2018 HSC Biology Exam Paper Solutions (with Option Topics)

The control is a plant cutting set up in the cylinder with water but kept in total darkness.

Variables to keep constant include: temperature, humidity, plant species, amount of water provided.

Cohesion of water molecules to one another is a mechanism for the movement of water through xylem vessels during the process of transpiration. As one water molecule moves up the xylem, cohesion causes the water molecules below it to follow.

Those individuals that are heterozygous for the co-dominant allele are more likely to survive and pass the allele on to their offspring, increasing the frequency of this allele in the population.

Individuals that are homozygous are infertile and cannot pass the allele on to their offspring, but because individuals that are heterozygous have an advantage they are likely to survive, reproduce, and pass the allele on to the next generation. Thus, despite the disadvantage produced by the allele it is still maintained within the population.

DNA polymerase enzymes then travel along each strand adding complimentary nucleotides until two identical DNA copies have been formed, each containing half of the original strand.

There is one maternally inherited chromosome and one paternally inherited. This is visible in the model as chromosomes with different colours.

During prophase I the homologous chromosomes join together (synapsis) and swap segments in a process known as crossing over. The result is chromosomes with both maternal and paternal segments, visible as different coloured segments of chromosomes in the model. This results in new combinations of alleles.
In metaphase I the homologous chromosomes line up in pairs along the metaphase plate. Then in Anaphase I each chromosome in the pair moves (randomly) to opposite ends of the cell.

In telophase the nuclear envelope dissolves and the chromosomes begin to decondense into chromatin. Cytokinesis completes the division of the cell into two unique daughter cells.

The work of Louis Pasteur and Robert Koch identified microbes as the cause of disease. Koch developed postulates to establish which specific microbe was associated with a disease. Pasteur experimented with vaccinations and developed a method of heating food products to kill off contaminating microbes (now known as pasteurisation). Once microbes were identified as the cause, efforts could focus on preventing the spread of microbes in order to prevent disease.

Hygiene practices were introduced to encourage medical practitioners, food preparers and the general public to wash their hands regularly to prevent the spread of microbes. In many countries water filtration systems were introduced to provide clean drinking water to the public via methods such as filtration and chlorination. Quarantine requirements were developed to prevent the spread of diseases in plants, animals and people through international travel (e.g. shipping ports, airports, border crossings).

An important technique to prevent disease is vaccination. Once a microbe has been identified as the cause of a disease, the antigens from its’ surface can be isolated and used in a vaccination. Once injected into the body, the immune system learns to recognise the antigen and produce memory cells which will be activated if the pathogen is encountered. The introduction of wide-scale vaccination campaigns in the 20th century lead to a dramatic decrease in deaths from diseases such as polio which now only occurs in around 3 countries. Smallpox has now been completely eradicated saving millions of lives. Diseases such as measles, mumps and rubella are now very rare in developed countries with childhood vaccination schedules. This has had a major impact on the life expectancy of people at birth.

New treatments for disease such as antivirals and antibiotics have had a huge impact. When antibiotics were first introduced in the 1940s they could easily cure what were once deadly bacterial infections. Antibiotics have saved millions of lives, but in recent years many bacteria have become resistant to antibiotics and new treatments must be found. As a result recent strategies have shifted from treatment to prevention.

There was a dramatic increase in expected life span over the 20th century. This is particularly evident in new-borns, where the life expectancy increased from around 47 years to 73 years. This increase is due to biological developments in identification, treatment and prevention of disease during this time.

The occurrence of colour vision corresponds to their use of colour in the environment. For example, planarians live in a dark environment and hence do not require any colour vision. They possess a basic eye-cup structure to only detect direction and intensity of light, but there is no need for colour as a form of visual communication, hence there is no need for colour vision.

In contrast, bees are insects which possess compound eyes composed of a large number of singular photoreceptor units called ommatidia. Each ommatidia has its own lens, pigment and receptor cells. This results in trichromatic colour vision (UV, blue, green, orange but not red). Colour and UV is used as the means by which flowers are able to “communicate their whereabouts” to the bees for pollination. Honey bees detect a portion of the ultraviolet spectrum in addition to the blue-green end of the visible spectrum, ranging from 300-700 nm. Thus, colour vision in bees allows the organisms to quickly locate pollen and nectar.

Snakes are able to detect light from 480-850 nm which includes the infrared part of the spectrum. As infrared light is produced by heat, snakes can use this sense to detect warm blooded prey at night when hunting.

