Genetics

Question 1

Differences between eukaryote- and prokaryote chromosomes

Answer 1

Eukaryote chromosomes:

• contain a linear DNA molecule
• associated with histone proteins
• no plasmids
• two or more different chromosomes

Prokaryote chromosomes:

• consist of a circular DNA molecule
• naked - no associated proteins
• plasmids often present
• one chromosome only

Question 2

Autoradiography and chromosomes

Answer 2

The technique of autoradiography combined with electron microscopy is used to find where radioactively labelled substances are located in cells. Notable discoveries where the shapes of chromosomes (either circular or linear).

Question 3

Chromatids

Answer 3

Eukaryote chromosomes are only visible during mitosis. In prophase they condense and in metaphase reach their minimum length, consisting of sister chromatids. They 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.

Question 4

Genome

Answer 4

The whole genetic information of an organism, hence equal to the amount of DNA in one set of chromosome. (Can be measured in millions of base pairs (bp) of DNA. The genome size varies considerably.

Question 5

Homologous chromosomes

Answer 5

Chromosomes of one particular type have the same genes may not have the same alleles of those genes.

These homologs, are a set of one maternal and one paternal chromosomes that pair up with each other inside a cell during meiosis. These copies have the same genes in the same loci where they provide points along each chromosome which enable a pair of chromosomes to align correctly with each other before separating during meiosis

• Can be found in eukaryotic diploid cells.

Question 6

Haploid and Diploid

Answer 6

Diploid = nucleus contains pairs of homologous chromosomes (e.g. most cells, zygote cell)

Haploid = nucleus contains only one chromosome type (e.g. gametes such as sperm)

Question 7

Sex chromosome

Answer 7

The 23. pair of chromosomes in humans determines gender.

Two X chromosomes (larger chromosome) = female

One X and Y chromosome (smaller chromosome) = male

Question 8

Karyotype

Answer 8

The number and type of chromosomes in a cell or organism. It displays chromosomes in decreasing length. Commonly used for deducing sex and diagnosing conditions due to chromosome abnormalities.

Question 9

Meiosis and sexual life cycles

Answer 9

Sexual life cycles include fertilization in which a male and a female gamete fuse to produce a zygote.

Meiosis is the process that halves chromosome number and allows a sexual life cycle with fusion of gametes.

• A diploid nucleus divides twice to produce four haploid nuclei. The DNA of the chromosomes is replicated in interphase, before first devision, so each chromosome consists of two sister chromatids. It follows two divisions to halve the chromosome number twice.

The haploid number of chromosomes is represented by “n” so the diploid number is “2n”.

Meiosis 1 = Reduction Division (homologs seperate)

Meiosis 2 = Meiotic Division (as it is identical to a mitosis division) (sister chromatids seperate)

• Dividing into four cells gives an evolutionary advantage since it results a more genetical diverse landscape.

Question 10

Meiosis and genetic variation

Answer 10

1. Random orientation of pair of homologous chromosomes in metaphase I when moving to the poles. This produces different chromosome combinations and therefore different combination of alleles.
2. Random crossing over during prophase 1: as homologous chromosomes pair up, parts of non-sister chromatids can be exchanged between them. This produces chromatids with new combinations of alleles and can occur at all sections of a chromosome.

Question 11

Non-disjunction and down syndrome

Answer 11

In anaphase, chromosomes that should separate and move to opposite poles during meiosis do not and instead move to the same pole, which can happen at the first or second devision of meiosis.

Gametes with too few chromosomes usually quickly die but those with too many sometimes survive (Down syndrome). Chances increase with the age of the parents.

This can be tested through amniocentesis or chorionic villus sampling.

If non-junction occurs in anaphase1, all cells are affected, if it happens in anaphase2, half of the four haploid cells are affected.

Question 12

Principle of inheritance: Mendel’s 3:1 ratio

Answer 12

When crossing two varieties of pea together, the offspring (F1 generation) had the same characteristic as one of the parents.

Their offspring contained both of the original partial types in a 3:1 ratio. The parents (F0) are homozygous because they have two of the same allele. F1 plants are heterozygous because they have two different alleles. Only one character is developed because that parent that gave the dominant allele masks the effect of the other parent’s recessive allele.

*One quarter* (hence 3:1 ratio) of the F2 generation have two recessive alleles and so show the character caused by this allele. This can be seen in a Punnett grid.

Question 13

Genotype

Answer 13

the alleles possessed by an organism

Question 14

Phenotype

Answer 14

the (observable) characteristics of an organism

Question 15

Cystic Fibrosis and Huntington’s Disease

Answer 15

All chromosomes apart form sex chromosomes are autosomes.

Genetic diseases can be predict, are rare, and due to recessive alleles of autosomal genes, e.g. cystic fibrosis (coding for a chloride channel). Usually neither parent has the disease but they both are carriers.

