Topic 3: Genetics

Question 1

What is DNA?

Answer 1

DNA is the genetic blueprint which codes for, and determines, the characteristics of an organism

Question 2

Where is DNA packed and organized into?

Answer 2

Chromosomes

Question 3

What is a gene?

Answer 3

a sequence of DNA that encodes for a specific trait (traits may also be influenced by multiple genes)

Question 4

What is a locus?

Answer 4

The position of a gene on a particular chromosome

Question 5

What is an allele?

Answer 5

alternative forms of a gene that code for the different variations of a specific trait (ex: the gene that codes for eye colour has different shades/pigments)

As alleles are alternative forms of the one gene, they possess very similar gene sequences:
- Alleles only differ from each other by one or a few bases

Question 6

What is a gene mutation?

Answer 6

a change in the nucleotide sequence of a section of DNA coding for a specific trait

• new alleles are formed by mutation

Question 7

What can gene mutations be?

Answer 7

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

Detrimental: mutations truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait

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

Question 8

What is sickle cell anemia?

Answer 8

Is an example of a disorder caused by a gene mutation.

The disease arose from a base substitution mutation - where a single base was changed in the gene sequence

Question 9

Causes of sickle cell anemia?

Answer 9

results from a change to the 6th codon for the beta chain of haemoglobin

DNA: The DNA sequence changes from GAG to GTG on the non-transcribed strand (CTC to CAC on the template strand)
mRNA: The mRNA sequence changes from GAG to GUG at the 6th codon position
Polypeptide: The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine (Glu to Val)

Question 10

Consequences of sickle cell anemia?

Answer 10

Alters the structure of haemoglobin, causing it to form insoluble fibrous strands
- this insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to constantly feel tired

Changes red blood cell shape to a sickle shape
- the sickle may form clots within the capillaries, blocking blood supply to vital organs
- sickle cells are destroyed more rapidly than regular cells, leading to lower blood cell count (anaemia)

Question 11

What is a genome?

Answer 11

The totality of genetic information of a cell, organism, or organelle

This includes all genes as well as non-coding DNA sequences (e.g. introns, promoters, short tandem repeats, etc.)

Question 12

What does the human genome consist of?

Answer 12

46 chromosomes
~3 billion base pairs
~ 21,000 genes

Question 13

What was the human genome project?

Answer 13

aim was to sequence the human genome
- it showed that humans shared the majority of their sequence, with short nucleotide polymorphisms contributing diversity

The completion of the project led to:
- mapping: number, location, size and sequence of human genes is now established
- screening: allowed for the production of specific gene probes to detect sufferers and carriers of genetic diseases
- medicine: discovery of new proteins have lead to improved treatments
- ancestry: Comparisons with other genomes have provided insight into the origins, evolution and migratory patterns of man

Question 14

gene comparisons between different species:

Answer 14

bacteria: ~4200
chicken: ~17, 000
human: ~ 21,000
water flea: ~

Question 14

gene comparisons between different species:

Answer 14

bacteria: ~ 4200
chicken: ~17, 000
human: ~ 21,000
water flea: ~ 31, 000
rice: ~ 38,000

The number of genes present in an organism will differ between species and is not a valid indicator of biological complexity

Question 15

How are the number of genes in a genome usually predicted?

Answer 15

by identifying sequences common to genes
- expressed sequence tags (ESTs) or sequences that are homologous to known genes

• presence of pseudogenes and transposons make accurate counts of unique gene numbers very difficult

Question 16

Explain prokaryotic genetics?

Answer 16

• prokaryotes do not have a nucleus
  (instead genetic material is found free in the cytoplasm in a region called a nucleoid)
• genetic material of a prokaryote consists of a single chromosome consisting of a circular DNA molecule (genophore)

-DNA of prokaryotic cells is naked - not associated with proteins for additional packaging

-may also have additional circular DNA molecules called plasmids

Question 17

What is a plasmid?

