Chromosomes, genes and proteins

1. Overview

This topic explores the molecular basis of inheritance, explaining how genetic information is stored, organized, and used by the body. It bridges the gap between the physical structure of DNA in the nucleus and the production of proteins that determine an organism's characteristics and control all cellular functions.

Key Definitions

• Chromosome: A thread-like structure of DNA, carrying genetic information in the form of genes.
• DNA (Deoxyribonucleic acid): The chemical substance that makes up chromosomes and stores the genetic code.
• Gene: A length of DNA that codes for a specific protein.
• Allele: An alternative form of a gene (e.g., the gene for eye color has alleles for blue or brown).
• Haploid Nucleus: A nucleus containing a single set of chromosomes (found in gametes).
• Diploid Nucleus: A nucleus containing two sets of chromosomes (found in body cells).
• Protein: A macromolecule made of amino acids, whose structure and function are determined by the gene sequence.

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Core Content

The Relationship Between DNA, Genes, and Chromosomes

• Inside the nucleus of every eukaryotic cell are chromosomes.
• Chromosomes are made of a chemical called DNA.
• Along the length of each chromosome are specific sections called genes.
• Each gene acts as an instruction manual for building one specific protein.

Inheritance of Sex in Humans

• Humans have 23 pairs of chromosomes. The 23rd pair determines the biological sex of the individual.
• Females have two of the same sex chromosomes: XX.
• Males have two different sex chromosomes: XY.
• Mechanism of Inheritance:
  • All female gametes (eggs) contain an X chromosome.
  • Male gametes (sperm) can contain either an X or a Y chromosome.
  • If an X-sperm fertilizes an egg, the zygote is XX (Female).
  • If a Y-sperm fertilizes an egg, the zygote is XY (Male).
• There is always a 50% (1:1) probability of a child being male or female.

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Extended Content (Extended Only)

DNA and Protein Structure

• The Genetic Code: DNA is made of a sequence of bases. The specific sequence of bases in a gene determines the sequence of amino acids used to build a protein.
• Protein Shape: Different sequences of amino acids cause the protein chain to fold in different ways. This gives every protein a unique 3D shape, which is essential for its function.

How DNA Controls the Cell

DNA does not "do" the work itself; it provides the instructions for proteins. These proteins control cell function in several ways:

1. Enzymes: Act as biological catalysts for metabolic reactions.
2. Membrane Carriers: Control which substances enter and leave the cell.
3. Receptors: Allow the cell to receive signals, such as neurotransmitters or hormones.

Protein Synthesis (Step-by-Step)

1. The Template: The DNA (gene) is too large to leave the nucleus, so it stays protected inside.
2. mRNA (Messenger RNA): The cell makes a "copy" of the gene called mRNA.
3. Transport: The mRNA molecule moves out of the nucleus and into the cytoplasm.
4. The Ribosome: The mRNA passes through a ribosome (the protein factory).
5. Assembly: The ribosome reads the base sequence on the mRNA and assembles amino acids in the correct order to form a protein.

Gene Expression

• Almost all body cells in an organism contain the exact same genes.
• However, a skin cell looks and functions differently than a liver cell. This is because many genes are not expressed (turned off).
• Cells only "switch on" and make the specific proteins they need to carry out their specialized functions.

Diploid and Haploid Nuclei

• Diploid (2n): Nuclei with two sets of chromosomes. In humans, the diploid number is 46 (23 pairs). In a diploid cell, there is a pair of each type of chromosome.
• Haploid (n): Nuclei with one set of chromosomes. In humans, the haploid number is 23. These are only found in gametes (sperm and egg) so that when they fuse at fertilization, the diploid number is restored.

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Key Equations & Ratios

• Diploid number in humans: $2n = 46$
• Haploid number in humans: $n = 23$
• Sex determination ratio: $1 : 1$ (50% XX, 50% XY)

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Common Mistakes to Avoid

• ❌ Wrong: Thinking that DNA is a protein.
• ✅ Right: DNA is the instruction (code) used to build a protein.
• ❌ Wrong: Saying that different cells in the body (like brain cells vs. heart cells) have different DNA.
• ✅ Right: All body cells have the same DNA, but they express different genes.
• ❌ Wrong: Confusing the terms gene and allele.
• ✅ Right: A gene is the category (e.g., Eye Color), an allele is the version (e.g., Blue eyes).

