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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 are noted when they are used. Note
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Biology ReviewMitosis and Meiosis
http://o.quizlet.com
2
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 are noted when they are used.
Note
3
Outline
• Mitosis• Cancer• Meiosis• Chromosomal disorders
4
Mitosis
http
://w
ww
.uq.
edu.
au
5
Reproduction
• 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.
6
Cell Division
• Our skin has an outer layer of dead epithelial cells—underneath are layers of living epithelial cells dividing and undergoing chemical reac-tions.
• 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, called mitosis, is a lifelong process for tissue growth and repair.
7
Human Skin
http://publications.nigms.nih.gov
http://www.web-books.com
8
Genetic Transmission
• 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.
9
Asexual Reproduction
• 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.
10
Asexual Reproduction (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 (called replication) is somewhat different for single-cell organisms such as bacteria.
11
Sexual Reproduction
• Sexual reproduction requires the fertilization of an egg by a sperm to form genetically-unique offspring.
• 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.
Computer-generated imagehttp://neurophilosophy.files.wordpress.com
Egg and sperm
12
Can you describe similarities and differences between asexual and sexual reproduction?
13
Genome
• A genome is a complete set of an organism’s genes—about 25,000 in humans.
• Most of the genome is found 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.
http
://w
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bio-
iitd.
res.
in
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Chromosomes
• Chromosomes are long DNA molecules bearing most of an 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 consist of chromatin and DNA tightly packed in protein molecules.
• The proteins help condense and organize the chromosomes, and control gene activity.
Chromatin in packed form, computer-generated imagehttp://www.cgl.ucsf.edu
15
Chromosomes Prior to Mitosis
• 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.
http
:/z.a
bout
.comNucleus of a chrysanthemum
(plant) cell
16
Sister Chromatids
• A cell duplicates all of its chromosomes through the process of DNA replication, to be discussed in the next lecture, before mitotic cell divi-sion 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.
17
Sister Chromatids (continued)
http://www.cbs.dtu.dk
Sister chromatidsjoined at the centromere
18
Chromatid Separation
• The sister chromatids separate from each other during mitosis to form an identical chromosome for 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.
19
Cell Cycle—Interphase
http://bhs.smuhsd.org
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Cell Cycle
• The rate at which cells divide depends on their role within the organism.• Some cells can 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.
21
Interphase
• A cell is generally in interphase for the large majority of its entire lifespan.• The phases of cell division make-up the process of mitosis, which occurs
after an interphase period.
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)
22
Cell Cycle—Mitotic- or M-Phase
http://bhs.smuhsd.org
23
Mitotic- or M-Phase
• 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.
24
Mitotic or M-Phase (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.
• Mitosis and cytokinesis produce two genetically-identical daughter cells.
25
Accuracy
• 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.
26
What could happen if mitosis was not typically a highly-accurate process?
27
Stages of Mitosis
• Although mitosis is a continuum of cell division activity, four stages are commonly described: – Prophase– Metaphase– Anaphase– Telophase
28
Mitosis in Onion Root Cells
http://www.sep.alquds.edu
1. Interphase (G2)2. Prophase3. Metaphase4. Anaphase5. Telophase
Mitosis consists of phases 2 through 5.
The process, except for the type of cytokinesis, is the same in plant and
animal cells.
29
Why Examine Onion Root Cells?
• 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.
http
://w
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.myt
inyp
lot.c
o.uk
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Interphase
http://www.microscopy-uk.org.uk
http://www.sep.alquds.edu
31
Interphase
• Late interphase is the period when the parent 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 syn-thesis during cell division.
• In very late interphase (G2), the cytoplasm has two centrosomes and pairs of centrioles.
32
Prophase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
33
Early Prophase
• In early prophase, obvious changes begin to appear in the nucleus and cytoplasm of the parent cell.
• 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 (centromere).
• The mitotic spindle forms with microtubules that extend from the centro-somes.
• The microtubules are constructed on protein molecules.
34
Mitotic Spindle
http
://w
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.orn
l.gov
http://mcb.berkeley.edu
Centrosome (both are shown)
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
35
Late Prophase
• In late prophase, the nuclear envelope breaks-up, enabling the micro-tubules of the mitotic spindle to reach the chromosomes.
• Some of the microtubules attach to the duplicated 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 parent cell.
