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Topic 3
Vocabulary
asexual reproduction – only one parent, reproduction by mitosis or Binary fission
Biotechnology--The application of the principles of engineering and technology to the life sciences;
bioengineering
body cell— (soma cell) any cell in the body that is not a sex cell
Bond--a mutual attraction between two atoms
Centriole—one of two tiny structures in an animal cell that helps form spindle fibers
Centromere—connection between two homologous chromosomes
Cytokinesis—division of the cytoplasm
Chromosome-- threadlike structure within the nucleus containing the genetic information that is
passed from one generation of cells to the next
Chromatid—one side of a homologous pair of chromosomes
Clone-genetically identical organism produced by a single cell
DNA-- deoxyribonucleic acid, the hereditary material in humans and almost all other organisms
Egg-- female gamete that is haploid
Expressed—able to be seen in offspring
Gene-- sequence of DNA that codes for a protein and thus determines a trait.
genetic engineering-- process of making changes in the DNA code of living organisms
genetic recombination—recombination of genes during sexual reproduction
Heredity-- the passing of traits to offspring
Cell cycle-- series of events that cells go through as they grow and divide
Mitosis—division of nucleus creating diploid cells
Mutation-- change in a DNA sequence that affects genetic information
Replicate— copying process by which a cell duplicates its DNA
selective breeding-- method of improving a species by allowing only those individual organisms with
desired characteristics to produce the next generation
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sexual reproduction-- process by which two cells from different parents unite to produce the first
cell of a new organism
Sperm—male sex cell that is haploid
Spindle-- fanlike microtbule structure that helps separate the chromosomes during mitosis
Subunit—parts DNA can be broken down into: Deoxyribose sugar, Phosphate, nucleic acid (A,T,G,C)
Template—RNA that codes for proteins
Traits-- specific characteristic that varies from one individual to another
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Heredity and Genes
Heredity is the passing of genetic information from
one generation to the next through reproduction.
The hereditary information (DNA) is organized in
the form of genes located in the chromosomes of
each cell.
Chromosomes are found in the cell nuclei, and
contain the DNA molecules. It is the DNA molecules
that carry the genetic information of the cell.
A human cell contains many thousands of genes in
its nucleus, and each gene carries a separate piece
of coded information.
The traits inherited by an individual can be determined by one pair of genes or by several
pairs of genes. A single gene pair can sometimes influence more than one trait.
Methods of Reproduction
There are two common methods of
reproduction: asexual and sexual.
Asexual reproduction -- involves
oneparent or individual (often a single-celled organism)
all the genetic instructions (genes) come from one individual or parent.
offspring are usually identical to the parent
3
Identical genetic copies are known as clones
Examples: Bacteria, yeast, vegetative propagation, tubers, bulbs, runners,
budding
4
The division of the nucleus during the M phase of the cell cycle is called
___________________.
Interphase is divided into 3 phases
1. ____________________
2. _____________________
3. ____________________
During the G1 phase
____________________________________________
During the S phase _____________________________________________
During the G2 phase ____________________________________________
The 4 phases of Mitosis are:
1.
2.
3.
4.
_________________________________
_________________________________
_________________________________
_________________________________
In pant cells a _____________________ forms
midway of the cell.

Sexual reproduction-- involves two
parents.
In organisms that reproduce sexually,
two parents are required to produce
offspring. Each parent produces sex
cells.
Sperm are the sex cells produced by
the male;
Egg is the sex cell produced by the female.
5
genes in body cells-cells other than sex cells occur
in pairs, but each sex cell contains only one gene from each pair. The offspring
that results from sexual reproduction therefore receives half of its genetic
information from the female parent (via the egg) and half from the male
parent (via the sperm).
Chromosome Number
All organisms have different numbers of chromosomes.
A body cell in an adult fruit fly has 8 chromosomes: 4 from the
fruit fly's male parent, and 4 from its female parent.
These two sets of chromosomes are homologous.
Each of the 4 chromosomes that came from the male parent has a
corresponding chromosome from the female parent.
A cell that contains both sets of homologous chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.
For Drosophila, the diploid number is 8, which can be written as 2N=8.
The gametes of sexually reproducing organisms contain only a single set of chromosomes,
and therefore only a single set of genes.
