UNIT 2: Genetic Processes
Chapter 4: Cell Division and Reproduction
How do the processes of mitosis and meiosis
explain heredity and genetic variation?
Chapter 5: Patterns of Inheritance
Chapter 6: Complex Patterns of Inheritance
UNIT 2 Chapter 4: Cell Division and Reproduction
4: Cell Division and Reproduction
Five dogs have been cloned from the genetic information
from Trakr, a search and rescue dog.
What are the ethical issues
related specifically to animal
cloning?
Are the ethical issues related
to cloning copies of a gene the
same as those related to cloning
an entire organism?
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
4.1 Cell Division and Genetic Material
Genetics is the field of biology that involves the study of how
genetic information is passed from one generation of organisms
or cells to the next. Understanding genetics begins with
understanding cellular processes. The cell theory, developed in
the mid-1800s, states that:
• All living things are composed of one or more cells.
• Cells are the smallest units of living organisms.
• New cells come only from pre-existing cells by cell
division.
It follows from the third postulate that traits must be passed from
a parent cell to new daughter cells.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
The Cell Cycle
A somatic cell is a plant or animal cell found in the body of an organism. All
somatic cells go through cell cycles. As one cell completes a cycle, it
becomes two cells. The duration of the cell cycle depends on the cell type
and the organism. For most healthy, actively dividing animal cells, the cycle
lasts 12 to 24 hours.
In multicellular organisms, cell division (cell cycling) has three functions:
• growth of the organism
• repair of tissues and organs
• maintenance to replace dead cells
Specific checkpoints in the cell cycle monitor growth to ensure the cycle
continues when it should and stops when it should. Regulation is the key to
preventing uncontrolled and rapid growth, such as cancerous growth.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Stages of the Cell Cycle
There are three main stages of the cell cycle:
• interphase: growth and intense cell activity
• mitosis: cell’s nucleus and genetic material divide
• cytokinesis: division of the cell cytoplasm and creation of
new cells
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Interphase
During interphase, the cell carries out its normal functions, as
it grows and makes copies of its genetic material.
Interphase is divided into three phases: Growth 1 (G1),
Synthesis (S), and Growth 2 (G2).
• G1 is the major period of growth.
• S phase is when DNA (also called chromatin) is replicated.
• G2 involves further growth and molecule synthesis.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Mitosis
Before it divides, the cell must undergo mitosis,
which is the separation of the cell’s replicated
genetic material. Mitosis involves the following
structures:
• chromosome: a structure in the nucleus that
contains DNA
• sister chromatid: one of two chromosomes
that are genetically identical and held together at
the centromere
• centromere: the region where two sister chromatids are held
together in a chromosome
• spindle fibre: a microtubule structure that facilitates the
movement of chromosomes within a cell
• centrosome: a structure that helps to form the spindle fibres
UNIT 2 Chapter 4: Cell Division and Reproduction
Mitosis
Here we see mitosis summarized in its four stages.
Section 4.1
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Cytokinesis
The process of cytokinesis is different in different cell types.
In animal cells, an indentation forms in the cell membrane
along the equator of the cell. It deepens as the cytoplasm
divides equally, and the cell pinches off into two cells. This
is accomplished by means of microfilaments constricting.
Cytokinesis begins with a
furrow that pinches the cell
and eventually splits the two
cells apart. This transmission
electron micrograph shows
two identical kidney cells
forming. Magnification: 1700x
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Cytokinesis
In plant cells, the rigid cell wall does not pinch inward.
Instead, a new structure called a cell plate forms between the
daughter nuclei. A cell wall forms on each side of the cell
plate.
Prokaryotic cells complete cell division with binary fission
since they lack a nucleus. DNA is pulled apart, and the cell
separates into two prokaryotic cells.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
The Structures of Genetic Material
DNA is comprised of nucleotides, each of which is
made up of a sugar (deoxyribose), a phosphate group,
and a base. There are four bases: adenine (A), guanine
(G), cytosine (C), and thymine (T). Across the middle of
the helix, A pairs with T, and C pairs with G. DNA is
shaped like a long, spiraling double helix.
