How you became you
Life of a cell from the time it is first formed
until its own division into two daughter cells
The Cell Cycle
Reproduce by cell division
Unicellular organisms
100 µm
(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).
Reproduce by a type of cell division known
as Binary Fission
Bacterial chromosome replicates and two
daughter chromosomes actively move apart
Binary Fission
Origin of replication
E. coli cell Bacterial Chromoso me Cell wall Plasma Membrane Two copies of origin Origin Origin Chromosome replication begins.
Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.
1
Replication continues. One copy of the origin is now at each end of the cell.
2
Replication finishes. The plasma membrane grows inward, and
new cell wall is deposited.
3
Two daughter cells result.
Depend on cell division for three
main things:
◦
Development from a fertilized cell
◦
Growth
◦
Repair
Multicellular Organisms
20 µm 200 µm
(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).
(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).
Results in two genetically identical
daughter cells
Before division occurs, daughter cells
need to have an exact copy of the
genetic material (DNA)
DNA contains the blueprint (genome)
DNA molecules are packaged into
chromosomes
◦ Eukaryotes: Chromatin {DNA + Protein (histones)}
◦ Animals: Somatic Cells have 2 sets of chromosomes
◦ Gametes: Have 1 set of chromosomes
What is a gene?
A sequence of nucleotides that provides the
cell with instructions to make a RNA or a protein
◦ Average is 1000-4000 nucleotides per gene
Genes influence how cells, tissues and
organs appear
What is a chromosome?
Chromosome: where cells package DNA
Chromatin: Strings of DNA and associated
proteins called histones
◦ State of the DNA inside the nucleus when the cell is not dividing
Why is Chromatin condensed during cell
How many chromosomes do you
have?
Most humans have 23 pairs of
chromosomes
◦ Called a homologous pairs ◦ Autosomes: 1-22
◦ Sex Chromosomes: X and Y
Gametes: contain a single set of 23
chromosomes
Chromosome Structure
Consist of two thin rod-like structures of
DNA called sister chromatids
◦ Exact replicas of each other copied during DNA Replication
Before Cell Division
DNA is replicated and the chromosomes
condense
Each duplicated chromosome has two
Before Cell Division
0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatid s Centromeres Sister chromatids
A eukaryotic cell has multiple chromosomes, one of which
is represented here. Before duplication, each
chromosome has a single DNA molecule.
Once duplicated, a chromosome consists of two sister chromatids connected at the centromere.
Each chromatid contains a copy of the DNA molecule.
Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells.
Eukaryotic Cell Division consists of:
◦ Mitosis: Division of the Nucleus
◦ Cytokinesis: Division of the Cytoplasm
In Meiosis
◦ Sex cells are produced after a reduction in chromosome number
But…that’s next semester…
Regulation system for the cell division
process
Consists of the Mitotic phase and Interphase
The Cell Cycle
INTERPHASE
G1 S(DNA synthesis)
G2
The cell spends most of the time in
this phase
“Living” phase of the cell
Three main stages:
◦
G
1phase
◦
S phase
◦
G
2phase
Chromosomes are indistinct
Nucleolus may be visible
Centrioles are present
The cell grows and functions normally
Protein synthesis occurs
More organelles are produced
Until the cell divides it will stay in this phase
The cell duplicates its DNA
Cell resumes its growth in preparation for
division
Checks for any errors in the duplicated DNA
Mitosis and Cytokinesis occur
Consists of five phases:
◦
Prophase
◦
Prometaphase
◦
Metaphase
◦
Anaphase
◦
Telophase
Centrosomes
◦ Where the spindle arises from Centrioles
◦ Responsible for organization of the microtubules that form the spindle
Asters
◦ Star shaped system of microtubules formed around each centrosome
Kinetochores
◦ Responsible for attachment of the chromosomes and the microtubules that are part of the spindle
Mitotic Spindle
◦ Microtubules that control chromosome movement during Mitosis
Phases of Mitosis
G2 OF
INTERPHASE PROPHASE PROMETAPHASE
Centrosomes (with centriole
pairs) Chromatin(duplicated ) Early mitotic spindle Aster Centromer e Fragments of nuclear envelope Kinetochor e
Nucleolus Nuclearenvelop e Plasma membran e Chromosome, consisting
of two sister chromatids
Phases of Mitosis
Centrosome at one spindle pole
Daughter chromosomes
METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS
Some spindle microtubules attach to the
kinetochores of chromosomes and move them to the metaphase plate
Mitotic spindle: A closer
look
Centrosome Aster Sister chromatids Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Centrosome ChromosomesMicrotubules 0.5 µm
1 µm
In animal cells, a process called cleavage
occurs.
