Topic 3: Genetics

Question 1

gene

Answer 1

a heritable factor that consists of a length of DNA and influences a specific characteristic.

Question 2

chromosones and genes of humans

Answer 2

-23 pairs of chromosones and 23,000 genes

Question 3

where are genes located

Answer 3

-occupies a specific position on one type of chromosome
-this position is called a locus.

Question 4

alleles

Answer 4

-specific form of gene
-gregor mendel explained inheritance in pea plants in terms of different heritable factors.

Question 5

mutation

Answer 5

-new alleles are formed by mutation
- are random changes to the sequence of bases in genes.
-most signficant type is base substitution

Question 6

base substitiuon mutation

Answer 6

-where one base in a sequence is replaced with another.
-can only change one codon in the mRNA script.

Question 7

somatic mutations

Answer 7

-occurs in nongermline tissues
-cannot be inheritated
-mutation in tumor only

Question 8

germline mutation

Answer 8

-present in egg or sperm
- can be inheritated -all cells affected in offspring

Question 9

Silent mutation

Answer 9

one base is substituted for another but the affected codon has the same amino acid

Question 10

missense mutation

Answer 10

one base substituted for another the affected codon now codes for a different amino acid

Question 11

nonsense mutation

Answer 11

one base substituted for another, the affected codon is now a stop codon

Question 12

sickle cell anaemia

Answer 12

-caused by a mutation of the gene that codes for the alpha-globin polypeptide in hemoglobin.
-The hemoglobin molecules that contain the incorrect alpha-globin stick together when oxygen concentration is low, forming rigid bundles that can distort the shape of the red blood cell.
-Distorted red blood cells cause damage to tissues and may only last as little a 4 days (c.f. 120 days). The body cannot replace RBCs fast enough, so anemia develops.

Question 13

difference between sickle cell anaemia and normal cells

Answer 13

The symbol for the alpha-globin gene is Hb.
Most humans have the normal allele, HbA. When this allele is transcribed, the sixth codon is GAG, which codes for glutamic acid.
In the HbS mutation, CTC becomes CAC in the DNA sequence. The sixth codon becomes GUG, which codes for valine.
The presence of valine instead of glutamic acid is what causes the hemoglobin molecules to stick together.

Question 14

inheritated sickle cell

Answer 14

This mutation is only inherited if it occurs (or already exists) in a germ line cell of the ovary or testis.

Question 15

genome

Answer 15

genome, is the whole of the genetic information of an organism.
human contain 46 chromosomes, plus their mitochondrial DNA.
plants. chromosomes and mitochondrial DNA and chloroplast DNA.
- prokaryotes= circular chromosomes and plasmids.

Question 16

the human genome project

Answer 16

-the aim of finding the base sequence of the entire human genome.

Question 17

Findings of the human genome project

Answer 17

-the current estimate of 23,000
-most are not transcribed, only 1% of the genome codes for proteins.
-remaining 99%, contain sequences that affect gene expression.
-comparisons between genomes of other organism reveal details of evolutionary history, and some evolutionary relationships have been revised on this comparison.

Question 18

sanger institute

Answer 18

-in 1999, first draft was completed in june 2000
- in 2014, one human genome can be sequenced in 90 mintues.

Question 19

techniques used for genome sequences

Answer 19

-a sequence of a genome, is broken into short lengths and each of these lengths is sequenced separately.
single stranded copies of DNA are made using DNA polymerase, a small proportion of non-standard nucleotides are included to bring copying to a premature end. gel electrophoresis, is used to discover length of bases.

Question 20

sanger sequences

Answer 20

-involves four separate lanes, one for each base.
gel electrophoresis used to sperate DNA fragments.

Question 21

bacterial chromosomes

Answer 21

-singular circular DNA molecule includes all the genes needed for all the basic life processes of the cell.
-only one chromosome, meaning only one copy of each gene.
Not associated with proteins, so described as ‘naked DNA”

Question 22

Replication of bacterial cells

Answer 22

-replicate the DNA of the chromosomes so that two identical copies are present within the cell, moved to opposite poles and the cell splits in two. process is called binary fission

Question 23

Plasmids

Answer 23

-found within some prokaryotes and very rare in eukaryotes.
-small extra DNA molecules
-Replicate independently of the chromosomes, multiple copies can be present.
-transferred from on cell to another and sometimes even between cells of a different species.
-include genes that give survival advantage
-antibiotic resistance genes are often found on plasmids., this resistance can be transferred through cells, this would not be possible if the genes were on the chromosone.

