17.1 Chromosomes, Genes and Proteins Revision Notes

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.