These cells are haploid. Haploid cells are represented by the symbol N.
For Drosophila, the haploid number is 4, which can be written as N=4.
What happens during the process of meiosis?
Meiosis is a process of reduction division in which the number of chromosomes per cell
is cut in half through the separation of homologous chromosomes in a diploid cell.
Meiosis involves two divisions, meiosis I and meiosis II.
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.
Meiosis I
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Cells undergo a round of DNA replication, forming duplicate chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
When homologous chromosomes form tetrads in meiosis I, they exchange portions of their
chromatids in a process called crossing-over.
Crossing-over produces new combinations of alleles.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward opposite ends of the cell.
Nuclear membranes form.
The cell separates into two cells.
The two cells produced by meiosis I have chromosomes and alleles that are different from
each other and from the diploid cell that entered meiosis I.
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A
B
C
D
E
Meiosis II
The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, neither cell goes through chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.
A to B Meiosis I results in two haploid (N) daughter cells, each with half the number of
chromosomes as the original cell.
C
The chromosomes line up in the center of cell.
D The sister chromatids separate and move toward opposite ends of the cell.
E Meiosis II results in four haploid (N) daughter cells
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Gamete Formation
In male animals, meiosis results in four equal-sized gametes called sperm.
In many female animals, only one egg results from meiosis. The other three cells, called
polar bodies, are usually not involved in reproduction.
How is meiosis different from mitosis?
Mitosis results in the production of two genetically identical diploid cells. Meiosis
produces four genetically different haploid cells.
Mitosis

Cells produced by mitosis have the same number of chromosomes as the original cell.

Mitosis allows an organism to grow and replace cells.

Some organisms reproduce asexually by mitosis.
Meiosis
•
Cells produced by meiosis have half the number of chromosomes as the
parent cell.
•
These cells are genetically different from the diploid cell and from each
other.
•
Meiosis is how sexually reproducing organisms produce gametes.
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Genetic Recombination When a sperm and egg combine to form a new cell with a
complete set of genetic instructions, a unique combination of genes results. The term
for this is genetic recombination. This unique combination of thousands of genes
produces an offspring that may resemble either or both parents in many ways but will
not be identical
to either of them.
Do Questions 1-7
The Genetic Code
The inherited instructions (genes) that are passed from
parent to offspring exist in the form of a chemical code
called the genetic code. It is contained in the DNA
molecules of all organisms. DNA molecules resemble a
flexible, twisted ladder formed from many smaller
repeating units, as shown in Figure 3-2.
DNA Structure
Like other large molecules of life, the DNA molecule is made of
thousands of smaller sections called subunits.
Each subunit has three chemical parts: a sugar, a phosphate, and
a base.
The bases are represented by of a DNA molecule the letters A,
G, C, and T.
The four subunits of DNA molecules are arranged in pairs, each
subunit forming one side and half of
one rung of the "twisted ladder."
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Watson and Crick's model of DNA was a double helix, in
which two strands were wound around
each other.
Watson and Crick discovered
that hydrogen bonds can form
only between certain base
pairs—adenine and thymine, and guanine and cytosine.
This principle is called base pairing.
Once the chemical and structural properties of DNA were discovered by scientists, it
became clear how this molecule could contain a kind of message that functions as a code.
The specific sequence of bases in a DNA molecule forms a coded message. The message of
a single gene is often a sequence of hundreds of bases. The code for an entire human is
estimated to be around 3 billion base pairs!
DNA Replication
DNA and Chromosomes
In prokaryotic cells, DNA is located in the
cytoplasm.
Most prokaryotes have a single DNA molecule
containing nearly all of the cell’s genetic
information.
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Many eukaryotes have 1000 times the
amount of DNA as prokaryotes.
Eukaryotic DNA is located in the cell
nucleus inside chromosomes.
The number of chromosomes varies widely
from one species to the next.
Eukaryotic Chromosome Structure
Eukaryotic chromosomes contain DNA wrapped around proteins called histones. The strands
of nucleosomes are tightly coiled and supercoiled to form chromosomes.
New Origin
Strand al
strand
Gr
Gr
ow
ow
th
th
Replic
ation
Fork
Replic
ation
Fork
What happens during DNA replication?