Question 24

draw a plasmid

Answer 24

ON NOTION

Question 25

Eukaryote chromosomes

Answer 25

-linear DNA molecules associated with histone proteins
-composed of two molecules:
DNA – an immensely long double-stranded DNA molecule
Protein – histone proteins that are globular in shape and have DNA coiled around them

during interphase, genes that are required are uncoiled.
-the process of anaphase, is supercoiling in which the chromosones become shorter and fatter

Question 26

difference between chromosomes

Answer 26

-variations in length, banding and pattern and the position of the centromere
-every gene occupies a specific locus on one type of chromosone, therefore a specific sequence of genes arranged in a set order along the linear DNA molecule.

Question 27

Homologous chromosomes

Answer 27

-carry the same sequence of genes but not necessarily the same alleles of those genes.
-in organisms that sexually reproduce, homologous chromosomes cannot be identical.

Question 28

Haploid nuclei

Answer 28

-have one chromosome of each pair

Question 29

diploid nuclei

Answer 29

-have pairs of homologous chromosomes.
-Many animals and plants consist entirely of diploid cells, with the exception of the gametes they produce for sexual reproduction.

Question 30

Chromosome numbers

Answer 30

The number of chromosomes is a characteristic feature of members of a species.
Organisms with a different number of chromosomes are unlikely to be able to interbreed.

Question 31

Sex determination

Answer 31

The X chromosome, which is relatively large and has a centromere near the middle
The Y chromosome, which is much smaller and has a centromere near the end
-all other chromosomes are called autosomes and do not affect sex

Question 32

Y chromosomes

Answer 32

-Unique genes to the Y:
One gene, the SRY gene, codes for a protein called ‘testis determining factor’, or TDF. The TDF protein initiates the development of male features, including testes and testosterone production.
A fetus with XX has no SRY gene, so does not produce TDF protein, so does not develop testes. Ovaries are formed instead, along with ‘female’ sex hormones.

Question 33

Karyotype

Answer 33

Karyotype: A property of a cell – the number and type of chromosomes present in the nucleus.

Question 34

kayrogram

Answer 34

a photgraph or diagram of the chromosomes present in the nucleus of a cell

Question 35

How they first measured the length of DNA within E’coli

Answer 35

1.E. coli cells were grown for two generations in a medium containing “tritiated thymidine” – tritium is a radioactive isotope of hydrogen. The DNA produced by replication in E. coli cells will be radioactively labelled. Thymine was used so that only DNA would be labelled.
2.Cells were placed on a dialysis membrane and lysed with lysozyme enzymes.
3.The surface of the dialysis membrane was coated in photographic emulsion and left for two months.
4.High-energy electrons released by decay of tritium atoms left dark spots on the film.
5.After two months, the film was developed and examined under a microscope. The dark spots revealed the position of the DNA.

Question 36

Meiosis

Answer 36

One diploid nucleus divides by meiosis to produce four haploid nuclei.
In the first division, meiosis I, a single diploid cell produces two nuclei that are haploid. These haploid nuclei have only one of each type of chromosome, not two.
In the second division, meiosis II, each haploid cell produces two more haploid cells.
The number of chromosomes does not decrease again in meiosis II, because when sister chromatids are separated in anaphase II, we call them chromosomes!

Question 37

Replication before meiosis

Answer 37

DNA is replicated before meiosis so that all the chromosomes consist of two sister chromatids.
DNA is always replicated in the S phase of the cell cycle, many hours before mitosis or meiosis begins.
During the early stages of meiosis, the chromosomes shorten due to supercoiling.
Now that the chromosomes are visible, it becomes apparent that each consists of two identical DNA molecules known as ‘sister chromatids’.

Question 38

Bivalents formation and crossing over

Answer 38

The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
A pair of homologous chromosomes is called a bivalent (or a tetrad). Each bivalent has four DNA molecules.
The pairing of homologous chromosome is unique to meiosis (it does not occur in mitosis).
Condensation is the compacting of a DNA molecule by coiling around histone proteins then coiling the resulting fibre again and again.

Question 39

Orientation of bivalents

Answer 39

-random and is not influecned by other bivalents.
- 2^n
For humans 223 = 8 388 608 possible unique combinations!

