Genetics Of Living Systems Flashcards
what are mutations
a random error in DNA nucleotide - an alteration in the nucleotide sequence of a cell in a living organism
what is the most common mutation
loss of purine or pyrimidine base
what are the effects of mutations
beneficial, neutral and harmful
beneficial: mutations offer a selective advantage to an individual and is the driving force behind natural selection
neutral: has no effect so the mutation occurs in the non coding part of DNA or is silent
harmful: thalassemia, sickle cell, cystic fibrosis (deletion), Huntington disease. Huntington is an expanding triple nucleotide repeat (CAG-CAG-CAG-CAG)
what are point mutations
changes in an individual gene due to miscopying of one of more nucleotides
occurs at a single point in the nucleotide sequence
there can be more than one point mutation in a sequence
examples:
- substitution mutations
- insertion mutations
- deletion mutations
what are substitution mutations
result in a change of one base
results in one triplet code being changed
what are insertion mutations
when a new base is inserted into DNA base sequence
this changes the triplet code at the point of mutation, but can have a FRAMESHIFT EFFECT on bases further down the sequence
what are deletion mutations
when a random base is DELETED from the nucleotide base sequence
like insertions they change the triplet code at the point of mutation but also have a frameshift effect
what can substitution mutations be further categorised into
silent mutations
nonsense mutations
missense mutations
what are silent mutations
a silent mutation occurs when substitution of a base still codes for the same amino acid as the original base
possible due to the DEGENERATE nature of genetic code
mutation has no effect on the final polypeptide
what are nonsense mutations
occurs when a substitution results in a premature ‘stop codon’ being coded for
leading to premature end to synthesis of polypeptide
very unlikely that final protein would function normally
stop codons are - UAA, UAG and UGA
what is thalassemia
autosomal recessive blood disease caused by nonsense mutation
sufferers have reduced rate or no synthesis of one of the beta globin chains that make up haemoglobin = anaemia
what are missense mutations
occur when a change in base leads to a different amino acid being coded for
polypeptide will have a single amino acid that is different
the effect of mutation is determined by the role of the amino acid in the final polypeptide
what is sickle cell disease
caused by missense mutation in the haemoglobin gene
the hydrophilic amino acid GLUTAMIC ACID is replaced by the hydrophobic amino acid valine
valines form bonds with themselves that stick haemoglobin molecules together, producing long chains of stuck-together haemoglobin molecules
this distorts the shape of the red blood cell and decreases its elasticity
when is vitamin D synthesised
when skin is exposed to sunlight and through diet
what is melanin and what does it do
a skin pigment that protects cells from harmful effects of UV and reduces vitamin D production
what is the cradle of mankind in Ethiopia
early humans that had dark skin
what is the result of genetic mutations in skin
lighter skin
less melanin
light skin does not shield against harmful UV - causes skin cancer
however, vitamin D can be synthesised with less sunlight
therefore suited migration to more temperature climes as increased melanin not needed to protect against sun
skin can synthesise enough vitamin D from low UV levels
when is high melanin and low melanin advantageous
high melanin in hotter climates
low melanin in colder climates
Who discovered that some genes are only turned on when conditions require the proteins they code for
What did they use this information for
Jacob and Monod
Used it to suggest that the control of enzyme levels in all cells occurs through regulation of transcription
What is e.coli capable of synthesising
A variety of different enzymes depending on their environment
What does lactose permease do
Transports lactose into the cell
What does Beta-galactosidase do
Catalyses hydrolysis of lactose into glucose and galactose
Why do E.coli only produce enzymes needed to metabolise lactose when lactose is present in substrate
Because it would be a waste to produce
Therefore expression of these proteins is induced by the presence of lactose
Describe the regulation of the lac operon
Structural genes encode proteins involved in the uptake and utilisation of lactose
Regulatory gene encodes a protein which controls when the structural gene is expressed (on/off switch)
RNA polymerase binds to the promoter region to initiate transcription of gene
MRNA is translated by ribosomes to produce a repressor protein
The repressor protein has two binding sites
- one site binds to lactose
The other site binds to LacO (operator region)
What does LacI encode for
A repressor protein
What does LacO do
The operator region of DNA
What does LacZ encode for
Beta-galactosidase
What does LacY code for
Permease proteins
When does the Lac operon switch off
When lactose is ABSENT the repressor protein binds to LacO
This blocks the promotor region (LacP) so that RNA cannot bind to it and the structural genes cannot be transcribed
When does the lac operon switch on
When lactose is PRESENT it binds to the lactose-binding site on repressor protein
This changes the shape of the DNA binding site so that it cannot bind to LacO
This allows RNA polymerase to bind to lacP so that the structural genes can be transcribed
What is an inducible operon
Lactose induces transcription of the enzyme that break it down
It is called the inducer
A mutation that renders the regulatory gene of a repressible operon non-functional would result in…?
