🧬 Chapter 17.1: Chromosomes, Genes and Proteins
Hello Biologists! This chapter is the absolute foundation of *Inheritance*. If you understand chromosomes and genes, you unlock the whole topic! We are going deep inside the nucleus to see how your traits are coded and built. Don't worry if this seems tricky at first; we will break it down using simple analogies!
1. The Blueprint of Life: DNA, Chromosomes, and Genes (Core)
Imagine the nucleus of your cell is a library. This library holds all the instructions needed to build and run you.
1.1 Chromosomes and DNA
- Chromosomes are long, thread-like structures found in the nucleus of almost all your cells.
- They are primarily made up of a huge chemical molecule called DNA (Deoxyribonucleic acid).
- DNA is the *complete instruction manual* for an organism.
1.2 Genes
- A gene is defined as a length of DNA that codes for a specific protein.
- Analogy: If the DNA is a huge cookbook, a single gene is a *specific recipe* (e.g., the recipe for making the protein called Insulin).
- These proteins are essential! They include enzymes, hormones, structural components (like collagen), and carriers in the cell membrane.
1.3 Alleles
- Chromosomes usually come in pairs (one from your mother, one from your father). This means you get two copies of every gene.
- An allele is an alternative form of a gene.
- For example, the gene for eye colour might have two alleles: one for brown eyes and one for blue eyes. They are different versions of the same instruction.
Quick Review: Core Concepts
Chromosome = Structure made of DNA.
Gene = Section of DNA that codes for a protein.
Allele = A version of that gene.
2. Chromosome Number: Diploid vs. Haploid (Extended)
The number of chromosomes in a cell tells us a lot about its job.
2.1 Diploid Nucleus (2n)
- A diploid nucleus contains two sets of chromosomes.
- In a diploid cell, there is a pair of each type of chromosome (one set from Mum, one set from Dad).
- This is characteristic of *most body cells* (somatic cells), such as skin, liver, and nerve cells.
- Humans have 23 pairs of chromosomes, totalling 46 chromosomes in a diploid cell. (2n = 46)
2.2 Haploid Nucleus (n)
- A haploid nucleus contains a single set of chromosomes.
- Haploid cells only have one copy of each chromosome type.
- This is characteristic of gametes (sex cells: sperm and egg).
- Humans have 23 *single* chromosomes in a haploid cell. (n = 23)
Memory Aid:
Diploid = Double (two sets).
Haploid = Half (single set).
3. The Role of DNA in Controlling Cell Function (Extended)
3.1 DNA and Protein Function
- DNA controls cell function by controlling the production of proteins.
- Proteins are the workhorses of the cell. DNA ensures the cell makes the right ones at the right time.
- Proteins controlled by DNA include: Enzymes (for metabolism), Membrane carriers (for transport), and Receptors (for communication).
3.2 DNA Base Sequence and Protein Shape
- The DNA molecule is like a sequence of letters (called bases).
- The sequence of bases in a gene determines the specific sequence of amino acids that will be linked together.
- Once the amino acid chain is built, it naturally folds up. This folding creates the protein's unique 3D shape.
- Different sequences of amino acids give different shapes to protein molecules. The shape is absolutely vital for the protein's function (e.g., the active site of an enzyme must be the right shape to fit the substrate).
3.3 Step-by-Step: How a Protein is Made
Proteins are built in the cytoplasm, but the DNA blueprint stays safely locked in the nucleus. The cell uses a messenger molecule (mRNA) to carry the instructions.
- The gene coding for the protein remains in the nucleus (DNA is too large and important to leave).
- A copy of the gene is made: this copy is called messenger RNA (mRNA). (It's a small, disposable copy.)
- The mRNA molecules are made in the nucleus and move to the cytoplasm.
- The mRNA passes through ribosomes (the protein-building factories).
- The ribosome assembles amino acids into protein molecules.
- The specific sequence of amino acids is determined precisely by the sequence of bases in the mRNA (which came directly from the gene).
Common Confusion Alert: You DO NOT need to know the details of transcription or translation (the specific steps of copying and reading the code). You only need to know this overall pathway and the roles of DNA, mRNA, and ribosomes.
3.4 Gene Expression
- Every cell in your body (except gametes) contains the same set of 46 chromosomes and therefore the same genes.
- However, a skin cell is very different from a liver cell because many genes are not expressed (not turned on) in a particular cell.
- The cell only makes the specific proteins it needs for its job. A nerve cell doesn't need to make the protein that carries oxygen in the blood, so that gene is switched off.
Key Takeaway: Genes and Proteins
The DNA sequence dictates the amino acid sequence, which dictates the protein shape, which dictates the protein's function (e.g., enzyme activity or cell structure).
4. Describing the Inheritance of Sex in Humans (Core)
In humans, 22 of the 23 pairs of chromosomes are called *autosomes*. The 23rd pair are the sex chromosomes, which determine whether you are male or female.
4.1 Sex Chromosomes
- Females have two identical sex chromosomes: XX
- Males have two different sex chromosomes: XY
4.2 The Cross
Sex is determined at fertilisation when the gametes (sperm and egg) fuse.
Parents' Genotypes:
Female (XX) x Male (XY)
Gametes Produced:
Female produces only X eggs.
Male produces 50% X sperm and 50% Y sperm.
Possible Offspring (The Punnett Square):
| Sperm (X) | Sperm (Y) | |
|---|---|---|
| Egg (X) | XX (Female) | XY (Male) |
The Results:
- There is a 50% chance of the offspring being XX (Female).
- There is a 50% chance of the offspring being XY (Male).
- Did you know? It is the sperm from the father that determines the sex of the baby, as the mother can only donate an X chromosome.
🌟 Comprehensive Review Box (17.1)
The Building Blocks:
DNA: Molecule making up chromosomes.
Gene: Section of DNA coding for a protein.
Allele: Alternative form of a gene.
Diploid (2n): Body cells (46 chromosomes in humans).
Haploid (n): Gametes (23 chromosomes in humans).
Function (Extended):
1. Gene base sequence dictates the amino acid sequence.
2. Amino acid sequence dictates the final 3D shape and function of the protein (e.g., enzyme).
3. Protein synthesis involves DNA (nucleus) -> mRNA (messenger) -> Ribosome (assembly).
Sex Inheritance (Core):
Males are XY. Females are XX. The resulting ratio of offspring sex is 1:1 (50% Female, 50% Male).
You've successfully covered the core concepts of molecular inheritance! Keep practising those definitions!