Genetics SL Flashcards
Gene
A sequence of DNA that encodes for a specific trait
Locus
The position of a gene on a particular chromosome
Allele
Alleles are alternative forms of a gene that code for the different variations of a specific trait
Gene mutation
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
Neutral mutations
Neutral mutations have no effect on the functioning of the specific feature (silent mutations)
Detrimental mutations
truncate the gene sequence (nonsense mutations) to block the normal function of a trait
Beneficial mutations
Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
Cause of Sickle Cell Anaemia
Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin
Polypeptide: The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine (Glu to Val)
Consequence of Sickle Cell Anaemia
The amino acid change (Glu → Val) alters the structure of haemoglobin, causing it to form insoluble fibrous strands
The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired
The formation of fibrous haemoglobin strands changes the shape of the red blood cell to a sickle shape
The sickle cells may form clots within the capillaries, blocking blood supply to vital organs and causing myriad health issues
The sickle cells are also destroyed more rapidly than normal cells, leading to a low red blood cell count (anaemia)
What is a genome?
The genome is the totality of genetic information of a cell, organism or organelle
What is The Human Genome Project (HGP) and what were the 4 outcomes?
The Human Genome Project (HGP) was an international cooperative venture established to sequence the human genome
The HGP showed that humans share the majority of their sequence, with short nucleotide polymorphisms contributing diversity
The completion of the Human Genome Project in 2003 lead to many outcomes:
Mapping – The number, location, size and sequence of human genes is now established
Screening – This has allowed for the production of specific gene probes to detect sufferers and carriers of genetic diseases
Medicine – The discovery of new proteins have lead to improved treatments (pharmacogenetics and rational drug design)
Ancestry – Comparisons with other genomes have provided insight into the origins, evolution and migratory patterns of man
Where is genetic material in prokaryotes
Genetic material is found free in the cytoplasm in a region called the nucleoid
What is the genetic material in prokaryotes?
The genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore)
The DNA of prokaryotic cells is naked – meaning it is not associated with proteins for additional packaging
(Prokaryotic cells may possess additional circular DNA molecules called plasmids)
What are plasmids?
Plasmids are small, circular DNA molecules that contain only a few genes and are capable of self-replication
Organisation of eukaryotic chromosomes
DNA is complexed with eight histone proteins (an octamer) to form a complex called a nucleosome
Nucleosomes are linked by an additional histone protein (H1 histone) to form a string of chromatosomes
These then coil to form a solenoid structure (~6 chromatosomes per turn) which is condensed to form a 30 nm fibre
These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin
Chromatin will then supercoil during cell division to form chromosomes that are visible (when stained) under microscope
What is a chromosone and describe its structure
Chromosomes are linear molecules of DNA that are compacted during cell division (mitosis or meiosis)
Each chromosome has a constriction point called a centromere, which divides the chromosome into two sections (or ‘arms’)
The shorter section is designated the p arm and the longer section is designated the q arm
What are homologous chromosomes?
Maternal and paternal chromosome pairs are called homologous chromosomes
Homologous chromosomes are chromosomes that share:
The same structural features (e.g. same size, same banding patterns, same centromere positions)
The same genes at the same loci positions (while the genes are the same, alleles may be different)
Why must Homologous chromosomes be separated in gametes prior to reproduction?
Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation
What is a diploid nucleus?
Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 2n)
These nuclei will possess two gene copies (alleles) for each trait
All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
Diploid cells are present in most animals and many plants
What is a haploid nucleus?
Nuclei possessing only one set of chromosomes are haploid (symbolised by n)
These nuclei will possess a single gene copy (allele) for each trait
How is sex determined in humans?
Sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes)
Females possess two copies of a large X chromosome (XX)
Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY)
With is a Autosome?
A chromosone which doesnt determin sex
What is a karyotype?
Karyotypes are the number and types of chromosomes in a eukaryotic cell
How do you make a karyogram?