How are nucleotides arranged within the double helix?
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
DNA Replication
When DNA is replicated during interphase, the double helix
unwinds and each strand of DNA serves as a template for a
new strand. Each new double helix contains one original
strand and one new strand.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Chromosomes
There is no relationship between the number of
chromosomes an organism has in its cells and
the complexity of the organism.
For humans, there are two sets of 23 chromosomes in the
somatic cells; one set from the father and one set from the
mother. The sets are homologous; they contain the same
sequence of genes (traits). They also have the same
length, location of the centromere, and stain banding
pattern. However, they can contain different alleles (forms)
of a gene.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Chromosomes
One of the pairs of human chromosomes is called the sex
chromosomes as they determine the sex of the individual. The
two chromosomes, called X and Y, are in fact not homologous.
XX = female
XY = male
The rest of the chromosomes are called autosomes, and each
one has a true homologous pair.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1
Karyotypes
A karyotype is a person’s particular set of chromosomes. The
chromosomes are collected and stained when a cell is in
metaphase so they appear as sister chromatid “Xs.”
This is a human karyotype. The chromosome pairs are arranged and numbered in order of their
length, from longest to shortest. The sex chromosomes are placed last in a karyotype. Note that
the banding patterns between homologous chromosomes are different in this image because of
the type of dye that was used.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.1 Review
Section 4.1
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
4.2 Sexual Reproduction
Sexual reproduction requires two parents and produces genetically distinct
offspring. It involves the fusion of a male reproductive cell with a female
reproductive cell. These are called gametes, and the cell that results from the
fertilization is called a zygote.
The zygote has the same number of chromosomes as a somatic cell. To
achieve this, each gamete only carries one set of homologous chromosomes.
A cell with one set is haploid (n); a cell with two sets is diploid (2n). The
human diploid number is 2n = 46.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Meiosis
The process that produces gametes with a haploid number
of chromosomes is called meiosis. It has two outcomes:
• genetic reduction: a form of cell division that produces
daughter cells with half the number of chromosomes of the
parent cell
• genetic recombination: the products of meiosis have
different combinations of alleles
Like mitosis, meiosis involves a precise sequence of events
that can be grouped into four distinct phases: prophase,
metaphase, anaphase, and telophase. Meiosis, however,
involves two complete cycles of the four phases, called
meiosis I and meiosis II.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Meiosis
Some key events in meiosis I and II are as follows:
• interphase: the replication of the DNA
• prophase I: the homologous chromosomes line up side-byside in an alignment called synapsis. The homologous
chromosomes are held tightly along their lengths while some
segments are exchanged. This increases genetic diversity.
• metaphase I: homologous chromosomes are lined up together
along the equator.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Meiosis
• anaphase I: homologous chromosomes are separated
• telophase I: the homologous chromosomes uncoil and spindle
fibres disappear
• followed by cytokinesis, in which a nuclear membrane forms
around each group of chromosomes, creating haploid cells
• prophase II and metaphase II: similar to mitosis
• In anaphase II, sister chromatids are separated
• telophase II and cytokinesis create four haploid cells
UNIT 2 Chapter 4: Cell Division and Reproduction
Meiosis
Meiosis involves two
complete cycles of four
phases. Notice that each
cell contains some
chromosomes from the
mother (yellow), some
chromosomes from the
father (blue), and some
chromosomes with
segments that have
been exchanged (yellow
and blue).
Magnification: 200x
Section 4.2
UNIT 2 Chapter 4: Cell Division and Reproduction
Comparing
Mitosis
and
Meiosis
Can you summarize the differences between
mitosis and meiosis in terms of number of
divisions, genetic material, and purpose?
Section 4.2
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Gamete Formation in Animals
Spermatogenesis is the process that produces sperm in male
animals; oogenesis produces eggs in females.
