CH1: THE GENETIC APPROACH TO BIOLOGY
Sept/1/11 – pg 1-29
1.1 Genetics and the Inducing mutations, localizing them on chromosomes, and combining them in
Questions of Biology different ways is a way to understand the roles of genes in affecting structure and
function of an organism
o Mutations in fruit flies supported the claim of evolutionists and anatomists
that wings, legs, and antennae in different organisms are variations on a
basic animal design and could be converted from one another in evolution.
- DNA replication, mutation, and the processing of the information specified by the
DNA continue through the life of the organism.
1.2 The Molecular Basis - Structure of organisms and their active physiological processes are based for the most
of genetic Information
part on proteins, which are synthesized based on genetic information found in DNA.
- There are 4 kinds of nucleotides in DNA: adenine (A), thymine (T), guanine (G),
and cytosine (C)
- DNA is composed of 3 nucleotide chains held together by complementary pairing of
A with T and G with C.
- There are 4 properties that characterize genetic information:
1. Diversity of structure
- Nucleotides can be in any order and DNA can be any length
2. Ability to replicate
- Pairing of the complementary strands results in 2 identical copies of the original
double helix being produced.
- When an incorrect base is put in or bases are lost/duplicated.
- New copy of DNA and all succeeding copies will be different causing a heritable
4. Translation into form and function
- Given sequence of A,T,G, and C must be used by the cell to create protein molecules
Specifying the amino acid
with particular sequences of amino acids.
sequence of a protein
Transcription - Turns a nucleotide sequence in the DNA into messenger RNA (mRNA)which is
composed of the molecule ribonucleic acid (RNA).
o RNA is different from DNA b/c it has ribose in backbone instead and has
nucleotide uracil (U) instead of (T)
1. DNA double helix separated into 2 single strands and one of them is the template for
building a complementary RNA sequence.
2. RNA produced is then altered (i.e cutting out introns) to produce a mature mRNA,
aka the transcript
- mRNA has 3 functions:
1. Increases number of copies of the genetic information available to the cell at any
time – amplification.
2. Relieves traffic congestion –RNA goes out into the cytoplasm
3. Stability and lifetime of message molecules act as controls on how much a
particular protein will be produced. 2
Translation - Production of a chain of amino acids based on sequence of nucleotides in mRNA
- Nucleotide sequence in mRNA is read in successive groups of 3 nucleotides along the
mRNA chain called a codon.
- There are 4x4x4 = 64 different codon triplets and only 20 amino acids, more than 1
codon corresponds to each amino acid.
1. transfer RNAs (tRNAs) contain an anticodon – the complement of the codon
for that amino acid that pairs with the appropriate codon on the mRNA. When the
tRNA attaches to its amino acid, it is charged.
2. A ribosome hooks up the amino acids on the tRNA in the order specified by the
3. Ribosome moves onto next codon on mRNA until it reaches a stop codon and
then it detaches to be used again.
From polypeptide - The primary structure that results from the above synthesis is not a protein.
protein - For any given amino acid sequence, there are several alternative stable foldings which
are dependent on conditions in the cell and presence of other molecules.
Gene Regulation - We also need a mechanism to regulate at what times and places in the organism the
information in the genome is to be translated.
- RNA polymerase is used to find the beginning of the DNA sequence and proceeds
along the gene hooking up successive ribonucleotides.
- Negative control: Molecules can bind to the controlling region of DNA at the
beginning of a gene and block movement of RNA polymerase to prevent
- Positive control: A molecule that normally sits on the DNA and prevents
transcription may be caused to fall off the DNA by some other molecule inside the
cell so transcription can proceed.
o Ex. When there’s no lactose, transcription is blocked by repressor. When
1.3 The Program of lactose is present, it binds to repressor, causing it to fall off transcription
The Necessity of Variation - Genetic polymorphism is when differences between members of a population are
- Genetic variation can also be produced in the lab using high energy radiation or
- Genetic method can be used to study the evolutionary events that have resulted in the
great diversity of form and function among living forms.
Starting with Variation:
Forward Genetics - Phenotypes are observed variations in morphology or physiology (i.e. albinism)
- Forward genetics searches for a genetic difference that causes a phenotypic
difference such as looking for patterns of inheritance in the descendants of crosses
between individuals with different phenotypes.
- Forward genetics also looks for abnormal variant for which almost all individuals
have a normal form.
o Wild type: normal form, mutants: exceptions.
- Most phenotypic characters are affected by a number of interacting pathways of
physiology or development, each pathway requiring multiple genes. 3
- The products of multiple genes are active in pathways that determine biological
properties such as eye color. To separate out the effects of multiple genes, crosses are
used to create individuals with mutations in different combinations of genes.
- Last stage of forward program is to characterize the DNA of the different variants
(alleles) of the genes and explain the differences in structure, function, amounts, and
localization within the organism
**Refer to flow diagram pg.13
Starting with DNA: - Reverse genetics starts with known genetic changes and looks at the changes in the
Reverse Genetics organism that result.
- Advantage of this is that large numbers of mutations of specific kinds can be tailor
- Another form of reverse genetics uses available information on genomes of a variety
of related species to see how they differ.
o Ex. Comparing humans and chimpanzees.
1.4 Methodologies Used 1. Isolation of mutations affecting the biological process under study.
in Genetics - Each mutant gene reveals a genetic component of the process and together the mutant
genes show the range of proteins that interact in that process.
2. Analysis of progeny of crosses between mutants and wild type
- Used to identify genes and alleles, chromosomal locations, and inheritance patterns.
3. Genetic analysis of cell’s biochemical processes
- Studying the ways genes are relevant to chemical reactions.
- Want to find out how cellular chemistry is disturbed by the mutant allele.
4. Microscopic analysis
- Labelling genes and gene products so that their locations can be easily visualized
under the microscope.
5. Direct Analysis of DNA
- Using gene cloning – amplifying a gene to produce a pure sample for analysis.
- Can do this by inserting the gene into a small bacterial chromosome and allowing
bacteria to copy the inserted DNA
- Nucleotide sequence of cloned gene can be determined.
- Genomics: The study of the structure, function, and evolution of whole genomes.
o Comparative genomics: describes the differences and similarities in the
Detecting specific genomes of species that are related to different degrees
molecules of DNA, RNA, o Bioinformatics: computational analysis of information content of genomes.
Probing - Most extensively used method for detecting specific macromolecules in a mixture.
- A mix of macromolecules is exposed to a molecule – the probe – that will bind only
with the sought after macromolecule.
- Probe is labelled in some way by a radioactive atom or fluorescent compound so that
the site of binding can be easily detected.
Probing for specific
DNA - Cloned genes can act as a probe for finding segments of DNA by binding based on