Cell Division - Joint School District No. 2

viewkickapooBiotechnologie

12 déc. 2012 (il y a 4 années et 11 mois)

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Discovering Cells

Key Concepts
: After reading the information below, answer the following questions in your science notebook

as
completely as you can with the information you are given using complete sentences.

1.

What are cells?

2.

How did the invention of the

microscope contribute to knowledge about

living things?

3.

What is the cell theory?

4.


How do microscopes produce magnified images?


All living things are made of
cells. Cells are the basic units of structure and function in living things
. Most
cells are too s
mall to be seen with the naked

eye.


The invention of the microscope made it possible for people to discover and learn about cells.

A
microscope

is an instrument that makes

small objects look larger. Some microscopes do this by using lenses to
focus

light.

A simple light microscope contains only one lens. A light microscope

that has more than one lens is
called a compound microscope.


One of the first people to observe cells was Robert Hooke. In 1663, Hooke

observed the structure of a thin
slice of cork usi
ng a compound microscope he

had built himself. At about the same time, Anton van Leeuwenhoek
built

simple microscopes and used them to observe tiny objects. Leeuwenhoek

called the single
-
celled organisms
he saw animalcules.


In 1838, Matthias Schleiden con
cluded that all plants are made of cells.

The next year, Theodor Schwann
concluded that all animals are also made of

cells. In 1855, Rudolf Virchow proposed that new cells are formed
only from

existing cells. Schleiden, Schwann, Virchow, and others helped
develop
the cell theory
. The
cell theory
states: All living things are composed of cells; cells are the basic unit of structure and function in living things;
all cells

are produced from other cells.

For a microscope to be useful, it must combine two impor
tant

properties

magnification and resolution.
Magnification is the ability to

make things look larger than they are.
The lenses in light microscopes

magnify an
object by bending the light that passes through them
. A lens

that magnifies is thicker in the
center than at the
edges and is called a convex

lens. Because a compound microscope uses more than one lens, it can

magnify an
object more than a simple microscope. The total magnification of

a compound microscope is equal to the
magnifications of the two
lenses

multiplied together. The ability to clearly distinguish the individual parts of

an object is called resolution.
Resolution is another term for the sharpness of

an image.

Since the 1930s, scientists have developed different types of electron

microsco
pes.
Electron microscopes
use a beam of electrons instead of light

to produce a magnified image
. Because they use tiny electrons to
produce

images, the resolution of electron microscopes is much better than the

resolution of light microscopes.


Looking Ins
ide Cells

Key Concepts

1.

What role do the cell wall and cell membrane play in the cell?

2.

What are the functions of cell organelles?

3.

How are cells organized in many
-
celled organisms?


A cell is very small. Inside a cell are even smaller structures called
organelles,
which carry out specific
functions within the cell. The
cell wall
is a rigid layer

of nonliving material that surrounds the cells of plants
and some other

organisms.
A plant’s cell wall helps to protect and support the cell.
The cell wall is ma
de of
a strong, flexible material called cellulose, and many materials can pass through it.

In cells that do not have cell walls, the
cell membrane
is the outside boundary that separates the cell
from its environment. All cells have cell membranes. In cell
s with cell walls, the cell membrane is located just
inside the cell wall.
The cell membrane controls what substances come into and

out of a cell.

The nucleus is a large, oval structure that acts as the “brain” of the cell.
You can think of the nucleus
as
the cell’s control center, directing all of the

cell’s activities.
The nucleus is surrounded by a protective
membrane called the nuclear envelope. Materials pass in and out of the nucleus through small openings, or
pores, in the nuclear envelope. Inside th
e nucleus, strands of chromation contain genetic material. Also, the
nucleolus is the place where

ribosomes are made.

The
cytoplasm
is the region between the cell membrane and the nucleus. Many cell organelles are
found in the cytoplasm. The
mitochondria a
re known

as the “powerhouses” of the cell because they
convert energy in food

molecules to energy the cell can use to carry out its functions.
Passageways called
the
endoplasmic reticulum carry proteins and other materials from one

part of the cell to anot
her.
Small,
grainlike bodies called
ribosomes function as

factories to produce proteins.
Collections of sacs and tubes
called
Golgi bodies

receive proteins and other newly formed materials from the endoplasmic

reticulum,
package them, and distribute them t
o other parts of the cell.
The Golgi bodies release materials outside the
cell. In plants and some other

organisms, large, green structures called
chloroplasts capture energy from
sunlight and use it to produce food for the cell.
Large water
-
filled sacs ca
lled

vacuoles
are
the storage areas
of cells.
A vacuole stores food and other

materials needed by the cell. Small, round structures called
lysosomes contain chemicals that break down certain materials in the cell.
Plants and animals contain many
cells. In
a many
-
celled organism, the cells

are often quite different from each other and are specialized to
perform specific

functions.
In many
-
celled organisms, cells are often organized into tissues, organs, and
organ systems.



