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Cellular Basis of Reproduction and Inheritance
Chapter 12 and 13
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
Explain how meiosis provides possibilities for genetic recombination
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
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
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
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
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
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
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
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)
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
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
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
mitotic (division) phase divided into two steps mitosis-nuclear division cytokinesis-cytoplasmic division result is two daughter cells with identical
chromosmes
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
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
Cytokinesis differs in plants and animals in animals, ring of microfilaments contracts
around periphery of cell forms cleavage furrow that eventually divides
cytoplasm
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
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
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
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
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
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
Common treatments for cancer radiation-disrupts normal processes of cell
division; cancer cells more susceptible chemotherapy-disrupt cell division
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
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
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
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
meiosis results in four daughter cells with half number of chromosomes as parent cells only occurs in diploid cells
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
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)
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
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