DNA Replication
Each strand of the DNA double helix has all the information needed to reconstruct the
other half by the mechanism of base pairing.
In most prokaryotes, DNA replication begins at a single point and continues in two
directions.
12
In eukaryotic chromosomes, DNA replication occurs at hundreds of places. Replication
proceeds in both directions until each chromosome is completely copied.
The sites where separation and replication occur are called replication forks.
Proteins and Cell Functioning
The work of the cell is carried out by
the many types of molecules the cell
assembles (synthesizes). Many of these
molecules are proteins. Protein
molecules are long chains. They are
formed from various combinations of
20 kinds of amino acids arranged in a
specific sequence. The sequence of
amino acids in a particular protein
influences the shape of the molecule.
This is because some of the amino acid
parts are attracted to (and may bond
with) other amino-acid parts of the
chain. The connections that form between different parts of the chain cause it to fold and
bend in a specific way. The final folded shape of the protein enables it to carry out its
function in the cell. Many proteins made by a cell become enzymes that regulate chemical
reactions.
Remember that an enzyme can interact with a specific molecule because their shapes
correspond.
Some of the proteins made in cells become parts of organelles, such as the cell membrane.
Other proteins include the hormone insulin or the many antibodies that bind to antigen
molecules on pathogens. The color of your eyes and skin are also the result of proteins
synthesized by your body.
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The DNA-Protein Connection
RNA and Protein Synthesis
Genes are coded DNA instructions that control the production of proteins.
Genetic messages can be decoded by copying part of the nucleotide sequence from DNA
into RNA.
RNA contains coded information for making proteins.
The Structure of RNA
RNA consists of a long chain of nucleotides.
Each nucleotide is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base.
There are three main differences between RNA and DNA:
•
•
•
The sugar in RNA is ribose instead of deoxyribose.
RNA is generally single-stranded.
RNA contains uracil in place of
thymine.
What are the three main types of RNA?
There are three main types of RNA:
•
• messenger RNA
• ribosomal RNA
• transfer RNA
Messenger RNA (mRNA) carries copies of
instructions for assembling amino acids into
proteins.
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Ribosomes are made up of proteins and ribosomal RNA (rRNA).
During protein construction, transfer RNA (tRNA) transfers each amino acid to the
ribosome.
What is transcription?
Transcription
RNA molecules are produced by copying part of a nucleotide sequence of DNA into a
complementary sequence in RNA. This process is called transcription.
Transcription requires the enzyme RNA polymerase.
During transcription, RNA polymerase binds to DNA and separates the DNA strands.
RNA polymerase then uses one strand of DNA as a template from which nucleotides
are assembled into a strand of RNA.
RNA polymerase binds only to regions of DNA known as promoters.
Promoters are signals in DNA that indicate to the enzyme where to bind to make RNA.
The Genetic Code
The genetic code is the “language” of mRNA instructions.
The code is written using four “letters” (the bases: A, U, C,
and G).
A codon consists of three consecutive nucleotides on
mRNA that specify a particular amino acid.
15
Each codon specifies a particular amino acid that is to be placed on the polypeptide chain.
Some amino acids can be specified
by more than one codon.
There is one codon AUG that can
either specify the amino acid
methionine or serve as a “start”
codon for protein synthesis.
There are three “stop” codons that
do not code for any amino acid.
These “stop” codons signify the end
of a polypeptide.
Translation
Translation is the decoding of an mRNA message into a polypeptide chain (protein).
Translation takes place on ribosomes.
During translation, the cell uses information from messenger RNA to produce proteins.
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Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it
attaches to a ribosome.
Translation begins when an mRNA molecule
attaches to a ribosome.
As each codon of the mRNA molecule moves
through the ribosome, the proper amino acid is
brought into the ribosome by tRNA.
In the ribosome, the amino acid is transferred to the growing polypeptide chain.
Each tRNA molecule carries only one kind of amino acid.
In addition to an amino acid, each tRNA molecule has
three unpaired bases.
These bases, called the anticodon, are complementary
to one mRNA codon.
The ribosome binds new tRNA molecules and amino
acids as it moves along the mRNA.
The process continues until the ribosome reaches a
stop codon.