Chemical Compounds in Cells

Key
Concepts

1.

What are elements and compounds?

2.


How is water important to the function of cells?

3.


What are the main kinds of organic molecules in living things?


An element is any substance that cannot be broken down into simpler

substances.
The smallest unit
of an
element is called an atom. The elements

found in living things include carbon, hydrogen, oxygen, nitrogen,

phosphorus, and sulfur.
When two or more elements combine chemically,

they form a compound.
The
smallest unit of many compounds is called a

molecule.



Water is a compound made up of hydrogen and oxygen.
Most chemical

reactions within cells could
not take place without water.
Water also helps

cells keep their size and shape and keeps the temperature of
cells from

changing rapidly.



Many o
f the compounds found in living things contain the element

carbon. Most compounds that
contain carbon are called organic compounds.

Carbohydrates, proteins, lipids, and nucleic acids are
important groups of

organic compounds in living things.
Compounds tha
t do not contain the

element carbon are called inorganic compounds.



A
carbohydrate
is an energy
-
rich organic compound made of the

elements carbon, hydrogen, and
oxygen. Sugars and starches are examples

of carbohydrates. Carbohydrates are important comp
onents of some
cell

parts, including cell walls and cell membranes.



Fats, oils, and waxes are all
lipids.
Lipids are energy
-
rich organic

compounds made of carbon,
hydrogen, and oxygen. Lipids contain more

energy than carbohydrates. Cells store energy i
n lipids for later
use.

Proteins
are large organic molecules made of carbon, hydrogen, oxygen,

nitrogen, and, in some cases,
sulfur. Protein molecules are made up of

smaller molecules called
amino acids.
Proteins make up much of the

structure of cells. An
enzyme
is a type of protein that speeds up a chemical

reaction in a living thing. Without
enzymes, many chemical reactions that

are necessary for life would either take too long or not occur at all.


Nucleic acids
are very long organic molecules made of
carbon, oxygen,

hydrogen, nitrogen, and
phosphorus. Nucleic acids contain the instructions

that cells need to carry out all the functions of life. There
are two kinds of

nucleic acids: DNA and RNA. Deoxyribonucleic acid, or
DNA,
is the genetic

material tha
t
carries information about an organism that is passed from

parent to offspring and directs all of the cell’s
functions. Ribonucleic acid, or

RNA,
plays an important role in the production of proteins. RNA is found

in the cytoplasm as well as in the nucleu
s.




The Cell in Its Environment

Key Concepts

1.

How do most small molecules cross the cell membrane?

2.

Why is osmosis important to cells?

3.

What is the difference between passive transport and active transport?


The cell membrane is
selectively permeable,
which

means that some

substances can pass through it
while others cannot. Oxygen, food molecules,

and waste products all must pass through the cell membrane.
Substances

that can move into and out of a cell do so by one of three methods: diffusion,

osmosis, or
active transport.



Diffusion is the main method by which small molecules move across

the cell membrane.
Diffusion
is the process by which molecules tend to

move from an area of higher concentration to an area of
lower concentration.

The concentration of

a substance is the amount of the substance in a given

volume.
Diffusion is caused by molecules moving and colliding. The

collisions cause the molecules to push away from
one another and spread

out. Molecules diffuse through the cell membrane into a cell w
hen there is a

higher
concentration of the molecules outside the cell than inside the cell.

The diffusion of water molecules through a
selectively permeable

membrane is called
osmosis. Because cells cannot function properly

without
adequate water, many cel
lular processes depend on osmosis.
In

osmosis, water molecules move by
diffusion from an area where they are

highly concentrated through the cell membrane to an area where they
are

less concentrated.



