Topic 3: Genetics

Question 1

Prokaryote and eukaryote chromosomes

Answer 1

In a prokaryote there is one chromosome consisting of a circular DNA molecule.
The DNA is naked - not associated with proteins.
Some prokaryotes have plasmids, which are much smaller extra loops of DNA.
There are 4 differences between the chromosomes of eukaryotes and prokaryotes -
Eukaryote chromosomes - a linear DNA molecule, associated with histone proteins, no plasmids, two or more different chromosomes.
Prokaryote chromosomes - a circular DNA molecule, naked DNA (no associated proteins), plasmids often present, one chromosome only.

Question 2

Autoradiography and chromosomes

Answer 2

The technique of autoradiography combined with electron microscopy has been used by biologists from the 1940s onwards to find where radioactively labelled substances are located in cells.
Thin sections of cells are coated with photographic film and left in dark. When viewed with a microscope both the structure of cells in the section and black dots in the film are visible.
Each black dot shows where a radioactive atom decayed and gave out radiation, which acts like light on the film.
John Cairns adapted this technique to research the chromosomes of E. Coli, a prokaryote.
He grew E. Coli in a medium containing radioactively labelled thymine, so its DNA became labelled but not RNA.
He placed cells on a membrane and digested their cell walls, allowing the DNA to spill out over the membrane.
He coated the membrane with a photographic film and left it in dark for two months.
Film was developed - lines of black dots showed the position of the DNA molecules from E. Coli.
Cairns found that the DNA molecules were circular and 1,100 um long, despite the E. coli cells only being 2 um long.
Other researchers found that DNA in eukaryotes is linear.

Question 3

Chromatids

Answer 3

• Eukaryote chromosomes are only easily visible during mitosis.
• In prophase, they decondense and in metaphase they reach their minimum length.
• Each chromosome in prophase and metaphase of mitosis consists of two structures known as sister chromatids.
• Each contain a DNA molecule that was produced by replication during interphase, so their base sequences are identical.
• Sister chromatids are held together by a centromere.
• At the start of anaphase, the centromere divides and allows the chromatids to become separate chromosomes.

Question 4

Genomes

Answer 4

Genome - the whole set of the genetic information in an organism.
- The size of a genome is therefore the total amount of
- DNA in one set of chromosomes in that species.
It can be measured in millions of base pairs (bp) of DNA.
- Genome size does not decide the organism’s complexity.

Question 5

Homologous chromosomes

Answer 5

Prokaryotes only have one chromosome but eukaryotes have different chromosomes that carry different genes.
In humans, for example, there are 23 different chromosome types each of which carries a different group of genes.
All the chromosomes of one particular type are homologous - although they have the same genes in the same sequence they may not have the same alleles of those genes.

Question 6

Haploid and diploid

Answer 6

Most plant and animal cells have a diploid nucleus - pairs of homologous chromosomes.
Some cells have a haploid nucleus - only one chromosome of each type.
Gametes such as the sperm and egg cells of humans are haploid.
Two haploid gametes fuse together during fertilisation to produce one diploid cell - the zygote.
This divides by mitosis to produce more diploid body cells with the same number of chromosomes.

Question 7

Chromosome numbers

Answer 7

The nr of chromosomes is a characteristic feature of members of a species.
Usually the nr of quoted is the diploid nr, as that is how many chromosomes are present in normal body cells.
The diploid nr varies considerably - some species have fewer large chromosomes and others have a greater nr of small chromosomes.

Question 8

Sex chromosomes

Answer 8

The twenty third pair of chromosomes in humans determines whether an individual is male or female. There are two types of sex chromosomes, a larger X and a smaller Y. Either XX or XY.

Question 9

Karyotypes and karyograms

Answer 9

Karyotype - the nr and type of chromosomes present in a cell or organism.
Karyogram - a photograph in which the chromosomes of an organism are shown in homologous pairs of decreasing length. To study the karyotype and identify conditions, and the sex of the organism.

Question 10

Fusion of gametes and variation

Answer 10

• When the gametes fuse together during fertilisation, the alleles from two different parents are brought together in one new individual.
• This promotes genetic variation - fertilisation is a random process - any gamete produced by the father could fuse with the gamete produced by the mother.
• Species that reproduce sexually thus generate genetic variation by both meiosis, and by random fusion of gametes.