The Roles of RNA and DNA
The cell uses the DNA “master plan” to prepare RNA “blueprints.” The DNA stays in the
nucleus.
The RNA molecules go to the protein building sites in the cytoplasm—the ribosomes.
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Genes and Proteins
Genes contain instructions for assembling proteins.
Many proteins are enzymes, which catalyze and regulate
chemical reactions.
Proteins are each specifically designed to build or operate a
component of a living cell.
The sequence of bases in DNA is used as a template for mRNA.
The codons of mRNA specify the sequence of amino acids in a
protein.
Do Questions 8-22
Mutations
• Mutations _are changes in the DNA sequence that affect genetic information.
•
Types of gene mutations (changes in a single gene):
–
–
Substitution--_one nucleic acid is substituted for another
•
Original strand: TAC GCA TGG
•
Mutated strand: TAC GTA TGG
Insertion—one nucleic acid is added in
•
Original strand: TAC GCA TGG
•
Mutated strand: TAT CGC ATG
Deletion – one nucleic acid is left out
•
Original strand: TAC GCA TGG
•
Mutated strand: TCG CAT GG
18
•
___insertion_________________and _______deletion__________cause the
biggest problems because everything shifts
–
This is called a frameshift mutation This makes the code unreadable
Example:
Normal DNATHEREDFOXATETHEBIGRAT
Insertion mutation-
THEEREDFOXATETHEBIGRAT
Deletion mutationTHEEDFOXATETHEBIGRAT
Types of chromosomal Mutations
(____change in whole chromosome_____):
DNA and Individuality Although an individual's body cells all originally come from a single
cell, the body is made up of many types of cells. Each body cell's nucleus-whether it is a
nerve cell, skin cell, or bone cell-has a complete set of identical genetic instructions for
that individual .
19
For years, scientists wondered how cells with identical genetic
instructions could be so different. The answer is that each
kind of cell uses only some of the genetic information it
contains. It uses only the instructions it needs to operate its
own kind of cell. For instance, information for building all of a
person's enzymes is coded in the chromosomes of each cell,
but a muscle cell uses only the specific enzymes that are
needed by a muscle cell. Both the internal and external
environment of the cell can influence which genes are
activated in that cell. Some of this influence may occur during
development, leading to the many different types of cells that
an organism needs.
The selective activation of genes in a cell may continue as
conditions change throughout life. For instance, chemical signals from within the cell or
from other cells may activate a particular gene. Hormones are one kind of molecule that can
activate parts of a cell's DNA code, leading to the production of a particular protein.
Although genes are inherited, an organism's environment can affect the way some genes
are revealed, or expressed, in the organism. For example, in some animals, such as the
Himalayan rabbit, the outside temperature can cause the activation or inactivation of the
genes for fur color. When the rabbit's body area is cold, black fur grows. If the same body
area becomes
warm, white fur grows instead. (See Figure 3-8.) The environment can also influence human
genes. Studies of identical twins (those with identical genetic information) who were raised
in different environments show that they have differences that can only be explained by
the influence of the environment on gene expression.
Do questions 23-31
20
Genetic Engineering
Selective Breeding
Question: What is the purpose of selective breeding?
Answer: Selective breeding allows only those organisms with desired characteristics to
produce the next generation.
Examples: Nearly all domestic animals and most crop plants have been produced by
selective breeding.
A) Hybridization
Hybridization is the crossing of dissimilar individuals to bring together the best of
both organisms.
Hybrids, the individuals produced by such crosses, are often hardier than either of
the parents.
B) Inbreeding
Inbreeding is the continued breeding of individuals with similar characteristics.
Inbreeding helps to ensure that the characteristics that make each breed unique will
be preserved.
Warning! Serious genetic problems can result from excessive inbreeding.
21
Increasing Variation
Question: Why might breeders try to induce mutations?
Answer: Breeders induce mutations to increase genetic variation in a population.
Mutations
Mutations occur spontaneously, but breeders can increase the mutation rate by using
radiation and chemicals.
Breeders can often produce a few mutants with desirable characteristics that are not
found in the original population.
A) Producing New Kinds of Bacteria
Introducing mutations has allowed scientists to develop hundreds of useful bacterial
strains, including bacteria that can clean up oil spills.