1.Harvesting cells (usually from a foetus or white blood cells of adults)
2.Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
3. The chromosomes are stained and photographed to generate a visual profile and the chromosomes are arranged into homologous pairs according to size
What. is a karyogram
The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram
The chromosomes of an organism are arranged into homologous pairs according to size
What is down syndrome? What. causes it? And what is the effect?
Down syndrome is a condition whereby the individual has three copies of chromosome 21 (i.e. trisomy 21)
It is caused by a non-disjunction event in one of the parental gametes
The extra genetic material causes mental and physical delays in the way the child develops
One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
The other parental gamete was normal and had a single copy of chromosome 21
When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21
Why may karyotyping be used prenatally?
Karyotyping will typically occur prenatally and is used to:
Determine the gender of the unborn child (via identification of the sex chromosomes)
Test for chromosomal abnormalities (e.g. aneuploidies or translocations)
What is Autoradiography? and how is it preformed? (EXTRA)
Measuring the length of DNA molecule
1.Cells are grown in a solution containing radioactive thymidine (tritiated thymidine – 3H-T)
2.Thymidine is incorporated into the chromosomal DNA of the cell,
3.The chromosomes are isolated by gently lysing the cells and fixing the chromosomes to a photographic surface
4.The surface is then immersed in a radioactively-sensitive emulsion containing silver bromide (AgBr)
5.The radiation released from the tritiated thymidine converts the Ag+ ions in silver bromide into insoluble metal grains
6.Excess silver bromide is washed away, leaving the silver grains to appear as small black dots
7.When the photographic film is developed, the chromosomal DNA can be visualised with an electron microscope
Why do infertile offspring such as mules come to be?
Chromosome number is a characteristic feature of members of a particular species
Organisms with different diploid numbers are unlikely to be able to interbreed (cannot form homologous pairs in zygotes)
In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)
What is meoisis?
Meiosis is the process by which sex cells (gametes) are made in the reproductive organs
It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei
Process of Meoisis HL
Meiosis I
The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
Meiosis II
The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
The final outcome of meiosis is the production of four haploid daughter cells
What are sister chromatids and how do they form?
During Interphase DNA is replicated (in the S phase) to produce two genetically identical copies
The two identical DNA molecules are identified as sister chromatids, and are held together by a single centromere
The sister chromatids are separated during meiosis II, following the separation of homologous chromosomes in meiosis I
How does crossing over occur?
In prophase I, homologous chromosomes undergo a process called synapsis, whereby they pair up to form a bivalent (or tetrad)
The homologous chromosomes are held together at points called chiasmata (singular: chiasma)
Crossing over of genetic material between non-sister chromatids can occur at these chiasmata
As a result of this exchange of genetic material, new gene combinations are formed on chromatids (recombination)
Once chiasmata are formed, the homologous chromosomes condense as bivalents and then are separated in meiosis
If crossing over occurs then all four haploid daughter cells will be genetically distinct (sister chromatids are no longer identical)
How do different chromosone combinations form?
During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements:
Maternal copy left / paternal copy right OR paternal copy left / maternal copy right
The orientation of pairs of homologous chromosomes is random, as is the subsequent assortment of chromosomes into gametes
The final gametes will differ depending on whether they got the maternal or paternal copy of a chromosome following anaphase I
As this random assortment will occur for each homologous pair, the number of possible gamete combinations are dependent on the number of homologous pairs
Gamete combinations = 2n (where n represents the haploid number)
Why must gametes be haploid?
In order to reproduce, organisms need to make gametes that are haploid (one copy of each chromosome)
Fertilisation of two haploid gametes (egg + sperm) will result in the formation of a diploid zygote that can grow via mitosis
If chromosome number was not halved in gametes, total chromosome numbers would double each generation (polyploidy)
What are the sources of genetic variation?
The three main sources of genetic variation arising from sexual reproduction are:
Crossing over (in prophase I)
Random assortment of chromosomes (in metaphase I)
Random fusion of gametes from different parents
What is non-disjunction?
Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)
How my non-disjunction occur?
Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected)
Why do chances of having a baby with genetic abnormalities increase with age?
Studies show that the chances of non-disjunction increase as the age of the parents increase
The risk of chromosomal abnormalities in offspring increase significantly after a maternal age of 30
There is a higher incidence of chromosomal errors in offspring as a result of non-disjunction in meiosis I
What is Chorionic villi sampling?
Chorionic villi sampling involves removing a sample of the chorionic villus (placental tissue) via a tube inserted through the cervix
It can be done at ~11 weeks of pregnancy with a slight risk of inducing miscarriage (~1%)
What is Amniocentesis ?
Amniocentesis involves the extraction of a small amount of amniotic fluid (contains fetal cells) with a needle
It is usually conducted later than CVS (~16 weeks of pregnancy) with a slightly lower risk of miscarriage (~0.5%)
How did Gregor Mendel develope the principles of inheritance by performing experiments on pea plants
First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)
As a result of these experiments, Mendel discovered the following things:
When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants
When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1
E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1
What conclusions did Mendel draw?
-Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)
-Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles)
-Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)
-Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
-For each factor, one version is dominant over another and will be completely expressed if present
Law of Segregation
When gametes form, alleles are separated so that each gamete carries only one allele for each gene
Law of Independent Assortment:
The segregation of alleles for one gene occurs independently to that of any other gene, however, this does not hold true for genes located on the same chromosome
Principle of Dominance
Recessive alleles will be masked by dominant alleles, but some genes show co-dominance or incomplete dominance
How do gametes contain only one allele of a gene?
During meiosis I, homologous chromosomes are separated into different nuclei prior to cell division
As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs
Consequently, as gametes contain only one copy of each chromosome they therefore carry only one allele of each gene
homozygous gene
If the maternal and paternal alleles are the same
heterozygous gene
If the maternal and paternal alleles are different
hemizygous gene
Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene
What is genootype?
The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype
What is phenotype?
The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype
What determines phenotype?
The phenotype is determined by both the genotype and environmental influences
What is Co-dominance?
Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual
Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect
How is human blood catagorised?
Human red blood cells can be categorised into different blood groups based on the structure of a surface glycoprotein (antigen)
How do A, B and O allelws modify red blood cells?
The A and B alleles are co-dominant and each modify the structure of the antigen to produce different variants
The O allele is recessive and does not modify the basic antigenic structure
Why are blood transfusions sometimes not possible? In which cases is it possible? In which is it not?
As humans produce antibodies against foreign antigens, blood transfusions are not compatible between certain blood groups
AB blood groups can receive blood from any other type (as they already possess both antigenic variants on their cells)
A blood groups cannot receive B blood or AB blood (as the isoantigen produced by the B allele is foreign)
B blood groups cannot receive A blood or AB blood (as the isoantigen produced by the A allele is foreign)
O blood groups can only receive transfusions from other O blood donor (both antigenic variants are foreign)
What would happen if an incompatible blood transfusion were to happen?
Surface antigens would interact with antibodies resulting in agglutination (blood clumping) and then hemolysis (destruction of RBC)
When do autosomal recessive genetic disease occur?
An autosomal recessive genetic disease will only occur if both alleles are faulty
What is a carrier of a genetic disease?
An autosomal recessive genetic disease will only occur if both alleles are faulty
Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers
When do autosomal dominant genetic diseases occur?
An autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder
How does a co-dominant genetic diseases differ from dominant genetic diseases
If a genetic disease is caused by co-dominant alleles it will also only require one copy of the faulty allele to occur
However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele
Discuss Cystic fibrosis
-Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
-Individuals with cystic fibrosis produce mucus which is unusually thick and sticky
-This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
-Heterozygous carriers who possess one normal allele will not develop disease symptoms
Discuss Huntington’s disease
-Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
-The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)
-More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats)
-When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration
-This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years)
-Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia
Why are genetic diseases so rare? Which are more common?