36
Metaphase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
37
Metaphase
• In metaphase, the mitotic spindle is fully formed, and the chromosomes are positioned along the equator of the parent cell.
• Other microtubules attach to the two sister chromatids of each chromo-some to pull them toward the opposite poles of the cell.
• For a time, a tug-of-war keeps the chromosomes positioned about mid-way between the two poles of the cell.
38
Anaphase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
39
• In anaphase, the sister chromatids of each chromosome pair suddenly separate.
• Each sister chromatid is now considered to be a daughter chromosome.
Anaphase
40
• 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 cyto-kinesis.
Anaphase (continued)
41
Telophase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
42
Telophase
• Telophase begins when the two chromosomes reach the opposite poles of the elongated parent cell.
• 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.
43
Cytokinesis in Animal Cells
• In cytokinesis, a ring of microfilaments in the cytoplasm produces a cleavage furrow in the elongated parent 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.
44
Cytokinesis in Animal Cells (continued)
Actin molecules in the process of pinching-off the parent cell to form two daughter cells.
http
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pres
sion
s.co
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Cell Control Cycle System
• The timing of mitosis is precisely controlled in eukaryotic cells to grow and maintain 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.
46
Off-State
• The cell cycle normally halts at the G1 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 G0.
47
Cancer
Cancerous squamous cell with cross-sectional cuthttp://www.wellcome.ac.uk
48
When Things Go Wrong
• 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.
49
Benign Tumors
• A benign tumor remains at its original site, although it may cause problems if it grows.
• Benign tumors of the brain can be dangerous because the cranial cavity is enclosed.
• The growth can damage the delicate tissues due to increased intra-cranial pressure and mechanical deformation.
50
Malignant Tumors
• A growth or lump resulting from reproduction of cancer cells is known as a malignant tumor.
• Like benign tumors, malignant tumors displace normal tissue as they grow larger.
Lung cancer cellshttp://www.oralcancerfoundation.org
51
Malignant Tumors (continued)
• 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 organism dies.
52
More on Cancer
• Cancer is a collection of diseases in which cells are no longer effec-tively controlled by the processes that normally limit division during mitosis.
• Cells divide excessively as if there were no stop signal—cancerous cells may also exhibit other unusual behaviors.
• The absence of a normal cell cycle control system is due to changes in some genes, or possibly in the way that certain genes are expressed.
53
Oncogenes and Proto-Oncogenes
• A gene that causes a cell to be cancerous is known as an oncogene or tumor gene.
• A normal gene that has the potential to become an oncogene is called a proto-oncogene.
• A proto-oncogene results from mutations that produce changes in gene expression.
54
Genes and Growth Factors
• Many of the genes involved in cancer code for growth factors—proteins that stimulate cell division in the cell control cycle.
• These proteins normally keep the rate of mitotic cell division at the right level.
• Uncontrolled cell growth can occur when the synthesis of these proteins malfunctions.
Cell control cycle
http://www.answers.com
Mitotic phase
G2 phase
S phase
G1 phase
55
Tumor Suppressor Genes
• Other genes may inhibit uncontrolled cell division by suppressing the division and growth of cancerous cells.
• Tumor suppressor genes are a promising focus of research for cancer treatments.
A protein produced by a tumor-suppressor gene shown
surrounding a segment of DNA.
Computer-generated imagehttp://www.cosmosmagazine.com
56
Colon Cancer
• Almost 150,000 people in the United States were diagnosed with colon or rectal cancer in 2003.
• Colon cancer—a well-understood type of human cancer—illustrates a key principle of how cancer develops:
More than one mutation is usually needed to produce a full fledged-cancer cell.
Colon cancer cellsfalse-color electron micrographhttp://www.wellcome.ac.uk
57
Progressive Mutations
• Colon cancer begins as unusually-frequent mitotic division of normal- appearing cells in the lining of the colon wall.
• Cell changes result in DNA mutations at this initial stage and at the later stages too.
• The number of progressive mutations before the cancer is evident—at least four—explains why some cancers can take a long time to develop.
• The cancerous cells are grossly altered in their physical appearance by their fourth mutation.
58
Progressive Mutations (continued)
http://science.kennesaw.edu
Normal cell
59
Role of Heredity
• Cancer is a genetic disease (but usually not inherited) since it results from mutations in the DNA.
