Download All life is based on the same genetic code

The Code of Life
Gregor Mendel 1822-1884; an austrian
monk who performed the first
comprehensive and systematic genetic
experiments.
Certain strains of peas bred true – tall
plants give tall plants
Short peas bred gave short plants
A Mendelian Genetic Primer
Genes come in pairs
that separate in the
formation of gametes.
The members of the pair may
be identical (homozygous) or
non-identical (heterozygous).
Each form of a particular
gene is an allele.
Genes, Alleles, and Chromosomes
Genes, Alleles, and Chromosomes
Mendel’s hypothesis
Each adult possessed
two sets of genes, one
contributed from each
parent.
Mendel characterized
genes as dominant or
recessive
Mendels genetic
model served provide
an understanding of
years of data collected
on pea plant breeding
Mendel’s Monohybrid
Cross – P to F1
Simple cross
• Each parent is
homozygous
• All the offspring are
heterozygous – and
all look alike
A Punnett square,
something we’ll
cover in a moment.
Second
generation cross
•
•
•
•
•
•
Parents are heterozygous
The offspring includes:
One homozygous dominant
One homozygous recessive
Two heterozygous
The phenotype ratio is 3
dominant to one recessive
expression
Phenotype vs Genotype
Years of
experimentation
on pea plants
lead to an
understanding of
Genetic
consistency
Characters
investigated by
Mendel
Dihybrid Cross
Principles of
Segregations and
Independent
Assortment.
Why Did Mendel Conclude That The
Inheritance of one Trait is Independent
of Another?
The alternative
and incorrect
hypothesis:
dependent
inheritance.
Because it’s the
only way to explain
the pattern of
inheritance.
Principle of Segregation Demystified
Principle of Segregation Demystified
Segregation
The principle of segregation is explained by the behavior of homologous
chromosomes at meiosis.
Genetics works for people too
• In the cross Aa x Aa,
where A is a
dominant allele for
(standard)
pigmentation
• a is a recessive allele
for no pigmentation
(albinism), ¾ of
offspring will be wild
type and ¼ will be
albino.
Multiple Alleles
• Many genes are
present in 3 or more
versions (alleles) –
this is known as
multiple alleles.
• The human ABO
blood group is
determined by three
alleles (IA, IB, and i)
of a single gene.
A Molecular Perspective of Genes
The answer to the question “What’s a
gene?” depends on we’re interested in.
At the molecular level: a gene is a sequence of
DNA capable of producing some element of
biological function.
• Biological function:
• It may be an observable trait,
(like skin color),
• A cellular property, ( cell cycle),
• A molecular property, like the
three dimensional shape of a
protein.
Alleles at the Molecular Level
Each form of a gene is an allele.
The standard (wild type) and altered
(mutant) forms of the gene associated
with hemoglobin and sickle cell anemia
provide an example.
The DNA sequences of both alleles of the
“hemoglobin gene” are 99.9% identical – a
single nucleotide difference makes for a
single amino acid difference, which makes
for a difference in protein shape, function
and, ultimately, phenotype.
normal
red blood
cell
sickled
red blood
cell
What’s So Special About DNA?
DNA is one of the most boring macromolecules imaginable its made of only four building blocks and has a perfectly
monotonous structure.
Worse yet, DNA just sits there - it doesn’t catalyze reactions
or build the cell or organism.
So, what’s so good about DNA?
The answer lies in DNA’s ability to
store and copy information.
Building DNA Building Blocks
DNA is Made of Two Long Chains of Nucleotides Joined by
DNA is Made of Two Long Chains of Nucleotides Joined by Hydrogen Bonds
Hydrogen Bonds
A Nucleotide
G and C are
complementary as
are A and T
Two Views ofTwo
theViews
Double
Helix
of the
Double Helix
Simple As It IsBase
in Principle,
Complementary
Pairing Allows
Each
of DNARequires
to Serve as a
DNAStrand
Replication
Template
for DNA That
Replication
Many Enzymes
Work
Coordinately
DNA is a perfect
illustration of function
following form
(structure dictates
function).
DNA polymerases are the first
and foremost of the replication
enzymes.
DNA Replication – Something
Old and Something New In Each
Daughter Molecule
Accidents Happen With Some “Accidents” (Base Mismatches)
Leading to Mutation
A mutation is a heritable change in DNA sequence.
Mutations due to replication errors only happen once
in every billion replicated nucleotides.
DNA Damage is Often the Root Cause of Mutation
DNA Damage is Often the Root Cause of Mutation
DNA is chemically altered (i.e. damaged) spontaneously and by
DNA is chemically
altered (i.e. damaged) spontaneously and by chemicals and
chemicals
and radiation.
radiation.
Mutation asas
Villain
Mutation
Villain
Cancerous growths that
Cancerous growths that
result
from loss of a protein
result from loss of a
that
polices
forDNA
errors.
protein
thatDNA
polices
for errors.
Cancer Incidence Increases Sharply with Age
Cancer Incidence Increases Sharply with Age
The increase is due at least in part to the age-related accumulation of
The increase
is due
at least
in part to the age-related accumulation of multiple
multiple
mutations
in single
cells.
mutations in single cells.
Genetics and Evolution
• While mutations can be seen as Villains – they
are also the “hero’s” of evolution
• It is only by mutation that genetic variations are
added to a population- leading to long term
change over time
• The effects of evolution are felt by individuals,
but it is the population as a whole that actually
evolves. Evolution is simply a change in
frequencies of alleles in the gene pool of a
population.
Hardy Weinberg Equilibrium:
Evolution will not take place if these
conditions are met:
•
•
•
•
•
•
1. mutation is not occurring
2. natural selection is not occurring
3. the population is infinitely large
4. all members of the population breed
5. all mating is totally random
6. everyone produces the same number of
offspring
• 7. there is no migration in or out of the
population