Transcript CH 12 - Pegasus @ UCF

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
 up
carried on single circular DNA molecule
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
 cell
attached to adjacent parts of membrane
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
 most
of dividing cell
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
systems
mediated by either blood or lymph

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
 sex
(2n)
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
n chromosomes, there are 2n different
combinations of half pairs
 for
humans, 223 different combinations
 there are 223x223 combinations possible at
fertilization (64 billion)
 for
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