Mitotic cell division

Download Report

Transcript Mitotic cell division

CH. 9
The Continuity of Life: Cellular Reproduction
Chapter 9 At a Glance
• 9.1 Why do Cells Divide?
• 9.2 What Occurs During the Prokaryotic Cell Cycle?
• 9.3 How is the DNA in Eukaryotic Chromosomes
• 9.4 What Occurs During the Eukaryotic Cell Cycle?
• 9.5 How Does Mitotic Cell Division Produce Genetically
Identical Daughter Cells?
• 9.6 How is the Cell Cycle Controlled?
CSI: Body, Heal Thyself
On 1/18/09, in the first quarter of a NFL playoff game, the Pittsburg Steelers’ star wide
receiver Hines Ward sprained a ligament in his right knee. In Ward’s words, “It was a severe
injury, maybe a four or six week injury.” But only 2 weeks later, he played in the Super
Bowl, catching two passes during the Steelers victory over the Arizona Cardinals.
Healing injured ligaments is a complicated process. Ligaments consist mostly of
specialized proteins, including collagen and elastin, organized in a precise orderly
arrangement that provides both strength and flexibility. A relatively small number of cells,
which originally produced these proteins are scattered throughout the ligament. When a
ligament is sprained, broken blood vessels leak blood into the site of injury. Platelets, a type
of blood cell, help to form clots, preventing further bleeding, and release a number of
different proteins collectively called growth factors into the injured ligament. The growth
factors attract various types of cells to the site of injury and stimulate cell division in both
these new cells and the existing cells in the ligament. These cells produce new ligament
proteins. Ideally, the injured ligament gradually develops the correct protein composition and
arrangement, returning to its original size, strength, and flexibility. Unfortunately, this
process is slow. To make things worse, it isn’t always completely successful.
CSI: Body, Heal Thyself
So how could Hines Ward play football in just 2 weeks? Of course, he had the best
treatment that money could buy. Part of that treatment was platelet-rich plasma (PRP)
therapy. In PRP therapy, blood is taken from a patient, centrifuged, and a portion rich in
platelets is injected directly into the wound site. Because the patient’s own cells are used,
there is no risk of rejection or introducing disease organisms from a donor’s blood. Although
PRP therapy for sports injuries is still considered experimental, studies in mice, rats, rabbits,
horses, and humans have found that it stimulates faster and more complete healing for a wide
range of damaged tissues, including bone, skin, tendons, ligaments, and muscle.
As you read this chapter, consider the following questions:
1. How do growth factors cause cells to divide?
2. Why don’t cells in undamaged tissues divide rapidly all the time?
3. Why are the dividing cells genetically identical to the cells they came from and, in fact,
genetically identical to most of the rest of the cells of the body?
9.1 When Do Cells Divide?
 to repair cuts / broken bones
 to make more blood cells
 to create a new lining of your stomach
 to make new sperm and egg cells
9.1 Why Do Cells Divide?
1. Too many demands on the nucleus
2. Can’t exchange nutrients and wastes fast enough
 Cell division: cells reproduce and produce
2 daughter cells that are identical to the
parent cell
• specific sequence of nucleotides in genes
spells out instructions for making proteins
3. Required for growth and development
a. Eukaryotic cells use mitotic cell division to grow or increase in numbers
b. Daughter cells may differentiate becoming specialized for specific functions
Cell Cycle – repeating pattern of dividing, growing, differentiating, and dividing again
Stem cells –
a) self renewal – retain ability to divide
b) ability to differentiate into a variety of cell types (1 daughter differentiates, the other
remains a stem cell)
c) Can produce any of the
specialized cell types of the
entire body
Embryonic stem cells - have
capacity to produce any cell in
human body (totipotent) – grown in lab
Adult stem cells – like embryonic stem cells
but have a limited potential
Case Study: Body, Heal, Thyself
Like platelet-rich plasma therapy, stem cell injection is a potential treatment for
wounds. For horses with many types of leg injuries, a veterinarian can take a
small sample of a horse’s own fat and send it to a company called Vet-Stem,
which isolates stem cells from the fat. Vet-Stem, sends the stem cells back to the
vet, who injects them into the horse’s injured leg. The stem cells divide and
differentiate, producing specialized cells that repair the damaged structures.
In 2010, professional baseball pitcher Bartolo Colon received similar stem-cell
therapy for a shoulder injury that never fully healed after rotator cuff surgery. In
2011, he was back pitching for the New York Yankees, with a 90+ mph fastball.