Question 40

Halving the chromosome numbers in meiosis

Answer 40

The chiasmata holding the chromosomes of a bivalent together release, allowing the two homologous chromosomes to move to opposite poles. This separation is called disjunction.
Only one of each type of chromosome arrives at a given pole, therefore, each resultant daughter cell has half the original number of chromosomes (is haploid).

Question 41

Meiosis I - Stages

Answer 41

Prophase I
Chromosomes coil up tightly (supercoil) and become visible under a light microscope
Homologous chromosomes pair up and crossing over occurs (the point of cross over is known as the chiasma, plural is chiasmata)
Nuclear membrane disintegrates and the centrioles travel to the poles of the cell

Metaphase I
Microtubules form a spindle and the spindle fibres attach to the centromeres of the chromosomes
Pairs of homologous chromosomes align along the equator

Anaphase I
Spindle fibres shorten pulling paired homologous chromosomes in opposite directions
Paired homologous chromosomes are separated and pulled to opposite poles so that each pole contains one chromosome of each pair.

Telophase I
A nuclear membrane forms around the chromosomes at each pole and chromosomes uncoil
The cell undergoes cytokinesis to form two daughter cells
Forms two haploid cells
At the end of telophase I the cells may enter a short interphase period or proceed directly to meiosis II. DNA is not replicated before meiosis II.

Question 42

Meiosis II - Stages

Answer 42

Prophase II
Chromosomes coil up again
Centrioles move to the cell poles
Nuclear membrane disintegrates

Metaphase II
Spindle fibres attach to the centromeres
Chromosomes align along the equator

Anaphase II
Spindle fibres shorten
Centromeres split
Chromatids of each chromosome travel to opposite poles

Telophase II
Nuclear membrane forms around the chromatids at each pole, once the membrane is formed, each chromatid is then called a chromosome.
Both cells undergo cytokinesis to form four cells. Chromosomes uncoil, and nucleoli form

Question 43

Obtaining cells from a fetus

Answer 43

Methods used to obtain cells for karyotype analysis e.g. chorionic villus sampling and amniocentesis and the associated risks.
Amniocentesis and CVS are invasive sampling procedures, used to obtain cells from a fetus in utero

Question 44

Non-disjunction and down syndrome

Answer 44

Down syndrome is caused by the presence of an extra chromosome 21 (trisomy 21).
Klinefelter’s syndrome is caused by the presence of an extra X chromosome (∴ XXY), while Turner’s syndrome is caused by the absence of a second X or a Y chromosome (∴ XO).
Studies showing age of parents influences chances of non-disjunction.
Trisomy 21 is the most common chromosomal abnormality.

Question 45

Mendel and the principle of inheritance

Answer 45

-experiments with large number of peas when crossed.
-living organisms pass on characteristics to heir offspring

Question 46

Gametes

Answer 46

-contain one allele of each gene,
-in humans, sperm cells are small and motile, white eff cells are much larger and cannot move by themselves.

Question 47

Zygotes

Answer 47

fusion of gametes result in diploid zygotes with two alleles of each gene.

Question 48

Possible allele combinations

Answer 48

. The gene for Rhesus factor in humans has two alleles, Rhesus positive Rh+ and Rhesus negative Rh–.
There are three possible allele combinations:
Rh+ Rh+ / Rh+ Rh– / Rh– Rh–

Some genes have more than two alleles. The human gene for ABO blood groups has three alleles, IA, IB and i.
There are six possible allele combinations:
Three homozygous genotypes: IA IA / IB IB / i i
Three heterozygous genotypes: IA IB / IA i / IB i

Question 49

Co-dominance

Answer 49

Co-dominance is different to incomplete dominance, and can be distinguished via the phenotype of heterozygous individuals.
In co-dominance, both phenotypes are expressed as they would be in a homozygous individual. The ABO blood group gene works this way.

Question 50

ABO blood groups

Answer 50

Inheritance of ABO blood groups.
One gene determines the blood group of a person, and this gene has three alleles – IA, IB and i. Neither IA nor IB is dominant over the other, so a person with the genotype IA IB has blood group AB.
The i allele is recessive to both IA and IB, so blood group O is only possible if a person has the genotype i i.

