Cellular Basis of Reproduction and Inheritance Chapter 12 and 13

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

F Explain how meiosis provides possibilities for genetic recombination

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

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

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

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

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

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

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

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

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

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

n Cell cycle divided into several steps u interphase represents 90% or more of cycle time F G 1 -cell increases in size and increases supply of proteins and organelles F S-DNA synthesis occurs F G 2 -cell prepares for division, increases supply of proteins necessary for division

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

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

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

n Cytokinesis differs in plants and animals u in animals, ring of microfilaments contracts around periphery of cell F forms cleavage furrow that eventually divides cytoplasm

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

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

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

Growth Factors n Three major check points in cell cycle u G 1 of interphase u G 2 of interphase u M phase n Release of growth factor at each of these checkpoints allows cell cycle to continue

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

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

n Common treatments for cancer u radiation-disrupts normal processes of cell division; cancer cells more susceptible u chemotherapy-disrupt cell division

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

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

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

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

u meiosis results in four daughter cells with half number of chromosomes as parent cells F only occurs in diploid cells

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

u for n chromosomes, there are 2 n different combinations of half pairs F for humans, 2 23 different combinations F there are 2 23 x2 23 combinations possible at fertilization (64 billion)

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

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