Genetics SL

Question 1

Gene

Answer 1

A sequence of DNA that encodes for a specific trait

Question 2

Locus

Answer 2

The position of a gene on a particular chromosome

Question 3

Allele

Answer 3

Alleles are alternative forms of a gene that code for the different variations of a specific trait

Question 4

Gene mutation

Answer 4

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait

Question 5

Neutral mutations

Answer 5

Neutral mutations have no effect on the functioning of the specific feature (silent mutations)

Question 6

Detrimental mutations

Answer 6

truncate the gene sequence (nonsense mutations) to block the normal function of a trait

Question 7

Beneficial mutations

Answer 7

Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait

Question 8

Cause of Sickle Cell Anaemia

Answer 8

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)

Question 9

Consequence of Sickle Cell Anaemia

Answer 9

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)

Question 10

What is a genome?

Answer 10

The genome is the totality of genetic information of a cell, organism or organelle

Question 11

What is The Human Genome Project (HGP) and what were the 4 outcomes?

Answer 11

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

Question 12

Where is genetic material in prokaryotes

Answer 12

Genetic material is found free in the cytoplasm in a region called the nucleoid

Question 13

What is the genetic material in prokaryotes?

Answer 13

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)

Question 14

What are plasmids?

Answer 14

Plasmids are small, circular DNA molecules that contain only a few genes and are capable of self-replication

Question 15

Organisation of eukaryotic chromosomes

Answer 15

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

Question 16

What is a chromosone and describe its structure

Answer 16

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

Question 17

What are homologous chromosomes?

Answer 17

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)

Question 18

Why must Homologous chromosomes be separated in gametes prior to reproduction?

Answer 18

Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation

Question 19

What is a diploid nucleus?

Answer 19

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

Question 20

What is a haploid nucleus?

Answer 20

Nuclei possessing only one set of chromosomes are haploid (symbolised by n)

These nuclei will possess a single gene copy (allele) for each trait

Question 21

How is sex determined in humans?

Answer 21

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)

Question 22

With is a Autosome?

Answer 22

A chromosone which doesnt determin sex

Question 23

What is a karyotype?

Answer 23

Karyotypes are the number and types of chromosomes in a eukaryotic cell

Question 24

How do you make a karyogram?

Answer 24

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

Question 25

What. is a karyogram

Answer 25

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

Question 26

What is down syndrome? What. causes it? And what is the effect?

Answer 26

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

Question 27

Why may karyotyping be used prenatally?

Answer 27

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)

Question 28

What is Autoradiography? and how is it preformed? (EXTRA)

Answer 28

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

Question 29

Why do infertile offspring such as mules come to be?

Answer 29

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)

Question 30

What is meoisis?

Answer 30

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

Question 31

Process of Meoisis HL

Answer 31

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

Question 32

What are sister chromatids and how do they form?

Answer 32

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

Question 33

How does crossing over occur?

Answer 33

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)

Question 34

How do different chromosone combinations form?

Answer 34

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)

Question 35

Why must gametes be haploid?

Answer 35

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)

Question 36

What are the sources of genetic variation?

Answer 36

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

Question 37

What is non-disjunction?

Answer 37

Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)

Question 38

How my non-disjunction occur?

Answer 38

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)

Question 39

Why do chances of having a baby with genetic abnormalities increase with age?

Answer 39

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

Question 40

What is Chorionic villi sampling?

Answer 40

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%)

Question 41

What is Amniocentesis ?

Answer 41

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%)

Question 42

How did Gregor Mendel develope the principles of inheritance by performing experiments on pea plants

Answer 42

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

Question 43

What conclusions did Mendel draw?

Answer 43

-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

Question 44

Law of Segregation

Answer 44

When gametes form, alleles are separated so that each gamete carries only one allele for each gene

Question 45

Law of Independent Assortment:

Answer 45

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

Question 46

Principle of Dominance

Answer 46

Recessive alleles will be masked by dominant alleles, but some genes show co-dominance or incomplete dominance

Question 47

How do gametes contain only one allele of a gene?

Answer 47

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

Question 48

homozygous gene

Answer 48

If the maternal and paternal alleles are the same

Question 49

heterozygous gene

Answer 49

If the maternal and paternal alleles are different

Question 50

hemizygous gene

Answer 50

Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene

Question 51

What is genootype?

Answer 51

The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype

Question 52

What is phenotype?

Answer 52

The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype

Question 53

What determines phenotype?

Answer 53

The phenotype is determined by both the genotype and environmental influences

Question 53

What is Co-dominance?

Answer 53

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

Question 54

How is human blood catagorised?

Answer 54

Human red blood cells can be categorised into different blood groups based on the structure of a surface glycoprotein (antigen)

Question 55

How do A, B and O allelws modify red blood cells?

Answer 55

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

Question 56

Why are blood transfusions sometimes not possible? In which cases is it possible? In which is it not?

Answer 56

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)

Question 57

What would happen if an incompatible blood transfusion were to happen?

Answer 57

Surface antigens would interact with antibodies resulting in agglutination (blood clumping) and then hemolysis (destruction of RBC)

Question 58

When do autosomal recessive genetic disease occur?

