Ch 1 · Chromosomes, Genes and Proteins · O Level Biology 5090
Topic 1 of 4 in Inheritance — O Level Biology 5090 — 58 lessons.
Introduction
Every inherited characteristic, from blood group to the enzymes in saliva, is written in DNA and read out as proteins. This lesson establishes the vocabulary the rest of the module depends on: chromosome, gene, allele, haploid and diploid. It also covers how sex is inherited and, for candidates, how a gene's base sequence becomes a protein and why cells with identical genes behave differently. Learn the Cambridge definitions word for word.
Key ideas — Chromosomes, genes and alleles
A chromosome is a thread-like structure of DNA, carrying genetic information in the form of genes. Chromosomes lie in the nucleus and become visible only when a cell divides.
🧑🏫 Teacher's explanation
Think of your body as a massive, complex construction project. The chromosomes are the heavy-duty filing cabinets located in the office (the nucleus) that hold all the blueprints. Inside these cabinets, you have individual instruction manuals called genes. Each manual tells the workers exactly how to build one specific part, like a window or a door. However, sometimes there are different versions of the same manual—one might say 'install a wooden door' and another might say 'install a metal door'. These different versions are called alleles.
📖 Definitions
🔍 Worked example — Easy
A human somatic (body) cell contains 46 chromosomes. How many chromosomes would be found in a human gamete (sperm or egg cell)?
🔍 Worked example — Standard
Explain the difference between a gene and an allele.
🔍 Worked example — Exam-style
A student observes a cell under a microscope and counts 23 chromosomes. The student concludes the cell is a human skin cell. Evaluate this statement.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) Describe how a specific protein is produced from a gene. [3] [4 marks]
Key ideas — Haploid and diploid nuclei
A haploid nucleus contains a single set of chromosomes. In humans this is 23 chromosomes, and the only haploid cells are the gametes (sperm and egg cells).
🧑🏫 Teacher's explanation
Imagine you are collecting trading cards. A diploid nucleus is like having two complete sets of a card collection—one set from your father and one set from your mother. Because you have two of every card, they come in matching pairs. If you lose one card from each pair, you still have a collection, but it's incomplete.
📖 Definitions
🔍 Worked example — Easy
A human cell is observed under a microscope. If the cell is a somatic (body) cell, calculate the number of chromosomes present using the notation 2n.
🔍 Worked example — Standard
A species of plant has a diploid number (2n) of 20. Determine the number of chromosomes found in the pollen grain of this plant.
🔍 Worked example — Exam-style
A researcher observes a cell from a new organism. The cell contains 14 chromosomes arranged in 7 homologous pairs. State whether this cell is haploid or diploid and show your reasoning.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the number of chromosomes in a gamete of this species. [1] (b) Explain why it is necessary for gametes to be haploid. [2] [3 marks]
Key ideas — Inheritance of sex
One of the 23 pairs in a human diploid cell is the pair of sex chromosomes. Females have two X chromosomes (XX); males have one X and one much smaller Y chromosome (XY). The other 22 pairs are autosomes.
🧑🏫 Teacher's explanation
Think of your DNA as a massive library of instruction manuals. The chromosomes are the individual books, and the genes are the specific recipes within those books. In humans, most of these books come in identical pairs (one from your mom, one from your dad). However, one specific pair—the sex chromosomes—is different. It's like having a special set of blueprints that determines the gender of the building.
📖 Definitions
🔍 Worked example — Easy
A human diploid cell contains 46 chromosomes. How many chromosomes are present in a human gamete (haploid cell)?
🔍 Worked example — Standard
A man with XY sex chromosomes marries a woman with XX sex chromosomes. Use a Punnett square to determine the ratio of male to female offspring expected.
🔍 Worked example — Exam-style
In a large population, the probability of an offspring being male is 0.5. If a couple has 4 children, what is the probability that all 4 are female? (Express as a fraction or decimal)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the number of chromosomes found in a human gamete. [1] (b) Explain why the sex of a child is determined by the sperm rather than the egg. [2] [3 marks]
Key ideas — From gene to protein
the genetic code and protein synthesis. DNA is built from four bases (A, T, C and G). The sequence of bases in a gene is the genetic code: it specifies the order in which amino acids are joined to make one specific protein. A change in the base sequence can change the protein.
🧑🏫 Teacher's explanation
Think of your DNA as a massive, master architectural blueprint kept safely inside a high-security vault (the nucleus). You wouldn't want to carry the original blueprint to a messy construction site where it might get stained or torn. Instead, you make a quick photocopy (mRNA) of just the specific page needed for one window (a gene). This photocopy is carried out to the construction site (the ribosome), where workers read the instructions to stack bricks (amino acids) in a very specific order to build the wall (the protein).
📖 Definitions
🔍 Worked example — Easy
A human diploid cell contains 46 chromosomes. How many chromosomes would be found in a human gamete (sperm or egg cell)?
🔍 Worked example — Standard
Describe the process of protein synthesis in a cell, starting from the gene in the nucleus.
🔍 Worked example — Exam-style
Explain why a skin cell and a muscle cell, which contain the same DNA, can have different functions.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain how this change in the base sequence can result in a non-functional protein. [2] (b) State the term used to describe the different forms of a gene. [1] [3 marks]
Key ideas — Gene expression
why cells differ. Every body cell is produced by mitosis from the zygote, so all body cells contain the same genes. Cells differ because many genes are not expressed: a gene is expressed only when its protein is being made. A cell makes only the specific proteins it needs for its function. A red blood cell precursor expresses the haemoglobin gene; a pancreas cell expresses the insulin gene; a muscle cell expresses the genes for contractile proteins. Other genes stay switched off, present but not copied into mRNA. This selective expression produces specialised cells from identical instructions.
🧑🏫 Teacher's explanation
Imagine you have a massive cookbook that contains every single recipe for every dish a restaurant could ever make. Every chef in the kitchen has the exact same cookbook in front of them. However, a pastry chef only follows the dessert recipes, while a grill chef only follows the meat recipes. Even though they both have the same book, they produce completely different menus because they only 'express' (read and use) specific pages.
📖 Definitions
🔍 Worked example — Easy
A student observes a skin cell and a pancreatic cell under a microscope. State the difference in their genetic content and explain why they function differently.
🔍 Worked example — Standard
Explain why a muscle cell contains the gene for insulin production but does not actually produce insulin.
🔍 Worked example — Exam-style
A researcher finds that a cell has 23 chromosomes. Determine if this cell is haploid or diploid and explain the implications for its protein synthesis capability compared to a somatic cell.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why these two cells have different functions. [2] (b) State the process by which the zygote produces these two different types of cells. [1] [3 marks]
Worked example
(a) A gene is a length of DNA that codes for a protein [B1]; an allele is an alternative form of that gene [B1].
🔍 Worked example — Easy
A student is asked to identify the difference between a gene and an allele. State the difference clearly.
🔍 Worked example — Standard
Calculate the number of chromosomes in a human diploid cell and a human haploid gamete.
🔍 Worked example — Exam-style
Using a genetic diagram, explain why the probability of a child being male or female is approximately 1:1.
⚠️ Common errors
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