Water Properties
State the exact type of bond within 1 water molecule and shortly explain its properties
Water (H₂O) is made of two hydrogen atoms covalently bonded to one oxygen atom.
Oxygen is more electronegative and pulls shared electrons closer, making water a polar molecule:
Oxygen side: Slightly negative (δ⁻)
Hydrogen side: Slightly positive (δ⁺)
The water molecule is a polar molecule: The opposite ends have opposite charges
Explain the property of water of being a universal solvent. What does it depend on?
Water’s ability to dissolve substances depends on on its polarity:
olar molecules will dissolve in water because they are “hydrophilic” and can form hydrogen bonds with water.
State the monomer of nucleic acids, and draw it with labeled parts
Nucleotide.
(should be drawn correctly) Nucleotides consist of:
One of the five nitrogeneous bases
A five-carbon pentose sugar
A phosphate group
Define DNA Replication and give 3 reasons why is it important
DNA replication is the process of producing exact copies of DNA.
1. Reproduction: In both unicellular and multicellular organisms, DNA replication is essential before cell division to pass genetic information to offspring.
2. Growth: New cells for body growth require exact DNA copies.
3. Tissue Replacement: Damaged or old cells are replaced with new ones having the same DNA.
Define gene expression and state its 2 main stages
is the process by which the information contained within a gene (DNA) is used to produce a functional gene product, such as a protein or a functional RNA molecule.
THIS PROCESS OF PROTEIN SYNTHESIS OCCURS IN TWO STAGES:
Transcription – DNA is transcribed and an mRNA molecule is produced
Translation – mRNA (messenger RNA) is translated and an amino acid sequence is produced
State the type of bond formed between water molecules and the property that allows this bond to be formed
Polarity allows water molecules to form hydrogen bonds with each other
Hydrogen bonds form between water molecules – weak alone, but strong collectively – giving water life-supporting properties:
State 3 types of solutions in relation to concentration differences, and describe the movement of water
A solution with higher osmolarity (more solute, less water) = hypertonic
A solution with lower osmolarity (less solute, more water) = hypotonic
Solutions with equal osmolarity = isotonic
When tissue is placed in a hypotonic solution (lower solute concentration outside):
When bathed in a hypertonic solution (higher solute concentration outside):
No movement in isotonic solution
State 2 main nucleic acids, and explain 5 differences between them
DNA and RNA
DNA CONTAINS DEOXYRIBOSE SUGAR, WHILE RNA CONTAINS RIBOSE SUGAR
DNA is double-stranded, RNA is single-stranded
DNA - thymine, RNA - uracil
DNA stores genetic information, RNA is involved in protein synthesis
DNA is mainly in the nucleus, RNA - in the nucleus and cytoplasm (e.g., ribosomes)
Define the term "semi-conservative", and explain why is this important for DNA replication to be semi-conservative
Semi-conservative means that one strand of the 'parent' DNA is kept in the 'daughter' molecule. The other half is determined by the code on the template strand and is built up from free nucleotides in the nuclear space around the chromosomes
It ensures that the new cells produced during cell division inherit all their genes with the correct sequence of DNA bases from their parent cells
Explain the meaning and the importance of "switching" the genes on and off
Not every protein is needed in every cell
As a result, our specialised cells have a way of switching certain genes off or on to match the requirements of the cell. This is called gene expression
Genes that are expressed are 'switched on' and undergo the process of transcription and translation
Genes that are not expressed are 'switched off' or silenced, and do not go through the process of transcription and/or translation
It controls which proteins are produced according to the cell’s needs, allows cells to respond to changes in the environment or developmental signals and saves energy by only producing necessary proteins
State the properties of water that allow the water movement inside the xylem vessels and explain how
1. Cohesion - Cohesive forces form an unbroken column in xylem vessels.
2. Adhesion - adhesive forces allow water to stick to the walls of the xylem
Define a contractile vacuole, give an example of organism that has it and explain its function.
Freshwater unicellular organisms (e.g., Amoeba proteus) lack cell walls and are constantly taking in water by osmosis. They use contractile vacuoles to pump excess water out and prevent bursting.
The contractile vacuole helps prevent excessive water influx that could harm and cause rupture (lysis) to the cell
Explain how is the RNA polymer formed
(type of reaction, which parts are joined and the type of bond formed)
RNA nucleotides join via a condensation reaction.
The 5′ phosphate of one nucleotide bonds with the 3′ carbon of the ribose on the next.
This forms a phosphodiester bond and releases water as a byproduct.
State the 2 main enzymes involved in DNA replication, and explain their roles
Helicase and DNA Polymerase
Helicase first unwinds the DNA, by flattening out its helical structure, then causes the hydrogen bonds to break between pairs of bases, exposing bases on either strand
DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template
mRNA has a site to which a ribosome can bind and a sequence of codons that specifies the amino acid sequence of the polypeptide.
Transfer RNA (tRNA) molecules translate the base sequence of mRNA to an amino acid sequence. They have an anticodon of three bases that bind to a codon on mRNA via complementary base pairing. tRNA molecules carry amino acids corresponding to their codon
The ribosomes are composed of rRNA and act as the binding site for mRNA and tRNA. They catalyse the peptide bonds between amino acids of the polypeptide.