Alleles that adversely affects survival and hence the capacity to reproduce is unlikely to be passed on to offspring
Recessive conditions tend to be more common, as the faulty allele can be present in carriers without causing disease
Dominant conditions may often have a late onset, as this does not prevent reproduction and the transfer of the faulty allele
What is sex linkage?
Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)
Why are most sex linked conditions related to the X chromosone?
The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)
The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp ; ~ 2,000 genes)
Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome
Why are x-linked recessive traits more common in males
Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits
X-linked recessive traits are more common in males, as the condition cannot be masked by a second all
Discuss Haemophilia
Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired
The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time
Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.g. haemophilia A = factor VIII)
Discuss Red-Green Colour Blindness
Red-Green Colour Blindness
Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues
This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome
Red-green colour blindness can be diagnosed using the Ishihara colour test
What is gene mutation?
A gene mutation is a change to the base sequence of a gene that can affect the structure and function of the protein it encodes
What causes gene mutation?
Mutations can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements
What are some mutagens?
Radiation – e.g. UV radiation from the sun, gamma radiation from radioisotopes, X-rays from medical equipment
Chemical – e.g. reactive oxygen species (found in pollutants), alkylating agents (found in cigarettes)
Biological Agents – e.g. bacteria (such as Helicobacter pylori), viruses (such as human papilloma virus)
What is a mutagene?
Agents which increase the rate of genetic mutations are called mutagens, and can lead to the formation of genetic disease
What is a carcinogen?
Mutagens which lead to the formation of cancer are more specifically referred to as carcinogens
How do you draw a pedigree chart?
A pedigree is a chart of the genetic history of a family over several generations
Males are represented as squares, while females are represented as circles
Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected
A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line
Generations are labeled with roman numerals and individuals are numbered according to age (oldest on the left)
What is PCR ?
The polymerase chain reaction (PCR) is an artificial method of replicating DNA under laboratory conditions
The PCR technique is used to amplify large quantities of a specific sequence of DNA from an initial minute sample
Each reaction cycle doubles the amount of DNA – a standard PCR sequence of 30 cycles creates over 1 billion copies (230)
Stages of PCR
PCR occurs in a thermal cycler and uses variations in temperature to control the replication process via three steps:
1.Denaturation – DNA sample is heated to separate it into two single strands (~95ºC for 1 min)
2.Annealing – DNA primers attach to the 3’ ends of the target sequence (~55ºC for 1 min)
3.Elongation – A heat-tolerant DNA polymerase (Taq) binds to the primer and copies the strand (~72ºC for 2 min)
Once large quantities of DNA have been created, other laboratory techniques are used to isolate and manipulate the sequences
DNA seperation by Gel electrophoresis
DNA Separation
-DNA may be cut into fragments using restriction endonuclease – different DNA samples will generate different fragment lengths
-Fragments separate because DNA is negatively charged due to the presence of a phosphate group (PO43–) on each nucleotide
-DNA samples are placed into an agarose gel and fragment size calculated by comparing against known industry standards
-Specific sequences can be identified by incorporating a complementary radiolabelled hybridisation probe, transferring the separated sequences to a membrane and then visualising via autoradiography (Southern blotting)
Protein Separation by Gel electrophoresis
Protein Separation
-Proteins may be folded into a variety of shapes (affecting size) and have positive and negative regions (no clear charge)
-Proteins must first be treated with an anionic detergent (SDS) in order to linearise and impart a uniform negative charge
-Protein samples are placed into a polyacrylamide gel and sizes compared against known industry standards
-Separated proteins are transferred to a membrane and then target proteins are identified by staining with specific monoclonal antibodies (Western blotting)
Describe Gel electrophoresis generally
Samples are placed in a block of gel and an electric current is applied which causes the samples to move through the gel
Smaller samples are less impeded by the gel matrix and hence will move faster through the gel
This causes samples of different sizes to separate as they travel at different speeds
How is DNA profiling done?