Question 11

Meiosis and genetic variation

Answer 11

2 processes in Meiosis promote genetic variation:

1. Random orientation of pairs of homologous chromosomes in Metaphase I:
   For each pair of chromosomes there are two possible orientations. Orientation is random and does not influence other pairs - different combinations (of alleles) can be produced. Nr of possible combinations is 2 to the power of n in humans where n is 23 - over 8 million combinations per parent.
2. Crossing over in Prophase I:
   In the early stages of meiosis, homologous chromosomes pair up and parts of the non-sister chromatids can be exchanged between them - crossing over. It produces chromatids with a new combination of alleles. Random where along the chromosome the exchange occurs.

Question 12

Non-disjunction and Down syndrome

Answer 12

Sometimes, chromosomes do not separate either in Anaphase I or Anaphase II and instead move to the same pole - non-disjunction.
Results in gametes with either one chromosome too many or too few.
Non-disjunction can cause Down syndrome - Trisomy 21 - 3 copies of chromosome 21.
Chance of having a child with Down Syndrome increases with mother’s age.

Two methods to test chromosomes of an unborn child:

1. Amniocentesis - sample of amniotic fluid is taken around the fetus by inserting a needle into the mother’s uterus.
2. Chorionic villus sampling - cells are removed from fetal tissues in the placenta called chorionic villi using a needle. Has a higher risk of miscarriage than amniocentesis. Both have a small risk of causing infections.

Question 13

Mendel and quantitative methods

Answer 13

• Father of genetics - crossed varieties of pea plants with different traits and found principles of inheritance.
• Large numbers of seeds - confident that the ratio was 3:1.
• Repeated many times and got the same ratio.

Question 14

Explaining the 3:1 ratio

Answer 14

1. Mendel crossed two varieties of pea together and found that all of the F1 offspring had the same characteristic as one of the parents.
2. He then crossed the F1 offspring together and they contained both the original parental types in a 3:1 ratio.
3. Each pea plant has two alleles of a gene that affect the phenotype.
4. The parents are homozygous as they had two of the same allele.
5. The F1 offspring were heterozygous as they had two different alleles.
6. The F1 plants all have the character of one of the parents as that parent had the dominant allele and in a heterozygote, the dominant overrules the recessive.
7. One quarter of the F2 generation had two recessive alleles and showed the character caused by this allele.

Question 15

Autosomal: Cystic Fibrosis and Huntington’s Disease

Answer 15

The principles by Mendel also act in humans and help predict some genetic diseases.
Many genetic diseases are due to autosomal genes (not on sex chromosomes).

Cystic fibrosis:
Caused by a recessive allele of a gene coding for a chloride channel.
Usually neither parent has the disease but they are both carriers of the recessive allele (heterozygotes) - 25% probability for the child to have the disease.

Huntington’s disease:
Caused by a dominant allele of a gene coding for a protein called huntingtin. Develops in adulthood.
Probability of a parent with Huntington’s passing it on to a child is 50%.

Question 16

Autosomal: Cystic Fibrosis and Huntington’s Disease

Answer 16

The principles by Mendel also act in humans and help predict some genetic diseases.
Many genetic diseases are due to autosomal genes (not on sex chromosomes).

1. Cystic fibrosis:
   Caused by a recessive allele of a gene coding for a chloride channel.
   Usually neither parent has the disease but they are both carriers of the recessive allele (heterozygotes) - 25% probability for the child to have the disease.
2. Huntington’s disease:
   Caused by a dominant allele of a gene coding for a protein called huntingtin. Develops in adulthood.
   Probability of a parent with Huntington’s passing it on to a child is 50%.

Question 17

Sex-linked diseases

Answer 17

• Sex-linkage - association of a characteristic with the sex of the individual as the gene controlling the trait is on a sex chromosome.
• X chromosome is larger and has many important genes on it.
• Males get only one copy of X - a conditions due to this gene are more frequent in males than in females as females can also be carriers but males cannot.
• Red-green colour blindness and Hemophilia are two examples that are due to recessive alleles of sex-linked genes.

Question 18

Inheritance of blood groups

Answer 18

Four blood groups exist in ABO system: A, B, AB and O.

1. Co-dominance - both IA and IB alleles are dominant. If both present they both affect the phenotype as co-dominant (AB blood group).
2. Multiple alleles - if there are more than two alleles of a gene they are called multiple alleles - the gene for ABO blood groups has 3 alleles - IA, IB and i. (ii is O blood group).

