Biology Review
Mitosis and Meiosis
http://o.quizlet.com
Note
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.
2
Outline
•
•
•
•
Mitosis
Cancer
Meiosis
Chromosomal disorders
3
http://www.uq.edu.au
Mitosis
4
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.
5
Cell Division
•
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, called mitosis, is a lifelong process
for tissue growth and repair.
6
Human Skin
http://publications.nigms.nih.gov
http://www.web-books.com
7
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.
8
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.
9
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 singlecell organisms such as bacteria.
10
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.
Egg and sperm
Computer-generated image
http://neurophilosophy.files.wordpress.com
11
Can you describe similarities and differences between
asexual and sexual reproduction?
12
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://www.scfbio-iitd.res.in
•
13
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 image
http://www.cgl.ucsf.edu
14
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.
Nucleus of a chrysanthemum
(plant) cell
http:/z.about.com
•
15
Sister Chromatids
•
A cell duplicates all of its chromosomes through the process of DNA
replication 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.
16
Sister Chromatids (continued)
Sister chromatids
joined at the
centromere
http://www.cbs.dtu.dk
17
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.
18
Cell Cycle—Interphase
http://bhs.smuhsd.org
19
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.
20
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)
21
Cell Cycle—Mitotic- or M-Phase
http://bhs.smuhsd.org
22
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.
23
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.
24
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.
25
What could happen if mitosis was not typically a highlyaccurate process?
26
Stages of Mitosis
•
Although mitosis is a continuum of cell division activity, four stages are
commonly described:
–
–
–
–
Prophase
Metaphase
Anaphase
Telophase
27
Mitosis in Onion Root Cells
http://www.sep.alquds.edu
1.
2.
3.
4.
5.
Interphase (G2)
Prophase
Metaphase
Anaphase
Telophase
Mitosis consists of phases 2 through 5.
The process, except for the type of
cytokinesis, is the same in plant and
animal cells.
28
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://www.mytinyplot.co.uk
•
29
Interphase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
30
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 synthesis during cell division.
•
In very late interphase (G2), the cytoplasm has two centrosomes and
pairs of centrioles.
31
Prophase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
32
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 centrosomes.
•
The microtubules are constructed on protein molecules.
33
Mitotic Spindle
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
http://www.ornl.gov
http://mcb.berkeley.edu
34
Late Prophase
•
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 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.
35
Metaphase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
36
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 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 of the cell.
37
Anaphase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
38
Anaphase
•
In anaphase, the sister chromatids of each chromosome pair suddenly
separate.
•
Each sister chromatid is now considered to be a daughter chromosome.
39
Anaphase (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.
40
Telophase
http://www.sep.alquds.edu
http://www.microscopy-uk.org.uk
41
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.
42
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.
43
http://www.molecularexpressions.com
Cytokinesis in Animal Cells (continued)
Actin molecules in the process of pinching-off the parent
cell to form two daughter cells.
44
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.
45
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.
46
Cancer
Cancerous squamous cell with cross-sectional cut
http://www.wellcome.ac.uk
47
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.
48
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 delicate tissues of the brain due to increased
intracranial pressure and mechanical deformation.
49
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 cells
http://www.oralcancerfoundation.org
50
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.
51
More on Cancer
•
Cancer is a collection of diseases in which cells are no longer effectively 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.
52
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.
53
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.
Mitotic phase
G2 phase
G1 phase
Cell control cycle
S phase
http://www.answers.com
54
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 tumorsuppressor gene shown
surrounding a segment of
DNA.
Computer-generated image
http://www.cosmosmagazine.com
55
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 cells
false-color electron micrograph
http://www.wellcome.ac.uk
56
Progressive Mutations
•
Colon cancer begins as unusually-frequent mitotic division of normalappearing 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.
57
Progressive Mutations (continued)
Normal cell
http://science.kennesaw.edu
58
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 malignant tumor starts.
•
Genetic mutations are not passed from the parents to the child if they
do not affect zygotes (eggs or sperm).
59
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 additional mutations.
60
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 mutations.
61
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.
62
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)
63
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.
64
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.
65
Two past cultural icons—Lauren
Bacall and James Dean
http://uberoriginal.blogspot.com
http://blog.beliefnet.com
Glamorous?