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Exam Tips

• Command Word "Define": When defining a gene or allele, use the exact wording provided in the syllabus to ensure full marks.
• Sex Linkage: If asked to show the inheritance of sex, always use a Punnett square. Label the gametes (X, X and X, Y) and the resulting genotypes (XX, XY).
• Protein Synthesis sequence: Remember the order: DNA → mRNA → Ribosome → Protein.
• The "Shape" connection: In the extended paper, always link the "sequence of amino acids" to the "specific shape" of the protein. This is a common marking point.
• Chromosome Numbers: Be careful with the distinction between "23 chromosomes" (haploid) and "23 pairs" (diploid). Misreading this is a common source of lost marks.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A scientist is studying the protein composition of a cell membrane.

(a) Define the term 'gene'. [2]

(b) State the role of a gene in the production of a cell membrane protein. [1]

(c) Explain how different sequences of amino acids lead to different protein shapes. [2]

Worked Solution:

(a)

1. A gene is a length of DNA. This establishes the basic material.
2. ...that codes for a protein. This specifies the function of the DNA length.

How to earn full marks:

• Mention that a gene is a length of DNA.
• Mention that the DNA sequence codes for a protein.

(b)

1. The gene contains the instructions for the sequence of amino acids to be used in the protein. This directly links the gene to the protein's composition.

How to earn full marks:

• Explicitly state that the gene provides the instructions (or code) for the amino acid sequence.

(c)

1. The sequence of amino acids determines the order of the amino acids in the polypeptide chain. This links the amino acid sequence to the polypeptide structure.
2. Different amino acids have different properties (e.g., hydrophobic/hydrophilic), leading to different folding patterns and thus different 3D shapes. This explains how the amino acid properties influence the final shape.

How to earn full marks:

• State that the amino acid sequence determines the order of amino acids in the chain.
• Explain that different amino acids have different properties that cause different folding.

Common Pitfall: Students often forget to mention that a gene is a length of DNA, and just say it is DNA. Also, when explaining protein shape, be sure to link the different properties of amino acids to the folding process.

Exam-Style Question 2 — Extended Response [8 marks]

Question:

A farmer is using selective breeding to improve the wool quality of his sheep. The gene for wool fiber thickness has two alleles: T (thick wool) and t (thin wool).

(a) Define the term 'allele'. [1]

(b) State the possible genotypes for the wool fiber thickness gene in these sheep. [2]

(c) Explain why the farmer might choose to only breed sheep with the TT genotype. [3]

(d) Explain how the T allele codes for thick wool. [2]

Worked Solution:

(a)

1. An allele is an alternative form of a gene. This is the standard definition.

How to earn full marks:

• State the definition clearly.

(b)

1. TT Homozygous dominant genotype.
2. Tt Heterozygous genotype.
3. tt Homozygous recessive genotype.

How to earn full marks:

• List all three possible genotypes, using the correct notation.

(c)

1. Sheep with the TT genotype will always produce offspring with at least one T allele. This explains the inheritance pattern.
2. If the farmer breeds sheep with Tt genotype, some offspring may inherit the tt genotype and have thin wool. This explains the risk of breeding heterozygotes.
3. By selecting TT, the farmer ensures that all offspring have at least one T allele, maximizing the probability of thick wool in future generations. This explains the long-term benefit of selecting the homozygous dominant genotype.

How to earn full marks:

• Mention that TT sheep only pass on the T allele.
• Explain that breeding Tt sheep can result in thin-wool offspring.
• Explain the long-term benefit of selecting TT sheep.

(d)

1. The T allele is a length of DNA that codes for a protein involved in wool fiber production. This links the allele to protein synthesis.
2. This protein might be an enzyme that increases the production of keratin (a wool protein), or a structural protein that contributes to the thickness of the wool fiber. This provides specific examples of how the protein could influence wool thickness.

