Chromosomes are made of DNA, and DNA carries genes. Genes come in different versions called alleles, and one pair of chromosomes decides whether you are male or female. Extended: how a gene's bases set the order of amino acids in a protein, why cells switch most genes off, and what haploid and diploid mean.
Chromosomes are made of DNA, which contains genetic information in the form of genes. A gene is a length of DNA that codes for a protein, and an allele is an alternative form of a gene. Females are XX and males are XY. Extended: the sequence of bases in a gene sets the sequence of amino acids in a protein, and that sequence sets the protein's shape.
Fig. 1: zooming in from a cell to its nucleus, a chromosome and the DNA of one gene
Section 01
Chromosomes, DNA and genes.
Use the zoom chips in the sim to go from a whole cell to its nucleus, one chromosome, the DNA double helix and finally two alleles of one gene.
Chromosome: a thread-like structure in the nucleus, made of one very long molecule of DNA. A human body cell has 46 chromosomes.
DNA: the chemical that carries genetic information, in the form of genes.
Gene: a length of DNA that codes for a protein.
Allele: an alternative form of a gene. The sim's alleles B and b differ in just one base, so they code for slightly different proteins, for example giving a different flower colour.
Examiner trap
Do not mix up gene and allele. The gene is the instruction for a feature such as flower colour; the alleles are the different versions of that instruction, such as purple and white.
Section 02
Chromosome sets and sex.
Compare a female body cell, a male body cell, an egg and a sperm, then press 20 children (XX x XY) a few times and watch the numbers of girls and boys.
Sex chromosomes: females have two X chromosomes (XX); males have one X and one much smaller Y (XY).
Gametes: every egg carries an X. Half the sperm carry an X and half carry a Y, so the sperm decides the sex of the child.
Chance: each child has a 1 in 2 (50%) chance of being a girl or a boy. A real family of 20 is rarely exactly 10 : 10.
Extended.Diploid nucleus: two sets of chromosomes, a pair of each type. A human diploid cell has 23 pairs (46). Haploid nucleus: a single set of chromosomes, as in an egg or sperm (23).
Examiner trap
The mother cannot decide the sex of a child: all her eggs carry an X. Whether the child is XX or XY depends on which sperm fertilises the egg.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the sim's zoom, what are chromosomes made of?
Quick check+10 XP
The sim shows two alleles, B and b. How do they differ?
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Pressing 20 children (XX x XY) gives 12 girls and 8 boys. Why not exactly 10 : 10?
Quick check+10 XP
Extended: in the protein sim, what travels from the nucleus to the ribosome?
Section 03
From gene to protein.
Extended. Press Make the protein and follow the message from the gene in the nucleus to a ribosome in the cytoplasm. Then press Change one base and make it again.
The gene stays in the nucleus. A copy of it is made as a molecule of messenger RNA (mRNA), inside the nucleus.
The mRNA moves out of the nucleus into the cytoplasm and passes through a ribosome.
The ribosome assembles amino acids into a protein molecule. The sequence of bases in the mRNA sets the sequence of amino acids.
Shape: different sequences of amino acids give protein molecules different shapes. Changing one base in the sim changes one amino acid, and the chain folds into a different shape.
DNA controls the cell by controlling which proteins it makes, including enzymes, membrane carriers and receptors for neurotransmitters.
Examiner trap
The DNA itself never leaves the nucleus. It is the mRNA copy that travels to the ribosome.
Section 04
Same genes, different jobs.
Extended. Click the muscle cell, the pancreas cell and the nerve cell in turn and see which of the six genes are lit up.
Same genes: most body cells in an organism contain the same genes.
Not all expressed: many genes in a particular cell are switched off, because the cell only makes the specific proteins it needs.
In the sim: all three cells express respiration enzymes; the pancreas cell also makes insulin and amylase, the muscle cell makes contraction proteins and the nerve cell makes neurotransmitter receptors. None of them uses the haemoglobin gene, although every one has it.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which is the best definition of a gene?
Exam style+20 XP
A couple already have three sons. What is the chance that their next child is a girl?
Exam style+20 XP
Extended: one base in a gene is changed. Why might the enzyme it codes for stop working?
Exam style+20 XP
Extended: a liver cell does not make insulin although it has the insulin gene. This is because:
Skill unlocked
Chromosomes, genes and proteins, mastered.
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