The movement of dissolved materials through a cell m
embrane without

using cellular energy is called
passive transport.
Diffusion and osmosis are

both types of passive transport. When a cell needs to take in
materials that

are in higher concentration inside the cell than outside the cell, the

movement of the

materials
requires energy.
Active transport
is the

movement of materials through a cell membrane using cellular
energy. The

main difference between passive transport and active transport is that
active

transport requires
the cell to use its own energy
while passive transport

does not.
Cells have several ways of moving
materials by active transport.

In one method, transport proteins in the cell membrane “pick up” molecules

outside the cell and carry them in. Another method of active transport is

engulfin
g, in which the cell
membrane wraps around, or engulfs, a particle

and forms a vacuole within the cell.

Most cells are very small. One reason is related to the fact that all

materials move into and out of cells
through the cell membrane. Once a

molecule en
ters a cell, it is carried to its destination by a stream of moving

cytoplasm. In a very large cell, streams of cytoplasm must travel farther to

carry materials from the cell
membrane to all parts of the cell.


Photosynthesis

Key Concepts

1.

How does the sun
supply living things with the energy they need?

2.

What happens during the process of photosynthesis?



The sun is the source of energy for most living things. All cells need energy

to carry out their
functions. The process by which a cell captures the
energy

in sunlight and uses it to make food is called
photosynthesis.




Nearly all living things obtain energy either directly or indirectly from

the energy of sunlight
captured during photosynthesis.
Plants, such as

grass, use energy from the sun to mak
e their own food
through the process

of photosynthesis. An organism that makes its own food is called an

autotroph.
An
organism that cannot make its own food is called a

heterotroph.
Many heterotrophs obtain food by eating
other organisms.

Photosynthesis i
s a complex process.
During photosynthesis, plants

and some other
organisms use energy from the sun to convert carbon

dioxide and water into oxygen and sugars.
Photosynthesis takes place in

two stages: (1) capturing the sun’s energy and (2) producing sugar
s. In

plants,
this energy
-
capturing process occurs mostly in the leaves. The

chloroplasts in plant cells give plants their
green color. The green color

comes from
pigments,
colored chemical compounds that absorb light. The

main
photosynthetic pigment in ch
loroplasts is
chlorophyll.
Chlorophyll

captures light energy and uses it to power
the second stage of photosynthesis

to produce sugars. The cell needs two raw materials for this stage: water

(H
2
O) and carbon dioxide (CO
2
).

Plant roots absorb water from the soil, and

the water then moves up to the
leaves. Carbon dioxide enters the plant

through small openings on the undersides of the leaves called
stomata.
Once

in the leaves, the water and carbon dioxide move into the
chloroplasts.

Inside the chloroplasts, the water
and carbon dioxide undergo a complex

series of chemical reactions and produce two important products of

photosynthesis: sugar and oxygen. Plant cells use sugar for food and to

make other compounds, such as
c
ellulose. Plant cells also store sugar for

later use. Oxygen exits the leaf through the stomata. Almost all of the
oxygen

in Earth’s atmosphere was produced by living things through photosynthesis.

The events of
photosynthesis can be summed up by the follo
wing chemical

equation:

light energy

6 CO
2
+ 6 H
2
O


C
6
H
12
O
6
+ 6 O
2

carbon dioxide water a sugar oxygen


Respiration

Key Concepts

1.

What events occur during respiration?

2.

What is fermentation?

Cells store and use energy in a way that is similar to the way you

deposit and

withdraw money from a
savings account. When you eat a meal, you add to

your body’s energy savings account. When your cells need
energy, they make

a withdrawal by breaking down the carbohydrates in food to release energy.

The process by which c
ells obtain energy from glucose (a type of sugar)

is called
respiration. During
respiration, cells break down simple food

molecules such as sugar and release the energy they contain.
Because

living things need a continuous supply of energy, the cells of al
l living things

carry out respiration
continuously. The term
respiration
also is used to mean

breathing, that is, moving air in and out of your lungs.
To avoid confusion,

the respiration process that takes place inside cells sometimes is called

cellular
re
spiration. The two kinds of respiration are related. Breathing

brings oxygen into your lungs, and oxygen is
necessary for cellular

respiration to occur in most cells.

Like photosynthesis, respiration is a two
-
stage process. The first stage

takes place in t
he cytoplasm of
the organism’s cells. There, glucose

molecules are broken down into smaller molecules. Oxygen is not
involved

in this stage of respiration, and only a small amount of energy is released.

The second stage of
respiration takes place in the mi
tochondria. There, the

small molecules are broken down into even smaller
molecules. These

chemical reactions require oxygen, and a great deal of energy is released.