Question 19

Causes of mutations

Answer 19

• Mutations - random changes to the base sequence of a gene.
• Substitutions, deletions, insertions.
• A source of genetic variation.

Increased risk due to two types of mutagens:

1. High energy radiation
2. Mutagenic chemicals

Chernobyl released more radioactive material but caused fewer deaths than Hiroshima as isotopes spread over a wider area and had longer half-lives.

Question 20

Nuclear accident at Chernobyl

Answer 20

Accident and explosions of a nuclear reactor.
Radioactive substances were released and spread over large parts of Europe in 1985.
- 28 workers died within three months. Increased cases of leukemia.
- Concentrations of radioactive iodine rose - damage to thyroid glands.
- Bioaccumulation caused high levels of radioactive fish.
- Small increase in the risk of cancer and genetic disease.
- 4km squared of pine forest turned ginger brown and died.

Question 21

Nuclear bombing of Hiroshima

Answer 21

Atomic bomb in 1945.
Devastation.
Long-term effects of radiation - cancers.
First ten years most.
Stillbirths, malformation and death.
Likely to have been some mutations but the numbers are too small for statistical significance.
Stigmatisation in marrying.

Question 22

Number of genes

Answer 22

Genetics - the study of variation and inheritance.
Basic unit of inheritance - gene.
A gene - a heritable factor that consists of a length of DNA and affects a specific characteristic.
Every gene occupies a specific position on a chromosome.
Haemoglobin gene on the end of the short arm of chromosome 11.
Animal or plant nucleus contains thousands of genes.
Bacteria have fewer genes than eukaryotes.
Some other animals have fewer or more than humans.
Plants may seem less complex but some have more genes than humans.

Question 23

Alleles

Answer 23

Allele - different versions of a gene.
Alleles may influence the same trait, occupy the same position on a type of chromosome and have base sequences that differ from each other by one or only a few bases.
Haemoglobin - second base in the sixth codon is Adenine in the commonest allele of the gene - less common is Thymine which causes the genetic disease sickle cell anemia.

Question 24

Sickle-Cell Anemia

Answer 24

Genetic disease.
Second base, sixth codon - adenine to thymine - glutamic acid to valine.
Mutation is a substitution in the HBB gene that codes for the beta polypeptide of haemoglobin (has 146 amino acids).
Sickle cells carry oxygen less efficiently but can give resistance to malaria - useful.

Question 25

Gene loci and protein products

Answer 25

Locus - particular position of a gene on homologous chromosomes.
Loci can be found using the OMIM website.
Locus and protein product.
11p15.4 - gene is on chromosome 11, p - short arm, and 15.4 - region.

Question 26

PCR

Answer 26

PCR - Polymerase Chain reaction - to amplify the amounts of DNA.
Cycle of stages to produce many copies of a DNA molecule.
Millions of copies in a few hours when small quantities are found in a sample and larger are needed for analyses - from semen, blood, other tissue or long-dead specimens.
1. Temperature increased to 73 degrees celsius to encourage Taq polymerase to replicate the two strands starting at the primer.
2. DNA is heated to 95 degrees celsius to separate the two strands.
3. Temperature is reduced to 53 degrees celsius to allow primers to bind to both strands of DNA next to the sequence to be copied.

Question 27

Gel electrophoresis

Answer 27

Gel electrophoresis - method of separating mixtures of proteins or fragments of DNA which are charged.

Question 28

Gel electrophoresis

Answer 28

Gel electrophoresis - method of separating mixtures of proteins or fragments of DNA which are charged.
The mixture is placed on a thin sheet of gel which acts like a molecular sieve - an electric field is applied to the gel by attaching electrodes to both ends.
Depending on the charge of the particles, they move towards one or the other end.
The rate of movement depends on the size of the molecules - small molecules move faster than larger ones.

Question 29

Gene sequencing technology

Answer 29

Many developments in scientific research follow improvements in technology.
In some cases research projects stimulate improvements in technology.
Methods for finding the base sequence of genes were developed in the 1970s and the technology has improved since then.
Sequencing the entire human genome seemed impossible, but the improvements in technology helped the Human Genome Project to be finished in 10 years (in 2000).
Hundreds of prokaryotes and over a hundred of eukaryotes sequenced by 2014.
The 1000 Plant genomes project was on its way as well.

