http://o.quizlet.com

Much of the text material is from, “Essential Biology with
Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon
(2004 and 2008). I don’t claim authorship. Other sources were also used and are noted.
2

•
•
•
Mitosis
Meiosis
Chromosomal disorders
3

4

• Reproduction is often associated with the formation of new offspring.
• it also occurs in most (but not all) types of cells for tissue growth and repair.
5

• Our skin has an outer layer of dead epithelial cells —underneath are layers of living epithelial cells dividing and undergoing chemical reactions.
• New epithelial cells move toward the skin surface to replace cast-off dead cells.
• New cells are also formed in our tissues to help heal wounds when we are injured.
• This form of cellular reproduction, known as mitosis, is a lifelong process for tissue growth and repair.
6

http://publications.nigms.nih.gov
http://www.web-books.com
7

•
•
The two daughter cells are identical to each other and the parent cell when a cell divides through mitosis.
In this context, daughter —the term used by biological scientists—does not imply gender.
• The parent cell duplicates its set of chromosomes before it divides into two daughter cells.
• During cell division, identical sets of chromosomes (genetic material) are distributed to the daughter cells.
8

• Single-cell organisms, such as amoeba, reproduce through simple cell division.
• The offspring are genetic replicas of the one parent.
• The process is known as asexual reproduction since it does not involve fertilization of an egg by a sperm.
9

(continued)
• In asexual reproduction, the parent and offspring have identical genetic material.
• The process that enables both cell division and asexual reproduction is mitosis.
• The division process is somewhat different for single-cell organisms such as bacteria.
10

• Sexual reproduction to form genetically unique offspring requires the fertilization of an egg by a sperm.
•
•
The production of egg and sperm cells involves a form of cell division known as meiosis.
The two types of cell division —mitosis and meiosis—are part of the lives of all sexually-reproducing organisms.
Egg and sperm
Computer-generated image http://neurophilosophy.files.wordpress.com
11

• A genome is a complete set of an organism’s genes—about 25,000 in humans.
•
•
Almost of the genome is located in the chromosomes in the cell nucleus.
The genes are formed from the nucleotide pairings in the cell’s DNA.
• A few genes are found on small DNA fragments in the mitochondria.
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•
•
Chromosomes are long DNA molecules bearing most of the organism’s genes.
The number of chromosomes varies by species —human somatic (body) cells usually have 46, dog cells have 78, and mouse cells have 40.
• Chromosomes are made-up of chromatin and DNA packed in protein molecules.
• The proteins help condense and organize the chromosomes, and control gene activity.
Chromatin in packed form, computer-generated image http://www.cgl.ucsf.edu
13

• For much of a cell’s lifecycle, the chromosomes are a mass of long fibers much longer than the diameter of the cell nucleus if they were stretched-out.
• When a cell prepares to divide, the chromatin fibers coil up and form compact chromosomes.
• The chromosomes are visible under a light microscope as shown below.
• When a cell is not preparing to divide, the chromosomes are too thin to be visible under a light microscope.
Nucleus of a chrysanthemum
(plant) cell
14

• A cell duplicates all of its chromosomes through the process of DNA replication, to be discussed in the next lecture, before mitotic cell division begins.
• Each chromosome now has two identical copies called sister chromatids
(the term does not imply gender).
• The sister chromatids are joined at their waists at a junction known as a centromere.
15

(continued)
Sister chromatids http://www.cbs.dtu.dk
16

• The sister chromatids separate from each other during mitosis to form an identical chromosome in each daughter cell.
• A dividing human somatic cell typically has 46 duplicated chromosomes.
• Each daughter cell receives a complete, identical set of chromosomes.
• The two daughter cells will each have 46 single chromosomes to form 23 pairs.
17

http://bhs.smuhsd.org
18

• The rate at which cells divide depends on their role within the organism.
• Some cells divide as often once a day, others less often, and others such as muscle cells and neurons usually not at all.
• The cell cycle is a sequence of events from the time a new cell is formed until it divides and forms two daughter cells.
19