Answer 58

An autosomal recessive genetic disease will only occur if both alleles are faulty

Question 59

What is a carrier of a genetic disease?

Answer 59

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

Question 60

When do autosomal dominant genetic diseases occur?

Answer 60

An autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder

Question 61

How does a co-dominant genetic diseases differ from dominant genetic diseases

Answer 61

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

Question 62

Discuss Cystic fibrosis

Answer 62

-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

Question 63

Discuss Huntington’s disease

Answer 63

-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

Question 64

Why are genetic diseases so rare? Which are more common?

Answer 64

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

Question 65

What is sex linkage?

Answer 65

Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)

Question 66

Why are most sex linked conditions related to the X chromosone?

Answer 66

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

Question 67

Why are x-linked recessive traits more common in males

Answer 67

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

Question 68

Discuss Haemophilia

Answer 68

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)

Question 69

Discuss Red-Green Colour Blindness

Answer 69

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

Question 70

What is gene mutation?

Answer 70

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

Question 71

What causes gene mutation?

Answer 71

Mutations can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements

Question 72

What are some mutagens?

Answer 72

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)

Question 73

What is a mutagene?

Answer 73

Agents which increase the rate of genetic mutations are called mutagens, and can lead to the formation of genetic disease

Question 74

What is a carcinogen?

Answer 74

Mutagens which lead to the formation of cancer are more specifically referred to as carcinogens

Question 75

How do you draw a pedigree chart?

Answer 75

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)

Question 76

What is PCR ?

Answer 76

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)

Question 77

Stages of PCR

Answer 77

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

Question 78

DNA seperation by Gel electrophoresis

Answer 78

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)

Question 79

Protein Separation by Gel electrophoresis

Answer 79

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)

Question 80

Describe Gel electrophoresis generally

Answer 80

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

Question 81

How is DNA profiling done?

Answer 81

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

Question 82

How is DNA profiling done in Forensic investigation

Answer 82

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

Question 83

How is DNA profiling used for Paternity Testing

Answer 83

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

Question 84

How do genes determine a specific trait?

Answer 84

A gene determines a particular trait by encoding for a specific polypeptide in a given organism

Question 85

How is gene modification carried out by gene transfer between species?

Answer 85

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

Question 86

Steps of gene transfer into bacteria

Answer 86

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

Question 87

Explain Step 1 of gene transfer: Isolating gene and vector

Answer 87

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

Question 88

Explain Step 2 of gene transfer: Digestion with Restriction Enzymes

Answer 88

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

Question 89

Explain Step 3 of gene transfer: Ligation of Vector and Insert

Answer 89

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

Question 90

Explain Step 4 of gene transfer: Selection and Expression

Answer 90

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

Question 91

GMOs pros

Answer 91

-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)

Question 92

GMOs cons

Answer 92

-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

Question 93

What are clones?

Answer 93

Clones are groups of genetically identical organisms or a group of cells derived from a single original parent cell

Question 94

How are clones made?

Answer 94

-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

Question 95

Describe nuclear transfer

Answer 95

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

Question 96

Natural methods of cloning

Answer 96

-vegetative propagation
-Binary fission
-Budding
-Fragmentation
–Pathogenesis

Question 97

Binary fission

Answer 97

The parent organism divides equally in two, so as to produce two genetically identical daughter organisms

Question 98

Budding

Answer 98

Cells split off the parent organism, generating a smaller daughter organism which eventually separates from the parent

Question 99

Fragmentation

Answer 99

New organisms grow from a separated fragment of the parent organism

Question 100

Pathogenesis

Answer 100

Embryos are formed from unfertilised ova (via the production of a diploid egg cells by the female)

Question 101

Vegetive propagation

Answer 101

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)

Question 102

Human methods of cloning

Answer 102

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

Question 103

Animal cloning

Answer 103

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

Question 104

Reproductive cloning

Answer 104

Reproductive cloning: If the embryo is implanted into the uterus of a surrogate, a new cloned organism will develop

Question 105

Therapeutic cloning

Answer 105

Therapeutic cloning: Embryonic cells can be induced to differentiate to create specific tissues or organs for transplantation

Question 106

somatic cell nuclear transfer (SCNT)

Answer 106

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)

Question 107

What is bacterial conjugation and what dpes it allow?

Answer 107

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

Question 108

Compare prokaryotic and eukaryotic DNA

Answer 108

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

Question 109

What are chiasmata?

Answer 109

Homologous chromosomes are held together at points called chiasmata (singular: chiasma)

Question 110

Compare and contrast meiosis with Mitosis

Answer 110

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

Question 111

How do we distinguish if genes are linked are unlinked?

Answer 111

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

Question 112

Distinguish between mono and poly - genic traits

Answer 112

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

Question 113

What are the types of selection?

Answer 113

Stabalising
Directional
Disruptive

Question 114

State and describe the different forms of reproductive isolation

Answer 114

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)

Question 115

State and discuss the two forms of speciaction

Answer 115

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

Question 116

Discuss polyploidism

Answer 116

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

Question 117

At what rates may speciation occur?

Answer 117

Phyletic Gradualism (Linear)
Punctuated Equilibrium (Steps)