Within the non-coding regions of an individual’s genome there exists satellite DNA – long stretches of DNA made up of repeating elements called short tandem repeats (STRs)
As individuals will likely have different numbers of repeats at a given satellite DNA locus, they will generate unique DNA profiles.
Thus through a comparison we can DNA profile
How is DNA profiling done in Forensic investigation
Suspects should be a complete match with the DNA sample taken from the crime scene if a conviction is to occur
The number of loci used to generate a unique profile depends on the size of the population being compare
How is DNA profiling used for Paternity Testing
Children inherit half their chromosomes from each parent and thus should possess a combination of parental fragments
In other words, all fragments produced in the child should also be produced by either the mother or father
How do genes determine a specific trait?
A gene determines a particular trait by encoding for a specific polypeptide in a given organism
How is gene modification carried out by gene transfer between species?
Because the genetic code is (almost) universal, an organism can potentially express a new trait if the appropriate gene is introduced into its genome
The transfer of genes between species is called gene modification, and the new organism created is called a transgenic
Steps of gene transfer into bacteria
1.Isolation of gene and vector (by PCR)
2.Digestion of gene and vector (by restriction endonuclease)
3.Ligation of gene and vector (by DNA ligase)
4.Selection and expression of transgenic construct
Explain Step 1 of gene transfer: Isolating gene and vector
Step 1: Isolating gene and vector
DNA can be isolated from cells by centrifugation – whereby heavier components such as nuclei are separated
The gene of interest can then be specifically amplified via the polymerase chain reaction (PCR)
Gene sequences can also be generated from mRNA using reverse transcriptase – these DNA sequences (cDNA) lack introns
A vector is a DNA molecule that is used as a vehicle to carry the gene of interest into a foreign cell
Bacterial plasmids are commonly used as vectors because they are capable of autonomous self-replication and expression
These plasmids may be modified for further functionality (e.g. selection markers, reporter genes, inducible expression promoters)
Other types of vectors include modified viruses and artificial chromosomes
Explain Step 2 of gene transfer: Digestion with Restriction Enzymes
Step 2: Digestion with Restriction Enzymes
In order to incorporate a gene of interest into a vector, both must be cut with restriction enzymes at specific recognition sites
Restriction enzymes cleave the sugar-phosphate backbone to generate blunt ends or sticky ends (complementary overhangs)
Scientists will often cleave the vector and gene with two different ‘sticky end’ restriction endonucleases (double digestion) to ensure the gene is inserted in the correct orientation and to prevent the vector from re-annealing without the desired insert
Explain Step 3 of gene transfer: Ligation of Vector and Insert
Step 3: Ligation of Vector and Insert
The gene of interest is inserted into a plasmid vector that has been cut with the same restriction endonucleases
This occurs because the sticky ends of the gene and vector overlap via complementary base pairing
The gene and vector are then spliced together by the enzyme DNA ligase to form a recombinant construct
DNA ligase joins the vector and gene by fusing their sugar-phosphate backbones together with a covalent phosphodiester bond
Explain Step 4 of gene transfer: Selection and Expression
Step 4: Selection and Expression
The recombinant construct (including the gene of interest) is finally introduced into an appropriate host cell or organism
This process can be achieved in a variety of ways and is called transfection (for eukaryotes) or transformation (for prokaryotes)
Antibiotic selection is commonly used in order to identify which cells have successfully incorporated the recombinant construct
The plasmid vector contains an antibiotic resistance gene, so only transgenic cells will grow in the presence of antibiotic
Transgenic cells, once isolated and purified, will hopefully begin expressing the desired trait encoded by the gene of interest
GMOs pros
-grow in a wider range of environments (e.g. drought / frost / salinity resistance)
-produce greater yields
-genes which slow the rate of spoiling, leading to longer shelf lives for GM foods
-may possess resistance to certain viruses or produce toxins to pests (reducing need for the use of pesticides)
GMOs cons
-potentially reduce biodiversity in a region by competing with indigenous plant life
-proteins or toxins produced by GM crops could negatively affect certain organisms within the ecosystem
-formation of herbicide-resistant weeds and grasses
What are clones?