• The phases of cell division make-up the process of mitosis, which occurs after an interphase period.
• A cell is generally in interphase for the large majority of its entire lifespan.
Interphase = the interval in the cell cycle between two cell divisions when the individual chromosomes cannot be distinguished, interphase was once thought to be in resting phase but it is far from a time of rest for the cell. It is the time when DNA is replicated in the cell nucleus.
(http://www.medterms.com)
20

http://bhs.smuhsd.org
21

• The portion of the cell cycle when the cell divides is called the mitotic or
M-phase.
• The M-phase has two overlapping components: mitosis and cytokinesis.
• In mitosis, the duplicated chromosomes are evenly distributed to the two daughter cells.
22

(continued)
• At the end of the M-phase, each connected daughter cell has a nucleus and organelles.
• In cytokinesis, the cytoplasm of the parent cell is divided in two individual compartments of plasma membrane to produce two distinct and separate daughter cells.
• In brief, mitosis and cytokinesis produce genetically-identical daughter cells.
23

• Mitosis is an accurate mechanism for allocating genetic material to two daughter cells.
• In yeast (eukaryotic) cells chromosomal errors occur about once in every 100,000 cell divisions.
24

• Although mitosis is a continuum of cell division activity, four stages are commonly described:
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Prophase
Metaphase
Anaphase
Telophase
25

http://www.sep.alquds.edu
1. Interphase (G
2
)
2. Prophase
3. Metaphase
4. Anaphase
5. Telophase
Mitosis consists of phases 2 through 5.
The process, except for the type of cytokinesis, is the same in plant and animal cells.
26

•
•
• Onion root cells are often used in demonstrating mitosis because they have large chromosomes which take stain well to enhance their visual appearance.
Mitosis in onion root cells can be observed through a light microscope.
The process of mitosis —but not cytokinesis—is identical in both plants and animals.
27

http://www.microscopy-uk.org.uk
http://www.sep.alquds.edu
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• Late interphase is the period when a cell synthesizes new molecules and organelles.
• The chromosomes are duplicated, although they cannot be visually distinguished since they are still loosely packed in chromatin fibers.
• The nucleolus is visible, and it is producing ribosomes for protein synthesis during cell division.
• In very late interphase (G2), the cytoplasm has two centrosomes and pairs of centrioles.
29

http://www.microscopy-uk.org.uk
http://www.sep.alquds.edu
30

• In early prophase, prominent changes begin to appear in the nucleus and cytoplasm.
• The chromatin fibers coil and become thick enough to be seen through a light microscope.
• Each individual chromosome appears as two identical sister chromatids joined at their waists.
• The mitotic spindle forms with microtubules that extend from the centrosomes.
31

Centrosome (both are shown) http://mcb.berkeley.edu
Centrosome = an organelle that serves as the main microtubule organizing center of the animal cell as well as a regulator of cell-cycle progression. http://en.wikipedia.org
32

• In late prophase, the nuclear envelope breaks-up, enabling the microtubules of the mitotic spindle to reach the chromosomes.
• Some of the microtubules attach to the chromosomes, and place them in an agitated (complex rocking) motion.
• Other microtubules make contact with microtubules from the opposite pole to position the chromosomes at the equator of the cell.
33

http://www.microscopy-uk.org.uk
http://www.sep.alquds.edu
34

• In metaphase, the mitotic spindle is fully formed, and the chromosomes are positioned along the equator.
• Microtubules attach to the two sister chromatids of each chromosome to pull them toward the opposite poles of the cell.
• For a time, a tug-of-war keeps the chromosomes positioned about midway between the two poles.
35

http://www.microscopy-uk.org.uk
http://www.sep.alquds.edu
36

• In anaphase, the sister chromatids of each chromosome pair suddenly separate.
• Each sister chromatid is now considered to be a daughter chromosome.
37