Two other
products of respiration are carbon dioxide and water.

Photosynthesis and respirat
ion can be thought of as
opposite processes.

Together, these two processes form a cycle that keeps the levels of oxygen

and carbon
dioxide fairly constant in the atmosphere.

Some cells obtain their energy through
fermentation,
an energyreleasing

process that does not require
oxygen.
Fermentation provides

energy for cells without using oxygen.
One type of fermentation occurs in

yeast and some other single
-
celled organisms. This process is sometimes

called alcoholic fermentation because
alcohol is o
ne of the products made

when these organisms break down sugars. Another type of fermentation

takes place at times in your body when your muscles run out of oxygen

for

example, when you’ve run as fast
as you could for as long as you could. One

product of th
is type of fermentation is an acid known as lactic acid.
When

lactic acid builds up, your muscles feel weak and sore.


Cell Division

Key Concepts

1.

What events take place during the three stages of the cell cycle?

2.

How does the structure of DNA help account
for the way in which DNA

copies itself?



The regular sequence of growth and division that cells undergo is known as

the
cell cycle.
The cell
cycle is divided into three main stages.



The first stage of the cell cycle is called
interphase. During
interphase, the

cell grows, makes a
copy of its DNA, and prepares to divide into two cells.

During the first part of interphase, the cell grows to
full size and produces all

the structures it needs. In the next part of interphase, the cell makes an exact

c
opy of
the DNA in its nucleus in a process called
replication.
At the end of

DNA replication, the cell contains two
identical sets of DNA.



Once interphase is complete, the second stage of the cell cycle begins.



Mitosis
is the stage during which the c
ell’s nucleus divides into two new nuclei.

During mitosis, one
copy of the DNA is distributed into each of the two

daughter cells.
Scientists divide mitosis into four parts,
or phases: prophase,

metaphase, anaphase, and telophase. During prophase, the thre
adlike

chromatin in the
cell’s nucleus condenses to form double
-
rod structures called

chromosomes.
Each identical rod in a
chromosome is called a chromatid. The

two chromatids are held together by a

structure called a centromere.
As the cell

progresses thr
ough metaphase, anaphase, and telophase, the chromatids

separate from each other
and move to opposite ends of the cell. Then two nuclei

form around the chromatids at the two ends of the cell.



After mitosis, the final stage of the cell cycle, called
cytokinesis,
completes

the process of cell
division.
During cytokinesis, the cytoplasm divides,

distributing the organelles into each of the two new
cells.
Each daughter cell

has the same number of chromosomes as the original parent cell. At the end of

cyt
okinesis, each cell enters interphase, and the cycle begins again. The length

of each stage and cell cycle
varies, depending on the type of cell.



DNA replication ensures that each daughter cell will have all of the genetic

information it needs to
carry
out its activities. The two sides of the DNA ladder

are made up of alternating sugar and phosphate
molecules. Each rung of the

DNA ladder is made up of a pair of molecules called nitrogen bases. There are

four kinds of nitrogen bases: adenine, thymine, gua
nine, and cytosine.

Adenine only pairs with thymine, and
guanine only pairs with cytosine. DNA

replication begins when the two sides of the DNA molecule unwind
and

separate. Next, nitrogen bases that are floating in the nucleus pair up with the

bases on ea
ch half of the
DNA molecule.
Because of the way in which the

nitrogen bases pair with one another, the order of the
bases in each new

DNA molecule exactly matches the order in the original DNA molecule.

Once the new
bases are attached, two new DNA molecule
s are formed.



Mendel’s Work

Key Concepts

1.

What were the results of Mendel’s experiments, or crosses?

2.

What controls the inheritance of traits in organisms?


Heredity
is the passing of physical characteristics from parents to offspring. Gregor Mendel was cu
rious
about the different forms of characteristics, or
traits,
of pea plants. Mendel’s work was the foundation of
genetics
, the scientific study of heredity.

A new organism begins to form when egg and sperm join in the process called
fertilization.
Before
fertilization can happen in pea plants, pollen must reach the pistil of a pea flower through pollination. Pea
plants are usually self
-
pollinating, meaning pollen from a flower lands on the pistil of the same flower. Mendel
developed a method by which he cr
oss
-
pollinated, or “crossed,” pea plants.

