Download Topic 2 & 3: Genetics Review

Topic 2 & 3: Genetics Review
Syllabus Statements
2.4.1 Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and
phosphate.
2.4.2 State the name of the 4 bases in DNA
2.4.3 Outline how the DNA nucleotides are linked together by covalent bonds into a
single strand.
2.4.4 Explain how a DNA double helix is formed using complimentary base pairing and
hydrogen bonds.
2.4.5 Draw a simple diagram of the molecular structure of DNA.
2.5.1 State that DNA replication is semi-conservative.
2.5.2 Explain DNA replication in terms of unwinding of the double helix and separation
of the strands by helicase, followed by formation of the new complementary strand
by DNA polymerase.
2.5.3 Explain the significance of complementary base pairing in the conservation of
the base sequence of DNA.
2.6.1 Compare the structure of RNA and DNA
2.6.2 Outline DNA transcription in terms of the formation of an RNA strand
complementary to the DNA strand by RNA polymerase.
2.6.3 Describe the genetic code in terms of codons composed of triplets of bases.
2.6.4 Explain the process of translation, leading to peptide linkage formation.
2.6.5 Define the terms degenerate and universal as they relate to the genetic code.
2.6.6 Explain the relationship between one gene and one polypeptide.
Outline DNA nucleotide structure in terms of
sugar (deoxyribose), base and phosphate.
Four DNA bases are
Thyamine
Adenine
Cytosine
Guanine
Nucleotide
Outline the bonding in the DNA
strand
Covalent bond
Structural Details
Antiparallel Strands
Complementary Base Pairing
What is meant by semiconservative
replication of DNA?
Complementary base pairing ensures that resulting strands are equal to initial
How Does replication happen?
a)
Helicase: which unwinds the DNA double helix and
separates the strands by breaking the hydrogen bonds
b) DNA Polymerase which links up the nucleotides to
form the new strand of DNA.
This 2nd step, after the unwinding and separation of
strands, involves having the single strands act as
templates for the new strands.
Free nucleotides are present in large numbers around the
replication fork. The bases of these nucleotides form
hydrogen bonds with the bases of the parent strand.
DNA polymerase is the main enzyme involved.
Daughter DNA molecules each rewind into a double helix.
List 3 ways RNA is different from
DNA
a) RNA nucleotides contain the sugar
ribose. Ribose has one more hydroxyl
than deoxyribose.
b) Uracil, a pyrimidine, is unique to RNA
and is similar to thymine (A, C, G, U).
c) RNA is single stranded.
Transcription
What is a codon?
• A 3 base sequence in
mRNA
• Each codes for a
particular amino acid
• 64 possible codons
• 20 amino acids
• Stops & starts also
Define
• Degenerate 
– Amino acids are coded for by multiple
different codon sequences. As many as 6
sequences in some cases for one amino acid
• Universal 
– DNA code is the same in all living things. The
gene for a bacterial polypeptide will create the
same polypeptide in any eukaryote
Translation Initiation
Translation Elongation
Translation Termination
So Here’s a DNA Strand
ATTCGGCCACATTTC
1. Write out the complementary strand
TAAGCCGGTGTAAAG
2. Write out the RNA transcript of the
original strand
UAAGCCGGUGUAAAG
3. Write out the first 3 tRNA anticodons
AUU CGG CCA
Syllabus Statements
• 3.1.1: State that eukaryotic chromosomes are
made of DNA and protein
• 3.1.2: State that in karyotyping chromosomes are
arranged in pairs according to their structure
• 3.1.3: Describe one application of karyotyping
(cross reference with 3.2.5)
• 3.1.4: Define gene, allele, genome
• 3.1.5: Define gene mutation
• 3.1.6: Explain the consequence of a base
substitution mutation in relationto the process of
transcription & translation, using the example of
sickle cell anemia
• Chromosomes = composed of DNA and
histone proteins in eukaryotes
Definitions
•
•
•
•
Gene =
Allele =
Genome =
Gene mutation =
Human Karyotype
• Can display an
individual’s somatic-cell
metaphase chromosomes
• Karyotype when arranged
in a standard sequence.
• Display of chromosomes
arranged by size, position
of centromere and
staining pattern.