Answer 17

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

• are present in some prokaryotic cells, not naturally present in eukaryotic cells

Question 18

What is bacterial conjunction?

Answer 18

bacterial cells may exchange plasmids via their sex pili

• this exchange of genetic material allows bacteria to evolve new features within a generation

(As plasmids can self-replicate and autonomously synthesise proteins, they are ideal vectors for gene manipulation in labs)

Question 19

Explain the organization of eukaryotic chromosomes?

Answer 19

1. DNA is complexed with 8 histone proteins to form a nucleosome
2. Nucleosomes are linked by an additional histone protein (H1) to form a string of chromatosomes
3. These then coil to form a solenoid structure (~6 chromatosomes per turn) which is condensed to form a 30nm fibre
4. These fibres then form loops, which are compressed and folded around a protein scaffold to form chromatin
5. Chromatin will then supercoil during cell division to form chromosomes that are visible under a microscope

Question 20

What are homologous chromosomes?

Answer 20

these chromosomes share: the same structural features , and the same genes at the same loci positions

Homologous pairs have a maternal and paternal copy

Question 21

What is a diploid nuclei?

Answer 21

nuclei processing pairs of homologous chromosomes (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 22

What is a haploid nuclei?

Answer 22

nuclei processing only one set of chromosomes

• These nuclei will possess a single gene copy (allele) for each trait
• All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
• Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)

Question 23

Sex Chromosomes

Answer 23

• 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)

The Y chromosome contains the genes for developing male sex characteristics (specifically the SRY gene)
- In its absence of a Y chromosome, female sex organs will develop
- The sex chromosomes are homologous in females (XX) but are not homologous in males (XY)

Hence the father is always responsible for determining the sex of offspring

Question 24

What is a karyotype and karyogram

Answer 24

the number and types of chromosomes in a eukaryotic cell – they are determined via a process that involves:

• Harvesting cells (usually from a foetus or white blood cells of adults)
• Chemically inducing cell division, then arresting mitosis while the chromosomes are condensed
• The stage during which mitosis is halted will determine whether chromosomes appear with sister chromatids or not

The chromosomes are stained and photographed to generate a visual profile that is known as a karyogram

Question 25

What is Autoradiography?

Answer 25

a technique for measuring the length of DNA molecules

Concluded:
- DNA replication involves formation of a replication bubble (and prokaryotic replication involves a single origin of replication)
- DNA replication is bi-directional

Question 26

Can organisms with different diploid numbers interbreed?

Answer 26

no/unlikely because they cannot form homologous pairs in zygotes

In cases where different species do interbreed, offspring are usually infertile (cannot form functional gametes)

For instance, a horse (diploid = 64) and a donkey (diploid = 62) may produce an infertile mule (non-diploid = 63)

Question 27

Genome Size

Answer 27

• Viruses and bacteria tend to have very small genomes
• Prokaryotes typically have smaller genomes than eukaryotes
• Sizes of plant genomes can vary dramatically due to the capacity for plant species to self-fertilise and become polyploid

-The largest known genome is possessed by the canopy plant Paris japonica – 150 billion base pairs
-The smallest known genome is possessed by the bacterium Carsonella ruddi – 160,000 base pairs

Question 28

What is meiosis?

Answer 28

The process in which sex cells are made in the reproductive organs
- involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei

The process of meiosis consists of two cellular divisions:
1. The first meiotic division separates pairs of homologous chromosomes to halve the chromosome number (diploid → haploid)
2. The second meiotic division separates sister chromatids (created by the replication of DNA during interphase)

Question 29

What happens during meiosis I?

Answer 29

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

Question 30

What happens during meiosis II?

Answer 30

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

Question 31

what is the final outcome of meiosis?

Answer 31

four haploid daughter cells

These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)

Question 32

explain the sexual lifecycle?

Answer 32

Most sexually reproducing organisms are diploid, meaning they have two copies of every chromosome (maternal / paternal copy)

In order to reproduce, these 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 33

3 main sources of genetic variation?