B) Producing New Kinds of Plants
Mutations in some plant cells produce cells that have double or triple the normal
number of chromosomes.
This condition, known as polyploidy, produces new species of plants that are often
larger and stronger than their diploid relatives.
22
Warning! Polyploidy in animals is usually fatal.
The Tools of Molecular Biology
Question: How do scientists make changes to DNA?
Answer: Scientists use their knowledge of the structure of DNA and its chemical
properties to study and change DNA molecules.
Scientists use different techniques to:
•
extract DNA from cells
•
cut DNA into smaller pieces
•
identify the sequence of bases in a DNA molecule
•
make unlimited copies of DNA
In genetic engineering, biologists make changes in the DNA code of a living organism.
DNA Extraction
DNA can be extracted from most cells by a simple chemical procedure.
The cells are opened and the DNA is separated from the other cell parts.
Cutting DNA
Most DNA molecules are too large to be analyzed, so biologists cut them into smaller
fragments using restriction enzymes.
Each restriction enzyme cuts DNA at a specific sequence of nucleotides.
Recognition Sequence
23
A restriction enzyme will cut a DNA sequence only if it matches the sequence
precisely.
Separating DNA
In gel electrophoresis, DNA fragments are placed at one end of a porous gel, and an
electric voltage is applied to the gel.
When the power is turned on, the negatively charged DNA molecules move toward
the positive end of the gel.
Gel electrophoresis can be used to compare the genomes of different organisms or
different individuals.
It can also be used to locate and identify one particular gene in an individual's
genome.
1) Restriction enzymes cut DNA into fragments.
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2) The DNA fragments are poured into wells on a gel.
3) An electric voltage is applied to the gel. This moves the DNA fragments
across the gel.
The smaller the DNA fragment, the faster and farther it will move across the
gel.
4) Based on size, the DNA fragments make a pattern of bands on the gel.
5) These bands can then be compared with other samples of DNA.
Using the DNA Sequence
Knowing the sequence of an organism’s DNA allows researchers to study specific genes, to
compare them with the genes of other organisms, and to try to discover the functions of
different genes and gene combinations.
Reading the Sequence
In DNA sequencing, a complementary DNA strand is made using a small proportion of
fluorescently labeled nucleotides.
Each time a labeled nucleotide is added, it stops the process of replication, producing
a short color-coded DNA fragment.
25
When the mixture of fragments is separated on a gel, the DNA sequence can be
read.
Cutting and Pasting
Short sequences of DNA can be assembled using DNA synthesizers.
“Synthetic” sequences can be joined to “natural” sequences using enzymes that splice
DNA together.
These enzymes also make it possible to take a gene from one organism and attach it
to the DNA of another organism.
Such DNA molecules are sometimes called recombinant DNA.
Jellycats
Lemid?
26
Making Copies
Polymerase chain reaction (PCR) is a technique that allows biologists to make copies
of genes.
A biologist adds short pieces of DNA that are complementary to portions of the
sequence.
1) DNA is heated to separate its two
strands, then cooled to allow the
primers to bind to single-stranded
DNA.
2) An enzyme called DNA
polymerase starts making copies of the region between the
primers.
Transforming Bacteria
Question: What happens during cell
transformation?
Answer: During transformation, a cell
takes in DNA from outside the cell. The
external DNA becomes a component of the
cell's DNA.
Foreign DNA is first joined to a small, circular DNA molecule known as a plasmid.
Plasmids are found naturally in some bacteria and have been very useful for DNA
transfer.
The plasmid has a genetic marker—a gene that makes it possible to distinguish
bacteria that carry the plasmid (and the foreign DNA) from those that don't.
27
Example: Diabetic humans don’t produce enough insulin to control the amount of sugar in
their blood. We used to harvest insulin from horses. Now, however, thanks to gene splicing
we’re able to splice our genes for insulin production into bacteria. We grow the bacteria in
large vats and produce large amounts of human insulin for a fraction of the cost.
Transforming Plant Cells
Question: How can you tell if a
transformation experiment has been
successful?
Answer: If transformation is successful, the
recombinant DNA is integrated into one of
the chromosomes of the cell.
Methods:
A) In nature, a bacterium exists that produces tumors in plant cells.