In males, the spermatogonia reproduce by mitosis and then
meiosis, starting at puberty.
In females, the oogonia reproduce by mitosis before birth.
They begin meiosis but stop at prophase I. Each month after
puberty, one cell completes meiosis. However, the cytoplasm
is unequally distributed, and only one cell matures, not four.
How do these differences change the storage and
production of sperm and egg?
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Gamete Formation in Animals
Analyze the figures below to observe the differences between
spermatogenesis and oogenesis.
UNIT 2 Chapter 4: Cell Division and Reproduction
Multiple Births
Compare the different scenarios and their outcomes.
Section 4.2
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Genetic Variation
During meiosis, genetic variation is ensured in two ways: independent
assortment and crossing over.
In a process called independent assortment, gametes are created that carry
different combinations of maternal and paternal chromosomes. This occurs
during metaphase I, when each homologous chromosome is randomly
oriented towards one of the poles. This alone can produce over 8 million
different chromosome combinations.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Genetic Variation
In a process called crossing over, genetic material between
maternal and paternal chromosomes is exchanged. This
occurs during prophase I in which non-sister chromatids
exchange genetic material in multiple sections.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Errors in Chromosome Structure
When chemical bonds are re-formed during crossing over, errors
can occur that can result in the following:
Is it possible to infer which type of error would
cause the most life-threatening symptoms?
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Errors in Chromosome Number
Sometimes non-disjunction occurs, where homologous
chromosomes or sister chromatids do not separate during
meiosis (anaphase I or anaphase II). Non-disjunction produces
gametes that have too many or too few chromosomes. Trisomy
21, or three copies of chromosome 21, is an example of nondisjunction. This results in Down Syndrome. Monosomy is the
term used for a single chromosome copy error.
UNIT 2 Chapter 4: Cell Division and Reproduction
Errors in Chromosome Number
Section 4.2
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2
Prenatal Genetic Testing
Pregnant women of all ages may request or be referred for
prenatal genetic testing. If referred, procedures are covered by
the Ontario Heath Insurance Plan. Deciding to have prenatal
testing can be a difficult decision since there are many factors
to consider, including the possibility of pregnancy termination
with some procedures.
UNIT 2 Chapter 4: Cell Division and Reproduction
Prenatal Genetic Testing
Section 4.2
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.2 Review
Section 4.2
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3
4.3 Reproductive Strategies and Technologies
Methods Used in Agriculture
1.
2.
3.
Selective breeding: the process of breeding plants and animals for
desirable traits. While sometimes imprecise, this strategy has produced
many varieties of plants and animals.
Artificial insemination: the transfer of (often processed) semen into a
female’s reproductive tract. As breeders make high-quality sperm from
choice males widely available, stock improves.
Embryo transfer: fertilizing an egg artificially and then transferring it
into a recipient female. Embryos can be shipped more easily than
animals.
The Appaloosa, bred for its
leopard-spotted coat and
striped hooves, is now one
of the most popular breeds
of horses.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3
Assistive Reproductive Technologies
for Humans
1.
2.
3.
Artificial insemination: sperm is collected and concentrated, then
introduced into a woman’s vagina.
In vitro fertilization (IVF): immature eggs are retrieved, joined with
sperm in the lab, and embryos are inserted into the woman’s uterus. This
is an option for women with blocked Fallopian tubes.
Pre-implantation genetic diagnosis: As an additional step to IVF, one of
the cells of an embryo is removed and tested for specific genetic
disorders before it is implanted in the uterus.
After the sperm and egg
are put together in
laboratory glassware, they
are incubated together for
about 18 hours to allow
fertilization.
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3
Cloning
Gene cloning involves manipulating DNA to produce multiple copies
of a gene or another segment of DNA in foreign cells. The production
of the protein insulin is an example of this process, which is as
follows:
1.
2.
3.
4.
5.