Question 51

ABO blood groups (glycoprotein)

Answer 51

All three alleles code for the production of a glycoprotein in the membrane of red blood cells.
The glycoprotein is also referred to as a blood group antigen. Antigens are molecules that can trigger the production of antibodies.
The i allele does not alter the basic glycoprotein, whereas the IA and IB alleles change the glycoprotein by adding molecules.
A person will produce antibodies against whichever glycoprotein they do not have on their own red blood cells.

Question 52

Change to Glycoproteins based on genetype

Answer 52

ON notion

Question 53

Test-crosses

Answer 53

A test-cross is used to determine if an organism with the dominant phenotype but unknown genotype is homozygous dominant or heterozygous.

Question 54

Sex-linked genes

Answer 54

Thomas Morgan’s work on Drosophila revealed this type of inheritance.
Drosophila melanogaster, or fruit flies, are small flies that complete their life cycle in 12 days. They have 8 chromosomes, including one pair of sex-determining chromosomes (XX for female, XY for male).
Drosophila are highly appropriate for crossing experiments because large numbers of offspring can be obtained quickly.

Question 55

Red-green colour blindness

Answer 55

-Example of a sex-linked trait
-Photoreceptor proteins are made by cone cells in the retina of the eye, and allow the cells to detect specific wavelengths of visible light.
Red-green colour-blindness is caused by a recessive allele for the gene that codes for one of the photoreceptor proteins. This gene is found on the X chromosome.
A male’s X chromosome is inherited from their mother.

Question 56

Haemophilia

Answer 56

Haemophilia is a life-threatening genetic condition. Most cases of haemophilia are due to an inability to make Factor VIII.
Factor VIII is a protein involved in the clotting of blood.
When a person with haemophilia suffers an injury, treatment involves a transfusion of Factor VIII purified from donated blood.
The gene for Factor VIII is located on the X chromosome, and the allele that causes haemophilia is recessive.

Question 57

Pedigree chart

Answer 57

-Females are shown as circles
Males are shown as squares
Shapes are shaded to indicate a person affected by a trait
Roman numerals indicate generations, Arabic numbers identify individuals within a generation

Question 58

Incomplete dominance

Answer 58

In incomplete dominance, an intermediate phenotype is produced, which is in between the phenotypes possible in homozygous individuals.

Question 59

Genetic disease in humans

Answer 59

Medical research has already identified more than 4000 genetic diseases, with many more yet to be discovered.
Fast and cheap genome sequencing is producing significant quantities of data for comparison.
It is estimated that an individual is likely to be a carrier for between 75 and 200 genetic diseases, though they will only produce a child with one of these genetic diseases if their partner is also a carrier for that same disease.

Question 60

Cause of mutation

Answer 60

Radiation and mutagenic chemicals increase the mutation rate and cause genetic disease and cancer

Question 61

Mutation (radiation)

Answer 61

Radiation increases the mutation rate if it has enough energy to cause chemical changes in the DNA.
Gamma rays, X-rays, short-wavelength UV rays and alpha particles from radioactive isotopes are all mutagenic.

Question 62

Mutagenic chemical

Answer 62

Mutagenic chemicals cause chemical changes to DNA.
Benzo[a]pyrene, nitrosamides and vinyl chloride are mutagenic (and found in tobacco smoke). Mustard gas, a chemical weapon used in WWI, is also mutagenic.

Question 63

The effects of Cystic fibrosis

Answer 63

-due to a recessive allele on the CFTR gene on chromosome 7.
-results in chloride channels being produced that do not function properly.
-sweat containing excess amounts of sodium chloride is produced and digestive juices and mucus are secreted with insufficient sodium chloride. resulting is mot enough water moving for osmosis.
-sticky mucus builds in the lungs causing infections and the pancreatic duct is usually blocked so digestive enzymes are not able to reach the small intestine.

Question 64

The effect of huntingstons disease

Answer 64

-due to a dominant allele of the HTH gene. gene is lovated on chromosome 4.
0causes degenerative changes in the brain. symptons usually start when a person is between 30 and 50 years old. a person typically has a 20 year life span after symptoms begin to display.
-typically succumbing to heart failure, pneumonia or some other infectious disease.

Question 65

Steps of DNA profiling

Answer 65

1, A sample of DNA is obtained from a person, a crime scene or a recent fossil.
2. Variable sequences (STRs) in the DNA are amplified by PCR, using primers that bind to the DNA just upstream from the STR location.
3. The DNA fragments are separated by gel electrophoresis depending on their length.
4. The pattern of bands is the individual’s DNA profile. DNA profiles of different individuals can be compared.

