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 Organized?•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

1.specific sequence of nucleotides in genes spells out instructions for making proteins

chromosomes

DNA

genes

3. Required for growth and developmenta. Eukaryotic cells use mitotic cell division to grow or increase in numbersb. 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 cellsbut have a limited potential (multipotent)

Case Study: Body, Heal, ThyselfLike 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. https://www.youtube.com/watch?v=aQ8mC61Zhmg – 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 chromosomes

each human chromosome contains 1 DNA double helix, 50-250 mill nucleotides long

wound around proteins (Histones) and coil even further 1,000x shorter

Figure 9-5 The principal features of a eukaryotic chromosome during cell division

gene loci

centromere

telomeres

A eukaryotic chromosome (one DNA double helix)before DNA replication

A eukaryotic chromosome after DNA replication

Separated sister chromatids become independentchromosomes

Duplicated chromosome(two DNA double helices)

centromere

sisterchromatids

independent daughter chromosomes, eachwith one identical DNA double helix

9.3 How is DNA in Eukaryotic Chromosomes Organized?

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)

XYsXX

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 division

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 division

• cytoplasm is divided roughly equally between 2 daughter cells

Prophase: • duplicated chromosomes condense• nuclear envelope breaks down• chromosomes hook onto microtubule zip-line

Metaphase: • chromosomes line up on the equator of the cell

Anaphase: • Sister chromatids separate and

pulled to opposite poles of the cell

Telophase: • Nuclear envelope reforms around each

group of chromosomes• Chromosomes unwind back to extended

state

https://www.youtube.com/watch?v=gwcwSZIfKlM – Amoeba Sisters

Cell plate

https://www.youtube.com/watch?v=mzeowbIxgwI – actual footage

offspring (daughter) cells are about equal in size

each daughter cell receives ½ thecytoplasm & organelles of the parent cell

each daughter receives an IDENTICAL copy of the parent cell’s chromosomes (DNA)

Results of Mitosis

Case Study: Body, Heal ThyselfThe 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, orif mitotic cell division sent random numbers and typesof chromosomes into the daughter cells. Some of thedaughter cells might not contain all the genes needed toform the various cell types that are required to repairdamaged tissues. Worse yet, some daughter cells might have genetic changes that would cause cancer, in which cells divide uncontrollably.

Case Study: Body, Heal ThyselfWhen 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 platelet-rich 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 ThyselfHines 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 onlywhen they bind to other proteins called cyclins

(interstitialfluid)

cyclin-dependentkinase (Cdk)

Cyclin bindsto Cdk

cyclin

Cyclin activatesCdk; active Cdkstimulates DNAreplication

plasmamembrane

Cyclins aresynthesizedgrowth factor

receptor

Growth factorbinds to its receptor

(cytosol)

growth

factor

Cell division occurs when

1) growth factors bind to cell surface proteins – leading to cyclin synthesis

2)cyclins bind to and activatespecific CDKs

3) CDKs stimutate synthesis and activity of proteins that are required for DNA synthesis (allowing it to enter S phase)

Other CDKs become activated during G2 and mitosis to promote events in those phases

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.

metaphase anaphase:Are all of the chromosomesattached to the spindle andaligned at the equator?

G1 S: Is the DNA

the DNA beencompletelyand accuratelyreplicated?

intact and suitablefor replication?

G2 M: Has

Checkpoints regulate progress through the cell cycle.

While CDKs drive the cell cycle, multiple checkpoints ensure that

1. Cell successfully completes DNA synthesis during interphase

2. Proper chromosome movements occur during mitotic cell division

3 Major checkpoints1. G1 S2. G2 Mitosis3. Metaphase Anaphase

Cancer1. uncontrollable cell growth

https://www.youtube.com/watch?v=IeUANxFVXKc (normal vs. cancerous cell division)