Answer 33

• Crossing over (in prophase I)
• Random assortment of chromosomes (in metaphase I)
• Random fusion of gametes from different parents

Question 34

What is non disjunction?

Answer 34

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

Question 35

explain how Down syndrome occurs?

Answer 35

Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)

-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

The risk of chromosomal abnormalities in offspring increase significantly after a maternal age of 30

Question 36

What did Mendel discover?

Answer 36

-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

-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

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)

Question 37

What rules can be established from Mendels conclusions?

Answer 37

1. Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
2. Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene*
3. Principle of Dominance: Recessive alleles will be masked by dominant alleles†

Question 38

What are gametes?

Answer 38

haploid sex cells formed by the process of meiosis – males produce sperm and females produce ova

● Gametes are haploid so contain one allele of each gene.
● Male gamete is smaller than the female one.
● It is motile, whilst the female is immobile/restricted in its movement.
● Male and females make an equal contribution because each gamete is
haploid and come together to form a diploid cell, zygote.

Question 39

Combination of genes/types of zygosity:

Answer 39

If the maternal and paternal alleles are the same, the offspring is said to be homozygous for that gene

If the maternal and paternal alleles are different, the offspring is said to be heterozygous for that gene

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

Question 40

Explain the 3 modes of inheritance

Answer 40

dominant (Rr - R is dominant)
codominant
recessive

Dominant alleles mask the effects of recessive alleles but co-dominant
alleles have joint effects.
● Dominant alleles would be expressed in the phenotype.
● Co-dominant alleles: Cw – White flowers. Ck – Red flowers. CwCk – Pink
flowers.
● Dominant alleles code for an active protein whereas recessive alleles code
for non-functional proteins.

Question 41

genotype vs phenotype

Answer 41

The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype
-The genotype of a particular gene will typically be either homozygous or heterozygous

The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype
-The phenotype is determined by both the genotype and environmental influences

Question 42

What is cystic fibrosis

Answer 42

caused by a recessive allele

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 43

What is Huntington’s disease?

Answer 43

caused by chromosomes, besides the sex chromosomes, are called autosomal diseases.

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 44

What causes genetic diseases?

Answer 44

Genetic diseases are caused when mutations to a gene (or genes) abrogate normal cellular function, leading to the development of a disease phenotype

Genetic diseases can be caused by recessive, dominant or co-dominant alleles

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

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

If a genetic disease is caused by co-dominant alleles it will also only require one copy of the faulty allele to occur

Question 45

Examples of other genetic diseases:

Answer 45

● Red-green colorblindness and haemophilia are examples of sex-linked diseases.
● Huntington’s disease is caused by a dominant allele, and is autosomal.
● Sickle cell anaemia is caused by recessive allele HbS
● Genetic diseases are very rare to inherit and only 75 to 200 alleles among 25,000 genes in the human genome code for any known genetic diseases.

Question 46

Factors which can cause gene mutations:

Answer 46

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 47

Explain Red and Green Colour Blindness

Answer 47

● Genes are located on the chromosome.
● Caused by a recessive allele for a photoreceptor protein-coding gene.
● Cone cells in retina are coded for by these.
● Detects specific wavelength ranges of visible light.
● If males inherit a chromosome carrying this recessive allele, they will be
colour-blind.
● Females are less likely to inherit colour blindness unless both their parents are either carriers or affected.

Question 48

Pedigree Chart

Answer 48

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 49

What is PCR and what are the three steps/stages?

Answer 49

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

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)

Question 50

What is gel electrophoresis?

Answer 50

Gel electrophoresis is a laboratory technique used to separate and isolate proteins or DNA fragments based on mass / size:

● Separates charged molecules in an electric field according to size and charge.
● Gel is immersed in conducting fluid and an electric field is applied.
● Negatively and positively charged molecules will move in opposite directions. Proteins can be separated according to charge.
● DNA are all negatively charged but are too long to move through the gel so are broken up into smaller fragments by restriction endonucleases.
● Smaller fragments will move further than larger ones in a set amount of time, as they move faster.
● Hence electrophoresis can distinguish molecules by size.

