • http://openstaxcollege.org/l/how_cells_dvid2

18. This karyogram shows the chromosomes of a female human immune cell during mitosis. (Credit: Andreas Bolzer, et al.)

19. Autosomes vs. Sex Chromosomes • All humans have 22 pairs of autosomes. • Females have a pair of X chromosomes and males have an X and a Y chromosome.

20. Errors in Meiosis • The orchestration of the process of meiosis is complex and often fails. • About 50% of spontaneous abortions and miscarriages are thought to have their origins in meiotic mistakes. • Nondisjunction is the name for failures in meiosis.

22. Nondisjunction • Mistakes can happen in either division. • In both males and females, nondisjunction seems to increase with age. • Nearly always, extra or missing chromosomes are fatal during embryogenesis. • However, an extra copy of chromosome 21 (Down syndrome) and X and Y are tolerated.

23. The incidence of having a fetus with trisomy 21 increases dramatically with maternal age.

24. CONCEPT IN ACTION • Visualize the addition of a chromosome that leads to Down syndrome in this video simulation. • http://openstaxcollege.org/l/down_syndrome2

25. Embryonic inactivation of one of two different X chromosomes encoding different coat colors gives rise to the tortoiseshell phenotype in cats. (Credit: Michael Bodega)

26. X Chromosome Inactivation • Males have one X chromosome and females have two. • In early development, females randomly shut down one of their X chromosomes leading to a mosaicism that can be detected in a calico cat.

27. This individual with cri-du-chat syndrome is shown at various ages: (A) age two, (B) age four, (C) age nine, and (D) age 12. (Credit: Paola CerrutiMainardi)

28. An (a) inversion occurs when a chromosome segment breaks from the chromosome, reverses its orientation, and then reattaches in the original position. A (b) reciprocal translocation occurs between two nonhomologous chromosomes and does not cause any genetic information to be lost or duplicated. (Credit: modification of work by National Human Genome Research Institute (USA))

29. LTS Activity • http://outreach.letstalkscience.ca/component/zoo/item/diy-activities-2.html?Itemid=652