(continued)
• Motor proteins in the microtubules ratchet the daughter chromosomes to the opposite poles of the parent cell.
• The microtubules shorten in length to help bring the chromosomes closer to each pole.
• Other microtubules, not attached to the chromosomes, lengthen and push the poles farther apart to elongate the parent cell in preparation for cytokinesis.
38

http://www.microscopy-uk.org.uk
http://www.sep.alquds.edu
39

• Telophase begins when the two chromosomes reach the opposite poles.
• Two nuclear envelopes form, the chromosomes uncoil, and the mitotic spindle disappears.
• Mitosis is now complete.
• Cytokinesis, the division of the parent cell into two daughter cells, takes place at the end of telophase.
40

• In cytokinesis, a ring of microfilaments in the cytoplasm produces a cleavage furrow in the elongated cell.
• This furrow encircles the equator of the cell midway between the two poles.
•
•
The ring, consisting of the protein molecule, actin, contracts like the pulling of a drawstring, deepening the furrow and pinching the parent cell in two.
Actin is also responsible for muscle contractions —it acts like a ratchet device.
41

(continued)
Actin molecules pinching-off the parent cell in cytokinesis to form two daughter cells.
42

• The timing of mitosis is precisely controlled in eukaryotic cells to grow and maintain healthy tissues.
• The events of the cell cycle are directed by a cell cycle control system made-up of special proteins within the cell.
• The proteins integrate information from the cell environment, and send start and stop signals via signal transduction pathways at key points in the cell cycle.
43

• The cell cycle normally halts at the G
1 stage unless it receives a signal to proceed.
• If a signal does not arrive, the cell cycle will switch to a permanent off state, such as in mature muscle cells and neurons, which don’t divide.
• These cells are said to remain in G
0
.
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•
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Cells can reproduce at the wrong time and too often if the cell cycle control system malfunctions.
The result may be a tumor —an abnormal mass of cells that can be either benign or malignant.
45

• A benign tumor remains at its original site, although it can cause problems if it grows.
• Benign tumors of the brain, however, can be dangerous because the cranial cavity is enclosed.
• Growth can damage the delicate tissues through increased pressure and mechanical deformation.
46

• A growth or lump resulting from the reproduction of cancer cells is known as a malignant tumor.
• Like benign tumors, malignant tumor displace normal tissue as they grow larger.
Lung cancer cells http://www.oralcancerfoundation.org
47

(continued)
•
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Malignant cancer cells can spread to adjacent tissues and other parts of the body.
This spread —known as metastasis—occurs through the blood vessels and lymphatic system.
• Malignant cancer cells may continue to metastasize until the host organism dies.
48

(continued)
• Cancer claims the lives of about one out of every five people in the
United States and other industrialized nations.
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Cancer is the result of a severely malfunctioning cell cycle control system.
The cells divide excessively as if there were no stop signal
—cancerous cells may also exhibit other unusual behaviors.
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Cancers are named based on where they originate.
Liver cancer, for example, originates in the liver —it may remain there or metastasize to other tissues.
• Cancers can be grouped into four broad categories based on their sites of origin:
-
-
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Carcinomas —external or internal coverings of the body such as the skin or intestines.
Sarcomas —tissues that support the body including bone and skeletal muscle.
Leukemias —blood-forming tissues including bone marrow.
Lymphomas —lymph nodes.
50

• The major types of cancer treatment are surgery, radiation therapy, and chemotherapy.
•
•
The treatments can be used individually or in combination.
Surgery is often a first step —less invasive surgical techniques are being introduced.
• Radiation therapy can often destroy malignant cells with their high rate of mitotic cell division, while leaving healthy cells with their lower rate of division intact.
• The side effects of radiation treatment can include hair loss and nausea.
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•
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Chemotherapy also disrupts the high rate of mitotic division in cancer cells.
Anti-mitotic drugs disrupt the formation of the mitotic spindle prior to cell division.
• Other anti-mitotic drugs freeze the mitotic spindle so that mitotic cell division cannot continue.
• Many of these drugs are produced from plants found in tropical and temperate rain forests, which are endangered due to over-cutting and clear-cutting.
52