Explain the terms hydrophobic and hydrophilic, and give the description of these types of substances
Hydrophobic Substances do not dissolve in water, dissolve better in non-polar solvents like acetone.
Non-polar molecules, such as fats and oils, repel water.
Hydrophilic Substances:
Attracted to water, include polar molecules (e.g., glucose) and charged particles (e.g., sodium and chloride ions)
Define turgor pressure, explain why is it created, and which types of cells experience this phenomenon.
What is the importance of 'turgid cells'?
Turgor pressure, also known as pressure potential or hydrostatic pressure, is the pressure exerted by the cell’s fluid content (e.g., water) against the cell wall.
When plant cells are in a hypotonic environment (lower solute outside), water moves into the cell.
Water fills the cytoplasm and pushes the plasma membrane against the cell wall.
This pressure is called turgor pressure.
The cell becomes turgid (swollen) – a healthy state.
Turgid cells help support stems and leaves, allowing plants to stand upright.
Explain the importance of complementary base pairing and the consequences of its absence
Complementary base pairing ensures genetic consistency during DNA replication and allows easy separation for replication or transcription.
Without complementary base pairing:
DNA wouldn’t replicate accurately → mutations
mRNA could carry incorrect instructions
Proteins might form incorrectly → malfunctioning cells
It is the foundation for genetic fidelity, heredity, and gene expression.
Explain the gel electrophoresis technique - how are the strands separated and what is the application of this method
During electrophoresis, the molecules are separated with an electric current according to their size or mass and their net charge. DNA is negatively charged, so it moves toward the positive electrode. Smaller fragments move faster and travel farther through the gel than larger ones.
Applications:
DNA profiling (forensics)
Paternity testing
Genetic research and diagnosis
Comparing DNA samples (e.g., crime scene analysis)
Explain how is the DNA stability ensured, its importance, and what could be the consequences of stability loss
The stability is ensured by the sugar-phosphate backbone and hydrogen bonds between nucleotides.
The fact that DNA is stable and doesn’t change its code easily is important for the conservation of the original code.
THE STABILITY OF DNA MAY BECOME COMPROMISED BY FREE RADICALS, CHEMICALS, CIGARETTE SMOKE OR EXPOSURE TO UV OR NUCLEAR RADIATION. THIS DAMAGE CAN LEAD TO A (HARMFUL OR BENEFICIAL) MUTATION
State and describe 4 main physical properties of water
Buoyancy - the upward force exerted by a fluid (liquid or gas) on an object immersed in it.
Viscosity - A measure of a fluid's resistance to flow, determined by the internal friction between its molecules.
Thermal conductivity - the rate at which heat transfers through a substance.
Specific heat - the amount of energy required to raise the temperature of 1 gram of a substance by 1°C
Explain how does the reaction of plant and animal cells differ when placed into the solutions with different tonicity, and which structure dictates this differences?
Hypotonic solution:
Water enters the cell. Animal cells may burst; plant cells become turgid.
Isotonic solution:
No net water movement. Animal cells stay normal; plant cells become flaccid.
Hypertonic solution:
Water leaves the cell. Animal cells shrink (crenation); plant cells undergo plasmolysis.
The cell wall is the structure that dictates these differences.
State and explain at least 4/7 properties of genetic code
Triplet – Each amino acid is coded by a sequence of three nucleotides (a codon).
Universal – The same codons code for the same amino acids in almost all organisms.
Degenerate (redundant) – One amino acid can be coded by more than one codon.
Unambiguous – Each codon codes for only one specific amino acid.
Non-overlapping – Codons are read one after another, not sharing nucleotides.
Continuous (comma-less) – The code is read without gaps or punctuation.
Has start and stop codons – AUG starts translation; UAA, UAG, UGA stop it.
Explain the Polymerase Chain Reaction (name 3 main stages), and its application
The three stages are:
Denaturation – the double-stranded DNA is heated to 95°C which breaks the hydrogen bonds that bond the two DNA strands together
Annealing – the temperature is decreased to between 50 - 60°C so that primers can anneal to the ends of the single strands of DNA
Elongation / Extension – the temperature is increased to 72°C for at least a minute, as this is the optimum temperature for Taq polymerase to build the complementary strands of DNA to produce the new identical double-stranded DNA molecules
Applications of PCR (Polymerase Chain Reaction):
Medical diagnosis: Detects pathogens (viruses, bacteria) and genetic mutations.
Forensics: Amplifies tiny DNA samples from crime scenes.
Paternity testing: Produces enough DNA for comparison.
Research & biotechnology: Clones genes and analyzes DNA sequences.
Compare and contrast transcription and translation
Transcription and translation are both enzyme-controlled steps of gene expression and both involve RNA.
In transcription, a DNA strand acts as a template and RNA polymerase builds an mRNA molecule from RNA nucleotides; this takes place in the nucleus (in eukaryotic cells).
In translation, the mRNA acts as a template and the ribosome joins amino acids together to form a polypeptide (protein); this occurs in the cytoplasm on ribosomes.