Question 66

Restriction endonuceleases

Answer 66

Enzymes that recognise a specific DNA sequence and ‘cut’ by breaking the covalent bond between adjacent nucleotides.

Question 67

dna ligase

Answer 67

An enzyme that catalyses the formation of a covalent bond between adjacent nucleotides. Normally involved in DNA replication on the lagging strand.

Question 68

Steps of Genetic modification

Answer 68

1. identify the target gene and select restriction endonuclease
2. cut the target gene and plamid with the same restriction endonuclease
3. allow annealing of sticky ends of gene and plasmid
4. re-join the sugar phosphate backbone with DNA ligase

Question 69

Genetic modification: Golden rice

Answer 69

-TGO
-aims to improve health outcomes for people in developing countries.
Vitamin A deficiency is a significant cause of blindness in children.
Two genes from daffodil plants and one from a bacterium were transferred to a variety of rice.
The three added genes activate a metabolic pathway that produces beta-carotene (β-carotene) in the endosperm of the seeds of the rice plants.
β-carotene is a precursor to vitamin A, so a diet that includes golden rice increases vitamin A intake and can improve vision in people with blindness caused by vitamin A deficiency.

Question 70

Transgenic plants: salt tolerant water

Answer 70

-high soil salinity leads to osmotic water loss from plant roots, causing salt and water stress, reducing growth.
-salt tolerant gene codes for a protein that removes sodium from the leaves.

Question 71

Transgenic plants: Bt cotton

Answer 71

Bt cotton has been genetically modified by the addition of two genes from the bacteria Bacillus thuringiensis.
The genes code for proteins that kill insect pests that eat parts of the plant by damaging their digestive system.
Using cotton that produces the Bt toxin reduces the use of sprayed pesticides, decreasing the environmental impact and saving money for farmers.

Question 72

analysising risks to monarch butterflies

Answer 72

-bt crops developed to reduce the use of chemical insecticides by developing strains of crop plants that produce an insecticide toxin.
A gene was transferred from Bacillus thuringiensis bacteria that codes for Bt toxin.
Bt toxin is a protein that is produced in all parts of the crop plants, including leaves, flowers and pollen. The toxin kills insects.
Monarch butterfly larvae feed on milkweed, which sometimes grows close to Bt corn crops.
Pollen from the Bt corn crops is wind-dispersed and can ‘dust’ surrounding plants.
Monarch larvae may therefore be at risk of poisoning by Bt toxin in the pollen.

Question 73

clones

Answer 73

are groups of genetically identical organims derives from a single original plant cell

Question 74

garlic natural cloning

Answer 74

A single garlic bulb grows leaves, which can produce organic materials so that a group of bulbs can grow. All the bulbs are identical to each other, so they are a clone.

Question 75

natural cloning of aphids

Answer 75

Female aphids can give birth to offspring that have been produced entirely from diploid egg cells that were produced by mitosis rather than meiosis. All offspring are a clone of their mother.

Question 76

cloning animal embroys

Answer 76

Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.
At the embryo stage, all cells are pluripotent (capable of developing into all types of tissue).
Fragmentation, or splitting of animal embryos is usually most successful at the 8-cell stage, before the cells start to differentiate and cease to be pluripotent.
Embryos can then be transplanted into surrogate mothers.

Question 77

method use to produce dolly

Answer 77

Adult cells were taken from the udder of a Finn Dorset ewe and grown of six days in a low-nutrient medium to inactivate genes.
Unfertilised egg cells were taken from the ovaries of a Scottish blackface ewe, and the nucleus was removed from each egg cell under a microscope.
One cultured adult cell was placed next to each enucleated egg cell, and a small electric pulse induced the two cells to fuse together to create a cell like a zygote that could grow into an embryo (10% success rate).
The 7-day-old embryos were inserted into surrogate ewes, just as is done in IVF. A normal gestation can then occur (3.4% success rate).

Question 78

Dolly

Answer 78

Production of cloned embryos by somatic-cell nuclear transfer (SCNT).
The production and birth of Dolly the sheep was a breakthrough in animal cloning.
Dolly was born on 5 July 1996 at the Roslin Institute in Edinburgh.
She lived to the age of 6 and died on 14 February 2003. She was euthanased after developing progressive lung disease and severe arthritis. Most Finn-Dorset sheep live to about 11-12 years.