•
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About 50 percent of cancers are thought to be related to lifestyle factors.
They include lack of physical exercise, poor diet, excessive exposure to UVB radiation in sunlight, and smoking.
•
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Many cancers are treatable if they are detected early.
Regular visits to a physician or health clinic can help identify tumors at the earliest stages for timely treatment.
• Websites and literature are available from various health organizations that discuss the risks and what can be done.
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54

We discussed asexual reproduction —now we cover some aspects of sexual reproduction, which we will return to later in the semester.
55

• All chromosomes —except X and Y on the 23rd pair in males—have a twin that is matched in size, shape, and bands.
• The pair are said to be homologous since each chromosome carries the same sequence of genes for controlling inherited characteristics.
• The multiple genes for eye color, for example, are found at identical locations in the homologous pairs.
• The instructions in each matching gene can be dominant or recessive since one is inherited from each parent.
56

• A typical body cell in humans, known as a somatic cell, usually (but not always) has 46 chromosomes.
• A light micrograph of the chromosomes can be made if the cell is opened during mitosis.
• The individual chromosomes can be arranged in an ordered array known as a karyotype.
57

http:www.biotechnologyonline.gov
Light micrograph of chromosomes during mitosis.
58

http://www.ucl.ac.uk
An ordered array of chromosomes.
59

• The 23rd pair —the sex chromosomes—determines the genetic sex of a human.
• Eggs carry an X chromosome.
• Sperm carry an X or Y chromosome, which determines the genetic sex of the embryo.
60

(continued)
• Genetic females usually, but not always, have two X chromosomes.
• Genetic males usually, but not always, have one X chromosome and one Y chromosome.
• The remaining 22 pairs, in both females and males, are the autosomes.
61

• Humans, and most animals, are diploid organisms because all somatic (body) cells contain paired sets of homologous chromosomes.
• The number of pairs is represented by n (in humans, n = 23 ).
• The number of chromosomes (46) is the diploid number, 2n .
• Exceptions to this rule are egg and sperm cells, known as gametes, which contain 23 individual chromosomes.
Di = two.
62

• Gametes (eggs and sperm) formed by meiosis in the ovaries and testes contain one member of each homologous chromosome pair.
• Gametes are haploid since they contain one-half the number of chromosomes found in body cells.
• The total number of chromosomes in human gametes (23) is called the haploid number, n .
Spermatozoa
(sperm)
63

• A sperm cell (spermatozoon) fuses with an egg cell (ovum) in the process of fertilization.
• Each gamete is haploid, and the fertilized egg (zygote) is diploid with the fusion of genetic material.
• In fusion, one member of each pair of homologous chromosomes is contributed by each parent.
Spermatozoa = plural; spermatozoon = singular.
64

Many sperm are present, but only one can fertilize the egg due to rapid biochemical changes in the plasma membrane of the egg once a sperm penetrates it.
65

• Mitotic cell division begins within hours of fertilization to assure that each somatic cell receives a complete copy of the 46 chromosomes.
• Every one of ~ 60 trillion cells in the human body can be traced to a single zygote.
66

The first day after fertilization — mitotic cell division has begun.
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At three days.
68

Eight-week-old human embryo http://library.thinkquest.org
Five-week-old human embryo
69

• Meiosis — the basis of sexual reproduction — resembles mitosis, but it has two additional aspects:
–
–
Halving of the number of chromosomes ( 2n is reduced to n ).
Exchange, or crossing-over, of genetic material between the homologous pairs of chromosomes.
• Gametes undergo two consecutive divisions in meiosis I and II.
• Four daughter cells result, each with one-half as many chromosomes
( n ) as the starting cell (2 n ).
Meiosis takes place exclusively in the testes and ovaries —mitosis occurs in somatic cells.
70