Question 30

DNA profiling

Answer 30

In the DNA of humans and other organisms there are loci in the chromosomes where instead of a gene consisting of a long sequence of bases there are much shorter sequences of three, four or five bases repeated many times - STR - Short tandem repeats.
At these STR loci, there are many different possible alleles that vary in the nr of repeats.
DNA profiling is used in forensic investigations or paternity testing.
STR alleles are used in DNA profiling.

1. A sample of DNA is obtained from a person - must not be contaminated.
2. DNA from a selection of STR loci is copied by PCR.
3. The copies of STR alleles made by PCR from one person’s DNA sample are separated by gel electrophoresis. The result is a pattern of bands. Two individuals are unlikely to have the same pattern of bands unless they are identical twins.

Question 31

Gene transfer using plasmids

Answer 31

Genetic modification - transfer of genes from one species to another.
GMO organisms - organisms that have had genes transferred to them.
The transfer of the gene for human insulin to bacteria.
Genes are transferred between species using a vector - small loop of DNA called a plasmid.
Two enzymes are used to insert genes into plasmids - restriction endonucleases cut DNA at specific base sequences and DNA ligase makes new sugar-phosphate bonds to link nucleotides together.
A plasmid with a gene from another species inserted is called a recombinant plasmid.
1. Messenger RNA coding for insulin is extracted from human pancreas cells that make insulin.
2. DNA copies of the mRNA are made using the enzyme reverse transcriptase.
3. Sticky ends are made by adding extra G nucleotides to the ends.
4. Plasmids are cut open using restriction enzymes.
5. Sticky ends are made by adding extra C nucleotides to the ends of the cut plasmid.
6. The insulin gene and the plasmid are mixed - link by complementary base pairing.
7. DNA ligase seals up the nicks in the DNA by making sugar-phosphate bonds.
8. The recombinant plasmids are mixed with the host cells - host cells absorb them.
9. The genetically modified E.coli are cultured in a fermenter.
10. The E.coli start to make human insulin which is extracted and purified for use by diabetics.

Question 32

Benefits and risks of genetic modification of crops

Answer 32

GMO crops are controversial.
A gene from a bacterium has been transferred to some varieties of corn - the gene codes for a bacterial protein called Bt toxin which kills insect pests feeding on the crops.
Benefits: higher crop yield, less land needed for crop production, less use of insecticides which are expensive and harmful.
Risks: Insects that are not pests may be killed, the transferred gene might spread to populations of wild plants by cross-pollination, the insect pests of corn may develop resistance to the Bt toxin.

Question 33

Clones and cloning

Answer 33

Clone - a group of genetically identical organisms derived from a single original parent cell.
Cloning - production of an organism that is genetically identical to another organism.
Asexual reproduction is a natural form of cloning used by many plants and some animals using mitosis.
Plants clone themselves by growing extra bulbs, tubers, runners.
Female aphids can give birth to young formed asexually from their own cells.
There are also methods of artificial cloning.
Cloning is useful to get more organisms with desirable traits.

Question 34

Artificial cloning of animals

Answer 34

1. Break up an embryo at an early stage when it consists of embryonic stem cells. Disadvantage - characteristics are unknown.
2. Somatic-cell nuclear transfer - nucleus is removed from an egg cell and replaced by a nucleus from a differentiated somatic body cell (used to produce Dolly the sheep).

Question 35

Cloning adult animals using differentiated cells (somatic cell nuclear transfer)

Answer 35

1. Cell taken from udder of donor adult and cultured in laboratory for six days.
2. Unfertilised egg taken from another sheep and nucleus removed.
3. Egg without a nucleus fused with donor cell using a pulse of electricity.
4. Embryo resulting from the fusion is transferred to the uterus of a third sheep.
5. Surrogate mother gives birth to lamb that is identical to the donor sheep that donated the udder cell.

Question 36

Investigating factors affecting rooting in stem cuttings

Answer 36

Stem cuttings - short lengths of stem that are used to clone plants artificially.
If roots develop from the stem, the cutting can become an independent new plant.
Some plant species root when the base of the cutting is placed in water but others root better when it is inserted into a solid medium.
Many factors affect root cuttings and whether they will form roots - independent variables (investigated).
IV could be the number of leaves left on the cutting, hormone rooting powder, warmth, whether a plastic bag is used.
DV could be whether any roots are formed, nr of roots.
All other factors - control variables - must be kept the same - cuttings from the same plant.
Repeats for reliability and avoidal of anomalous results.