Researchers can remove the tumor-producing gene found in this bacterium and insert
a piece of foreign DNA into the plasmid.
The bacteria with the recombinant plasmid can then be used to infect plant cells.
B) When their cell walls are removed, plant cells in culture will sometimes take up
DNA on their own.
C) DNA can also be injected directly into some cells.
Cells transformed by either procedure can then be cultured to produce adult plants.
Transforming Animal Cells
Many egg cells are large
enough that DNA can be
28
directly injected into the nucleus.
Enzymes may help to insert the foreign DNA into the chromosomes of the injected cell.
DNA molecules used for transformation of animal and plant cells contain marker genes.
DNA molecules can be constructed with two ends that will sometimes recombine with
specific sequences in the host chromosome.
The host gene normally found between those two sequences may be lost or replaced with a
new gene.
Transforming Bacteria
Question: What happens during
cell transformation?
Answer: During transformation, a cell
takes in DNA from outside the cell. The
external DNA becomes a component of the cell's DNA.
Foreign DNA is first joined to a small, circular DNA molecule known as a plasmid.
Plasmids are found naturally in some bacteria and have been very useful for DNA
transfer.
The plasmid has a genetic marker—a gene that makes it possible to distinguish
bacteria that carry the plasmid (and the foreign DNA) from those that don't.
Example: Diabetic humans don’t produce enough insulin to control the amount of sugar in
their blood. We used to harvest insulin from horses. Now, however, thanks to gene splicing
we’re able to splice our genes for insulin production into bacteria. We grow the bacteria in
large vats and produce large amounts of human insulin for a fraction of the cost.
29
Transgenic Organisms
An organism described as transgenic, contains
genes from other species.
Question: How are transgenic organisms useful to
human beings?
Answer: Genetic engineering has spurred the growth of biotechnology.
Transgenic Microorganisms
Transgenic bacteria produce important
substances useful for health and
industry. Transgenic bacteria have been
used to produce:
•
insulin
•
growth hormone
•
clotting factor
Transgenic Animals
Transgenic animals have been used to
study genes and to improve the food supply.
Mice have been produced with human genes that make their immune systems
act similarly to those of humans. This allows scientists to study the effects of
diseases on the human immune system.
Researchers are trying to produce transgenic chickens that will be resistant
to the bacterial infections that can cause food poisoning.
Transgenic Plants
Transgenic plants are now an important part of our food supply.
30
Many of these plants contain a gene that produces a natural insecticide, so
plants don’t have to be sprayed with pesticides.
Cloning
A clone is a member of a population of genetically identical cells produced from a
single cell.
In 1997, Ian Wilmut cloned a sheep called Dolly.
Researchers hope cloning will enable them to make copies of transgenic animals and
help save endangered species.
Warning! Studies suggest that cloned animals may suffer from a number of genetic
defects and health problems.
Do question 32-43
Human Chromosomes
Cell biologists analyze chromosomes by looking at karyotypes.
Cells are photographed during mitosis. Scientists then cut out the chromosomes
from the photographs and group them together in pairs.
A picture of chromosomes arranged in this way is known as a karyotype (below).
31
Two of the 46 human chromosomes are known as sex chromosomes, because they
determine an individual's sex.
 Females have two copies of an X chromosome. (XX)
 Males have one X chromosome and one Y chromosome. (XY)
If you look carefully at the karyotype above, you can see the sex chromosomes in the
lower right corner. Is this individual male or female?
Male
The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes.
Question: How is sex determined?
Answer: All human egg cells carry a single X chromosome (23,X).
Half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome
(23,Y).
About half of the zygotes will be 46,XX (female) and half will be 46,XY (male).
Males and females are born in a roughly 50 : 50 ratio because of the way in which
sex chromosomes segregate during meiosis.
So, the sex of the offspring depends upon whether the sperm cell is carrying an X
chromosome (female offspring) or the Y chromosome (male offspring).
Human Traits
32
In order to apply Mendelian genetics to humans, biologists must identify an inherited trait
controlled by a single gene.
1. They must establish that the trait is inherited and not the result of
environmental influences.
2. They have to study how the trait is passed from one generation to the
next.
Pedigree Charts
A pedigree chart shows the relationships within a family.