Mendel crossed two pea plants that differed in height. He crossed purebred tall plants with purebred
short plants. These parent plants, the P generation, were
purebred
because they always produced offspring
with th
e same trait as the parent.
In all of Mendel’s crosses, only one form of the

trait appeared in the F
1
generation. However, in the F
2
generation, the

“lost” form of the trait always reappeared in about one fourth of
the

plants.
From his results, Mendel reas
oned that individual factors, one from each parent, control the
inheritance of traits. Today, scientists call the factors that control traits
genes.
The different forms of a gene
are called
alleles
.
An organism’s traits are controlled by the alleles it inh
erits from its

parents. Some alleles are
dominant, while other alleles are recessive.
A
dominant allele
is one whose trait always shows up in the
organism when the allele is present. A
recessive allele
is hidden whenever the dominant allele is present. A
t
rait controlled by a recessive allele will only show up if the organism does not have the dominant allele.

In Mendel’s cross, the purebred tall plant has two alleles for tall stems. The purebred short plant has
two alleles for short stems. The F
1
plants ar
e all
hybrids:
they have two different alleles for the trait

one
allele for tall stems and one for short stems. Geneticists use a capital letter to represent a dominant allele and a
lowercase version of the same letter for the recessive allele.

Mendel’s di
scovery was not recognized during his lifetime. In 1900, three different scientists
rediscovered Mendel’s work. Because of his work, Mendel is often called the Father of Genetics.


Probability and Heredity

Key Concepts

1.

What is probability and how does it
help explain the results of genetic crosses?

2.

What is meant by genotype and phenotype?

3.

What is codominance?


Probability is a number that describes how likely it is that an event will occur.
The principles of
probability predict what is
likely
to occur, not

necessarily what
will
occur. For example, in a coin toss, the coin
will land

either heads up or tails up. Each of these two events is equally likely to

happen. In other words, there
is a 1 in 2 chance that a tossed coin will land

heads up, and a 1 in 2
chance that it will land tails up. A 1 in 2
chance can be

expressed as a fraction,
1
/
2
, or as a percent, 50 percent. The result of one coin

toss does not
affect the result of the next toss. Each event is independent of

another.

When Gregor Mendel analyzed
the results of his crosses in peas, he carefully counted all the offspring.
Over time, he realized that he could apply the principles of probability to his crosses. Mendel was the first
scientist to recognize that the principles of probability can be used
to predict the results of genetic crosses.

A tool that applies the laws of probability to genetics is a Punnett square. A
Punnett square
is a chart
that shows all the possible combinations of alleles that can result from a genetic cross. Geneticists use Pu
nnett
squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular
outcome. In a Punnett square, all the possible alleles from one parent are written across the top. All the
possible alleles from the other p
arent are written down the left side. The combined alleles in the boxes of the
Punnett square represent all the possible combinations in theoffspring.
In a genetic cross, the allele that each
parent will pass on to its

offspring is based on probability.

Tw
o useful terms that geneticists use to describe organisms are
genotype
and
phenotype. An
organism’s phenotype is its physical appearance, or visible traits. An organism’s genotype is its genetic
makeup, or allele combinations.
When an organism has two iden
tical alleles for a trait, the

organism is said to
be
homozygous
for that trait. An organism that has two

different alleles for a trait is said to be
heterozygous
for that trait.

For all of the traits in peas that Mendel studied, one allele was dominant

wh
ile the other was
recessive. This is not always the case. In an inheritance

pattern called
codominance
,
the alleles are neither
dominant nor recessive. As a result, both alleles are expressed in the offspring.
Codominant alleles

are written
as capital lett
ers with superscripts to show that neither is

recessive.


The Cell and Inheritance

Key Concepts

1.

What role do chromosomes play in inheritance?

2.

What events occur during meiosis?

3.

What is the relationship between chromosomes and genes?


In the early 1900s, sci
entists were working to identify the cell structures that carried Mendel’s hereditary
factors, or genes. In 1903, Walter Sutton observed that sex cells in grasshoppers had half the number of
chromosomes as the body cells. He also noticed that each grasshop
per offspring had exactly the same number
of chromosomes in its body cells as each of the parents. He reasoned that the chromosomes in body cells
actually occurred in pairs, with one chromosome in each pair coming from the male and the

other coming from th
e female.

From his observations, Sutton concluded that genes are located on chromosomes. He proposed the
chromosome theory of inheritance.
According to the chromosome theory of inheritance, genes are carried

from parents to their offspring on chromosomes.