Question 51

What is DNA profiling?

Answer 51

a technique by which individuals can be identified and compared via their respective DNA profiles (paternal testing, forensic investigation)

DNA is obtained from a known individual.
● Highly repetitive sequences sequences are selected and copied by PCR.
● Copied DNA is then split by restriction endonucleases.
● Fragments then undergo gel electrophoresis.
● Banding pattern produced is the individual’s profile.
● Profiles of different individuals can be compared.

Question 52

What is the process of gene transfer?

Answer 52

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

● Endonucleases cut a section of a plasmid, and cut the desired genes from a larger DNA molecule.
● The cutting process, leave single stranded sticky ends on both the plasmid
and on the genes from the DNA.
● Hydrogen bonds can form between the bases of the DNA genes and the
plasmid.
● DNA ligase seals the nicks in the sugar phosphate backbone after the
hydrogen bonds between bases have been formed.

Question 53

Risks of GMO (genetically modified) plants:

Answer 53

● Environmental benefit of GM crops:
o Use of GM crop varieties reduces the need for ploughing and
spraying crops, so less fuel is needed for farm machinery.

● Health benefits of GM crops:
o Nutritional value of crops can be enhanced by increasing the
vitamin content/ removing allergens that may be present in the
crop naturally.

● Agricultural benefits of GM crops:
o Varieties resistant to drought, cold and salinity can be produced,
expanding the range over which crops can be produced and
increasing yields.
● Environmental risks of GM crops:
o Non-target organisms could be affected by toxins that are intended
to control pests.
● Health risks of GM crops:
o Antibiotic resistance genes used as markers during gene transfer
could spread to pathogenic bacteria.

● Agricultural risks of GM crops:
o Pests may become resistant to toxins used in GM crops,
exacerbating the issue.

Question 54

Risks to monarch butterflies of BT corn

Answer 54

Gene transferred to corn from Bacillus thuringiensis that codes for Bt toxin:
● This kills butterflies, moths, flies, beetles and ants.
● Larvae of the Monarch butterfly feed on milkweed that often grows close
to corn,
● This milkweed could be dusted with the toxic pollen of Bt corn and the
larvae might be poisoned.

Question 55

What is cloning?

Answer 55

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

-Organisms that reproduce asexually will produce genetically identical clones
-Additionally, mechanisms exist whereby sexually reproducing organisms can produce clones (e.g. identical twins)

Question 56

How to clone adult embryos?

Answer 56

Process of splitting an embryo is called fragmentation.
● It can be done because embryos are pluripotent.
o Totipotent – Can form any cell type, as well as extra-embryonic
(placental) tissue (e.g. zygote)
o Pluripotent – Can form any cell type (e.g. embryonic stem cells)
o Multipotent – Can differentiate into a number of closely related
cell types (e.g. hematopoietic adult stem cells)
o Unipotent – Can not differentiate, but are capable of self renewal
(e.g. progenitor cells, muscle stem cells)
● This has been observed to occur in coral embryos.
● Each individual embryo would then be placed in a surrogate mother.
● This is most successful at the eight-cell stage of the blastocyst.

Question 57

Cloning animals using somatic cell transfer?

Answer 57

o Adult cells are taken from the udder of a sheep.
o In order to make the genes inactive and lose differentiation patterns, the cells are grown in a medium of low nutrients.
o Unfertilised eggs are taken from another sheep and the nuclei are removed.
o Cultured cells from the first sheep are placed inside the zona pellucida around the egg and using an electric pulse they are fused together.
o With a 10% success rate, the new embryo is then injected into a surrogate mother using IVF.

Question 58

Animal cloning methods:

Answer 58

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

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