• Most mutations leading to cancer arise in the organ where the malig-nant tumor starts.
• Genetic mutations are not passed from the parents to the child if they do not affect zygotes (eggs or sperm).
60
Role of Heredity (continued)
• In a small number of families, the mutations in one or more genes can be passed to their children and may increase their risk of certain types of cancer.
• The cancer usually does not occur unless the person acquires addi-tional mutations.
61
Breast Cancer
• One out of ten women in the United States will be diagnosed with breast cancer in their lifetimes.
• The large majority of cases have nothing to do with inherited muta-tions.
62
BRCA1 Gene
• A very small number of breast cancer cases, however, is related to mutations in the BRCA1 gene.
• Research suggests that protein encoded by the normal BRCA1 gene serves as a tumor suppressor.
• Clinical tests are available for detecting the presence of mutations in the BRCA1 gene.
• Few viable options currently exist if a positive test result is reported.
63
Cancer Risk
• Cancer is a leading cause of death in industrialized countries including the United States.
• Death rates for some types of cancer have declined, but the overall rate is on the rise.
• Cancer-causing agents—called carcinogens—lead to DNA changes and cellular mutations.
• In some instances, the mutagenic effects may require years of exposure to the carcinogen.
• Lifestyle factors have a role in at least 50 percent of all cases of cancer.
Mutagenic = something capable of causing a gene-change. Among the known mutagens are radiation, certain chemicals and
some viruses. (http://www.medterms.com)
64
Lifestyle Factors
• Some of the chemicals in first- and second-hand tobacco smoke are potent carcinogens.
• Excessive exposure to the UVB radiation in sunlight can cause skin cancer, or melanoma.
• Consumption of too much animal fat is associated with colon cancer— a reduction in fat consumption is a good idea for a number of health reasons.
• Consuming about 20 to 30 grams of plant fiber each day—about twice the U.S. average—can reduce the risk of colon cancer.
• Fruits and vegetables are good sources of soluble and insoluble fiber.
65
Lifestyle Factors (continued)
• Vitamins including C, E, and A may offer some protection against some cancers—however, some recent research suggests that this may be questionable.
• The role of diet in increasing the risk of some cancers is a focus of medical and public health research.
66
Glamorous?
http
://ub
eror
igin
al.b
logs
pot.c
om
Two past cultural icons—Lauren Bacall and James Dean
http
://bl
og.b
elie
fnet
.com
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Possible Outcome
http
://w
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.hom
e-ai
r-pu
rifie
r-ex
pert.
com
http
://w
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.esu
bulle
tin.c
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Healthy and cancerous lung tissues.
68
Cancer Incidence in the United States
Rank Cancer Known or likely carcinogen of factor Estimated
cases (2003)
Estimated deaths (2003)
1 Prostate Testosterone, possibly dietary fat 220,900 28,900
2 Breast Estrogen, possibly dietary fat 212,600 40,200
3 Lung Tobacco smoke 171,900 157,200
4 Colon and rectum High dietary fat, low dietary fiber 147,500 57,100
5 Lymphatic system Viruses for some types 61,000 24,700
6 Skin Ultraviolet light 58,800 9,800
7 Bladder Tobacco smoke 57,400 12,500
8 Uterus Estrogen 40,100 6,800
9 Kidney Tobacco smoke 31,900 11,900
10 Pancreas Tobacco smoke 30,700 30,000
11 Leukemias X-rays, benzene, viruses for some types 30,600 21,900
12 Ovary Large number of ovulation cycles 25,400 14,300
13 Stomach Table salt, tobacco smoke 22,400 12,100
14 Mouth and throat Tobacco including smokeless tobacco; alcohol 20,600 5,500
15 Brain / nervous system Physical trauma, x-rays 18,300 13,100
16 Liver Alcohol, hepatitis virus 17,300 14,400
17 Cervix Viruses, tobacco smoke 12,200 4,100
All other cancers 154,500 92,000
Totals 1,334,100 556,500
69
Cancer Types
• 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:- Carcinomas—external or internal coverings of the body such as the
skin or intestines.- Sarcomas—tissues that support the body including bone and skele-
tal muscle.- Leukemias—blood-forming tissues including bone marrow.- Lymphomas—lymph nodes.
