Biology 2: free practice, theory and problems

1. From gene to protein

What is it about?

DNA is the recipe, proteins do the work. The path from gene to protein has two steps: transcription and translation. This is called "the central dogma".

Concepts and formulas

• Transcription (in the nucleus): DNA → mRNA. RNA polymerase reads the template strand, and in RNA T is replaced by U.
• In eukaryotes introns are cut out, and exons are joined together (splicing).
• Translation (in the ribosome): mRNA → protein. Three bases at a time (a codon) code for one amino acid.
• tRNA has an anticodon that matches the codon and carries the right amino acid.
• Start codon: AUG. Stop codons: UAA, UAG and UGA.
• $4^3 = 64$ codons, but only 20 amino acids: several codons can give the same amino acid.
• Gene regulation: not all genes are on all the time. That is why a muscle cell and a nerve cell are different, even though they have the same DNA.

Example

DNA template strand: TAC GGA → mRNA: AUG CCU → amino acids: methionine–proline.

Practise from gene to protein in the app →

2. Biotechnology

What is it about?

Biotechnology is using living organisms and DNA to make products and solve problems: insulin from bacteria, gene therapy, CRISPR and DNA profiles in criminal cases.

Concepts and formulas

• PCR (polymerase chain reaction) copies a piece of DNA many times. After $n$ cycles there are $2^n$ copies.
• Gel electrophoresis sorts DNA fragments by size: small fragments travel furthest.
• Restriction enzymes cut DNA at specific sequences.
• Recombinant DNA: a gene is inserted into a plasmid and into a bacterium, which then makes the protein (for example insulin).
• CRISPR-Cas9 is a "gene scissors" that can edit DNA at a precise location.
• GMO: genetically modified organisms. In Norway they are strictly regulated.
• DNA profile: variation in short repeated sequences makes each person unique (except identical twins).

Practise biotechnology in the app →

3. The nervous system and hormones

What is it about?

The body is controlled by two systems: the nervous system (fast, electrical) and the endocrine system (slower, chemical via the blood). Together they keep the body in balance (homeostasis).

Concepts and formulas

• A nerve cell (neuron) has dendrites (in), a cell body, an axon (out) and synapses.
• Resting potential about $-70$ mV. An action potential is a rapid change where Na⁺ flows in and K⁺ flows out.
• In the synapse neurotransmitters (for example dopamine and serotonin) are released and bind to receptors on the next cell.
• Myelin insulates the axon and makes the signal much faster.
• Reflex: the signal goes via the spinal cord without going through the brain first, which is why it is so fast.
• Hormones: insulin lowers blood sugar, glucagon raises it. Adrenaline prepares the body for "fight or flight".
• Negative feedback: a deviation causes a response that counteracts it (blood sugar, temperature).

Practise the nervous system and hormones in the app →

4. The immune system

What is it about?

The immune system protects the body against bacteria, viruses and other foreign things. It has two parts: the innate (fast but general) and the adaptive (slower the first time, but precise and with memory).

Concepts and formulas

• First line of defence: skin, mucous membranes, stomach acid.
• Innate immunity: inflammation, fever and phagocytes (eating cells) that engulf invaders.
• Adaptive immunity: B cells make antibodies, T cells kill infected cells and direct the response.
• Antigen: what the immune system recognises (for example a protein on a virus).
• Memory cells make the body respond much faster the second time.
• Vaccine: a harmless antigen that teaches the body to recognise the pathogen without getting ill.
• Herd immunity: when enough people are immune, the infection does not spread, and those who cannot be vaccinated are protected.
• Antibiotics work on bacteria, not on viruses.

Practise the immune system in the app →