Cytokinesis: A Closer look
Cleavage furrow
Contractile ring of microfilaments
Daughter cells
100 µm
(a) Cleavage of an animal cell
(SEM)
Frequency of division varies with the type of
cell
The cell cycle is regulated by a molecular
control system
How do we know this?
Molecules in the cytoplasm regulate
progress through the cell cycle
Experimental Evidence
In each experiment, cultured mammalian cells at two different phases of the cell cycle were induced to fuse.
When a cell in the M phase was fused with a cell in G1, the G1 cell immediately began mitosis
—
a spindle formed and chromatin condensed, even though the chromosome had not been duplicated.
EXPERIMENTS
RESULTS
CONCLUSIONThe results of fusing cells at two different phases of the cell cycle suggest that molecules present in the cytoplasm of cells in the S or M phase control the progression of phases.
When a cell in the S phase was fused with a cell in G1, the G1 cell immediately entered the S phase—DNA was synthesized.
S
S S M M
M
G1 G1
Experiment 1 Experiment 2
Sequential events of the cell cycle
Checkpoints
Figure 12.14
Control system
G2 checkpoint M checkpoint
G1 checkpoint
G1
S
The cell cycle stops until it receives a go-ahead signal
Red Light; Green Light
G1 checkpoint
G1 G1
G0
(a) If a cell receives a go-ahead signal at the G1 checkpoint, the
cell continues on in the cell cycle.
(b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.
Located at the end of the cell cycle’s G1
phase
◦ Choices:
Division
Delay of Division
Enter a resting stage (G0 phase)
Located at the end of G2 phase
Checks the success of DNA Replication
Makes the decision if the cell is ready for
Mitosis
◦ Yes Goes through to the M phase
◦ No The cell stops and performs the necessary repairs
Metaphase checkpoint
◦ Is the Mitotic spindle formed properly?
◦ Are the chromosomes lined up on the plate correctly?
◦ Are the chromosomes attached to the microtubules?
Yes Go onto Anaphase
No Fix the problem or start the cell process of
apoptosis (Cell Death)
Cell Cycle Regulation
Late S phase: cyclins are
synthesized and it continues through G2
Cyclin combines with Cdk to form MPF
Promotes mitosis by phosphorylating various protein During Anaphase, the
Internal and External signal control the cell
cycle checkpoint
Growth factors play a large part in
stimulating cell division
◦ Molecular peer pressure
Normal Cells
Density-Dependent Inhibition
Anchorage Dependence
Crowded cells
stop dividing
Cells need to be
attached to a surface to divide
Cells anchor to dish surface and
divide (anchorage dependence).
When cells have formed a complete single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the remaining cells divide to fill the gap and then stop
Abnormal Cells
Cancer Cells
Do not exhibit Anchorage or
Density Dependent Inhibition
25 µm
Cancer cells do not exhibit anchorage dependence or density-dependent inhibition.
Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells.
(b)
Transformation: conversion of a normal cell
to a cancer cell
Benign tumor: remain at the original site
and do not move
Malignant tumor: have the ability to move
to other locations
They do not respond normally to the body’s
control mechanisms
Form Tumors
◦ Malignant tumors invade surrounding tissues and can metastasize
Exportation cancer cells to other parts of the body
where they can form secondary tumors
Cancer Cells
Chemotherapy drugs interfere with specific
steps of the cell cycle
◦ Taxol
Freezes the mitotic spindle by preventing it from
shortening
Radiation
Destroys the DNA of cancer cells
Most cancer cells have lost the ability to repair their DNA
when it is destroyed
