Download Cell Cycle

Chapters 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 Ch12/13
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

Cellular Basis of Reproduction
and Inheritance
Chapter 12 and 13
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 about 30,000-35,000 genes
 organized
into separate, linear chromosomes
 DNA
complexed with proteins
 Just prior to division, chromosomes become
visible
 remain
visible during division process
Somatic Cells


Somatic cells are body cells (not sex cells)
Ex. Hair cells
These cells need to contain the full set of chromosomes so
that all the directions for functions and activities of the cell
can be carried out.
Normally you inherit 23 chromosomes from each of your
parents

This complete set of chromosomes (46) is known as the
Diploid Number in Humans
Sex Cells (Gametes)

Sex cells are known as gametes

These cells have half of the number of chromosomes
that a body cell would have.

In humans this number is 23

Sooooo..Somatic (body) cells contain the diploid
number of chromosomes compared to sex cells
(haploid number)
 human
cells:
 somatic cells-46 chromosomes (2n=46)
 sex
cells-23 chromosomes (n=23)
What is a chromosome??????

Prior to cell division, chromosomes are duplicated
 visible
chromosomes consist of two identical sister
chromatids attached at centromere
 sister
chromatids are able to be separated…
 Once
sister chromatids separate they are again called
chromosomes
I
know you are all thinking:
WHAAAAAAATTTTTT????????
Lets tie it all together
Humans have 23 pairs of chromosomes
 They get numbers 1-23 from Mom and 1-23 from
Dad = 46
 These 46 chromosomes are found in somatic
cells
 Sex cells ( gametes) have only 23
 Each species has a specific diploid number

Cell Cycle

The cell cycle is like a “ alarm clock” that tells the
cell when it is time to do some essential activities
and when to divide.

It is regulated by many chemicals inside the cell.
Cell Cycle

Cell cycle results in cell division
many
cells in an organism divide on regular
basis
dividing cells undergo cycle: sequence of steps
repeated during each division
Cell Cycle Cont.

Cell cycle divided into several steps (phases)
 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, checks for DNA damage
 G0 –
cell stops progressing through cycle- will not divide
Cell Cycle

G0 = This is a very important phase of cellular
activity

The cell has the opportunity to stop progressing
towards division, or DNA synthesis

Why would this be important for a cell??????

Cells can phase into and out of G0 from several
other cell cycle phases, its like an escape hatch
Cell Cycle Cont.

Different cells are in various phases of cycle even in
same tissue

Also Different Tissues May Regulate Cycle Differently

Ex. Hair Divides Constantly

Nerve Tissue Never Divides In Adults

Adult Liver Tissue Does Not Divide, Except For Repair
Cell Cycle cont.

How does a cell progress through the cell cycle?

Many biochemicals stimulate the transition

One of them is a Kinase

A Kinase is an enzyme that catalyzes the transfer of
a phosphate group from ATP to another molecule.
How does a Kinase work ?

It works a bit like turning on a light switch….
A PO4 is taken off ATP:
 AT-PO4 - PO4 - PO4  AT-PO4 - PO4 + PO4

The PO4 is placed onto an enzyme, which activates
the enzyme
 The enzyme ( and many other chemicals) now tell
the cell to move to the next phase of its cell cycle

Soooooooooooooooooooooooooooo
 If
you are thinking…. Who cares????
 How
 Get
is this relevant to my life?????
ready to write down the ways!
Cyclins

Cyclins are special chemicals that make the cell
cycle go around

There are many different types
Cell Cycle cont.

A Cdk is a cyclin dependent kinase

MPF is a co- chemical that is attached to Cdk

These chemicals stimulate the transition to cell division.
When they are HIGH, the cell will divide

Why do we care about this?

BECAUSE CYCLIN AND CDK LEVELS ARE
ALTERED IN CANCER CELLS……..
Mitosis: Somatic Cell Division
 mitotic
(division) phase divided into two steps:
 mitosis-nuclear
division
 cytokinesis-cytoplasmic
 result
division
is two daughter cells with identical
chromosmes
Mitosis
Somatic cells in humans have 46 chromosomes
 At the end of mitosis will they be diploid or
haploid and why?????

Mitosis

Interphase: not part of division; Cell does other
work

Prophase (division beginning): 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; MAD
 Anaphase:
sister chromatids separate; migrate to poles
 Telophase:
reverse of prophase
 Cytokinesis:
 movement
division of cytoplasm
of chromosomes driven by addition or
subtraction of protein subunits to kinetochore 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
In Normal Cells

In mitotic normal mammal cells division only
occurs 20-50 times prior to cell death.

Telomeres are the “cell clocks” that govern cell
longevity

Telomeres shorten with each division; after about
fifty times they reach a critical length and a
division cessation signal is given
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
Density Dependent Inhibition
 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
Three Cell Cycle Checkpoints

Three major check points in cell cycle
 G1
of interphase
 G2 of interphase
 M phase

Release of growth factor/ chemical signals at each
of these checkpoints allows cell cycle to continue

The cell will ultimately divide if not halted at a
checkpoint
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
Cancer cells and telomerase

Keeps telomeres lengthened

Cells keep dividing; cells with short telomeres
should stop manufacturing this enzyme

Not so simple cells in mice lacking telomerase
also became cancerous
Common treatments for cancer:
 radiation-disrupts normal processes of cell division;
cancer cells more susceptible


chemotherapy-disrupt cell division
Cell Death

Cells die two ways:


Necrosis- from damage, poisons,starvation, hypoxia,
ATP depletion
Apoptosis- genetically programmed cell death;
often normal in developmental pathways
Apoptosis
sunburned cells

Also extends damage after a stroke

Cancer cells loose ability to carry out
apoptosis become a problem
Meiosis CH 13

Chromosomes are matched in homologous pairs
 share
shape, genetic loci; carry genes controlling same
traits- alleles
 each homolog 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
Question

Are X AND Y Homologous?
Gametes

Normal Gametes have single set of chromosomes- No
Pairs
 somatic

cells have two sets of homologues
diploid (2n)
 sex
cells(gametes) 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 ( embryo)
Meiosis

Meiosis reduces chromosome number from
diploid to haploid
 occurs
only in diploid cells destined to become
gametes
 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
PROPHASE I
METAPHASE I
2n- diploid
2n- diploid
nuc. memberane breakdown
homologs pair; synapsis,
chiasmata
DNA condenses
Spindle app. forms
Homologs aligned in
cell center =equatorial plate
(MAD genes)
ANAPHASE I
2n- diploid
TELOPHASE I
n- haploid
(end)
Each cell new
haploid
Homologs
pulled apart
Short interphase
no S phase-No
DNA synthesis
PROPHASE II
n- haploid
nuc. membrane breakdown
DNA condenses
Spindle app. forms
METAPHASE
II
n- haploid
Chromosomes aligned in
cell center = equatorial plate
ANAPHASE II
n- haploid
TELOPHASE II
n- haploid
The other cell from
Telophase I also divides
into 2 cells so
4 cells
total :each haploid
: 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
 Except starting cells are haploid
 mitosis results in two daughter cells with same
number of chromosomes as parent cells but meiosis
results in 4 haploid 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 that will become
gametes

Cells only run thru meiosis I and II ONCE
Why?

Independent assortment of chromosomes in
meiosis and random fertilization lead to varied
offspring
 during
prophase I each homologue pairs up with its
“partner of the same number”
 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 haploid 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
Visual Comparison of Mitosis and Meiosis