70
Surgery and Radiation Therapy
• 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.
71
Chemotherapy
• 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.
72
Early Detection and Intervention
• 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.
73
Do you think if enough money were devoted to cancer research, that an overall cure will be found?
74
Meiosis
http
://w
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ence
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.com
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Sexual Reproduction
We discussed asexual reproduction—now we cover some aspects of sexual reproduction, which we will return to later in the semester.
http
://ab
out.b
iolo
gy.c
om
76
Homologous Chromosomes
• 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.
• For example, the multiple genes for eye color are found at identical locations on the homologous pairs.
• The instructions can be dominant or recessive since one is inherited from each parent.
77
Karyotype
• A typical body cell in humans, known as a somatic cell, usually (but not always) has 46 chromosomes.
• We will cover some exceptions to the rule when will discuss chromosomal disorders.
• 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.
78
Unordered Chromosomes
http:www.biotechnologyonline.gov
Light micrograph of chromosomes during the earliest stages of mitosis.
79
Karyotype
An ordered array of chromosomes (size, shape, and banding).
http://www.ucl.ac.uk
80
Chromosomes
• 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.
81
Chromosomes (continued)
• Genetic females usually (but not always) have two X chromosomes.• Genetic males usually (but not always) have a X chromosome and a
Y chromosome.• The 23rd set is known as the sex chromosomes.• The remaining 22 pairs, in both females and males, are autosomes.
82
Diploid Number
• Humans, and most animals, are diploid organisms because all so-matic (body) cells contain paired sets of homologous chromosomes.
• The number of pairs is represented by n (in humans, n = 23 ).• The number of individual chromosomes (46) is the diploid number,
2n.
Di = two.
83
Haploid Number
• 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 chro-mosomes found in body cells.
• The number of chromosomes in human gametes (23) is the known as the haploid number, n.
http
://zo
olog
y.un
h.ed
u
Spermatozoa(sperm)
84
Fertilization
• A sperm cell (spermatozoon) fuses with an egg cell (ovum) in the process of fertilization.
• Each gamete is haploid and the fertilized egg (called a zygote) is diploid once the fusion of genetic material occurs.
• In fusion, one member of each pair of homologous chromosomes is contributed by each parent.
Spermatozoa = plural; spermatozoon = singular.
85
Sperm and Egg
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.
http
://nm
hm.w
ashi
ngto
ndc.
mus
eum
86
Mitotic Cell Division
• 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 ~ 70 trillion cells in the human body can be traced to a single zygote.
87
Two-Cell Stage
http
://w
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.mid
wes
ttiv.
com
The first day after fertilization—mitotic cell division has begun.
88
Eight-Cell Stage
http
://fig
.cox
.mia
mi.e
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At three days.
89
Continued Growth
http://library.thinkquest.org
Five-week-old human embryo
Eight-week-old human embryo
http
://nm
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ngto
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90
Meiosis
• 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.
• The 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 (2n).
Meiosis takes place exclusively in the testes and ovaries—mitosis occurs in somatic cells.
91
Meiosis (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 in asexual reproduction, the offspring will have substantial ge-netic variation.
92
Interphase and Meiosis I
http://www.mun.ca
93
Interphase
• 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.
http
://w
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.sin
auer
.com
94
Prophase I
• 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
95
Prophase I (continued)
• Crossing-over occurs to assure genetic variation from generation-to-generation 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 parent cell’s equator.
http://www.uic.edu
96
Metaphase I, Anaphase I, and Telophase I
http
://w
ww
.mun
.ca
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Metaphase I
• In metaphase I, the sister chromatids in the tetrad remain attached at their centromeres (waists).
• The tetrads are aligned on the 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.
http://www.uic.edu
98
Anaphase I
• In anaphase I, the microtubules in the spindles move the chromosomes toward the opposite poles of the parent cell.
• Unlike in mitosis, sister chromatids migrate as pairs rather than splitting up.
• The sister chromatids, each with unique genetic content, are separated from their homologous partners—this occurs after the process of cross-ing-over in prophase I.
http://www.uic.edu
99
Telophase I and Cytokinesis
• 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
100
Meiosis II
http
://w
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.mun
.ca
101
Meiosis II
• Meiosis II is similar to mitosis, but it starts with a haploid cell (n) rather than a diploid cell (2n).
