Chapter 3
The Biological Basis of Life
Chapter Outline
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The Cell
DNA Structure
DNA Replication
Protein Synthesis
What is a Gene?
Cell Division: Mitosis and Meiosis
New Frontiers
The Cell
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Cells are the basic units of life in all living
organisms.
In some forms, such as bacteria, a single cell
constitutes the entire organism.
Complex life forms, such as plants and animals,
are made up of billions of cells.
An adult human is made up of perhaps 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.
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.
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
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.)
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.
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.
Cell Division: Mitosis and
Meiosis
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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.
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.
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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
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
1.
2.
3.
4.
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
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.
Problems With Meiosis
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Errors in meiosis may lead to miscarriage.
Nondisjunction occurs when chromosomes don’t
separate during meiosis.
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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.
Mitosis and Meiosis
Compared
Abnormal Numbers of
Chromosomes
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Down's syndrome occurs because of
three copies of chromosome #21.
 Problems include mental retardation,
heart defects and respiratory infections.
Nondisjunction may occur in the X and Y
chromosomes and result in sterility.
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.
Human Genome Project
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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.
Scientists are still several years away
from identifying the functions of many of
the proteins produced by these genes.
Quick Quiz
1. The entire sequence of DNA bases on
the chromosome that code for a
particular polypeptide chain is a(n)
a) ribosome.
b) amino acid.
c) gene.
d) polypeptide chain.
Answer: c
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The entire sequence of DNA bases on
the chromosome that code for a
particular polypeptide chain is a gene.
2. 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.
3. 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:
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It produces gametes.
4. Chromosomes that carry genes for the
same traits are _________________
pairs.
Answer: homologous
• Chromosomes that carry genes for the
same traits are homologous pairs.
5. Somatic cell division is termed
_________________ .
Answer: mitosis
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Somatic cell division is termed mitosis.