CIE iGCSE Biology-17.1 Chromosomes, genes and proteins- Study Notes- New Syllabus
CIE iGCSE Biology-17.1 Chromosomes, genes and proteins- Study Notes – New syllabus
CIE iGCSE Biology-17.1 Chromosomes, genes and proteins- Study Notes -CIE iGCSE Biology – per latest Syllabus.
Key Concepts:
Core
- State that chromosomes are made of DNA, which contains genetic information in the form of genes
- Define a gene as a length of DNA that codes for a protein
- Define an allele as an alternative form of a gene
- Describe the inheritance of sex in humans with reference to X and Y chromosomes
Supplement
- State that the sequence of bases in a gene determines the sequence of amino acids used to make a specific protein (knowledge of the details of nucleotide structure is not required)
- Explain that different sequences of amino acids give different shapes to protein molecules
- Explain that DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters
- Explain how a protein is made, limited to:
• the gene coding for the protein remains in the nucleus
• messenger RNA (mRNA) is a copy of a gene
• mRNA molecules are made in the nucleus and move to the cytoplasm
• the mRNA passes through ribosomes
• the ribosome assembles amino acids into protein molecules
• the specific sequence of amino acids is determined by the sequence of bases in the mRNA
(knowledge of the details of transcription or translation is not required) - Explain that most body cells in an organism contain the same genes, but many genes in a particular cell are not expressed because the cell only makes the specific proteins it needs
- Describe a haploid nucleus as a nucleus containing a single set of chromosomes
- Describe a diploid nucleus as a nucleus containing two sets of chromosomes
- State that in a diploid cell, there is a pair of each type of chromosome and in a human diploid cell there are 23 pairs
Chromosomes, DNA, and Genes
✅ Key Idea:
Chromosomes are made of DNA, and this DNA carries genetic information in the form of genes.
📚 In Simple Terms:
- Inside the nucleus of almost every cell in your body, there are chromosomes.
- These chromosomes are long, coiled-up molecules of DNA.
- DNA stands for Deoxyribonucleic Acid – it’s the molecule that stores all your genetic instructions.
- Small sections of DNA are called genes.
- Each gene contains the code for making a particular protein, which helps build and control your body’s functions (like eye colour, height, or blood type).
🧠 Why This Matters:
- Genes are passed from parents to offspring and determine your inherited traits.
- If there’s a change (mutation) in the DNA, it can affect how your body works.
🔄 Summary Table:
| Term | What It Is |
|---|---|
| Chromosome | A structure in the nucleus made of coiled DNA |
| DNA | The molecule that stores genetic instructions |
| Gene | A section of DNA that codes for one specific protein |
What is a Gene?
✅ Key Definition:
A gene is a length of DNA that codes for a specific protein.
Explanation:
- Your DNA is like a giant instruction book made of chemical codes.
- A gene is like one full instruction or “recipe” in that book.
- It contains the code that tells the cell how to build a specific protein – and proteins are the building blocks that make your body work.
🔍 Where Are Genes Found?
- Genes are found on chromosomes inside the nucleus of each cell.
- Humans have about 20,000 to 25,000 genes.
📌 Why Are Genes Important?
- Each gene gives the cell a blueprint for a protein, which might control something like your eye colour, blood group, or ability to digest certain foods.
- Genes can be inherited from your parents.
🔄 Summary Table:
| Feature | Description |
|---|---|
| Gene | A segment of DNA coding for a protein |
| Function | Carries instructions to build proteins |
| Location | Found on chromosomes in the cell nucleus |
What is an Allele?
✅ Key Definition:
An allele is an alternative form of a gene.
📘 Explanation:
- A gene controls a particular characteristic (like eye colour).
- Each gene can exist in different versions, and these versions are called alleles.
- All alleles of a gene are found at the same position (locus) on a chromosome.
🔁 Example:
| Gene | Possible Alleles | What They Do |
|---|---|---|
| Eye colour | Brown allele (B), Blue allele (b) | Determine the colour of the eyes |
| Blood group | A, B, O alleles | Control your blood type |
- You inherit one allele from your mother and one from your father.
- If both alleles are the same → you are homozygous for that gene.
- If the alleles are different → you are heterozygous.