Genetic counselors analyze pedigree charts to infer the genotypes of family
members.
Genes and the Environment
Some obvious human traits are almost impossible to associate with single
genes.


Traits, such as the shape of your eyes or ears, are polygenic,
meaning they are controlled by many genes.
Many of your personal traits are only partly governed by genetics.
Human Genes
The human genome includes tens of thousands of genes.
It took us a long time to identify all of them, but in 2003, the DNA sequence of the human
genome was published.
In a few cases, biologists were able to identify genes that directly control a single human
trait such as blood type.
Blood Group Genes
33
Human blood comes in a variety of genetically determined blood groups.
A number of genes are responsible for human blood groups.
There are :
 ABO blood groups (Blood type A, B, AB, and O)
Phenotype
Genotype(s)
(Blood Type)
Antigen
Safe
(protein)
Transfusions
On Red Blood
To
From
A, AB
A,O
Cell
A
I AI A, I Ai
A
 Rh blood groups (Rh Positive and Rh Negative)
Every human has a blood type and an Rh factor. Do you know yours?
ABO blood group
• There are three alleles for this gene, IA, IB, and i.
• Alleles IA and IB are codominant.
• Allele i is recessive.
34
B
IBIB, IBi
B
B, AB
B,O
AB
I AI B
A and B
AB
A,B,AB,O
**(Universal
**
Recipient)
O
***(Universal
ii
None
A, B,
AB,O
O
***
Donor)
Rh blood group
The Rh blood group is determined by a single gene with two alleles—
positive and negative.
The positive (Rh+) allele is dominant, so individuals who are Rh+/Rh+ or
Rh+/Rh- are said to be “Rh-positive”.
Individuals with two Rh- alleles are said to be “Rh-negative”.
Recessive Alleles
The presence of a normal, functioning gene is revealed only when an abnormal
or nonfunctioning allele affects the phenotype.
Many disorders are caused by autosomal recessive alleles.
Dominant Alleles
The effects of a dominant allele are expressed even when the recessive allele
is present.
Three examples of genetic disorders caused by autosomal dominant alleles are
35
Codominant Alleles
Sickle cell disease is a serious disorder caused by a codominant allele.
Sickle cell is found in about 1 out of 500 African Americans.
From Gene to Molecule
Question: How do small changes in DNA cause genetic disorders?
Answer: In both cystic fibrosis and sickle cell disease, a small change in the DNA of a
single gene affects the structure of a protein, causing a serious genetic disorder.
Cystic Fibrosis
Cystic fibrosis is caused by a recessive allele.
Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their
lungs and breathing passageways.
The most common allele that causes cystic fibrosis is missing 3 DNA bases.
These DNA bases contain the code for a specific amino acid used in
making protein. This amino acid is called phenylalanine.
As a result, the amino acid phenylalanine is missing from the CFTR
protein.
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Normal CFTR is a chloride ion channel in cell membranes.
Abnormal CFTR cannot be transported to the cell membrane.
The cells in the person’s airways are unable to transport chloride ions.
As a result, the airways become clogged with a thick mucus.
Sickle Cell Disease
Sickle cell disease is a common genetic disorder found in African Americans.
It is characterized by the bent and twisted shape of the red blood cells. Normally these
cells are flattened and round, kind of like a Frisbee.
Normal
Sickle Cell
Hemoglobin is the protein in red blood cells that carries oxygen.
In the sickle cell allele, just one DNA base is changed.
As a result, the abnormal hemoglobin is less soluble than normal hemoglobin.
Low oxygen levels cause some red blood cells to become sickle shaped.
People who are heterozygous for the sickle cell allele are generally healthy and they are
resistant to malaria.
There are three phenotypes associated with the sickle cell gene.
An individual with both normal and sickle cell alleles has a different phenotype—
resistance to malaria—from someone with only normal alleles.
Sickle cell alleles are thought to be codominant.
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Sex-Linked Genes
In humans, the X chromosome and the Y chromosomes determine sex (XX=Female,
XY=Male).
Genes located on these chromosomes are called sex-linked genes.
More than 100 sex-linked genetic disorders have now been mapped to the X chromosome.
The Y chromosome is much smaller than the X chromosome and appears to contain only a
few genes.
Question: Why are sex-linked disorders more common in males than in females?