• Often made from
lymphocytes or
amniocytes.
Sickle Cell Disease
Sickle Cell Disease
• Replacement of #6 amino acid, glutamic
acid, with valine substitutes a non-polar
amino acid for a polar amino acid.
• #6 amino acid is on the OUTSIDE of the
hemoglobin molecule.
• Hydrophobic interaction set up by valine
causes change in shape of hemoglobin
which leads to sickling of the red blood
cell.
Syllabus Statements II
• 3.3.1: Define  genotype, phenotype, dominant
allele, recessive allele, codominant alleles, locus,
homozygous, heterozygous, carrier, test cross
• 3.3.2: Construct a Punnett grid
• 3.3.3: Construct a pedigree chart
• 3.3.4: State that some genes have more than two
alleles (multiple alleles)
• 3.3.5: Describe ABO blood groups as an example
of codominance and multiple alleles
• 3.3.6: Outline how sex chromosomes determine
gender by referring to the inheritance of X and Y
chromosomes in humans
Some Definitions
Dominant allele =
Recessive allele =
Codominant alleles =
Locus =
• 3.3.7: State that some genes are present on the X
chromosome and absent from the shorter Y
chromosome in humans.
• 3.3.8: Define Sex linkage
• 3.3.9: State two examples of sex linkage
• 3.3.10: State that human females can be
homozygous or heterozygous
• 3.3.11: Explain that female carriers are
heterozygous for X-linked recessive alleles
• 3.3.12: Calculate and predict the genotypic and
phenotypic ratios of offspring of monohybrid
crosses involving any of the above patterns of
inheritance
• 3.3.13: Deduce the genotypes or phenotypes of
individuals in pedigree charts
Figure 14.4 Mendel’s law of segregation (Layer 2)
Terms from Crosses
1. Homozygous: Having 2 identical alleles for
a given trait
•
True breeding, all gametes carry same allele
2. Heterozygous: Having 2 different alleles for
a given trait
•
Not true breeding, half of gametes carry one
allele, half carry the other
3. Phenotype: An organism’s expressed traits
•
F2 phenotypic ratio of 3:1 = 3 purple, 1 white
4. Genotype: An organism’s genetic make up
•
F2 genotypic ratio of 1:2:1 = 1PP, 2Pp, 1pp
Test Crosses
Test Cross: Breeding an organism of unknown
genotype with one that is known
homozygous recessive
e.g. P__ X pp : if all progeny purple, then ___
: if ½ purple, ½ white, then ___
Figure 14.9 Incomplete dominance in snapdragon color
Multiple Alleles
• More than 2 alternate forms of a gene
• E.g. = ABO blood group; produce 4 possible
phentypes (A, B, AB, O)
• A & B are 2 genetically determined
polysaccharides (A & B antigens) found on the
surface of RBC
• Three alleles for this gene: IA (A antigen), IB (B
antigen), and i (no antigen)
• Alleles IA & IB are codominant, and dominant to i
Figure 14.10 Multiple alleles for the ABO blood groups
Pedigree Rules
• By convention squares represent males
• Circles represent females
• Shaded symbols indicate individuals
showing the trait
• Horizontal line connecting male and
female signifies reproduction (or marriage)
• Offspring are listed below in birth order
Generally
• If a mother & father do not show a trait but
they have an offspring that does – usually
this indicates recessive inheritance
• If males show a trait more than females
and / or mothers pass the trait to their
sons it is generally sex linked inheritance
• Otherwise its dominance inheritance –
only specific example we discussed of this
was huntington’s chorea
Practice Problem
Sex linkage
• Sex linkage: In addition to determining
sex, the X and Y chromosomes contain
many genes not related to sex
• In humans sex linkage is usually
equivalant to X-linkage
• Examples
– Colorblindness: Xb presence relative to the
normal XB
– Hemophilia: Xh presence relative to the
normal XH
Hemophilia
•
A man with hemophilia ( a recessive, sex
linked condition Xh) has a daughter of normal
phenotype. She marries a man who is normal
for the trait
1. What is the probability that the daughter of this
mating will be a hemophiliac?
2. What is the probability for a son?
3. If the couple has 4 sons, what is the probability
that they will all be born with hemophilia?