🔬 In Summary:
| Term | Definition |
|---|---|
| Allele | An alternative form of the same gene |
| Gene | A section of DNA that codes for a protein |
Inheritance of Sex in Humans (X and Y Chromosomes)
✅ Key Concept:
In humans, sex is determined by the combination of X and Y chromosomes inherited from the parents.
🧬 Sex Chromosomes:
- Humans have 46 chromosomes (23 pairs).
- One of these pairs is the sex chromosomes:
- Females: XX
- Males: XY
The father determines the sex of the child because he can pass on either an X or a Y chromosome.
The mother only contributes an X.
👪 How Sex Is Inherited:
| Parent | Sex Chromosomes |
|---|---|
| Mother | Always provides an X chromosome |
| Father | Can provide an X or a Y chromosome |
Therefore:
- If the child inherits X from the father → XX → Female
- If the child inherits Y from the father → XY → Male
🧪 Inheritance Chart:
| Sperm (Father) | Egg (Mother) | Offspring’s Chromosomes | Sex |
|---|---|---|---|
| X | X | XX | Female |
| Y | X | XY | Male |
How Genes Determine Proteins
✅ Key Statement:
The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.
Gene → base triplets → amino acids → protein
Explanation:
- A gene is a section of DNA.
- DNA is made up of four bases: A, T, C, and G.
- The order of these bases tells the cell which amino acids to use and in what order.
- Each group of 3 bases (called a triplet or codon) codes for one amino acid.
- The amino acid sequence determines the protein’s structure and function.
🔄 Summary:
| Component | Role |
|---|---|
| Gene (DNA sequence) | Holds the code for building a protein |
| Base sequence | Determines the order of amino acids |
| Protein | Made by joining amino acids in correct order |
How Amino Acid Sequences Affect Protein Shape
✅ Key Concept:
Different sequences of amino acids result in different shapes of protein molecules, which determine the protein’s function.
Explanation:
- Proteins are made of long chains of amino acids.
- The sequence of amino acids is determined by the base sequence of a gene.
- After the chain is built, it folds into a 3D shape.
- This shape is what gives the protein its function.
🔄 Why Shape Matters:
- Changing the amino acid sequence alters how the chain folds.
- This creates a new shape – possibly non-functional or with a new function.
- The protein may:
- Bind to different molecules
- Fit specific receptors or active sites
- Operate in different parts of the cell or body
🧪 Examples:
| Protein | Function Depends On Shape |
|---|---|
| Enzyme | Has an active site that fits the substrate exactly |
| Antibody | Shape must match the antigen to bind and fight infection |
| Hormone (e.g., insulin) | Shape allows it to bind to receptors on target cells |
🧠 Summary:
Even a single change in the amino acid sequence can result in a non-functional or entirely different protein.
How DNA Controls Cell Function Through Proteins
Key Concept:
DNA controls cell function by controlling the production of proteins, including:
- Enzymes
- Membrane carrier proteins
- Receptors for neurotransmitters
How It Works:
- DNA carries the genetic code for making proteins.
- Each gene on the DNA has instructions to make one specific protein.
- Proteins are made by reading the base sequence in the gene and linking amino acids in the correct order.
🔄 Types of Proteins Controlled by DNA:
| Protein Type | Function in the Cell |
|---|---|
| Enzymes | Speed up chemical reactions (e.g., digestion, respiration) |
| Membrane Carriers | Transport specific substances across the cell membrane (e.g., glucose, ions) |
| Receptors | Detect signals like neurotransmitters or hormones and allow the cell to respond |
🔬 Why This Matters:
Because proteins do most of the work in the cell, DNA controls what the cell does, how it responds, and how it interacts with other cells.
🧠 Summary Line:
DNA is like a control center – it runs the cell by telling it which proteins to make and when to make them.
How a Protein Is Made?
✅ Step-by-Step Process:
1. The gene remains in the nucleus
- The DNA that codes for a specific protein stays safely inside the nucleus of the cell.
- DNA does not leave the nucleus.
2. Messenger RNA (mRNA) is a copy of the gene
- A short, single-stranded copy of the gene is made – this is called mRNA (messenger RNA).
- It carries the instructions from the gene.
3. mRNA is made in the nucleus and moves to the cytoplasm
- The mRNA leaves the nucleus through tiny pores in the nuclear envelope.
- It travels into the cytoplasm, where proteins are made.