Isolate the insulin gene segment.
Choose an appropriate vector, such as a bacterial plasmid.
Create recombinant DNA by inserting the insulin gene into the vector,
using molecular agents to cut and join pieces together.
Treat the bacterial cells so that they take in the recombinant DNA in a
process called transformation. Cells now make many copies of the
gene and thus produce a large amount of the protein.
Harvest the insulin.
UNIT 2 Chapter 4: Cell Division and Reproduction
Cloning
A gene or piece of DNA can be cloned. Many
copies of it or the protein product that the gene
codes for can be produced and isolated.
Section 4.3
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3
Cloning
Therapeutic cloning involves producing genetically identical cells
that are used to treat various diseases. The cloned cells are then used to
grow new tissues and organs.
Reproductive cloning also involves production of cell clones, but with
the aim of producing a genetically identical organism. Reproductive
cloning in animals is rarely successful.
Both use a process called somatic cell nuclear transfer (SCNT). In
this technique, an egg cell’s nucleus is removed and replaced with the
nucleus of a somatic cell of a donor.
UNIT 2 Chapter 4: Cell Division and Reproduction
Cloning
Therapeutic and reproductive cloning involve
inserting the nucleus from a somatic cell of the
donor into an egg cell that has had its nucleus
removed.
Section 4.3
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3
Cloning Controversy
Controversy surrounds both therapeutic and reproductive cloning as
people are unsure how the processes will be applied. Two issues arise
with the process of therapeutic cloning (somatic cell nuclear transfer
or SCNT) because:
• the process produces stem cells that could potentially be used to
create a human clone
• in some cases, the original cells used are embryonic
stem cells
Scientists have found a solution to the second issue. They have
discovered a way to use specialized adult (pluripotent) cells that have
been induced into a stem-cell-like state. Scientists continue their
research with all types of stem cells because of the potential impact on
human health and transplant acceptance.
UNIT 2 Chapter 4: Cell Division and Reproduction
Cloning Controversy
Stem cells can be stimulated to differentiate into
specific tissue types under the right conditions.
Potential applications for stem cells include
treating diseases and in regenerative medicine.
Section 4.3
Learn more!
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3
Transgenic Plant Applications
A transgenic organism, or genetically modified organism
(GMO), has had DNA from another species inserted into its
genetic material.
There are many applications for this technique
in the plant kingdom:
• to increase the plant’s resistance to herbicides, insects, or
viruses
• to produce medicinal proteins such as insulin from the
safflower plant for humans
• to increase the nutritional value of a plant such as golden rice
UNIT 2 Chapter 4: Cell Division and Reproduction
Transgenic Plant Applications
This transgenic product, golden rice, contains
four different foreign genes. Three of these
genes come from other plants, and one comes
from a fungus.
Section 4.3
UNIT 2 Chapter 4: Cell Division and Reproduction
Transgenic Animal Applications
There are some applications for this technique in the animal
kingdom:
• transgenic milk-producing animals
can produce medical proteins like
human growth hormone
• milk-producing animals can be
modified to secrete silk for
commercial use
• transgenic animals could
successfully serve as organ donors for
humans
Genetic engineering
can create transgenic
animals that secrete
human proteins or other
substances in their milk.
Section 4.3
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3
GMO Concerns
Many people have reservations about GMOs despite thorough
review processes. These concerns include:
• creation of super-weeds as species cross-reproduce, due to
their herbicide genes
• herbicide-resistant plants could encourage the use of stronger
herbicides
• not enough is known about the long-term effects of human
consumption of transgenic foods and medicine
• the amount of money spent may be greater than the overall
benefit
UNIT 2 Chapter 4: Cell Division and Reproduction
Section 4.3 Review
Section 4.1
UNIT 2 Supplemental Material
UNIT 2 STSE Feature
Stem cells from bone marrow or the central
nervous system (CNS) can be manipulated to
generate many cell types that can then be
transplanted to treat illness or repair damage.