Continuous transcription of the structural genes encode proteins
Describe gene expression in eukaryote
Eukaryotes are multicellular
So instead of having sections of DNA that are switched on and off, they have cells that have different sections of the DNA switched on off or
Cell differentiation
What are transcription factors
Proteins or non-coding pieces of RNA that work within the nucleus to control the expression of genes
How do transcription factors work in eukaryotes
Factors bind to their specific promotor region and either activate or repress RNA polymerase from binding to the DNA
What is post-transcriptional gene regulation
Splicing
- not all DNA codes for protein, some is essentially ‘junk’ DNA
- coding DNA = Exons
- non-coding DNA = introns
Non-coding is removed from the strand before translation occurs - splicing
Define monogenic inheritance
Give an example
The inheritance of characteristic controlled by a single gene
E.g. wing length in fruit flies
Who was Gregor Mendel?
What did he discover?
and why was his work important
An Austrian monk
In his garden he noticed how characteristics in plants were passed from one generation to the next
His research, published in 866 became the foundation of modern genetics
What did Mendel do a lot of experiments with and what was the conclusion of his results
Pea plants
Crossed a homozygous tall pea plant with a homozygous dwarf pea plant,
Continuously bred them and because dwarf offspring was passed from the original dwarf it only showed sometimes
This shows that genes are passed to offspring but some are dominant over others.
What are the three biological agents
Alkylating agents - methyl or ethyl groups are attached to bases resulting in incorrect base paring
Base analogs - incorporated into DNA in place of the usual base during replication, changing base sequence
Viruses - viral DNA may insert itself into a genome, changing the base sequence
Different mutations have a range of effects on the protein synthesised. What are these categorised as?
Amorph - mutation that results in the loss of function of a protein
Hypomorph - mutation that results in a reduction of function of a protein
Hypermorph - mutation that results in a gain in function of a protein
What are the four changes in chromosome structure?
Deletion - section breaks off and is lost within the cell
Duplication - sections get duplicated
Translocation - section of one chromosome breaks off and joins another non-homologous chromosome
Inversion - a section of chromosome breaks off, is reversed and then joins back onto the chromosome
What are the different ways (4) genes are regulated
Transcriptional - genes can be turned on or off
Post-transcriptional - mRNA can be modified which regulates translational and the types of proteins produced
Translational - translations can be stopped or started
Post-translational - proteins can be modified after synthesis which changes their functions
What are the ways in which transcription is controlled
Chromatin remodelling
Histone modification
Describe acetylation and phosphorylation in histone modification
Addition of acetyl or phosphate groups
Reduces the positive charge on the histones
Causes DNA to coil less tightly, allowing certain genes to be transcribed
Why does DNA coil around histones
Because histones are positively charged and DNA is negatively charged
Describe methylation in histone modification
Addition of methyl groups make histones more hydrophobic so they bind more tightly to each other
Causes DNA to coil more tightly and preventing transcription of genes
What are homeobox genes
A group of genes which all contain a homeobox
- a section of DNA 180 base pairs long coding for a part of the protein 60 amino acids long that is highly conserved (very similar) in plants, animals and fungi
Homeobox genes are regulatory genes
Why are homeobox genes regulatory genes
Because a part of the protein (homeodomain) binds to DNA and switches other genes on or off. Therefore, homeobox genes are regulatory genes
Why are homeobox genes regulatory genes
Because a part of the protein (homeodomain) binds to DNA and switches other genes on or off. Therefore, homeobox genes are regulatory genes
What are HOX genes
HOX genes are one group of homeobox genes that are only present in animals,
- responsible for the correct positioning of body parts
- found in gene clusters
- mammals have four clusters on different chromosomes
How are body plans usually represented
As cross sections through an organism showing the fundamental arrangement of tissue layers
What do HOX genes in the head and thorax control
Head - development of mouthparts
Thorax - development of wings, limbs or ribs
What are the three types of symmetry shown in body shapes
Radial symmetry - have no left or right sides, only top and bottom
Bilateral symmetry - organisms have both left and right sides and head and tail
Asymmetry is seen in sponges which have no lines of symmetry
Define diploblastic and triploblastic organisms
Diploblastic = two primary tissue layers
Triploblastic = three primary tissue layers
What are somites and what have the developed into
Somites are segments in the embryo that are directed by HOX genes to develop in a particular way depending on their position in the sequence.
