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Chapter 3
The Biological Basis of Life
Chapter Outline
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The Cell
DNA Structure
DNA Replication
Regulatory Genes
Protein Synthesis
What is a Gene?
Chapter Outline
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Mutation: When Genes Change
Chromosomes
Karyotyping Chromosomes
Cell Division
New Frontiers
Issue: Stem Cell Research: Promise and
Controversy
The Cell
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Cells are the basic units of life.
In some forms, such as bacteria, a single cell
constitutes the entire organism.
Complex life forms are made up of billions of
cells.
An adult human is made up of as many as
1,000 billion cells, all functioning in complex
ways that promote the survival of the individual.
Cells
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Life on earth can be traced back 3.7
billion years in the form of prokaryotic
cells.
Prokaryotic cells are single celled
organisms, such as bacteria and bluegreen algae.
Eukaryotic cells, structurally complex
cells, appeared 1.2 billion years ago.
Bacterium
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Each one of these
oval-shaped
structures is a singlecelled bacterium.
Structure of a Eukaryotic Cell
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The outer boundary of a cell is the cell
membrane.
Organelles are structures found in the
cytoplasm:
• Mitochondria produce energy.
• Ribosomes manufacture protein
The nucleus is surrounded by the
cytoplasm and contains chromosomes.
Structure of a Eukaryotic Cell
Types of Cells
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Somatic cells are components of body
tissue.
Gametes are sex cells.
• Ova are produced in female ovaries.
• Sperm are produced in male testes.
• A zygote is the union between a
sperm and an ovum.
Early DNA Researchers
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James Watson (left)
and Francis Crick in
1953 with their model
of the structure of the
DNA molecule.
Question
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An adult human is made up of
approximately how many cells?
a) 1000 billion
b) 10 million
c) One trillion
d) One billion
Answer: a
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An adult human is made up of
approximately 1000 billion cells.
DNA Structure
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Cellular function and an organism’s
inheritance depends on the structure and
function of DNA.
DNA is composed of two chains of
nucleotides.
A nucleotide consists of a sugar, a
phosphate, and one of four nitrogenous
bases.
Part of a DNA Molecule
DNA Structure
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Nucleotides form long chains.
The two chains are held together by
bonds formed on their bases with their
complement on the other chain.
Adenine (A) is the complement of
Thymine(T)
Guanine(G) is the complement of
Cytosine(C)
The DNA Replication Process
1.
2.
3.
Enzymes break the bonds between the
DNA molecule.
Two nucleotide chains serve as
templates for the formation of a new
strand of nucleotides.
Unattached nucleotides pair with the
appropriate complementary nucleotide
The DNA Replication Process
4.
5.
The result is two newly formed strands
of DNA.
Each new strand is joined to one of the
original strands of DNA.
DNA Replication
Hemaglobin Molecule
Hormones
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Substances (usually proteins) that are
produced by specialized cells.
Hormones travel to other parts of the
body, where they influence chemical
reactions and regulate various cellular
functions.
Regulatory Proteins
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Proteins that can bind to DNA and modify
the action of genes.
Many are active only during certain
stages of development.
Amino acids
Small molecules that are the
components of proteins.
Messenger RNA (Mrna)
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A form of RNA that’s assembled on a
sequence of DNA bases
It carries the DNA code to the ribosome
during protein synthesis.
Proteins
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The major structural components of tissue.
Enzymes are proteins that serve as catalysts,
initiating chemical reactions in the body.
Amino acids are the building blocks of protein.
Proteins differ according to number of amino
acids and the sequence in which they are
arranged.
Protein Synthesis
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Ribosomes help convert the genetic
message from the DNA into proteins.
Messenger RNA (mRNA) carries the
genetic message from the cell nucleus to
the ribosome.
Transfer RNA (tRNA),found in the
cytoplasm, binds to one specific amino
acid.
RNA and DNA
RNA differs from DNA in three important ways:
1. It’s usually single-stranded. (This is true of the
forms we discuss, but it’s not true for all.)