(continued)
• Meiosis is the basis of sexual reproduction in eukaryotic organisms
(animals, plants, and fungi).
• Each offspring inherits a unique combination of genes from the two parents.
• Unlike asexual reproduction, the offspring will have substantial genetic variation.
71

http://www.mun.ca
72

• In the interphase, before meiosis I begins, each chromosome of a homologous pair replicates to form two pairs of sister chromatids of identical genetic content.
• The two pairs of sister chromatids remain together as a tetrad until the end of meiosis.
73

• In prophase I, specialized proteins hold the tetrads together when the chromatin condenses.
• The chromatids of the homologous pairs in the tetrads exchange DNA segments in a process known as crossing-over.
http://www.uic.edu
74

(continued)
• Crossing-over occurs, to assure genetic variation from generation-togeneration, and between siblings.
• The process rearranges the genetic information from the two parents, as we will discuss.
• Spindles of microtubules form and the tetrads are moved toward the cell’s equator.
http://www.uic.edu
75

76

•
•
In metaphase I, the sister chromatids in the tetrad remain attached at their centromeres (waists).
The tetrads are aligned on the cell’s equator by the spindles anchored to the opposite poles of the cell.
• The spindle is arranged so that the homologous chromosomes of each tetrad can move to the opposite poles of the cell.
http://www.uic.edu
77

• In anaphase I, the microtubules in the spindles move the chromosomes toward the opposite poles of the cell.
• Unlike in mitosis, sister chromatids migrate as pairs rather than splitting up.
• The sister chromatids, each of unique genetic content, are separated from their homologous partners — this is after the process of crossingover in prophase I.
http://www.uic.edu
78

• In telophase I, the sister chromatids reach the poles as a haploid set since the chromosomes are still in duplicate form.
• Two haploid daughter cells with pairs of chromosomes are formed by cytokinesis at the end of telophase I.
• No further chromosome duplication occurs in the subsequent stages of meiosis II.
http://www.uic.edu
79

80

• Meiosis II is similar to mitosis, but it starts with a haploid cell ( n ) rather than a diploid cell (2 n ).
• The processes of prophase II, metaphase II, anaphase II, telophase II, and cytokinesis are very similar to what we discussed for mitosis.
•
•
Meiosis I results in two haploid daughter cells, while meiosis II doubles the number to four haploid daughter cells.
The haploid cells serve as the progenitors for eggs and sperm produced by the gonads.
Progenitor = predecessor.
81

•
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Mitosis enables growth, tissue repair, and asexual reproduction by the production of daughter cells that are genetically-identical to the parent cell.
Meiosis enables sexual reproduction by the production of geneticallyunique daughter cells called gametes (eggs or sperm).
• In mitosis and meiosis I, the chromosomes duplicate only once during the interphase.
82

(continued)
• Mitosis involves one division of the cell nucleus and cytoplasm to produce two diploid daughter cells (2 n ).
• Meiosis I and II involves two divisions of the cell nucleus and cytoplasm to produce four haploid daughter cells (4 n ).
• All events unique to meiosis (those not occurring in mitosis), happen in meiosis I.
83

• Independent assortment
• Crossing-over and genetic recombination
• Random fertilization
Because of these mechanisms, each offspring will have substantial genetic variation from her or his parents and all siblings except in instances of identical (monozygotic) twins.
84

• Each pair of homologous chromosomes in the tetrad orients itself independently during metaphase I.
•
•
•
•
The orientation is a matter of chance similar to flipping a coin.
The total number of unique chromosome combinations in a gamete is 2 n where n is the haploid number.
Since n = 23 in humans, over 2 23 combinations of pairings are possible.
Each gamete —egg or sperm—is therefore one of over eight million (8 x
10 6 ) combinations.
85

(continued) http://3.bp.blogspot.com
86

• At the time of crossing-over of genes, homologous chromosomes in a tetrad are closely paired all along their lengths.
• Due to this arrangement, a precise gene-by-gene alignment enables the exchange of genetic material.
• Crossing-over during prophase I provide vastly more possibilities for genetic variation between the parents and offspring.
87