Somites developed into individual vertebrae and associated structures
What animals have radial symmetry
Diploblastic animals like jellyfish
What is the role of mitosis and apoptosis in body plan development
Mitosis - increase the number of cells leading to growth
Apoptosis - removes unwanted cells and tissues
- cells undergoing apoptosis can also release chemical signals to stimulate mitosis and cell proliferation - remodelling tisssues
How are mitosis and apoptosis regulated
By HOX genes
What factors affect the expression of regulatory genes
Environment - internal and external
Stress - homeostatic balance within an organism is upset
External - change in light intensity or temperature
Internal - release of hormones or psychological stress
Drugs - e.g. thalidomide
Explain how thalidomide affected the activity of regulatory genes
- given to pregnant women to treat morning sickness
- prevented expression of HOX gene related to limb length
- thalidomide prevents the formation of networks of capillaries which are necessary for some tumours to grow and develop
how is gene expression regulated at the transcription level?
transcription factors
- proteins that can either initiate or inhibit the transcription of genes so that only certain parts of DNA are expressed
describe the function of the lac operon in LOW lactose concentrations
E.coli can use lactose as a respiratory substrate via use of an enzyme
when lactose conc is low, lac operon binds to the gene that makes the enzyme and inhibits its expression
describe the function of the lac operon in high lactose concentrations
lactose binds to operon, causing it to change shape and unattach from the gene.
allows RNA polymerase to bind and the gene to be expressed
how is development controlled by homeobox genes
code for transcription factors that activate genes when they’re needed during development of a zygote
what best describes an operon
a cluster of genes under control of a promotor
what feature of a pathogen could be altered by a mutation making a vaccine ineffective
shape of proteins/ antigens on plasma membrane
What type of adaptation is represented by tigers stripes
Anatomical
Describe and explain how a tiger with striped fur may have evolved from a non-striped ancestor
(Discuss the different types of genes that might be involved)
- Mutations (e.g. of pigment gene and of regulatory genes)
- selection pressure of prey availability
- adaptation helped tigers to hide from prey/ camouflage
- striped tigers had greater survival probability - more likely to reproduce
- beneficial alleles passed onto next generation
- allele frequency of relevant genes would have increased with each generation
- after many generations, all tigers of that population were striped
Roles of regulatory genes:
- regulatory genes control the pattern/ where pigments produced/ expression of other genes
- genes switched on or off during development
Outline how a dominant allele codes for the production of an enzyme and explain why the recessive allele codes for an enzyme with a different primary structure
Genetic code:
- DNA base sequence codes for amino acid sequence
- ref to mRNA base sequence
- triplet code/ 3 bases = 1amino acid
- degenerate code
- substitution could result in same amino acid
Transcription C:
- transcription then translation
- complementary base pairing
- synthesis of mRNA strand
- role of RNA polymerase
Translation L:
- mRNA binds to ribosome
- tRNA binds to mRNA
- tRNA brings specific amino acid
- mRNA translated into polypeptide
Effect of y allele
- substitution/ frameshift
- different base sequence of DNA
- different mRNA codon
- different tRNA anticodon
- tRNA brings different amino acids
- different sequence of amino acids
- amino acid sequence is primary structure
Describe how gene expression can be regulated after transcription
Primary mRNA is modified
Removal of introns to produce mature mRNA
Alternative splicing can produce different versions of mRNA
Protein must be activated by cAMP/ phosphorylation
Binding of cAMP alters shape of protein
Suggest and explain why lactose is unable to cross membranes
- too big
- unable to pass between phospholipids
Or
- no/small concentration gradient
- needs carrier protein/ pump
Define epistasis
Interaction between genes that influences a phenotype
Suggest how an allele may inhibit the expression of another allele (both dominant)
Allele 1 produces/ codes for repressor protein/ transcription factor
Product of allele 1 binds to promotor of allele 2/ mRNA/ ribosome
Product of allele 1 stops transcription/ translation of allele 2
Product of allele 1 inhibits the enzyme encoded by allele 2
Fill in the blanks:
Development of eukaryotic body plan is controlled by _____ genes
These genes code for proteins that are able to bind to ______and turn specific genes on and off and are known as ______factors. These proteins contain a sequence of base pairs that varies little between species within the animal, _______ or fungus______
Homeobox
DNA
Transcription
Plant
Kingdoms
