Genetics Review Spring 2015 – (This is not all-inclusive
but a good review!)
Central Dogma of Molecular Biology:
DNA ---transcription--- RNA ---- (AAs)--translation ---Proteins
DNA – chapter 12
 Role of DNA
o Store all genetic material, including the
instructions to make proteins
o Be able to copy that information for new cells
o Pass on the information to offspring
 Structure
o Made of nucleotides – phosphate, sugar, base
o Form of double helix – two strands that bond
together with bases in the middle and are
twisted.
o Structure was discovered by Watson and Crick.
o Bases are Adenine, Thymine, Guanine and
Cytosine. A bonds with T and G bonds with C
o Chargaff’s Rule: since A and T bond, they will be
found in the same concentrations in DNA. Same
with G and C. So … [A]=[T] and [G]=[C]
o Bases are held together by weak hydrogen
bonds
 Weak bonds allow them to come apart so
DNA can be copied
o Backbone (phosphate and sugar) is help by
strong covalent bonds
 Replication – process to make a full copy of DNA
1. DNA unwinds and unzips – used enzyme helicase
2. New DNA nucleotides are brought in to
complement the original strands – enzyme DNA
polymerase
a. A leading strand copies easily from the 5’ to 3’
end
b. The lagging strand also has to copy from the 5’
to 3’ end, but creates Okazaki fragments in
the process. These fragments have to be
linked together – by enzyme ligase.
3. DNA polymerase enzyme then proofreads the two
new segments*
*Each new segment has one original strand and
one new strand.
Base pairs – DNA to mRNA
A-U
T-A
G-C
C-G
Transcription – in nucleus – page 364 in text
1. DNA unwinds – helicase
2. DNA unzips – helicase
3. New RNA nucleotides come in and complement DNA
– RNA polymerase
4. Introns spliced out and stay in nucleus
5. Exons link together
6. Cap and tail added for protection
7. mRNA is done and leaves nucleus
Translation – in ribosome (in cytoplasm or on ER) - page
368 in text
1. mRNA leaves nucleus and goes to ribosome (made
of rRNA and proteins)
2. tRNA transfers the AA from cytoplasm to ribosome
3. codon of mRNA bonds with anticodon of tRNA
4. next tRNA brings next AA
5. the AAs peptide bond together
6. 1st tRNA releases its AA
7. Process continues until the mRNA codon is a Stop
Codon – no AA comes in
8. String of AAs (polypeptide chain) leaves ribosome,
folds special way and is now a protein!
DNA:
mRNA:
codons:
TACGGCCCCTACCCGATT
AUGCCGGGGAUGGGCUAA
AUG CCG GGA AUG GGC UAA
amino acids:
Met-Pro-Gly-Met-Gly- Stop
Protein: lactase – enzyme that breaks down lactose
1000s of AAs long, so how many DNA bases??
3DNA bases = 3mRNA bases = 1 codon = 1 AA
(don’t forget about introns/exons!)
DNA:
mRNA:
codons:
TACGGCTCTTACCCGATT
AUGCCGUGAAUGGGCUAA
AUG CCG UGA AUG GGC UAA
amino acids:
Met-Pro-Stop
Protein: lactase – IS NOT MADE!!!!!!! This is a mutation!
Mutation – change in DNA that affects protein
Can be positive, negative or neutral.
Example
Original DNA: TACGGCCCCTACCCGATT
mRNA:
AUGCCGGGGAUGGGCUAA
codons:
AUG CCG GGG AUG GGC UAA
amino acids:
Met-Pro-Gly-Met-Gly- Stop
The cat ate the rat .