Effective stem cell therapy? Or just more time to heal? No one knows for sure,
but MLB seems to believe that the therapy works, and has launched an
investigation into whether stem cell therapy might produce superhuman powers,
and hence should be banned as a performance-enhancing drug. – Dog Stem Cell Therapy
Cell division is required for sexual & asexual reproduction in eukaryotic organisms
1. Sexual reproduction occurs when offspring are produced from fusion of gametes
(sperm & eggs) from 2 adults
 Meiotic cell division (Meiosis) – produces the gametes that have half the genetic
information of their parent cells
(egg + sperm = zygote)
 Most multicellular organisms, some plants
(seeds) and fungi
2. Asexual reproduction occurs by a single parent without having a sperm fertilize an egg
 Mitotic cell division (Mitosis) – produces clones or offspring genetically identical
to the parent and to each other
 bacteria and single-celled organisms, some multi-cellular organisms (Hydra), many
plants and fungi
Hydra – budding
Planaria - regenerating
9.2 What Occurs During the Prokaryotic Cell Cycle?
 Prokaryotic division - binary fission
9.3 How is DNA in Eukaryotic Chromosomes Organized?
 Eukaryotic chromosomes – housed within
membrane bound nucleus
 Eukaryotic cells
– always have multiple chromosomes
- longer and have more DNA than prokaryotic
 each human chromosome contains
1 DNA double helix, 50-250 mill
nucleotides long
 wound around proteins (Histones) and
coil even further  1,000x shorter
9.3 How is DNA in Eukaryotic Chromosomes Organized?
Figure 9-5 The principal features of a eukaryotic chromosome during cell division
gene loci
A eukaryotic chromosome (one DNA double helix)
before DNA replication
Duplicated chromosome
(two DNA double helices)
A eukaryotic chromosome after DNA replication
independent daughter chromosomes, each
with one identical DNA double helix
Separated sister chromatids become independent
Telomeres (2) – repeated nucleotide sequence that are essential for chromosome stability
Centromere (1) –
1) temporarily holds 2 daughter DNA double helices together after DNA replication
2) attachment site for specific structures that move chromosomes during cell division
Sister chromatids – duplicated chromosome consisting of 2 identical DNA double helices
that are attached at the centromere
 Separate during Mitosis become an independent chromosome that winds up in one
of the two daughter cells
Eukaryotic organisms usually occur in pairs with similar genetic information
• Karyotype –picture showing an entire set of paired stained chromosomes from 1 cell
• Paired chromosomes (homologous chromosomes) - same length and same staining pattern
(same genes arranged in the same order)
• Cells with pairs of chromosomes are called
diploid (double)
• Human cell has 23 pairs of chromosomes –
46 total
• 22/23 pairs are autosomes
(similar appearance, similar DNA,
paired in diploid cells of both sexes)
• 23rd pair is sex chromosomes
(different in male and female)
9.4 What Occurs During the Eukaryotic Cell Cycle?
• Eukaryotic cell cycle consists of interphase and mitotic cell division
• Interphase = acquisition of nutrients, growth, and DNA replication
• G1 (growth phase 1): acquires
nutrients, grows to proper size,
and decides wether to divide
• S (synthesis phase): DNA synthesis
• G2 (growth phase 2): completion of
cell growth, makes protein for cell
9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?
Mitotic cell division (nuclear division): 1 copy of every chromosome and half of the
cytoplasm and organelles are distributed into the 2 daughter cells
• Mitosis: division of the nucleus
• Prophase • Metaphase
• Anaphase
• Telophase
• Cytokinesis: cytoplasmic
• cytoplasm is divided roughly
equally between 2 daughter
• duplicated chromosomes condense
• nuclear envelope breaks down
• chromosomes hook onto microtubule zip-line
• chromosomes line up on the equator of the cell
• Sister chromatids separate and
pulled to opposite poles of the cell
• Nuclear envelope reforms around each
group of chromosomes
• Chromosomes unwind back to extended
state – Amoeba Sisters – actual footage
Cell plate
Results of Mitosis
 offspring (daughter) cells are about
equal in size
 each daughter cell receives ½ the
cytoplasm & organelles of the parent
 each daughter receives an
IDENTICAL copy of the parent
cell’s chromosomes (DNA)
Case Study: Body, Heal Thyself
The precision of mitotic cell division is essential for wound repair, because it
ensures that the daughter cells are genetically identical to their parent cells.
