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Lecture PowerPoint to accompany
Molecular Biology
Fifth Edition
Robert F. Weaver
Chapter 1
A Brief History
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A Brief History
• What is molecular biology?
– The attempt to understand biological
phenomena in molecular terms
– The study of gene structure and function at
the molecular level
• Molecular biology is a melding of aspects
of genetics and biochemistry
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1.1 Transmission Genetics
• Transmission genetics deals with the
transmission of traits from parental
organisms to their offspring
• The chemical composition of genes was
not known until 1944
– Gene - genetic units
– Phenotype - observable characteristics
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Mendel’s Laws of Inheritance
• A gene can exist in different forms called
alleles
• One allele can be dominant over the other,
recessive, allele
• The first filial generation (F1) contains
offspring of the original parents
• If each parent carries two copies of a
gene, the parents are diploid for that gene
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Mendel’s Laws of Inheritance
• Homoozygotes have two copies of the same allele
• Heterozygotes have one copy of each allele
• Parents in 1st mating are homozygotes, having 2
copies of one allele
• Sex cells, or gametes, are haploid, containing only
1 copy of each gene
• Heterozygotes produce gametes having either
allele
• Homozygotes produce gametes having only one
allele
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Summary
• Genes can exist in several different forms or alleles
• One allele can be dominant over the other, so
heterozygotes having two different alleles of one
gene will generally exhibit the characteristic
dictated by the dominant allele
• The recessive allele is not lost; it can still exert its
influence when paired with another recessive allele
in a homozygote
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The Chromosome Theory of Inheritance
• Chromosomes are discrete physical entities
that carry genes
• Thomas Hunt Morgan used the fruit fly,
Drosophila melanogaster, to study genetics
• Autosomes occur in pairs in a given
individual (not the X or the Y chromosome)
• Sex chromosomes are identified as X and Y
– Females have two X chromosomes
– Males have one X and one Y chromosome
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Location of Genes on a Chromosome
• Every gene has its place, or locus, on a
chromosome
• Genotype is the combination of alleles
found in an organism
• Phenotype is the visible expression of the
genotype
– Wild-type phenotype is the most common or
generally accepted standard
– Mutant alleles are usually recessive
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Genetic Recombination and Mapping
• In early experiments genes on separate
chromosomes behaved independently
• Genes on the same chromosome behaved
as if they were linked
• This genetic linkage is not absolute
• Offspring show new combinations of
alleles not seen in the parents when
recombination occurs
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Recombination
• During meiosis, gamete formation,
crossing over can occur resulting in the
exchange of genes between the two
homologous chromosomes
• The result of the crossing-over event
produces a new combination of alleles
• This process is called recombination
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Genetic Mapping
• Morgan proposed that the farther apart
two genes are on a chromosome, the
more likely they are to recombine
• If two loci recombine with a frequency of
1%, they are said to be separated by a
map distance of one centimorgan (named
for Morgan)
• This mapping observation applies both to
prokaryotes and to eukaryotes
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Physical Evidence for Recombination
• Microscopic examination of the maize
chromosome provided direct physical
observation of recombination using easily
identifiable features of one chromosome
• Similar observations were made in
Drosophila
• Recombination was detected both
physically and genetically in both animals
and plants
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Summary
• The chromosome theory of inheritance holds that
genes are arranged in linear fashion on
chromosomes
• Certain traits tend to be inherited together when the
genes for those traits are on the same chromosome
• Recombination between two homologous
chromosomes during meiosis can scramble the
parental alleles to yield nonparental combinations
• The farther apart two genes are on a chromosome
the more likely it is that recombination will occur
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1.2 Molecular Genetics
• The Discovery of DNA: The general
structure of nucleic acids was discovered by the
end of the 19th century
– Long polymers or chains of nucleotides
– Nucleotides are linked by sugars through
phosphate groups
• Composition of Genes: DNA? RNA? Protein?
In 1944, Avery and his colleagues demonstrated
that genes are composed of DNA
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The Relationship between Genes
and Proteins
• Experiments have shown that a defective
gene gives a defective or absent enzyme
• This lead to the proposal that one gene is
responsible for making one enzyme
• Proposal not quite correct for 3 reasons:
1. One enzyme may be composed of several
polypeptides, each gene codes for only one
polypeptide
2. Many genes code for non-enzyme proteins
3. End products of some genes are not
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polypeptides (i.e. tRNA, rRNA)
Activities of Genes
Genes perform three major roles
• Replicated faithfully
• Direct the production of RNAs and
proteins
• Accumulate mutations thereby allowing for
evolution
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Replication
• Franklin and Wilkins produced x-ray
diffraction data on DNA, Watson and Crick
proposed that DNA is double helix
– Two DNA strands wound around each other
– Strands are complementary – if you know the
sequence of one strand, you automatically
know the sequence of the other strand
• Semiconservative replication keeps one
strand of the parental double helix conserved
in each of the daughter double helices
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Genes Direct the Production of
Polypeptides
• Gene expression is the process by which
a gene product is made
• Two steps are required
– 1. Transcription: DNA is transcribed into RNA
– 2. Translation: the mRNA is read or translated
to assemble a protein
– Codon: a sequence of 3 nucleic acid bases
that code for one amino acid within the mRNA
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Genes Accumulate Mutations
Genes change in several ways
• Change one base to another
• Deletions of one base up to a large
segment
• Insertions of one base up to a large
segment
• The more drastic the change, the more
likely it is that the gene or genes involved
will be totally inactivated
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Summary
• All cellular genes are made of DNA
arranged in a double helix
• This structure explains how genes replicate,
carry information and collect mutations
• The sequence of nucleotides in a gene is a
genetic code that carries the information for
making an RNA
• A change in the sequence of bases
constitutes and mutation, which can change
the sequence of amino acids in the genes
polypeptide product
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1.3 The Three Domains of Life
Current research theories support the
division of living organisms into three
domains
1. Bacteria
2. Eukaryota
3. Archaea
• Like bacteria as they are organisms without
nuclei
• More similar to eukaryotes in the context of
their molecular biology
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Archaea
Archaea live in the most inhospitable
regions of the earth
• Thermophiles tolerate extremely high
temperatures
• Halophiles tolerate very high salt
concentrations
• Methanogens produce methane as a
by-product of metabolism
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