66
http://www.esubulletin.com
http://www.home-air-purifier-expert.com
Possible Outcome
Healthy and cancerous lung tissues.
67
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 typ es
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
154,500
92,000
1,334,100
556,500
All other cancers
Totals
68
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 skeletal muscle.
- Leukemias—blood-forming tissues including bone marrow.
- Lymphomas—lymph nodes.
-
69
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.
70
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.
71
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.
72
Do you think if enough money were devoted to cancer
research, that an overall cure will be found?
73
http://www.scienceclarified.com
Meiosis
74
http://about.biology.com
Sexual Reproduction
We discussed asexual reproduction—now we cover some aspects
of sexual reproduction, which we will return to later in the semester.
75
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.
76
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.
77
Unordered Chromosomes
http:www.biotechnologyonline.gov
Light micrograph of chromosomes during the earliest stages of mitosis.
78
Karyotype
http://www.ucl.ac.uk
An ordered array of chromosomes (size, shape, and banding).
79
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.
80
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.
81
Diploid Number
•
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 individual chromosomes (46) is the diploid number,
2n.
Di = two.
82
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 chromosomes found in body cells.
•
The number of chromosomes in human gametes (23) is the known as
the haploid number, n.
Spermatozoa
(sperm)
http://zoology.unh.edu
•
83
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.
84
http://nmhm.washingtondc.museum
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.
85
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.
86
http://www.midwesttiv.com
Two-Cell Stage
The first day after fertilization—mitotic cell division has begun.
87
http://fig.cox.miami.edu
Eight-Cell Stage
At three days.
88
Continued Growth
http://library.thinkquest.org
Five-week-old human
embryo
http://nmhm.washingtondc.museum
Eight-week-old human
embryo
89
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.
90
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 genetic variation.
91
Interphase and Meiosis I
http://www.mun.ca
92
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://www.sinauer.com
•
93
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
94
Prophase I (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
parent cell’s equator.
http://www.uic.edu
95
http://www.mun.ca
Metaphase I, Anaphase I, and Telophase I
96
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
97
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 crossing-over in prophase I.
http://www.uic.edu
98
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
99
http://www.mun.ca
Meiosis II
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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.
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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 geneticallyunique daughter cells called gametes (eggs or sperm).
•
In mitosis and meiosis I, the chromosomes duplicate only once during
the interphase.
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Mitosis versus Meiosis (continued)
•
Mitosis involves one division of the cell nucleus and cytoplasm to produce 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.
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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.
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Independent Assortment
•
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 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.
107
Crossing-Over (continued)
Homologous
chromosomes
Tetrads
Haploid cells
http://regentsprep.org
108
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.
110
Can you describe the sources of genetic variation that
leads to substantial human diversity?
111
http://cdn.sheknows.com
Chromosomal Disorders
112
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)
Trisomy-21 karyotype
http://atlasgeneticoncology.org
Characteristic physical
features of a young child
with Down syndrome.
http://www.impaedcard.com
114
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
115
http://www.cdss.ca
Down Syndrome (continued)
http://www.downsyndrome.com
Many people with Down syndrome are
socially adept, able to hold jobs, and live
fulfilling lives.
116
http://www.icongrouponline.com
Resources and Support
117
Incidence (likelihood)
Maternal Age
p = 1 / 46
p = 1 / 2300
Age of mother at conception
http://fig.cox.miami.edu
(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.
118
http://embryology.med.unsw.edu.au
Amniocentesis can be performed
from about 15 weeks to term.
http://www.dkimages.com
Amniocentesis
119
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.
120
Nondisjunctions
http://www.uic.edu
Nondisjunctions occasionally occur during meiosis—they are the
chromosomal mechanism for Down syndrome and other trisomies.
121
Nondisjunctions (continued)
•
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.
122
Nondisjunctions (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 trisomy21, trisomy-18 (Edward syndrome), and some sex-linked chromosomal
disorders.
•
We will cover sex-linked chromosomal disorders later in the semester.
123
Edward Syndrome
http://www.slh.wisc.edu
The condition is also known as trisomy-18 due to a third number-18
chromosome.
124
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).
125
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 survive to term (birth).
126
Why Do Nondisjunctions Happen?
•
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.
127
Would you know where to find more information and
chromosomal disorders?
128