Classical and
Modern Genetics
Chapter 23
Great Idea:
All living things use the same genetic code to guide
the chemical reactions in every cell.
Chapter Outline
• Classical Genetics
• DNA and the Birth of Molecular Genetics
• The Genetic Code
Classical Genetics
Chapter 23- Part 1 Classical Genetics
Genetics got it’s start as the study of inheritance.
Charles Darwin proposed that favorable traits could
be passed from generation to generation resulting
in natural selection.
However, Darwin did not know how these traits
were passed on.
Chromosomes from the Indian muntjak
It remained for the
Austrian monk
Gregor Mendel, in
1865, to carry out
the definitive
experiments.
Mendel crossed tall and dwarf pea plants: all offspring
were tall.
Tall
Dwarf
x
F1
tall
Next, Mendel crossed some of these F1 plants among
themselves. Of these offspring (the F2 generation), about
3/4 of the plants were tall and 1/4 were dwarf.
tall
tall
x
F2
tall
tall
tall
dwarf
Mendel tested 6 other traits of pea plants:
traits for seed shape (wrinkled or smooth)
seed color (yellow or green), etc.
In each case, all of the F1 plants looked as though
they had inherited the trait of just one of their two
parents, but in the F2 generation both traits always
appeared -- and always in a 3 to 1 ratio.
The trait which was expressed in the F1 generation was
always about 3 times as numerous in the F2 generation
as was the other one which was hidden in the F1's.
Homozygous = same
Heterzygous = different
When both alleles for a trait are identical, say that the organism
is homozygous for that trait. When the 2 alleles are different, is
heterozygous.
TT = Homozygous
tall
Tt = Heterozygous
tall
Tall is dominant over dwarf; dwarf is said to be a recessive trait
(i.e. can only be expressed when there are two copies of it).
Homozygous = same
Tall
TT
Heterzygous =different
Dwarf
tt
Tall
Tt
Mendel's original cross produced only tall offspring:
However,
in the second generation the rules of probability dictate that
1/4 of the plants will be tt = dwarf and 3/4 will have at least
one T and hence be tall.
Mendel Studied Many Traits in Pea Plants--
Seed shape- smooth or wrinkled
Seed color- green or yellow
Pod shape- smooth or bumpy
Pod color- green or yellow
Flower location- at leaf or tip of branch
Many traits are passed on by genes.
The genes encode the information for proteins.
The genes are segments of DNA.
Mendel found that two factors determine traits.
These are alternate forms of genes- one from each
parent.
These are now called alleles.
Classical Genetics
• Mendel
– Basic laws of inheritance
– Classic pea plant experiments
• Purebred
• Hybrid
• Results
– First generation
– Second generation
• Gene
– Dominant
– Recessive
Rules of Classical Genetics
• Traits (genes) are passed from parent to
offspring
– mechanism unknown
• Two genes for each trait
– One from each parent
• There are dominant and recessive genes
– Dominant expressed
Alleles: two different forms of the gene.
For many hereditary traits, genes exist in two or more different
forms called alleles. On each pair of chromosomes, there is one
allele for a particular gene on each. ex. A, B, O blood groups. In
humans there are 3 alleles: A, B, and O.
Genotype
Phenotype
AO
BO
AB
OO
A
B
AB
O
Genotype- genetic composition
Phenotype- physical characteristics
Genotype AO
Phenotype A
BO
B
AB
AB
OO
O
Ex. ABO blood groups. A and B are codominant and O is
recessive.
Qualitative versus
Quantitative Genetics
• Qualitative
– observational
• Quantitative
– Predictive model
– Used to trace genetic
disease
DNA and the Birth of
Molecular Genetics
Nucleotides: The Building
Blocks of Nucleic Acids
• Nucleotide
– Three molecules
• Sugar
– DNA: deoxyribose
– RNA: ribose
• Phosphate ion
• Base
–
–
–
–
Adenine (A)
Guanine (G)
Cytosine (C)
Thymine (T)
DNA Structure
• Join nucleotides
– Alternating phosphate and
sugar
• DNA
– 2 strands of nucleotides
– Joined by base pairs
• Bonding pattern
– Adenine:Thymine
– Cytosine:Guanine
DNA Structure
RNA Structure
• Differences
– One string of nucleotides
– Sugar is ribose
– Thymine replaced by uracil
• Uracil (U) bonds with adenine
The Replication of DNA
• DNA replication
– Occurs before
mitosis & meiosis
• Process
– DNA double helix
splits
– New bases bond to
exposed bases
– Result
• Two identical DNA
strands
The Genetic Code
•
Transcription
of
DNA
Transcription
– Information transport
– Uses RNA
• Process
– Unzip DNA
– RNA binds to exposed bases
– RNA moves out of nucleus (mRNA)
The Synthesis of Proteins
• tRNA
– Reads message
– Structure
• Amino acid
• 3 bases
• Process
–
–
–
–
mRNA moves to ribosome
rRNA aligns mRNA and tRNA
tRNA matches codon on mRNA
Amino acid chain forms
• Basis for protein
Protein synthesis cont.
• One gene codes for one protein
• Protein drives chemical process in cell
• DNA
– Introns
– Exons
• All living things on Earth use the same genetic code
Mutations and DNA Repair
• Mutations
– Change in DNA of parent
– Causes
• Nuclear radiation
• X-rays
• UV light
• DNA Repair
– 10,000 ‘hits’ per day
– Cells repair damage
Why Are Genes Expressed?
• Gene control
–
–
–
–
Turning genes on and off
Each cell contains same genes
Not all cells have same function
Certain genes activated
• Scientists currently studying how
Viruses
• Virus
– Not alive
– No metabolism
– Cannot reproduce on own
• Structure
– Short DNA or RNA
– Protein coating
• How it works
–
–
–
–
Taken into cell
Takes over cell
Produces more copies
Kills cell
HIV
• Human Immunodeficiency
Virus (HIV)
–
–
–
–
–
Contains RNA
Codes back to DNA
DNA incorporated into cell
Makes new viruses
Cell dies
• Complex
– Two protein coats
• Outer coat fits T cell receptors
• Inner coat encloses RNA
Viral Epidemics
• Viruses
– Cannot use medication
– Use vaccination
• Viruses evolve rapidly
–
–
–
–
HIV
Influenza
SARS
Bird flu
DNA Fingerprinting
The polymerase chain
reaction (PCR) copies
a sequence of DNA.
(a) A strand of DNA
is mixed in solution
with DNA precursors
(nucleotides), a
primer that targets a
specific piece of
DNA, and an enzyme
(polymerase) that
helps to assemble
DNA. The mix is
heated to 200°F to
separate DNA
strands.
(b) When cooled to
140°F, primers attach
to the DNA strands.
(c) At 160°F
nucleotides begin to
attach to the DNA
strands.
(d) At the end you
have two copies of
the desired DNA.
DNA fingerprinting
requires
breaking DNA into
short
fragments, tagging
those
fragments with
radioactive
tracers, and then
mixing the
fragments in a gel.
In an electric field, smaller fragments move farther along the gel,
and the distribution of fragments can be recorded on a
photographic film
(b). Because each person’s DNA sequence is unique, each DNA
fingerprint is distinctive.
The steps in DNA fingerprinting