Imagine what might happen if DNA synthesis during
interphase did not copy all of there genes accurately, or
if mitotic cell division sent random numbers and types
of chromosomes into the daughter cells. Some of the
daughter cells might not contain all the genes needed to
form the various cell types that are required to repair
damaged tissues.
Worse yet,
some daughter cells
might have
genetic changes that
would cause
cancer, in which
cells divide
Case Study: Body, Heal Thyself
When Hines Ward sprained the ligaments in his knee, the normal heling process started
immediately. Platelets from damaged blood vessels accumulated in the injured ligament and
secreted growth factors, stimulating cell division. The cells produced new ligament proteins,
replacing those that were torn during the injury. Why, then, should platelet-rich plasma
therapy assist healing – shouldn’t platelets already present in the blood and leaking into the
wound be able to do the job?
In most cases, that’s exactly what happens. Nevertheless, PRP therapy can be valuable for
two major reasons. First, some tissues, particularly tendons, ligaments, and some bones, have
a rather sparse blood supply, so torn ligaments or tendons receive only a low dose of growth
factors. As a result, cell division occurs slowly, and the injured structures often never regain
their original strength. Incompletely healed tendons and ligaments may be chronically
painful, especially during and after exercise, and are prone to re-injury. Infection of plateletrich plasma can provide a larger dose of growth factors, which stimulates faster cell division
and more complete healing. In a study with horses, PRP treatment results in better healing of
tendon injuries including more and better-organized collagen protein, and greater strength.
Even issues with a good blood supply, such as skin, may benefit from an extra dose of growth
factors; some cosmetic surgeons use PRP therapy after facial surgery to improve healing.
Case Study: Body, Heal Thyself
Hines Ward’s experience is an example of a second potential benefit of PRP therapy: Faster
cell division may promote more rapid healing. No one, not even a weekend warrior or a
couch potato, wants to be out of action longer than necessary. The extra growth factors
supplied by PRP may speed up healing, even if the body would eventually have doen a
perfectly good job without the extra help. However, in most real-life cases, including Ward’s,
no one knows for sure if healing was really faster or better because PRP therapy stimulated
cell division in the injured ligament – there was, of course, no “control Ward,” who was not
receiving PRP for comparison.
1. Is PRP therapy a wonder treatment that should be routinely used for most injuries?
 isn’t always helpful (Achilles tendon)
 injection painful
 expensive ($500 – $1,000) and not covered by insurance
2. Should PRP be reserved for major injuries? Just for the wealthy?
3. Would you want PRP for a sprained knee?
9.6 How is the Cell Cycle Controlled?
The activities of specific proteins drive the cell cycle.
• Growth factors (hormone-like molecules) are released during development, after an
injury, to compensate for normal wear and tear
• Cell cycle is driven by proteins called cyclin-dependent kinases (CDKs)
• Kinases are enzymes that phosphorylate other proteins to
stimulate or inhibit the protein’s activity
• CDKs are ‘cyclin dependent’ because they are active only
when they bind to other proteins called cyclins
Cell division occurs when
1) growth factors bind to cell
surface proteins – leading to
cyclin synthesis
2) cyclins bind to and activate
specific CDKs
Growth factor
binds to its receptor
growth factor
3) CDKs stimutate synthesis
and activity of proteins that are
required for DNA synthesis
(allowing it to enter S phase)
Cyclins are
Cyclin activates
Cdk; active Cdk
stimulates DNA
kinase (Cdk)
Other CDKs become activated
during G2 and mitosis to
promote events in those phases
Cyclin binds
to Cdk
Go….Lick Yourself!
Why do dogs lick themselves? The saliva of dogs, like the saliva of most mammals
(including humans), contains enzymes antibacterial compounds epidermal growth factor
(EGF), and a variety of other growth factors. When a dog licks a wound, it not only cleans
out some of the dirt that may have enter3ed, but also leaves EGF and other growth factors
behind. The growth factors speed up the synthesis of cyclins, thereby stimulating the division
of cells that regenerate the skin, helping to heal the wound more rapidly.
Checkpoints regulate progress through the
cell cycle.
 While CDKs drive the cell cycle,
multiple checkpoints ensure that
M: Has
the DNA been
and accurately
Are all of the chromosomes
attached to the spindle and
aligned at the equator?
1. Cell successfully completes DNA
synthesis during interphase
2. Proper chromosome movements
occur during mitotic cell division
 3 Major checkpoints
1. G1  S
2. G2  Mitosis
3. Metaphase  Anaphase
 Cancer
1. uncontrollable cell growth (normal vs.
cancerous cell division)
S: Is the DNA
intact and suitable
for replication?