Some diseases are due to dominant alleles of autosomal genes, e.g. Huntington’s disease. This neurodegenerative disease codes for a protein (with an unknown function) and develops only during adulthood by which time an individual already had children.

Question 16

Sex linkage

Answer 16

The association of a characteristic with the sex of the individual, because the gene controlling the characteristic is located only on a sex chromosome.

There are very few genes on the Y chromosome, but the X chromosome is relatively large and has important genes on it. Hence, there are differences between males and females: e.g. hemophilia and red-green color blindness. Conditions like these are much more common in males because females are carriers of recessive alleles.

Question 17

Co-dominance (inheritance of blood groups)

Answer 17

When both alleles are co-dominant, they have joint effects on a heterozygous individual - blood group AB

Question 18

Multiple alleles (inheritance of blood groups)

Answer 18

More than two alleles of a gene exist – blood group ABO

Question 19

Mutation: causes, types, importance

Answer 19

Mutations randomly replace one base in a gene with a different base - base substitution. This is an important source of genetic variation, necessary for evolution to occur, though very few mutations prove to be beneficial, some causing genetic diseases or cancer.

Mutation rate is increases by two types of mutagen:

• high energy radiation (e.g. X-rays, alpha particles)
• mutagenic chemicals (e.g. nitrosamines in tabacco)

mutations:

silent – (no effect, because of more DNA codons code for one amino acid, see codon table)

mis-sense – (works incorrectly)

non-sense – (does not function)

Question 20

Alleles

Answer 20

A gene consists of a sequence of bases on a piece of DNA. There are different versions of some genes that have almost the same base sequence but differ in just one or a very small number of bases. These variant forms are called alleles.

However, these small changes can have significant consequences, as the genetic disease sickle-cell anemia shows.

Question 21

Sickle-cell anemia

Answer 21

A mutation occurring in hBB, the gene for the beta polypeptide of hemoglobin consisting of 146 amino acids. The effect on the phenotype makes cells carry oxygen less efficiently but can give resistance to malaria.

Question 22

Gene sequencing

Answer 22

Huge databases are used to log the locus of a gene (a particular position on homologous chromosomes) and its protein product.

Question 23

PCR - polymerase chain reaction

Answer 23

PCR consists of a cycle of stages carried out repeatedly to produce many copies of DNA - millions of copies can be produced in a few hours.

This amplification can be very useful for research.

1. The temperature is increased to 73°C, which encourages Taq DNA polymerase to replicate both strands, starting at the primer, producing two double-stranded copies of the original DNA.
2. DNA is heated to 95°C to separate the two strands.
3. The temperature is reduced to 53°C, which allows primers to bind to both strands of the DNA next to the sequence that is to be copied.

Question 24

Gel electrophoresis

Answer 24

A method of sporting mixtures of proteins or fragments of DNA, which are charged. The mixture is placed on a thin sheet of gel. An electric field is applied to the gel (via opposite electrodes) and wether the particles are positively or negatively charged, they move towards one of the electrodes or the other. The smaller molecules move faster and furhter.

Question 25

DNA profiling

Answer 25

There are loci in the chromosomes where short sequences which are repeated many times - “short tandem repeats (STR)”. The number of repeats and patterns are unique to every person (exceptions, like twins) and, hence, used for DNA profiling. The pattern is revealed through gel electrophoresis and PCR.

This is used in paternity- and forensic (crime court) investigations.

Question 26

Genetic modification, plasmids and uses

Answer 26

The transfer of genes from one species to another. Organisms tat have had genes transferred to them are called genetically modified organisms (GMO) or transgenic organisms.

Plasmids (a vector) is a small loop of DNA, in which two enzymes are used to insert genes:

restriction endonucleases cut DNA molecules at a specific base sequence and DNA ligase makes sugar-phospahre bonds to link nucleotides together and form continuous strands of nucleotides - a recombinant plasmid in a host cell.

This can be used to make bacteria produce certain substances, such as insulin, which can be purified and used by diabetics.

Question 27

Benefits of genetic modification of crops

Answer 27

• higher crop yields and thus more food for humans, due to less pest damage
• less land needed for crop production, some could become areas for wildlife conservation
• less use of insecticide sprays, which are expensive and can be harmful to farm workers and wildlife
• increased vitamin content
• decreased allergen or toxin content
• resistance to virus diseases
• increased tolerance to drought, flooding, cold or saline soils

Question 28

Possible limitations of genetic modification of crops

Answer 28

• insects that are not pests could be killed
• the transferred gene might spread to populations of wild plants by cross-pollination, making them also toxic to insects feeding them
• insects pests of corn may developed resilience to the toxin

Question 29

Clones

Answer 29

Genetically identical organisms derived from a single original parent, such as asexual reproduction.

When cloning artificially, it may be done by using embryonic stem cells or a somatic-cell nuclear transfer.