Clones are groups of genetically identical organisms or a group of cells derived from a single original parent cell
How are clones made?
-Stem cells can be artificially generated from adult tissue using a process called somatic cell nuclear transfer (SCNT)
-Organisms that reproduce asexually will produce genetically identical clones
Describe nuclear transfer
Somatic cell nuclear transfer is a method by which cloned embryos can be produced using differentiated adult cells
Somatic cells are removed from the adult donor and cultured (these cells are diploid and contain the entire genome)
An unfertilised egg is removed from a female adult and its haploid nucleus is removed to produce an enucleated egg cell
The enucleated egg cell is fused with the nucleus from the adult donor to make a diploid egg cell (with the donor’s DNA)
An electric current is then delivered to stimulate the egg to divide and develop into an embryo
The embryo is then implanted into the uterus of a surrogate and will develop into a genetic clone of the adult donor
Natural methods of cloning
-vegetative propagation
-Binary fission
-Budding
-Fragmentation
–Pathogenesis
Binary fission
The parent organism divides equally in two, so as to produce two genetically identical daughter organisms
Budding
Cells split off the parent organism, generating a smaller daughter organism which eventually separates from the parent
Fragmentation
New organisms grow from a separated fragment of the parent organism
Pathogenesis
Embryos are formed from unfertilised ova (via the production of a diploid egg cells by the female)
Vegetive propagation
Plants have the capacity for vegetative propagation, whereby small pieces can be induced to grow independently
This is because adult plants possess meristematic tissue capable of cellular differentiation (totipotent)
Human methods of cloning
Identical twins (monozygotic) are created when a fertilised egg (zygote) splits into two identical cells, each forming an embryo
Non-identical twins (dizygotic) are created when an unfertilised egg splits into two cells and each is fertilised by a different sperm
Identical twins will be clones of one another (genetically identical), while non-identical twins will share 50% of the same DNA
Animal cloning
At a very early stage, embryonic cells retain pluripotency (meaning they can divide and become any type of tissue)
These cells will differentiate to form all the different tissues comprising the organism
If these embryonic cells are separated artificially in the laboratory, each group of cells will form cloned organisms
This separation of embryonic cells can also occur naturally to give rise to identical (monozygotic) twins
The separated groups of cells are then implanted into the uterus of a surrogate to develop into genetically identical clones
Reproductive cloning
Reproductive cloning: If the embryo is implanted into the uterus of a surrogate, a new cloned organism will develop
Therapeutic cloning
Therapeutic cloning: Embryonic cells can be induced to differentiate to create specific tissues or organs for transplantation
somatic cell nuclear transfer (SCNT)
This involves replacing the haploid nucleus of an unfertilised egg with a diploid nucleus from an adult donor
The advantage of this technique is that it is known what traits the clones will develop (they are genetically identical to the donor)
What is bacterial conjugation and what dpes it allow?
Bacterial cells may exchange plasmids via their sex pili, in a process known as bacterial conjugation
This exchange of genetic material allows bacteria to evolve new features within a generation
Compare prokaryotic and eukaryotic DNA
Prokaryotic DNA:
Is found freely in the cytoplasm (within a region called the nucleoid)
Is naked (i.e. not bound with proteins and therefore doesn’t form chromatin)
Genomes are compact (contain little repetitive DNA and no introns)
Contains extra-chromosomal plasmids
Is circular in shape
Eukaryotic DNA:
Is contained within a nucleus
Is bound to histone proteins
Genomes contain large amounts of non-coding and repetitive DNA (including introns)
Do not contain plasmids (but organelles such as the mitochondria may contain their own chromosomes)
Are linear in shape
What are chiasmata?