• 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.
102
Mitosis versus Meiosis
• 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 genetically-unique daughter cells called gametes (eggs or sperm).
• In mitosis and meiosis I, the chromosomes duplicate only once during the interphase.
103
Mitosis versus Meiosis (continued)
• Mitosis involves one division of the cell nucleus and cytoplasm to pro-duce two diploid daughter cells (2n).
• Meiosis I and II involves two divisions of the cell nucleus and cytoplasm to produce four haploid daughter cells (4n).
• All events unique to meiosis (those not occurring in mitosis), happen in meiosis I.
104
Mechanisms of Genetic Variation
• Independent assortment• Crossing-over and genetic recombination• Random fertilization
Because of these mechanisms, offspring will display substantial genetic variation from her or his parents and all
siblings except in instances of monozygotic (identical) twins.
105
Independent Assortment
• Each pair of homologous chromosomes in the tetrad orients itself inde-pendently 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 2n
where n is the haploid number.• Since n = 23 in humans, over 223 combinations of pairings are possible.• Each gamete—egg or sperm—is therefore one of over eight million (8 x
106) combinations.
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Independent Assortment (continued)
http://3.bp.blogspot.com
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Crossing-Over
• At the time of crossing-over of genes during prophase I, 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 provides vastly more possibilities for genetic variation between parents and offspring.
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Crossing-Over (continued)
http://regentsprep.org
Haploid cells
Tetrads
Homologous chromosomes
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Genetic Recombination
• Chromosomes resulting from the process of 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.
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Random Fertilization
• A human egg (8 x 106 possibilities) when fertilized by a sperm (8 x 106 possibilities) will produce one of over 6.4 x 1013 possible combinations.
• The fertilization process adds a high degree of genetic variability to the offspring.
6.4 x 1013 = 64,000,000,000,000 possible combinations.
111
Can you describe the sources of genetic variation that leads to substantial human diversity?
112
Chromosomal Disorders
http
://cd
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ekno
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com
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Errors During Meiosis
• 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.
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Down Syndrome (Trisomy-21)
http://atlasgeneticoncology.org
Characteristic physical features of a young child
with Down syndrome.
Trisomy-21 karyotype
http://www.impaedcard.com
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Down Syndrome
• Characteristic features of Down syndrome include:– 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
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Down Syndrome (continued)
http
://w
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http://www.downsyndrome.com
Many people with Down syndrome are socially adept, able to hold jobs, and live
fulfilling lives.
117
Resources and Support
http
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grou
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Maternal Age
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.
Age of mother at conception (20 to 47 years)
http://fig.cox.miami.edu
Inci
denc
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kelih
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p = 1 / 2300
p = 1 / 46
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Amniocentesis
http://embryology.med.unsw.edu.au
Amniocentesis can be performed from about 15 weeks to term.
http
://w
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120
Chorionic Villus Sampling
http://www.contentanswers.com
Chorionic villus sampling (CVS) from the chorion can be performed earlier than amniocentesis starting at 11 to 14 weeks.
121
Nondisjunctions
Nondisjunctions occasionally occur during meiosis—they are the chromosomal mechanism for Down syndrome and other trisomies.
http://www.uic.edu
122
• The production of gametes (eggs and sperm) is the result of meiosis in the ovaries and testes.
• The spindle of microtubules usually distributes chromosomes to the daughter cells without error.
Nondisjunctions (continued)
123
• 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.
Nondisjunctions (continued)
124
Edward Syndrome
http://www.slh.wisc.edu
The condition is also known as trisomy-18 due to a third number-18 chromosome.
125
Edward Syndrome (continued)
• Edward Syndrome occurs about one in 3,000 conceptions, and one in 6,000 live 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 to 10 percent of babies with Edward Syndrome will survive the first year due to heart defects and severe breathing difficulties (known as apnea).
126
Edward Syndrome (continued)
• Major medical interventions are often withheld since the long-term prognosis is not good.
• Trisomy-18 is the result of a nondisjunction occurring during meiosis I. • Other trisomies can occur, but the embryo or fetus usually fails to sur-
vive to term (birth).
127
• 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.
• The mechanisms for nondisjunctions are well understood, although the reasons why they happen are not.
Why Do Nondisjunctions Happen?
128
Would you know where to find more information and chromosomal disorders?