Solution
•
•
P generation (man) =
(XH XH) x (XhY)
F1 generation
(daughter) =
(Xh XH) x (XHY)
1. 0%
2. 50%
3. ½ * ½ * ½ * ½
= 1/16
H
X
Y
XH
XH XH XHY
h
X
XH Xh XhY
Red-green Color Blindness
•
Red-green Color Blindness is caused by a sexlinked recessive allele (Xb). A color blind man
marries a woman with normal vision whose
father was color blind.
1. What is the probability that they will have a
color blind daughter? (this is 2 events 1st
daughter, then color blind)
2. What is the probability that thir first son will be
color blind?
Solution
•
•
•
Color blind man = XbY
Woman = XBXb
P generation =
(XBXb) x (XbY)
1. ¼
2. ½
b
X
Y
XB
XB Xb XBY
b
X
Xb Xb XbY
Sex Linkage: General Statements
1. Fathers pass X-linked alleles to only and all of
their daughters
2. Fathers cannot pass X-linked traits to their sons
3. Females receive 2 X chromosomes, one from
each parent
4. Mothers pass only 1 X chromosome (either
maternal or paternal homologue) to every
daughter and son
5. If a sex linked trait is due to a recessive allele, a
female will express that trait if and only if she is
homozygous
Syllabus Statements
• 3.2.1 State that meiosis is a reduction division in terms
of diploid and haploid numbers of chromosomes.
• 3.2.2 Define homologous chromosomes
• 3.2.3 Outline the process of meiosis, including pairing of
chromosomes followed by two divisions, which results in
four haploid cells.
• 3.2.4 Explain how the movement of chromosomes
during meiosis can give rise to genetic variety in the
resulting haploid cells.
• 3.2.5 Explain that non-disjunction can lead to changes
in chromosome number, illustrated by reference to
Down’s syndrome (trisomy 21).
• 3.2.6 State Mendel’s law of segregation. (Done with
genetics)
• 3.2.7 Explain the relationship between Mendel’s law of
segregation and meiosis (Done with genetics)
Sexual vs. Asexual
Reproduction
• Asexual: 1 individual is
sole parent
• Single parent passes on
all its genes to offspring
• Offspring genetically
identical to parent
• Results in clone; rarely,
genetic differences occur
as a result of mutation, a
change in the DNA
• Sexual: 2 parents give
rise to offspring
• Each parent passes on ½
its genes to offspring
• Offspring have unique
combination of genes
• Results in greater
variation; offspring vary
genetically from sibs and
parents
• Adaptive to changing
environments
Definitions
5.
6.
7.
Homologous Chromosomes: a pair of
chromosomes that have the same size,
centromere position and staining pattern.
With one exception, homologues carry the same
genetic loci.
X and Y chromosomes pair during meiosis but Y is
much smaller.
Autosome: chromosome that is not a sex
chromosome.
Sex chromosome: dissimilar chromosomes that
determine an individual’s sex.
Chromosomal pairs in the human karyotype are a
result of our sexual origins: 1 homologue is
inherited from each parent
Mitosis vs. Meiosis
Chromosomal Non-disjunction
and Down Syndrome: Errors in
Meiosis
• Sometimes chromosomes fail to disjoin during
either Meiosis I or Meiosis II. Instead they move
to the same pole.
• Non-separation of chromosomes is called nondisjunction and results in gametes with either
one chromosome too many or one chromosome
too few (frequently lethal).
• Gametes with 2 doses of a chromosome that
join with a gamete with the normal dose of
chromosomes now have 3 doses of this
chromosome (trisomy).
Nondisjunction in Meiosis I and
II
Syllabus Statements
3.4.1 State that PCR copies and amplifies minute quantities of nucleic acid
3.4.2 State that gel electrophoresis involves the separation of fragmented pieces of
DNA according to their charge and size.
3.4.3 State that gel electrophoresis of DNA is used in DNA profiling.
3.4.4 Describe 2 applications of DNA profiling.
3.4.5 Define genetic screening.
3.4.6 Discuss 3 advantages and/or disadvantages of genetic screening.
3.4.7 State that the Human Genome Project is an international cooperative venture
established to sequence the complete human genome.
