Chapter 3
The Biological Basis of Life
Chapter Outline
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The Cell
DNA Structure
DNA Replication
Protein Synthesis
Chapter Outline
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What is a Gene?
Mutation: When a Gene Changes
Chromosomes
Cell Division: Mitosis and Meiosis
New Frontiers
The Cell
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Cells are the basic units of life in all
living organisms.
Complex life forms, such as plants and
animals, are made up of billions of cells.
The cells of all living organisms share
many similarities as a result of their
common evolutionary past.
Cells
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Prokaryotic cells are single celled
organisms, such as bacteria and bluegreen algae.
Life on earth can be traced back 3.7
billion years in the form of prokaryotic
cells.
Eukaryotic cells, structurally complex
cells, appeared 1.2 billion years ago.
Structure of a Eukaryotic Cell
Two Types of Cells
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Somatic cells are the components of
body tissues.
Gametes are sex cells.
• Ova are egg cells produced in female
ovaries.
• Sperm are sex cells produced in male
testes.
• A zygote is the union between a sperm
and an ovum.
Part of a DNA Molecule
The DNA Replication Process
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2.
Enzymes break the bonds between
the DNA molecule.
Two nucleotide chains serve as
templates for the formation of a new
strand of nucleotides.
The DNA Replication Process
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5.
Unattached nucleotides pair with the
appropriate complementary nucleotide
The result is two newly formed strands
of DNA.
Each new strand is joined to one of
the original strands of DNA.
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. (There are 20 important amino acids.
 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.
Protein Synthesis:
Transcription
Protein Synthesis: Assembly
of an Amino Acid Chain
Protein Synthesis: Assembly
of an Amino Acid Chain
Protein Synthesis: Assembly
of an Amino Acid Chain
Protein Synthesis: Translation
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Decoding and implementing the genetic
message on the mRNA.
– 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.
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.
 Exons are DNA segments transcribed
into mRNA that code for specific
amino acids.
 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.
Help! Coding and Noncoding
DNA
A Human Chromosome
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 together at a
constricted area called 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.
– Homologous chromosomes carry genetic
information influencing the same traits.
– Homologous chromosomes are not
genetically identical.
Types of Chromosomes
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Autosomes - governs 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.
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.
Mitosis
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 spontaneous abortion
or miscarriage.
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 game containing one extra chromosome that
fuses with a normal gamete will produce a zygote
containing 47 chromosomes.