2. It contains a different type of sugar.
3. It contains the base uracil as a substitute for
the DNA base thymine. (Uracil is attracted to
adenine, just as thymine is.)
Question
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Which of the following is not true about
RNA?
a) It is single stranded.
b) Some forms of RNA are involved
with protein synthesis.
c) It has a different type of sugar than
DNA has.
d) It contains the base thymine.
Answer : d
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The following is not true about RNA.
It contains the base thymine.
Characteristics of the DNA
Code
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2.
The code is universal.
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The same basic messages apply to all lifeforms on the planet, from bacteria to oak
trees to humans.
The code is triplet.
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Each amino acid is specified by a sequence
of three bases in the mRNA (the codon),
which in turn is coded for by three bases in
the DNA.
Characteristics of the DNA
Code
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The code is continuous—without
pauses.
The code is redundant. While there are
20 amino acids, there are 4 DNA bases
and 64 possible triplets or mRNA
codons.
Protein Synthesis:
Transcription
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The process of coding a genetic message
for proteins by formation of mRNA.
A portion of the DNA unwinds and serves
as a template for the formation of a mRNA
strand.
Transcription
Protein Synthesis:
Translation
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The mRNA travels through the nuclear
membrane to the ribosome.
tRNAs arrive at the ribosome carrying their
specific amino acids.
The base triplets on the tRNA match up with the
codons on the mRNA.
As each tRNA line up in the sequence of
mRNA codons their amino acids link to form a
protein.
Assembly of an Amino Acid
Chain in Protein Synthesis
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As the ribosome
binds to the mRNA,
tRNA brings a
particular amino acid,
specified by the
mRNA codon, to the
ribosome.
Assembly of an Amino Acid
Chain in Protein Synthesis
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The tRNA binds to
the first codon while a
second tRNA–amino
acid complex arrives
at the ribosome.
Assembly of an Amino Acid
Chain in Protein Synthesis
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The ribosome moves
down the mRNA,
allowing a third amino
acid to be brought
into position by
another tRNA
molecule. Note that
the first two amino
acids are now joined
together.
Diagram of a DNA Sequence
Being Transcribed
Genes
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A gene is the entire sequence of DNA bases
responsible for the synthesis of a protein.
A mutation occurs when the sequence of
bases in a gene is altered.
Mutations may interfere with an organisms
ability to produce vital protein and may lead to a
new variety within the species, hence,
evolution.
Gene Structure
The gene consists of exons and introns.
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Exons are DNA segments transcribed
into mRNA that code for specific amino
acids.
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Introns are DNA sequences not
expressed during protein synthesis.
Homeobox Genes
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An evolutionarily ancient family of
regulatory genes that directs the
development of the overall body plan and
the segmentation of body tissues.
Hox Genes
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The differences in these vertebrae are caused by Hox
genes during embryonic development. The cervical
(neck) vertebrae (a) have characteristics that
differentiate them from the thoracic (b) and the lumbar
vertebrae (c) of the lower back.
Universal Genetic Code
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The DNA code of all life on earth is
composed of the same molecules and
carries on similar functions.
The universality of the genetic code
implies a common ancestry for all life on
the planet.
Organisms differ according to the
arrangement of the DNA.
Coding and Noncoding DNA
Sickle-cell Anemia
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A severe inherited hemoglobin disorder in
which red blood cells collapse when
deprived of oxygen.
It results from inheriting two copies of a
mutant allele.
This allele is caused by a single base
substitution in the DNA.
Gene Mutation
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Scanning electron
micrograph of a
normal, fully
oxygenated red blood
cell.
Scanning electron
micrograph of a
collapsed, sickle
shaped red blood cell
that contains HBS.
Symptoms in People With
Sickle-cell Anemia
Substitution of One Base at Position 6
Produces Sickling Hemoglobin
Chromosomes During Cell
Division
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Scanning electron
micrograph of human
chromosomes during
cell division.