Organisms produce sex cells during meiosis.
Meiosis
is the process by which the number of
chromosomes is reduced by half to form sex cells


sperm and eggs.
During meiosis, the chromosome pairs
separate and are

distributed to two different cells. The result
ing sex cells have only half as

many chromosomes
as the other cells in the organism.
When they combine, each sex cell contributes half the number of
chromosomes to produce offspring with the correct number of chromosomes.

Punnett squares show the results o
f meiosis. When chromosome pairs separate, so do the alleles
carried on the chromosomes. One allele from each pair goes to each sex cell.

Chromosomes are made up of many genes joined together like beads on a string.
Each chromosome
contains a large number
of genes, each gene

controlling a particular trait. Each chromosome pair has the same
genes. The

genes are lined up in the same order on both chromosomes. However, the

alleles for some of the
genes might differ from each other, making the

organism heterozy
gous for some traits. If the alleles are the
same, the

organism is homozygous for those traits.


The DNA Connection

Key Concepts

1.

What forms the genetic code?

2.

How does a cell produce proteins?

3.

How can mutations affect an organism?


Today, scientists know that genes control the production of proteins in the cells of an organism. Proteins
determine the size, shape, and other traits of organisms. Recall that chromosomes are composed mostly of
DNA. A DNA molecule is made up of four nitro
gen bases

adenine (A), thymine (T), guanine (G), and
cytosine (C).
The order of the nitrogen bases along a gene

forms a genetic code that specifies what type of
protein will be produced.

In the genetic code, a group of three DNA bases codes for one specifi
c amino

acid.

During protein synthesis, the cell uses information from a gene on a chromosome to produce a specific
protein.
Protein synthesis occurs on the

ribosomes in the cytoplasm of the cell. DNA, however, is located in
the cell

nucleus. Before protei
n synthesis occurs, a genetic “messenger,” called

ribonucleic acid or RNA, is
made based on a code in the DNA. RNA is

similar to DNA, except RNA has only one strand and it has uracil
instead of

thymine.

In the first step of protein synthesis, the DNA molec
ule “unzips” and directs the production of
messenger RNA. There are several types of RNA involved in protein synthesis.
Messenger RNA
copies the
coded message from the DNA in the nucleus, and carries it to the ribosomes in the cytoplasm.
Transfer RNA
carri
es amino acids and adds them to the growing protein.

Sometimes changes called mutations occur in a gene or chromosome.
Mutations can cause a cell to
produce an incorrect protein during protein

synthesis. As a result, the organism’s trait, or phenotype, may

be
different

from what it normally would have been.
If a mutation occurs in a body cell, the mutation affects only
the cell that carries it. However, if a mutation occurs in a sex cell, the mutation can be passed on to an
offspring and affect the offsprin
g’s phenotype. Some mutations are the result of small changes in

an organism’s hereditary material. Others occur when chromosomes don’t separate correctly during meiosis.

Some of the changes brought about by mutations are harmful to an organism. A few muta
tions,
however, are helpful, and still others are neither harmful nor helpful. A mutation is harmful if it reduces an
organism’s chance for survival and reproduction. Whether or not a mutation is harmful depends partly on the
organism’s environment. For ex
ample, a white lemur may not survive in the wild, but the mutation has no
effect on its ability to survive in a zoo.


Human Inheritance

Key Concepts

1.

What are some patterns of inheritance in humans?

2.

What are the functions of the sex chromosomes?

3.

What is the

relationship between genes and the environment?


Many human traits are controlled by a single gene with one dominant allele and one recessive allele. As
with tall and short pea plants, these human traits have two distinctly different phenotypes, or physical
appearances. For example, the allele for a wido
w’s peak, which is a hairline that comes to a point in the
middle of the forehead, is dominant over the allele for a straight hairline.

Some human traits are controlled by single genes with two alleles, and others by single genes with
multiple alleles. Sti
ll other traits are controlled by many genes that act together.
Height and skin color are both
examples of

human traits controlled by many genes. When more than one gene controls a

trait, there are many
possible combinations of genes and alleles. There is
an

enormous variety of phenotypes for height, for
example, and human skin color

ranges from almost white to nearly black, with many shades in between.

