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Review
Some terms
• Gene – section of DNA that encodes a
trait
– Trait – characteristic (Ex. Eye color)
• Allele – different varieties of a gene
– Example: brown eye color
• Because human cells are diploid, we
have 2 alleles for each gene
– alleles are the same – homozygous
– alleles are different – heterozygous
• Whatever combination of alleles are
present is called the genotype
• The outward appearance of those alleles
is called the phenotype
Punnett square
• Alleles can be either
– Dominant (is observed) – express as a capital letter
– Recessive (is not observed) – express as a lower case letter
• Example (Mendel’s experiment)
– Purple flowers (PP)
• What is the phenotype?
• What is the genotype?
– White flowers (pp)
• What is the phenotype?
• What is the genotype?
– What happens when we cross these two plants?
• Cross means to have an organism sexually reproduce with another
organism
• Punnett square helps us answer this question
Punnett square
P
P
p
Pp
Pp
p
Pp
Pp
• Separate the phenotype into the gametes it can produce
– PP -> P or P
– Pp -> p or p
• Gray boxes = parental generation (P generation)
• F1 generation = offspring of the parental generation
– What is the phenotype of the F1 generation?
– What is the genotype of the F1 generation?
• If you were to cross the F1 generation with itself = F2
generation
Punnett square
• Separate the phenotype into the gametes it can
produce
– Pp -> P or p
– Pp -> P or p
p
P
P
PP
Pp
p
Pp
pp
What are the phenotypes of the F2 generation?
What are the genotypes of the F2 generation?
What is the ratio of phenotypes?
What is the ratio of genotypes?
What is the probability that you will have a purple flower based on the F1 cross?
Mendel’s Laws
• 1st law – law of segregation
– We’ve already seen this when we made our
punnett square
– Alleles separate during the formation of gametes,
so that half will carry one copy and the other half
the other copy
• 2nd law – law of independent assortment
– Genes located on different chromosomes are
inherited independently of one another
– Example: eye color and hair color
Law of segregation
P
P
meiosis
P
p
p
p
Law of Independent Assortment
P
P
P
p
W
w
meiosis
W
W
p
p
w
w
P
P
P
p
w
W
meiosis
w
w
p
p
W
W
Some traits don’t follow Mendel’s laws
• Many traits are determined by multiple genes
= polygenic
– Example: height
– Distribution of phenotypes = continuous variation
Some traits don’t follow Mendel’s laws
• Pleiotropy = when one gene affects multiple
phenotypes
– Example: gene that makes phenylalanine hydroxylase
affects brain development AND skin color
• Incomplete dominance
– Both alleles are expressed
– The expression of one affects the expression of the
other
• Can think of this as phenotype averaging
– Example: japanese four o’clock flowers
• RR = red flower
• WW = white flower
Incomplete dominance
R
R
W
RW
RW
W RW
RW
• Separate the phenotype into the gametes
it can produce
– RR -> R or R
– WW -> W or W
• F1 generation = offspring of the parental
generation
– What is the phenotype of the F1 generation?
– What is the genotype of the F1 generation?
Some traits don’t follow Mendel’s laws
• Codominance
– Both alleles are expressed
– The expression of one does not affect the expression
of the other
– Example: human blood types
• AA = type A blood – galactosamine added to plasma
membrane of red blood cells
• BB = type B blood – galactose added to plasma membrane of
red blood cells
• AB = type AB blood – galactose and galactosamine added to
plasma membrane of red blood cells
• OO = type O blood – nothing is added to red blood cells
Codominance
A
AA
B
BB
A
B
AB
Chromosomes are responsible for all
this inheritance
• Sex-Linked traits
– Humans have 23 pairs of chromosomes
• 22 pairs of chromosomes are identical in their genes
(not their alleles) – called autosomes
• 1 pair is not identical and helps determine sex – called
sex chromosomes
– Males = XY
– Females = XX
karyotype
Determination of sex
X
X
X
XX
XX
Y
XY
XY
• Half male, half female
• Sex Linked traits – genes that are located on sex
chromosomes (Example: baldness)
– XB – full head of hair
– Xb - bald
B
Xb
X
XB
XBXB
XBXb
Y
XBY
Xb Y
Nondisjunction
• Nondisjunction – when chromosomes fail to separate
properly during meiosis
• This leads to abnormal number of chromosomes =
aneuploidy
– If chromosome is lost (one copy = monosomic) = individual does
not survive
– If chromosome is gained (3 copies = trisomic) = individual may
survive but only in a few cases and will be mentally impaired
• Example: Trisopy 21 (Down syndrome)
• Non disjunction of sex chromosomes
– XXY – Klinefelter Syndrome – sterile male with many female
characteristics
– X – Turner Syndrome- Mentally impaired females, short stature
Mutations
• Accidental change in the DNA sequence
– Breaks in DNA (X-rays)
– Copied incorrectly (UV light)
• Mutations often causes harmful effects = genetic
disorder
– More often recessive because function of normal
proteins is lost – so the good copy can make up for it
• Scientists use pedigrees to help determine how a
mutation is inherited
Two types of mutations
• Point mutation – One nucleotide is replaced
with a different nucleotide (Example: A is
replaced with G)
• Frame-shift mutation – A nucleotide is added
or removed (Example: ATGCGA becomes
ATGGA)
– Causes a bigger change usually because
everything downstream of the mutation is altered
Normal Replication
DNA
T A C G T A C G T A C G
A T G C A T G C A T G C
DNA
Point Mutation
DNA
T A C G T G C G T A C G
A T G C A T G C A T G C
DNA
Frame shift Mutation
DNA
T A C G T
C G T A C G
A T G C A T G C A T G C
DNA
Pedigree
Normal
female
•
Sickle Cell Anemia – point
mutation
Recessive disorder
– SS – normal
– Ss – normal
– Ss – sickle cell anemia – difficulties in transporting
oxygen to the body
– When we get to evolution, we’ll see there is an
advantage to having this allele
BREAK
What’s the genetic material
• Griffith experiment
1.