Answer: For a recessive allele to be expressed in females, there must be two copies of the
allele, one on each of the two X chromosomes.
However, males have just one X chromosome. Thus, all X-linked alleles are expressed in
males, even if they are recessive.
Colorblindness (X-linked)
Three human genes associated with color vision are located on the X
chromosome.
In males, a defective version of any one of these genes produces
colorblindness.
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Hemophilia (X-linked)
In hemophilia, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal
bleeding if bruised.
The X chromosome carries two genes that help control blood clotting.
A recessive allele in either of these two genes would produce hemophilia
in a male.
Duchenne Muscular Dystrophy (X-linked)
Duchenne muscular dystrophy is a sex-linked disorder that results in the
weakening and
loss of skeletal muscle.
It is caused by a defective version of the gene that codes for a muscle
protein.
Again, this gene is carried on the X chromosome.
X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in female cells, one X chromosome is randomly
switched off.
This chromosome forms a dense region in the nucleus known as a Barr body.
Barr bodies are generally not found in males because their single X chromosome is
still active.
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Chromosomal Disorders (Improper number of chromosomes)
Question: What problems does nondisjunction cause?
Answer: The most common error in meiosis occurs when homologous chromosomes fail to
separate.
This is known as nondisjunction, which means, “not coming apart.”
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into
gametes, and a disorder of chromosome numbers may result.
Down Syndrome
If two copies of an autosomal chromosome fail to separate during meiosis, an
individual may be born with three copies of a chromosome.
Down syndrome involves three copies of chromosome 21.
Down syndrome produces mild to severe mental retardation.
It is also characterized by:
• increased susceptibility to many diseases
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•
higher frequency of some birth defects
Sex Chromosome Disorders
In females, nondisjunction of sex chromosomes can lead to Turner’s syndrome.
A female with Turner’s syndrome has only one X chromosome.
Her genotype would simply be “X”.
Women with Turner’s syndrome are sterile.
In males, nondisjunction of sex chromosome causes Klinefelter’s syndrome.
The genotype of an affected male would be “XXY”.
The extra X chromosome interferes with meiosis and usually prevents
these individuals from reproducing.
Human DNA Analysis
There are roughly 6 billion base pairs in your DNA.
Biologists search the human genome using sequences of DNA bases.
Genetic tests are available for hundreds of disorders.
DNA testing can pinpoint the exact genetic basis of a disorder.
DNA Fingerprinting
DNA fingerprinting analyzes sections of DNA that have little or no known
function but vary widely from one individual to another.
Only identical twins are genetically identical.
DNA samples can be obtained from blood, sperm, and hair strands with tissue
at the base.
for
Chromosomes contain large amounts of DNA called repeats that do not code
proteins.
This DNA pattern varies from person to person.
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Procedure:
1. Restriction enzymes are used to cut the DNA into fragments
containing genes and repeats.
2. DNA fragments are separated using gel electrophoresis.
3. Fragments containing repeats are labeled.
4. This produces a series of bands—the DNA fingerprint.
Obviously, these two samples of DNA are form different people.
The Human Genome Project
Question: What is the goal of the Human Genome Project?
Answer: Begun in 1990, the Human Genome Project is an ongoing effort to analyze the
human DNA sequence. By 2000, it had produced a working copy of the genome. The Project
was successfully completed in 2003.
Research groups are analyzing the DNA sequence, looking for genes that may provide clues
to the basic properties of life.
Biotechnology companies are looking for information that may help develop new drugs and
treatments for diseases.
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A Breakthrough for Everyone
For the first time, data from publicly supported research on the human
genome have been posted on the Internet on a daily basis.
Everyone can read and analyze the latest genome data for themselves.
Gene Therapy
Question: What is gene therapy?
Answer: In gene therapy, an absent or faulty gene is replaced by a normal, working gene.
The body can then make the correct protein or enzyme, eliminating the cause of the
disorder.
Viruses are often used because of their ability to enter a cell’s DNA.
Procedure:
1. Virus particles are modified so that they cannot cause disease.
2. A DNA fragment containing a replacement gene is spliced to viral DNA.
3. The patient is then infected with the modified virus particles, which should
carry the gene into cells to correct genetic defects.
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