Some human traits are controlled by a single gene that has more than two alleles. Such a gene is said to have
multiple alleles

three or more forms of a gene that code for a single trait. An example of a human trait that is

controlled by a gene with multipl
e alleles is blood type. There are four main blood types

A, B, AB, and O

controlled by three alleles. The
sex chromosomes
are one of 23 pairs of chromosomes in each body cell.
The
sex chromosomes carry genes that determine whether a person is

male or femal
e. They also carry genes that
determine other traits.
If you are female, you have two X chromosomes. If you are male, you have an X and a

Y chromosome. Whether you inherited an X or Y chromosome from your father determines your sex.

Genes on the X and Y ch
romosomes are often called
sex
-
linked genes.
Traits controlled by sex
-
linked
genes are called sex
-
linked traits. Because

males have only one X chromosome, males are more likely than
females to

have a sex
-
linked trait that is controlled by a recessive allel
e. One example of

a sex
-
linked trait that
is controlled by a recessive allele is red
-
green

colorblindness. A
carrier
is a person who has one recessive
allele for a trait

and one dominant allele. Although a carrier does not have the trait, the

carrier can p
ass the
recessive allele on to his or her offspring. In the case of

sex
-
linked traits, only females can be carriers.

The effects of genes are often altered by the environment

the organism’s surroundings.
Many of a
person’s characteristics are determined by

an

interaction between genes and the environment.
Several genes
determine human height. However, environment also influences people’s heights. People’s diets can affect
their height. A poor diet can prevent a person from growing as tall as might be possib
le.






Human Genetic Disorders

Key Concepts

1.

What are two major causes of genetic disorders in humans?

2.

How do geneticists trace the inheritance of traits?

3.

How are genetic disorders diagnosed and treated?


A
genetic disorder
is an abnormal condition that a person inherits through genes or chromosomes.
Some
genetic disorders are caused by mutations in

the DNA of genes. Other disorders are caused by changes
in the overall

structure or number of chromosomes.

Cystic fibrosis is a
genetic disorder in which the body produces abnormally thick mucus in the lungs
and intestines, making it hard to breathe and digest food. The allele that causes cystic fibrosis is recessive.
Currently there is no cure for cystic fibrosis, although there a
re treatments to help control the symptoms.

Sickle
-
cell disease is a genetic disorder that affects hemoglobin, the protein in blood that carries
oxygen. People with sickle
-
cell disease suffer from lack of oxygen in the blood and experience pain and
weakness. The allele that causes sickle
-
cell diseas
e is codominant with the normal allele. People with two
sickle
-
cell alleles have the disease. People with one sickle
-
cell allele produce both normal and abnormal
hemoglobin but usually do not have symptoms of the disease. Currently there is no cure for sic
kle
-
cell disease.
However, treatments can lessen the pain and other symptoms.

Hemophilia is a genetic disorder in which the blood clots very slowly or not at all. People with the
disorder do not produce one of the proteins needed for normal blood clotting.

Hemophilia is caused by a
recessive allele on the X chromosome. Because it is a sex
-
linked disorder, it occurs more often in males than
in females. With treatment, people with hemophilia can lead normal lives.

Down syndrome is a genetic disorder that is d
ue to an extra copy of chromosome 21. Most often Down
syndrome occurs when the chromosomes fail to separate properly during meiosis. People with Down
syndrome have a distinctive physical appearance and some degree of mental retardation. Many people with
Do
wn syndrome lead full, active lives. Geneticists trace the inheritance of traits through several generations of
a family.
One

important tool that geneticists use to trace the inheritance of traits in humans is a

pedigree.
A
pedigree
is a chart or “family t
ree” that tracks which members have a particular trait.

Today, doctors use tools such as karyotypes to help diagnose genetic disorders. People with
genetic disorders are helped through medical care, education, job training, and other methods.
To detect
chr
omosomal disorders such as Down syndrome, a doctor

examines the chromosomes from a person’s cells.
The doctor uses a
karyotype,
or picture

of all the chromosomes in a cell, to examine the chromosomes. The
chromosomes are

arranged in pairs. A karyotype can
reveal whether a developing baby has the correct

number
of chromosomes in its cells.

A couple that has a family history or concern about a genetic disorder may turn to a genetic counselor
for advice. Genetic counselors help couples understand their chances

of having a child with a particular
genetic disorder. Genetic counselors use tools such as karyotypes, pedigree charts, and Punnett squares.



Advances in Genetics

Key Concepts



What are three ways of producing organisms with desired traits?



