Cellular Basis of Reproduction and Inheritance

1. Cellular Basis of Reproduction and Inheritance Chapter 12 and 13

2. Objectives • Describe binary fission in bacteria • Describe the structures that play roles in the mitotic phase of the cell cycle: the centrioles, spindle microtubules and chromosomes • Outline the phases of the cell cycle • Describe the factors that control cell growth and how cancer results from a breakdown of this control • Outline the general progression and overall results of meiosis, contrasting them with mitosis

3. Explain how meiosis provides possibilities for genetic recombination

4. Introduction • Life cycle is sequence of life forms from one generation to next • Sexual reproduction involves passing traits from two parents to next generation • Asexual reproduction involves passing traits from one parent to next generation • Cell division is basis of all processes that link phases of life cycle

5. Like beget like (more or less) • True only for organisms that reproduce asexually • single-celled organisms reproduce asexually by dividing in two • called binary fission • daughter cells receive identical copy of parent’s genes

6. offspring of multi-cellular organisms not genetically identical to parents • unique combination of parents traits • breeders of domestic plants and animals manipulate sexual reproduction by selecting offspring that exhibit desired traits

7. Cells arise from preexisting cells • cell reproduction called cell division • two roles • enables fertilized egg to develop through various stages to adult organism • ensures continuity from generation to generation

8. Binary Fission • Bacterial chromosomes • genes carried on single circular DNA molecule • up to 500x cell length • minimal packaging • complexed with few proteins and attached to plasma membrane at one point

9. Binary fission • prior to cell division, genome copied • copies attached to adjacent parts of membrane • cell elongation and new plasma membrane separates two genomes • plasma membrane pinches through cell

10. Eukaryotic Cell Division • Eukaryotes have large, complex, multiple chromosomes • human cells contain 50,000-100,000 genes • organized into separate, linear chromosomes • DNA complexed with proteins • Just prior to division, chromosomes become visible • remain visible during division process

11. Somatic (body) cells contain 2x chromosomes (diploid) compared to sex cells (haploid) • human cells: • somatic cells-46 chromosomes (2n=46) • sex cells-23 chromosomes (n=23)

12. Prior to cell division, chromosomes are duplicated • visible chromosomes consist of two identical sister chromatids attached at centromere • sister chromatids are divided among daughter cells (now chromosomes) • each cell gets identical set of chromosomes

13. Cell cycle results in cell multiplication • most cells in organism divide on regular basis • dividing cells undergo cycle-sequence of steps repeated during each division

14. Cell cycle divided into several steps • interphase represents 90% or more of cycle time • G1-cell increases in size and increases supply of proteins and organelles • S-DNA synthesis occurs • G2-cell prepares for division, increases supply of proteins necessary for division

15. mitotic (division) phase divided into two steps • mitosis-nuclear division • cytokinesis-cytoplasmic division • result is two daughter cells with identical chromosmes

16. Mitosis • While continuum, several established dividing points for cell cycle phases • Interphase: duplication of genetic material, ends with visible chromosomes • Prophase: mitotic spindle forms from MTOC’s; ends when chromatin coiled into chromosomes; nucleoli and nuclear membrane dissolved

17. Metaphase: spindle formed; chromosomes aligned single file with centromeres on metaphase plate • Anaphase: chromosomes separate; migrate to spindle poles • Telophase: reverse of prophase • Cytokinesis: division of cytoplasm • movement of chromosomes driven by addition or subtraction of protein subunits to kinetichore end of spindle microtubules

18. Cytokinesis differs in plants and animals • in animals, ring of microfilaments contracts around periphery of cell • forms cleavage furrow that eventually divides cytoplasm

19. in plants, vesicles containing cell wall material collect on spindle equator • vesicles fuse from inside out forming cell plate • cell plate gradually develops into new cell wall between new cells • membranes surrounding vesicles fuse to form new parts of plasma membranes

20. Factors Affecting Cell Division • Control of cell division important for proper growth, development and repair of organisms • growth factors regulate cell division • product of dividing cell • most plant and animal cells will not divide unless in contact with solid surface-anchorage dependence

21. division usually stops when single layer of cells formed and cells touch-density-dependent inhibition • due to depletion of growth factor proteins in cell mass

22. Growth Factors • Three major check points in cell cycle • G1 of interphase • G2 of interphase • M phase • Release of growth factor at each of these checkpoints allows cell cycle to continue

23. Cancer • Cancer cells not affected by growth factors that regulate density-dependent inhibition • malignant tumor-metastasize • benign-no metastasis • named for organ or tissue of origin • some cancer cells produce factors that keep them dividing

24. Benign tumor becomes malignant when cancerous cells from tumor mass spread to new sites and continue to proliferate • movement mediated by either blood or lymph systems

25. Common treatments for cancer • radiation-disrupts normal processes of cell division; cancer cells more susceptible • chemotherapy-disrupt cell division

26. Meiosis • Chromosomes are matched in homologous pairs • share shape, genetic loci; carry genes controlling same traits • each homologue inherited from separate parent • in humans, 22 pairs are autosomes, remaining pair sex chromosomes • female-two X chromosomes • male-one X and one Y chromosome

27. Gametes have single set of chromosomes • somatic cells have two sets of homologues • diploid (2n) • sex cells have one set of homologues • haploid (n) • produced by meiosis • sexual life cycle involves alternation between diploid and haploid • fusion of haploid gametes at fertilization results in diploid zygote

28. Meiosis reduces chromosome number from diploid to haploid • occurs only in diploid cells • preceded by single duplication of chromosomes • results in four haploid daughter cells • consists of two consecutive phases: • meiosis I-halving of chromosome number • meiosis II-separation of sister chromatids

29. Comparison of mitosis and meiosis • all unique events in meiosis occur in meiosis I • crossing over during prophase I • separation of homologous pairs during anaphase I • meiosis II virtually identical to mitosis • starting cells are haploid • mitosis results in two daughter cells with same number of chromosomes as parent cells • can occur in either diploid or haploid cells

30. meiosis results in four daughter cells with half number of chromosomes as parent cells • only occurs in diploid cells

31. Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring • during prophase I each homologue pairs up with its “other” • during anaphase I maternally and paternally inherited homologues move to one pole or other independently of other pairs

32. for n chromosomes, there are 2n different combinations of half pairs • for humans, 223 different combinations • there are 223x223 combinations possible at fertilization (64 billion)

33. Homologous chromosomes carry different versions of genes • Crossing over increases genetic variability • exchange of corresponding segments between two homologues • site of crossing over called chiasma • occurs between chromatids within tetrads as homologues pair up during synapsis

34. produces new combinations of genes-genetic recombination • can occur several times in variable locations • variability much greater than calculated • two individual parents can never produce identical offspring from separate fertilizations