Sound is a particularly versatile form of communication and mammals such as humans have ears made up of an outer, middle, and inner ear segment. The inner ear includes the cochlea that converts sound into mechanical vibration of the ear hairs (stereocilia) and then into electrical signals that are sent to the brain. However sound detection mechanisms vary widely between animals. Different animals have different ranges of detection. For example bats can detect between 10 000 and 150 000 Hz as they use ultrasound for echolocation. By contrast elephants hear between 5 – 12 000 Hz as they use infrasound for communication.

Insects do not have a cochlea but have a Tympanum membrane on the front legs or abdomen that detect vibrations in air and Chordotonal organs located in the legs that detect vibration through surfaces.

Fish have hard Otoliths that vibrate and cause hair cells to move, triggering sensory neurons. These are used for detection of high frequency sound while a Lateral line system (involving stereocilia in cupula) is used for detection of lower low frequency sounds. The swim bladder is used as a resonance chamber. Thus the models of hearing and vision in animals is very different to that of humans as animals have very different needs.

A base substitution will replace just one base on the DNA. This will change a single codon on the mRNA and one incorrect amino acid will be included in the polypeptide. In some cases, a silent mutation will occur and the codon will still code for the same amino acid.

or example, if the first base was changed from A to G the DNA sequence would read GTC AGT CGT ACG ATC GCA TA and the mRNA would read as CAG UCA GCA UGC UAG CGU AU. Only the first amino acid will be affected.

By contrast, a frameshift mutation will change all of the codons in the gene from the point where the mutation occurred, potentially resulting in many more amino acids being incorrectly coded. This is more likely to result in a dysfunctional protein.

For example, if a G was inserted at the beginning of the DNA sequence it would read GAT CAG TCG TAC GAT CGC ATA and the mRNA would read as CUA GUC AGC AUG CUA GCG UAU. All of the amino acids in this sequence would be incorrect and the protein product would likely be dysfunctional. (In fact the fourth codon is now a stop codon and the polypeptide will not be completed. This is a nonsense mutation.)

However, Lynch syndrome does not cause cancer in 100% of individuals that are carrying the allele so some individuals in the family that do not have cancer may still be carrying the allele. The female that was diagnosed with colon cancer at age 37 is likely to have inherited lynch syndrome from the mother who was carrying the allele but did not have cancer.

This means that 3/4 of the second generation had at least one allele for lynch syndrome and suggests that the parents in the first generation both were heterozygous for lynch syndrome.

1. A diploid somatic cell with 32 chromosomes is removed from the animal to be cloned.
2. An unfertilised haploid egg with 16 chromosomes is removed from a different individual of the same species.
3. The nucleus is removed from the egg, it is now enucleated (0 chromosomes).
4. The diploid somatic cell (32 chromosomes) is fused with the enucleated egg (0 chromosomes) using an electrical pulse.
5. The egg (32 chromosomes) is stimulated to begin cell division to form an embryo.
6. The embryo (32 chromosomes) is implanted in a surrogate that gives birth after normal gestation.
7. The DNA fingerprint of the offspring is compared to the DNA fingerprint of the somatic cell donor to confirm that they are identical.

The Watson and Crick model of DNA allows to understand the double helix structure of the DNA molecule and how DNA is composed of the four bases (Adenine, Thymine, Guanine and Cytosine). This allows us to examine the ‘code’ that makes up living things and was the first step in enabling researchers to manipulate that code. Beadle and Tatum established that sections of the DNA called ‘genes’ were providing the code for the production of proteins, or polypeptides more specifically.

Eventually the process of transcription and translation was discovered, whereby a gene is copied as mRNA which is then read by a ribosome. The ribosome assembles amino acids based on the code on the mRNA.

In order to research diseases, the genes that are responsible for them had to be identified. The human genome project successfully located all coding genes on the human genome. It did not identify what they did but enabled scientists to map the location of genes and begin the process of identifying what each gene does and its potential role in disease. Once a disease-causing gene is identified, DNA sequencing enables scientists to identify the exact code of a disease-causing gene such as muscular dystrophy.

This understanding of genes and DNA eventually lead to the development of transgenic organisms, where a gene from one species is inserted into the genome of another. For example, the muscular dystrophy gene can be removed from a human cell and inserted into the plasmid of a bacteria. The bacterium is allowed to reproduce by binary fission to produce many copies of the gene. The gene can then be inserted into a mouse egg nucleus, with the resulting offspring containing the gene in all of its cells.

Tissue cells can then be taken from the offspring and used to produce clones. The tissue cells are fused with enucleated egg cells, and thus are the only contributors of genetic information. All of the offspring will be identical to the transgenic tissue donor. This process requires an understanding of how genetic information is contained in the nucleus.

Models such as these are now regularly produced in order to study different human diseases. These models only exist due to the accumulation of knowledge about genes and gene technologies.