Note that these
chromosomes are
composed of two
strands, or two DNA
molecules.
Point Mutation
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A chemical change in a single base of a
DNA sequence.
Chromatin
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The loose, diffuse form of DNA seen when a
cell isn’t dividing.
When it condenses, chromatin forms into
chromosomes.
Chromosomes are discrete structures
composed of DNA and protein found only in
the nuclei of cells.
Chromosomes are visible under
magnification only during certain phases of
cell division.
Cell Division
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Cell division results in production of new
cells.
During cell division:
Cells are involved with normal cellular
and metabolic processes.
The cell’s DNA becomes tightly coiled.
DNA is visible under a microscope as
chromosomes.
Chromosome Structure
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A chromosome is composed of a DNA molecule
and associated proteins.
During normal cell functions, chromosomes
exist as single-stranded structures.
During cell division, chromosomes consist of
two strands of DNA joined at the centromere.
Since the DNA molecules have replicated, one
strand of a chromosome is an exact copy of the
other.
Chromosome Structure
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Centromere
The constricted portion of a
chromosome.
After replication, the two strands of a
double-stranded chromosome are
joined at the centromere.
Chromosome Structure
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Locus
The position on a chromosome where a
given gene occurs.
The term is sometimes used
interchangeably with gene.
Chromosomes and Genetics
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Each species is characterized by a
specific number of chromosomes.
Humans have 46 chromosomes.
Chromosome pairs are called
homologus:
These carry genetic information that
influences the same traits.
They are not genetically identical.
Types of Chromosomes
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Autosomes - govern all physical
characteristics except sex determination.
Sex chromosomes - X and Y
chromosome.
Mammal
females have two X
chromosomes.
Mammal males have one X and
one Y chromosome.
Standard Chromosomal
Complement in Various Organisms
Organism
Chromosome Number
in Somatic Cells
Chromosome
Number in Gametes
Human
46
23
Chimpanzee
48
24
Gorilla
48
24
Dog
78
39
Chicken
78
39
Standard Chromosomal
Complement in Various Organisms
Organism
Chromosome Number
in Somatic Cells
Chromosome
Number in Gametes
Frog
26
13
Housefly
12
6
Onion
16
8
Corn
20
10
Tobacco
48
24
Karyotyping Chromosomes
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A karyotype is the chromosomal complement of
an individual, or what is typical for a species.
Usually displayed in a photomicrograph, the
chromosomes are arranged in pairs and
according to centromere size and position.
Physicians and genetic counselors routinely
use karyotypes to help diagnose chromosomal
disorders in patients, or in prenatal testing to
identify chromosomal abnormalities in
developing fetuses.
Alleles
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Alternate forms of a gene.
Alleles occur at the same locus on paired
chromosomes and thus govern the same trait.
But because they’re different, their action may
result in different expressions of that trait.
The term is sometimes used synonymously with
gene.
Karyotype of a Male
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Chromosomes
arranged by
size and
position of the
centromere, as
well as by
banding
patterns.
Question
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There are basically two types of cells.
They are:
a) zygotes and nucleotides.
b) cytoplasmic and nuclear.
c) protein and mitochondrial.
d) somatic and gametes.
Answer: d
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There are basically two types of cells.
They are somatic and gametes.
Question
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The alternate form that a gene can take
or the different expressions of a gene is
a(n):
a) karyotype.
b) clone.
c) allele.
d) autosome.
Answer: c
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The alternate form that a gene can take or
the different expressions of a gene is a(n)
allele.
Mitosis
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Mitosis is cell division in somatic cells.
Mitosis occurs during growth and
repair/replacement of tissues.
The result of mitosis is two identical
daughter cells that are genetically
identical to the original cell.
Steps in Mitosis
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The 46 chromosomes line up in the
center of the cell.
The chromosomes are pulled apart at
the centromere.
The strands separate and move to
opposite ends of the dividing cell.
The cell membrane pinches in and two
new cells exist.
Mitosis
Mitosis
Meiosis
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Production of gametes (sex cells).