How to earn full marks:

• State that the T allele codes for a protein related to wool production.
• Give a specific example of the protein's function (e.g., enzyme, structural protein).

Common Pitfall: Many students confuse genotype and phenotype. Make sure you understand the difference! Also, when explaining how an allele codes for a trait, remember to mention the protein involved and its specific function.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

Human body cells contain 23 pairs of chromosomes.

(a) Define the term 'diploid nucleus'. [2]

(b) State the number of chromosomes in a human sperm cell. [1]

(c) Explain why it is important that gametes (sperm and egg cells) are haploid. [3]

Worked Solution:

(a)

1. A diploid nucleus is a nucleus containing two sets of chromosomes. This identifies the number of chromosome sets.
2. ...or chromosomes present in homologous pairs. This adds information about the arrangement of chromosomes.

How to earn full marks:

• Mention the presence of two sets of chromosomes.
• Mention homologous pairs.

(b)

1. $\boxed{23}$ Haploid number of chromosomes.

How to earn full marks:

• Correct number of chromosomes, no units required.

(c)

1. Gametes fuse during fertilisation to form a zygote. This establishes the basic process of sexual reproduction.
2. If gametes were diploid, the zygote would have double the normal number of chromosomes. This highlights the consequence of diploid gametes.
3. This would lead to genetic abnormalities or the failure of the zygote to develop. This explains the negative outcome.

How to earn full marks:

• State that gametes fuse to form a zygote.
• Explain that diploid gametes would result in a zygote with too many chromosomes.
• Explain that this can lead to genetic problems.

Common Pitfall: Students sometimes forget that the purpose of meiosis is to halve the chromosome number. Make sure you understand why maintaining the correct chromosome number is vital for healthy offspring.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A geneticist is studying a newly discovered protein in a species of plant. She knows that the protein is essential for the plant's ability to produce a red pigment in its flowers.

(a) State the type of molecule that makes up chromosomes. [1]

(b) Describe the process of how messenger RNA (mRNA) is produced from a gene. [3]

(c) Explain how the sequence of bases in the mRNA molecule determines the sequence of amino acids in the pigment production protein. [3]

(d) Suggest why most body cells in the plant contain the gene for this protein, but only petal cells produce it in large quantities. [2]

Worked Solution:

(a)

1. DNA This is the primary molecule.

How to earn full marks:

• State DNA.

(b)

1. The gene coding for the protein remains in the nucleus. This states the location of the gene.
2. Messenger RNA (mRNA) is a copy of the gene. Defines mRNA's role.
3. mRNA molecules are made in the nucleus. States the site of production.
4. ...and move to the cytoplasm. States the destination.

How to earn full marks:

• State that the gene is in the nucleus.
• State that mRNA is a copy of the gene.
• State that mRNA is made in the nucleus and moves to the cytoplasm.

(c)

1. The mRNA passes through ribosomes in the cytoplasm. This establishes the location of protein synthesis.
2. The ribosome 'reads' the sequence of bases on the mRNA. This explains how the information is accessed.
3. Each sequence of three bases (codon) codes for a specific amino acid. This explains the coding relationship.
4. The ribosome assembles amino acids into protein molecules according to the sequence of codons in the mRNA. This explains the assembly process.

How to earn full marks:

• State that mRNA passes through ribosomes.
• Explain that the ribosome reads the sequence of bases.
• Explain that each codon codes for a specific amino acid.
• Explain that the ribosome assembles amino acids according to the mRNA sequence.

(d)

1. All cells in the plant contain the same genes. This states the common genome.
2. However, only petal cells express the gene for the pigment production protein because they are the cells responsible for producing the flower's color. This links the gene expression to cell function.
3. Other cells do not need to produce large quantities of this protein, so the gene is not expressed in those cells. This explains the cell-specific expression.

How to earn full marks:

• State that all cells contain the same genes.
• Explain that only petal cells need to produce the pigment production protein.
• Explain that other cells do not need this protein.

Common Pitfall: Students often struggle to explain the link between the mRNA base sequence and the amino acid sequence. Remember that each codon (three bases) on the mRNA corresponds to a specific amino acid. Also, remember that gene expression is often cell-specific, meaning not all genes are active in every cell.