Homologous chromosomes are held together at points called chiasmata (singular: chiasma)
Compare and contrast meiosis with Mitosis
Similarities:
They are both preceded by interphase (which includes DNA replication)
They both divide according to a common pathway (prophase → metaphase → anaphase → telophase)
They both split their cells via cytokinesis
Differences:
Division – Mitosis involves only one cell division, but meiosis requires two cell divisions
Independent assortment – Homologous pairs are randomly separated into separate cells in meiosis, but not mitosis
Synapsis – Homologous pairs form bivalents in meiosis, but not mitosis
Crossing over – Non-sister chromatids of homologous pairs may exchange genetic material in meiosis, but not mitosis
Outcome – Mitosis results in the formation of two daughter cells, while meiosis produces four daughter cells
Ploidy – Daughter cells produced by mitosis are diploid, while daughter cells produced by meiosis are haploid
Use – Mitosis is used to clone body cells, while meiosis is used to generate sex cells (gametes)
Genetics – Cells produced by mitosis are genetically identical (clones), while cells produced by meiosis are genetically distinct
Mnemonic: Disco Pug
How do we distinguish if genes are linked are unlinked?
It is possible to infer whether two genes are linked or unlinked by looking at the frequency distribution of potential phenotypes
Offspring with unlinked genes have an equal possibility of inheriting any potential phenotypic combination
This is due to the random segregation of alleles via independent assortment
Offspring with linked genes will only express the phenotypic combinations present in either parent unless crossing over occurs
Consequently, the ‘unlinked’ recombinant phenotypes occur less frequently than the ‘linked’ parental phenotypes
A CHI-SQUARED test is required for statistical sagnificance
Distinguish between mono and poly - genic traits
Monogenic traits (characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes
Polygenic traits (characteristics controlled by more than two gene loci) tend to exhibit continuous variation, with an individual’s phenotype existing somewhere along a continuous spectrum of potential phenotypes
In the case of polygenic inheritance:
Increasing the number of loci responsible for a particular trait increases the number of possible phenotypes
This results in a phenotypic distribution that follows a Gaussian (bell-shaped) normal distribution curve
What are the types of selection?
Stabalising
Directional
Disruptive
State and describe the different forms of reproductive isolation
Temporal Isolation
Temporal isolation occurs when two populations differ in their periods of activity or reproductive cycles
Example: Leopard frogs and wood frogs reach sexual maturity at different times in the spring and hence cannot interbreed
Behavioural Isolation
Behavioural isolation occurs when two populations exhibit different specific courtship patterns
Example: Certain populations of crickets may be morphologically identical but only respond to specific mating songs
Geographic Isolation
Geographic isolation occurs when two populations occupy different habitats or separate niches within a common region
Example: Lions and tigers occupy different habitats and do not interbreed (usually)
State and discuss the two forms of speciaction
Allopatric Speciation
Allopatric speciation occurs when a geographical barrier physically isolates populations of an ancestral species
The two populations begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection
Eventually the two populations reach a degree of genetic divergence whereby they can no longer interbreed (speciation)
Sympatric Speciation
Sympatric speciation is divergence of species within the same geographical location (i.e. without a physical barrier)
Sympatric speciation may result from the reproductive isolation of two populations as a result of genetic abnormalities
Typically, a chromosomal error may arise which prevents successful reproduction with any organism lacking the same error
Discuss polyploidism
Fertile polyploid offspring require two polyploid parents (unless allopolyploidy occurs)
This is because reproduction with the original parent population results in offspring with an uneven number of chromosome sets
Consequently, polyploidy is far more common in plant species as they may lack separate sexes (self polination) or can reproduce asexually (vegative propagation)
Polyploid crops may be particularly desirable to farmers for a number of reasons:
-Allows for the production of seedless fruits as the polyploid are infertile
-Polyploid crops will typically grow larger and demonstrate improved longevity and disease resistance
Consequently, farmers may induce polyploidy in certain plant species by treating plants with certain drugs
At what rates may speciation occur?
Phyletic Gradualism (Linear)
Punctuated Equilibrium (Steps)