2 divisions result in 4 daughter cells.
Each daughter cell contains 23 chromosomes.
Resulting gamete may unite with another
gamete to create a zygote.
The zygote inherits the DNA, half from each
parent, to develop and function normally.
Meiosis
Meiosis
Mitosis and Meiosis
Problems With Meiosis
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Nondisjunction occurs when chromosomes
don’t separate during meiosis.
A gamete containing one less chromosome
that fuses with a normal gamete will produce
a zygote containing 45 chromosomes.
A gamete containing one extra chromosome
that fuses with a normal gamete will produce
a zygote containing 47 chromosomes.
Nondisjunction
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When chromosomes or chromosome strands
don’t separate during either of the two divisions.
The result is that one of the daughter cells
receives two copies of the affected
chromosome, while the other daughter cell
receives none.
If such an affected gamete unites with a normal
gamete, the resulting zygote will have 45 or 47
chromosomes.
Question
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Which of the following is true for
meiosis?
a) It has only one division that duplicates
the parent cell exactly.
b) It produces gametes.
c) When a mutation occurs it affects only
the individual.
d) It has no effect on evolution.
Answer: b
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The following is true for meiosis:
 It produces gametes.
Examples of Nondisjunction
in Sex Chromosomes
Chromosomal
Complement
Name
XXX
Trisomy X
XYY
XYY
syndrome
Incidence
Manifestations
1 per 1,000
female births
Slight increase in
sterility and mental
impairment. In cases
with more than three X
chromosomes, mental
retardation can be
severe.
1 per 1,000
Affected males are fertile
and tend to be taller than
average.
Examples of Nondisjunction
in Sex Chromosomes
Chromosomal
Complement
XO
Name
Turner
syndrome
Incidence
Manifestations
Short-stature, broad chests,
webbed neck and sterility.
1 per 10,000
Usually no mental
female births
impairment. Between 95
and 99% of affected fetuses
die before birth.
Examples of Nondisjunction
in Sex Chromosomes
Chromosomal
Complement
XXY
Name
Klinefelter
syndrome
Incidence
Manifestations
1 per 1,000
male births
Reduced testicular
development, reduced
facial and body hair,
breast development in
half of all cases, reduced
fertility.
Evolutionary
Significance of Meiosis
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Meiosis and sexual reproduction are
highly important evolutionary innovations.
Meiosis increases genetic variation at a
faster rate than mutation.
Offspring in sexually reproducing species
represent the combination of genetic
information from two parents.
DNA Fingerprints
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Eight DNA fingerprints,
one of which is from a
blood sample left at an
actual crime scene. The
other seven are from
suspects. By comparing
the banding patterns, it is
easy to identify the
guilty party.
Polymerase Chain Reaction
(PCR)
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Enables scientists to make of copies of DNA
samples which can then be analyzed.
Scientists use PCR to:
Examine nucleotide sequences in Neandertal
fossils and Egyptian mummies
Identify DNA sequences unique to an
individual.
Identify remains of victims of 911.
Exonerate people wrongly convicted of
crimes and imprisoned for years.
Recombinant DNA
Technology
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A process in which genes from the cell of
one species are transferred to somatic
cells or gametes of another species.
Nuclear Transfer Process
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The nucleus of an egg is drawn into a needle.
The enucleated egg with only the cytoplasm
remaining.
The nucleus of a skin cell from the individual
being cloned is injected into the egg.
Electric shock causes the nucleus of the skin
cell to fuse with the egg’s cytoplasm.
The egg begins to divide, and a few days later
the cloned embryo will be transferred into the
uterus of a host animal.
Nuclear Transfer Process
Human Genome Project
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A genome is the entire genetic makeup of an
individual or species.
In humans, it’s estimated that each individual
possesses 3 billion DNA nucleotides.
Effort begun in 1990 to sequence the entire
human genome, which consists of some 3
billion bases comprising approximately 25,000
to 30,000 genes.
The goal was achieved in 2003.