2.
3.
4.
Live S bacteria kill mice
Live R bacteria do NOT kill mice
Heat-killed S bacteria do NOT kill mice
Heat killed S bacteria + Live R bacteria??
Griffith Experiment
X
S
X
S
R
X
S
R
R was transformed by something in S
X
Avery and Hersey-chase Experiments
• 1944
– Avery – Repeated Griffith experiment, but they removed all the
protein from the dead S bacteria
• Still saw the same result, so it must not be the protein
• 1952
– Hershey-Chase
• Labeled DNA with radioactive phosphate
• Labeled proteins with radioactive sulfur
• Let viruses grow in the presence of either radioactive DNA or
radioactive protein
• Let the viruses infect bacterial cells
• Removed the viruses and just looked at the bacterial cells to see which
ones had radioactivity
– Bacteria that were exposed to viruses with radioactive DNA had radioactivity
– Bacteria that were exposed to viruses with radioactive protein did not have
radioactivity
Hershey-Chase experiment
P
S
protei
n
DNA is the genetic material
?
How does DNA replicate?
Structure of DNA helps
• Chargaff’s Rule
– Measured the amount of A,T,C,
and G
– Found that the amount of A
always equaled the amount of
T (same applied for C and G)
• Double helix discovered by
Rosalind Franklin, Francis
Crick, and James Watson
• Minor groove
• Major groove
How does DNA replicate?
Meselson-Stahl experiment
• 3 possible explanation for how DNA is replicated
– Conservative replication
• original DNA is split in half
• Each half is copied
• Original DNA is rejoined (both strands are old) and new copy is
joined together (both strands are new)
– Semiconservative replication
• Original DNA is split in half
• Each half is copied
• Original DNA stays with new copy (one strand is old and one is
new)
– Dispersive replication
• Original strand is split in half and then further divided into sections
• Sections are split between both old and new strands
3 possible explanations
Meselson-Stahl experiment
• Grow up some bacteria
• Let one round of replication happen with heavy DNA
• Let a second round of replication happen with normal
DNA (light DNA)
– If conservative
• First round – 2 types: heavy AND light
• Second round – 2 types: heavy and light
– If semiconservative
• First round – 1 type: average
• Second round – 2 types: light AND average
– If dispersive
• First round – 1 type: average
• Second round – 1 type: somewhere between average and light
Messelson-Stahl experiment
Normal DNA (light)
Heavy DNA (heavy)
1st round cells are made with heavy DNA
2nd round cells are made with light DNA
DNA replication
• Helicase – enzyme that unzips the DNA
– The region where the DNA is unzipped is called the replication
fork
• DNA polymerase – copies both strands of DNA at the same
time
– Needs a primer – section of DNA that starts the new strand
– Always copies from 5’ -> 3’
• One strand is easy to copy because replication is moving in the same
direction as DNA polymerase – this new strand formed is called the
leading strand
• One strand is difficult to copy because replication is moving opposite
to the direction of DNA polymerase – this new strand is called the
lagging strand
– Must be made in segments
– Segments are later joined together by another enzyme - DNA ligase
Normal Replication
5’
DNA
3’
T A C G T A C G T A C G
A T G C A T G C A T G C
DNA
3’
5’
Helicase is an enzyme that unzips the DNA
DNA polymerase copies the DNA
5’
DNA
3’
T A C G T A C G T A C G
5’
3’
Leading strand
DNA
helicase
T A C G T A C G T A C G
A T G C A T G C A T G C
DNA
3’
DNA polymerase
5’
Helicase is an enzyme that unzips the DNA
DNA polymerase copies the DNA
5’
DNA
3’
T A C G T A C G T A C G
Replication is moving the direction of the red arrow. DNA must be made from 5’ -> 3’
direction. Problem!! The orientation of the top strand doesn’t allow this.
5’
DNA
3’
T A C G T A C G T A C G
A T G C A T G C A T G C
DNA
3’
DNA polymerase
5’
Helicase is an enzyme that unzips the DNA
DNA polymerase copies the DNA
5’
DNA
T A C G T A C G T A C G
How could we get around this problem?
Would be easy if we could just flip it..so
that’s sort of what our cells do.
3’
1. DNA loops around
2. DNA primer is attached to DNA
3. DNA polymerase copies DNA – called
the lagging strand
4. Because of the loop, the DNA can only
be copied in segments – called Okazaki
fragments
5. DNA ligase joins the segments
together
DNA
ligase