3.4.8 Describe two possible advantageous outcomes of this project.
3.4.9 State that genetic material can be transferred between species because the
genetic code is universal.
3.4.10 Outline the basic technique used for gene transfer involving plasmids, a
host cell (bacterium, yeast or other cell), restriction enzymes (endonuclease)
and DNA ligase.
3.4.11 State 2 examples of the current uses of genetically modified crops or
animals.
3.4.12 Discuss the potential benefits and possible harmful effects of 1 example of
genetic modification.
3.4.13. Outline the process of gene therapy using a named example.
3.4.14 Define clone.
3.4.15 Outline a technique for cloning using differentiated cells.
3.4.16 Discuss the ethical issues of cloning in humans.
Explain the Following Techniques
• What does PCR do?
- copies and amplifies minute quantities of
nucleic acid
• What does gel electrophoresis do?
-separation of fragmented pieces of DNA
according to their charge and size.
- used in DNA profiling
What are some of the Applications
of DNA Profiling?
1.
2.
3.
4.
Forensic use
Diagnosis of genetic disorders.
Identifying individuals who died long ago.
Identifying animals that migrate in order to
estimate population size.
5. Identifying animal parts (distinguish from
common species vs. endangered species).
Contamination of samples by even a single cell
would totally throw off results.
What is Genetic Screening?.
Testing of an individual for the presence or absence of a gene
Advantages:
1. Fewer children with genetic
diseases are born because
people choose not to have
children or choose a partner
who is not a carrier for the
same allele.
2. Frequency of alleles causing
genetic disease can be
reduced.
3. Genetic disease can be
found and treated more
effectively (e.g. PKU,
Galactosemia)
Disadvantages:
1. Frequency of abortion may
increase.
2. Harmful psychological effects
may result from knowing that
you are a carrier/at risk of a
genetic disease.
3. Creation of a genetic
underclass (think of the movie
Gataca): refusal of jobs, life
insurance, health insurance;
less likely to find a partner.
What is the Human Genome
Project
- an international cooperative venture established to
sequence the complete human genome.
• Sequenced 3 billion bases and found 30,000
genes.
• Could lead to an understanding of many genetic
diseases and how genes control human
development.
• Could lead to the development of genomic libraries
and the production of gene probes to detect
sufferers and carriers of genetic disease.
• Allows the production of pharmaceuticals based on
DNA sequences (Designer drugs).
Describe the
basic technique
of gene transfer
List examples of GMO
a) Salt tolerance in tomato plants; delayed ripening
in tomatoes, frost resistance in tomatoes (trout
gene).
b) Herbicide resistance in crop plants
c) Factor IX (human blood clotting factor) formed in
sheep milk
d) transfer from cattle to chickens a gene for making
growth hormone.
e) Transfer of gene for human insulin to bacteria.
f) Transfer of frost resistance gene from trout into
tomato.
What are the potential risks &
benefits of GM? Bt Corn example
1) Less pest damage; higher crop
yields, reduction of food
shortages.
2) Less land needed for crop
production, more for wildlife
conservation.
3) Less use of insecticide sprays
which harm farm workers and
wildlife.
1.
2.
3.
Humans or farm animals
could eat genetically
modified maize and be
harmed by bacterial DNA or
Bt toxin.
Other non-pest insects could
be killed e.g. pollen blown
distances kill monarch
butterfly caterpillars.
Engineered crops could
cross species barriers.
Escape and spread and
compete with naturally
occurring varieties.
Outline the
process of gene
therapy
What is a clone?
A clone is a group of genetically identical
organisms or a group of cells artificially
derived from a single parent cell.
Outline the process of cloning
differentiated cells
Step 1: Udder cells were taken from a donor sheep. Cells
were cultured in low nutrient broth to make them switch
off their genes and become dormant.
Step 2: Unfertilized egg cells were taken from another
sheep and the nucleus removed from each egg cell
using a micropipette.
Step 3: Enucleate egg cells were fused with donor cells
using a pulse of electricity.
Step 4: Fused cells developed like zygotes and became
embryos which were implanted into another sheep who
became the surrogate mother.
Step 5: Lamb born was genetically identical to sheep
whose udder cells were used.
Is it right or wrong to clone
humans?