What is the
goal of the Human Genome Project?


For thousands of years, people have tried to produce plants and animals with desirable traits.
Selective
breeding, cloning and genetic engineering

are three methods for developing organisms with desirable
traits.

The proc
ess of selecting organisms with desired traits to be parents of the next generation is called
selective breeding.
People have used selective breeding with many different plants and animals. One selective
breeding technique is called inbreeding.
Inbreeding
involves crossing two individuals that have similar
characteristics. One goal of inbreeding is to produce breeds of organisms with specific traits. For example, by
only crossing horses with exceptional speed, breeders can produce purebred horses that can r
un very fast.
Unfortunately, inbreeding also increases the probability that organisms may inherit alleles that lead to genetic
disorders.

Another selective breeding technique is called hybridization. In
hybridization,
breeders cross two
genetically differe
nt individuals. The hybrid organism that results is bred to have the best traits from both
parents. For example, a farmer might cross corn that produces many kernels with corn that is resistant to
disease.

For some organisms, another technique, called clon
ing, can be used to produce offspring with desired
traits. A
clone
is an organism that is genetically identical to the organism from which it was produced. One
way to produce a clone of a plant is to cut and grow a small part of a plant, such as a leaf or
stem. Several
types of animals have been cloned in recent years. Another technique for producing organisms with desired
traits is called genetic engineering. In
genetic engineering,
genes from one organism are transferred into the
DNA of another organism.
Genetic engineering can produce medicines and improve food crops, and may
some day correct human genetic disorders. In a type of genetic engineering called
gene

therapy,
working
copies of a gene may be inserted directly into the cells of a person with a ge
netic disorder. Some people are
concerned about the long
-
term effects of genetic engineering.

A
genome
is all the DNA in one cell of an organism.
The main goal of the Human Genome Project
has been to identify the DNA sequence of every gene in the human gen
ome.
From the Human Genome
Project,

scientists hope to learn more about what makes the body work and what

causes things to go wrong. A
genetic technique called DNA fingerprinting

is used to identify people. No two people, except for identical
twins, have t
he same DNA.









Cancer

Guide for Reading

1.

How is cancer related to the cell cycle?

2.

What are some ways that cancer can be treated?


Cancer
is a disease in which cells grow and divide uncontrollably, damaging the parts of the body around
them. There are more than 100 types of cancer. Cancer can occur in almost any part of the body. Cancers are
often named by the place in the body where they be
gin. In the United States today, lung cancer is the leading
cause of cancer deaths among both men and women. Scientists think that cancer begins when something
damages a portion of the DNA in a chromosome. The damage causes a change in the DNA called a
mut
ation.
Cancer begins when mutations disrupt the normal cell cycle,

causing cells to divide in an uncontrolled
way.
Without the normal controls on the cell cycle, the cells grow too large and divide too often. As the cell
divides over and over, the repeated

divisions produce more and more abnormal cells. In time, these cells form
a tumor. A
tumor
is a mass of abnormal cells that develops when cancerous cells divide and grow
uncontrollably. Some of the cancerous cells may break off the tumor and enter the blo
odstream. In this way,
the cancer can spread to other areas of the body.

There are three common ways to treat cancer: surgery, radiation, and drugs that destroy the
cancer cells.
When a cancer is detected before it has

spread, surgery is usually the best t
reatment. If doctors
can completely

remove the cancerous tumor, a person may be cured. If, however, the cancer

has spread or the
tumor cannot be removed, doctors may use radiation.

Radiation consists of beams of high
-
energy waves. Fast
-
growing cancer cells

are more likely than normal cells to be destroyed by radiation.
Chemotherapy

is the use of drugs to treat a disease.

Cancer treatment drugs are carried throughout the body by the bloodstream. These drugs kill cancer
cells or slow their growth. Scientists
continue to look for new ways to treat cancer. If scientists can discover
how the cell is controlled, they may find ways to stop cancer cells from multiplying.

People can reduce their chances of developing cancer by avoiding smoking, eating a healthful die
t, and
protecting their skin from bright sunlight. When people repeatedly inhale tobacco smoke, lung cancer and
other forms of cancer may result. Eating more fruits and vegetables instead of fatty meats and fried foods may
help lower the risk for some type
s of cancers. Most skin cancers are caused by ultraviolet light in sunlight. If

people limit their exposure to bright sunlight, they can reduce their risk of getting skin cancer.