4. mRNA passes through ribosomes
- The ribosome is the protein-building machine in the cell.
- The mRNA attaches to a ribosome.
5. The ribosome assembles amino acids into a protein
- The ribosome reads the mRNA code three bases at a time (called a codon).
- It uses this code to join together amino acids in the correct order.
6. The sequence of amino acids is determined by the base sequence in the mRNA
- The order of bases in mRNA decides the order of amino acids.
- This order determines the shape and function of the final protein.
🧠 Final Summary:
DNA → mRNA → Ribosome → Protein
The gene stays in the nucleus, but a coded message (mRNA) is sent to the ribosome, which builds the protein using amino acids.
Gene Expression in Body Cells
Key Idea:
Most body cells in an organism contain the same genes, but not all genes are expressed in every cell — only the ones needed for that cell’s function.
📘 Explanation:
- Every cell in your body (e.g. skin cell, muscle cell, liver cell) contains the full set of genes (all the DNA).
- However, each cell type only “switches on” or expresses the genes it needs to do its specific job.
- The unneeded genes remain inactive or “switched off.”
🧪 Examples:
| Cell Type | Genes Expressed | Genes Not Expressed |
|---|---|---|
| Muscle Cell | Makes actin, myosin (for contraction) | Doesn’t make insulin or digestive enzymes |
| Pancreatic Cell | Makes insulin | Doesn’t make muscle proteins |
| Skin Cell | Makes keratin (for protection) | Doesn’t make liver detox enzymes |
- All body cells have identical DNA, but they function differently because they use different sets of genes.
- This selective gene expression is what allows cell specialisation.
What is a Haploid Nucleus?
✅ Key Definition:
A haploid nucleus contains a single set of chromosomes.
📘Explanation:
- Most body cells in humans have two sets of chromosomes – one from each parent – and are called diploid.
- In contrast, gametes (sperm and egg cells) have only one set of chromosomes. These cells have a haploid nucleus.
- This means they carry half the genetic information of a normal body cell.
🔢 In Humans:
- Total number of chromosomes in a diploid (body) cell = 46
- Number of chromosomes in a haploid (gamete) = 23
🧪 Examples of Haploid Cells:
| Cell Type | Number of Chromosomes | Haploid/Diploid |
|---|---|---|
| Sperm cell | 23 | Haploid |
| Egg cell | 23 | Haploid |
During fertilisation, the haploid sperm and haploid egg fuse to form a diploid zygote, restoring the full set of chromosomes.
What is a Diploid Nucleus?
✅ Key Definition:
A diploid nucleus contains two sets of chromosomes – one set inherited from each parent.
Explanation:
- Most cells in the human body (e.g. skin, muscle, liver cells) have a diploid nucleus.
- This means they carry two copies of each chromosome, arranged in pairs.
- These chromosome pairs are called homologous pairs – one from the mother and one from the father.
🔢 In Humans:
- Total chromosomes in a diploid cell = 46
- 23 pairs (one from each parent)
- Example: 23 from mother + 23 from father = 46 chromosomes total
🧪 Examples of Diploid Cells:
| Cell Type | Number of Chromosomes | Diploid or Haploid |
|---|---|---|
| Skin cell | 46 | Diploid |
| Liver cell | 46 | Diploid |
| Muscle cell | 46 | Diploid |
Diploid cells are the standard cells of the body.
They ensure that when gametes (haploid cells) fuse during fertilisation, the resulting zygote has the full set of chromosomes again (diploid).
Chromosomes in Diploid Cells
✅ Key Statement:
- In a diploid cell, there is a pair of each type of chromosome.
- In human diploid cells, there are 23 pairs of chromosomes (total = 46 chromosomes).
- A diploid cell has two of each chromosome – one from the mother, one from the father.
- These matched chromosomes are called homologous pairs.
- Humans have 23 types of chromosomes, so diploid cells have 23 pairs (23 × 2 = 46 chromosomes).
- This is the normal chromosome number in body cells (e.g. skin, liver, muscle).
🔢 Summary Table:
| Cell Type | Chromosomes | Pairs | Haploid or Diploid? |
|---|---|---|---|
| Human body cell | 46 | 23 pairs | Diploid |
| Human gamete | 23 | — | Haploid |
The diploid number (46 in humans) is restored at fertilisation when a sperm and egg (each with 23 chromosomes) combine.