(continued)
Homologous chromosomes
Tetrads
Haploid cells http://regentsprep.org
88

•
•
Chromosomes resulting from crossing-over are known as recombinant.
The genetic recombinations are different from the parent chromosomes.
• A single cross-over can affect many genes because most chromosomes have thousands of genes.
89

• A human egg (8 x 10 6 possibilities) when fertilized by a sperm (8 x 10 6 possibilities) will produce one of over 6.4 x 10 13 possible combinations.
• The fertilization process adds a high degree of genetic variability to the offspring.
6.4 x 10 13 = 64,000,000,000,000 possible combinations.
90

91

• Errors during meiosis can result in chromosomal disorders in humans.
• Chromosomal disorders often have a characteristic set of physical and mental signs.
•
•
The sum total (constellation) of these signs is known as a syndrome.
Just one —or even a few—characteristic signs do not necessarily make a syndrome.
A chromosomal or genetic disorder, no matter how startling, does not make the person abnormal and separate from other people.
We are all finding our way through this life, and we each face our own struggles and challenges.
92

Trisomy-21 karyotype http://atlasgeneticoncology.org
Characteristic physical features of a young child with Down syndrome.
http://www.impaedcard.com
93

• Characteristic features of Down syndrome include:
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–
–
–
–
–
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Fold of skins at the inner corner of the eye
Round face and flattened nose bridge
Small irregular teeth
Short stature
Heart defects
Susceptibility to some diseases
Sexual underdevelopment and sterility
Varying degrees of intellectual impairment
94

(continued) http://www.downsyndrome.com
Many people with Down syndrome are socially adept, able to hold jobs, and live fulfilling lives.
95

96

p = 1 / 46 p = 1 / 2300 http://fig.cox.miami.edu
Age of mother at conception
(20 to 47 years)
The risk of bearing a child with Down syndrome increases with the age of the mother, especially if she is in her late-30s or -40s.
Due to the potential risk, older parents may decide on prenatal genetic testing and counseling for Down syndrome and other chromosomal disorders.
97

http://embryology.med.unsw.edu.au
Amniocentesis can be performed from about 15 weeks to term.
98

http://www.contentanswers.com
Chorionic villus sampling (CVS) from the chorion can be performed earlier than amniocentesis starting at 11 to 14 weeks.
99

http://www.uic.edu
Nondisjunctions occasionally occur during meiosis —they are the chromosomal mechanism for Down syndrome and other trisomies.
100

(continued)
• The production of gametes (eggs and sperm) results from meiosis in the ovaries and testes.
• The spindle made-up of microtubules usually distributes chromosomes to the daughter cells without error.
101

(continued)
• On rare occasions, chromosomes may not separate completely during anaphase I in the ovaries.
• The result is an abnormal number of chromosomes such as in trisomy-
21, trisomy-18 (Edward syndrome), and some sex-linked chromosomal disorders.
• We will cover sex-linked chromosomal disorders later in the semester.
102

http://www.slh.wisc.edu
The condition is also known as trisomy-18 due to a third number-18 chromosome.
103

(continued)
•
•
–
–
–
–
–
Edward Syndrome occurs about one in 3,000 conceptions, and one in 6,000 live female and male births.
Characteristics features include:
Low birth weight
Small head and other characteristic facial features
Structural heart defects
Feeding and breathing difficulties
Developmental delays
• Only 5-10 percent of babies with Edward Syndrome survive the first year due to heart defects and severe breathing difficulties (known as apnea).
104

(continued)
•
•
• Since the long-term prognosis is not good —major medical interventions are often withheld.
Trisomy-18 is the result of a nondisjunction occurring during meiosis I.
Other trisomies can occur, but the embryo or fetus usually fails to survive to term (birth).
105

• The mechanisms for nondisjunctions are well understood, although the reasons why they happen are not.
• Egg cells are arrested in the middle of the meiosis process for as long as 40 or more years since meiosis I begins in the ovaries before birth.
106