Different types of mutations
1. Gene mutation (point mutation)
a. Substitution
DNA:
TACGGCTCCTACCCGATT
mRNA:
AUGCCGUGGAUGGGCUAA
codons:
AUG CCG UGG AUG GGC UAA
amino acids: Met-Pro-Stop
Mutation: The cat. (WHAT??? What did the cat do?????)
b. Insertion = frameshift mutation
Mutated DNA: TACGAGCCCCTACCCGATT
mRNA:
AUGCUCGGGGAUGGGCUAA
codons:
AUG CUC GGG GAU GGG CUA A
amino acids: Met-Leu-Gly-Asp-Gly- Leu
Mutation: The cMa tat eth era t (WHAT???)
c. Deletion = frameshift mutation
Mutated DNA: TAC___GCCCCTACCCGATT
mRNA:
AUG___CGGGGAUGGGCUAA
codons:
AUG CGG GGA UGG GCU AA
amino acids: Met-Arg-Gly-Try-AlaMutation: The ata tet her at (WHAT???)
2. Chromosomal mutation – (page 374 in text)
a. Deletion – all or part of chromosome is missing
Ex. – Human with 45 chromosomes – female with
only one X
Remember:
 Females have XX sex chromosomes – so
females have 2 copies of all genes on the X
chromosome
 Males have XY sex chromosomes - Y
chromosome has gene to create testicles
b. Duplication – extra chromosome or part of
chromosome
Ex. – Human with 3 copies of chromosome 21 –
Down Syndrome
c. Inversion – gene order is switched
This can turn genes on that should be off, or turn
genes off that should be on! Do you want your
tumor suppressor gene on (expressed) or off???
d. Translocation – like “crossing over” of nonhomologous chromosomes
Ex. Parts of chromosome 9 & 22 swap causing
leukemia
Some helpful vocab:
Chromosome - Lots of DNA and histone condensed
Chromatin = DNA and histone (page 280)
Chromatid = the 2 arms of a chromosome in
Prophase
Gene – segment of DNA that codes for a protein
Allele – different forms of a gene
Genotype – combination of alleles (one from mom, one
from dad)
Phenotype – how the genotype is expressed (or what the
protein does)
Ex – Gene for cystic fibrosis is found on a chromosome.
Everyone has 2 copies of the gene.
(FF) Homozygous Dominant - both copies of the allele
form the protein that allows ions to cross the membrane
– you are fine.
(Ff) Heterozygous - One copy forms the protein and one
doesn’t - you are fine because you form the protein.
However, you are a CARRIER of a bad variant of the gene
and can pass that to your kids.
(ff) Homozygous Recessive – both copies do not form the
protein, so you have no way to allow certain ions to cross
the membrane – you have cystic fibrosis.
What about sickle cell disease? Do you have the gene?
Yes!! Do you have the allele for the disease?
RR– red blood cells form normally. No problems.
Rr – some RBCs form in sickle shape but no problem.
In fact, for some reason, there is a resistance to
malaria associated with this genotype!
rr – RBCs form in sickle shape causing lots of pain,
especially in joints.
CF and SCD both follow Mendel’s Rule of Dominance.
Some alleles are dominant over others. Recessive alleles
are ONLY expressed if no dominant allele is present.
CF and SCD both are controlled by one gene.
But WAIT … there’s more!
Codominance – R (red) allele and W (white) allele are
both expressed equally. So
Genotype - RW
Phenotype – red and white stripes
Incomplete Dominance - R (red) allele and W (white)
allele are both expressed and BLEND together. So
Genotype - RW
Phenotype – pink
Polygenic Traits- characteristics controlled by more than
one gene. Ex. Skin color. Four genes – all
dominant/recessive.
AABBCCQQ – darkest color
aabbccQQ – lightest color
AaBbCcQQ – somewhere in between
AABBCCqq –albino because there is no “Q” to allow for
the expression of ANY color! (Qq allows for color
expression.)
Multiple Alleles – more than just 2 forms of the gene.
Ex. – Blood type (which is also codominant)
Allele A – produces an A protein
Allele B - produces a B protein
Allele O – does not produce a protein
Genotypes
Phenotypes
AA, AO
Produces A protein
BB, BO
Produces B protein
AB
Produces both A & B protein
OO
Produces NO protein
Punnett squares are used to determine the probability of
genotype of offspring.
Ex. What is the probability that parents with genotypes
AO and AB will produce offspring with the phenotype B
blood type? AO x AB
A
O
A
B