DNA and Protein Synthesis » Chromosomes and Genes

What you'll learn this session

Study time: 30 minutes

The structure and function of chromosomes
What genes are and where they're located
The relationship between DNA, genes and chromosomes
How genetic information is organised
The role of chromosomes in inheritance
How chromosomes determine sex in humans

Chromosomes: The Carriers of Genetic Information

Inside almost every cell in your body is a complete set of instructions for making you. These instructions are packaged into structures called chromosomes, which are found in the nucleus of your cells. Let's explore what chromosomes are and how they relate to genes and DNA.

Key Definitions:

Chromosome: A thread-like structure made of DNA and protein that carries genetic information.
Gene: A section of DNA that contains instructions for making a specific protein or controlling a characteristic.
DNA: Deoxyribonucleic acid, the molecule that contains the genetic code.
Genome: The complete set of genetic material in an organism.

🌍 Human Chromosomes

Humans have 46 chromosomes arranged in 23 pairs. One chromosome from each pair comes from your mother and one from your father. This is why you inherit traits from both parents!

🔬 Chromosome Structure

Chromosomes are made of DNA tightly wrapped around proteins called histones. This packaging helps fit the long DNA molecule into the tiny cell nucleus.

The DNA-Gene-Chromosome Relationship

Understanding the relationship between DNA, genes and chromosomes is crucial for grasping how genetic information is organised and passed on.

From DNA to Chromosomes: The Hierarchy

Think of your genetic information as being organised in a hierarchy:

🧬 DNA

The basic building block. DNA is a long molecule made of units called nucleotides (A, T, G and C).

📜 Genes

Sections of DNA that code for specific proteins or traits. Humans have about 20,000-25,000 genes.

✨ Chromosomes

Structures that contain many genes bundled together with proteins.

This relationship can be compared to how information is organised in a library:

DNA nucleotides are like the letters of the alphabet
Genes are like individual books (containing specific information)
Chromosomes are like bookshelves holding many books
The nucleus is like the entire library building

Did You Know? 💡

If you stretched out all the DNA from a single human cell, it would be about 2 metres long! Your body has trillions of cells, so all your DNA could stretch to the Sun and back multiple times.

Chromosomes and Cell Division

Normally, chromosomes exist as loosely arranged threads in the nucleus, making them difficult to see even with a microscope. However, during cell division, they condense and become visible as distinct X-shaped structures.

📊 Mitosis

When body cells divide, chromosomes are duplicated and distributed equally to the two new cells. This ensures each new cell has a complete set of genetic information.

🧝 Meiosis

When sex cells (eggs and sperm) are produced, the chromosome number is halved. This ensures that when fertilisation occurs, the normal chromosome number is restored.

Genes: The Units of Inheritance

Genes are the basic units of heredity. Each gene carries instructions for making a specific protein, which in turn affects a particular trait or characteristic.

Gene Structure and Function

Genes are made up of DNA sequences that follow a specific pattern. The genetic code uses the sequence of nucleotides (A, T, G and C) to specify which amino acids should be used to build proteins.

Key points about genes:

Genes can exist in different versions called alleles
Different alleles lead to variations in traits (like eye colour)
Some genes are dominant while others are recessive
Genes can be switched on or off in different cells

Case Study Focus: Gregor Mendel

Gregor Mendel, often called the "father of genetics," discovered the basic principles of inheritance by studying pea plants in the 1860s. He identified that traits were passed down in predictable patterns, which we now know is due to genes. Interestingly, Mendel made these discoveries without ever seeing a chromosome or knowing about DNA!

Sex Chromosomes: Determining Biological Sex

Of the 23 pairs of chromosomes in humans, 22 pairs are called autosomes and are the same in males and females. The 23rd pair consists of the sex chromosomes, which determine biological sex.

♀ Females

Females have two X chromosomes (XX).

♂ Males

Males have one X and one Y chromosome (XY).

During fertilisation:

If a sperm carrying an X chromosome fertilises the egg: XX = female
If a sperm carrying a Y chromosome fertilises the egg: XY = male

The Y chromosome contains a gene called SRY (Sex-determining Region Y) that triggers male development. Without this gene, female development occurs.

Chromosomal Disorders

Sometimes errors occur in the number or structure of chromosomes, leading to genetic disorders.

Examples of Chromosomal Disorders

These disorders occur when there are extra or missing chromosomes, or when parts of chromosomes are altered:

Down syndrome: Caused by an extra copy of chromosome 21 (trisomy 21)
Turner syndrome: Females with only one X chromosome (XO)
Klinefelter syndrome: Males with an extra X chromosome (XXY)

Karyotyping 🔍

Scientists can visualise and analyse chromosomes by creating a karyotype a photograph of all the chromosomes in a cell, arranged in pairs by size. Karyotyping can identify chromosomal abnormalities and is often used in prenatal testing.

Genes and Protein Synthesis: The Connection

The main function of genes is to control the production of proteins through a process called protein synthesis. This involves two main steps:

📝 Transcription

The DNA code is copied to make a molecule called messenger RNA (mRNA).

🏭 Translation

The mRNA code is read by ribosomes to assemble amino acids into proteins.

This process is how your genetic information (genotype) leads to your physical characteristics (phenotype). For example, genes coding for the protein melanin determine your skin, hair and eye colour.

Summary: The Big Picture

Let's tie everything together:

Chromosomes are structures in the cell nucleus that contain your genetic information
Genes are sections of DNA on chromosomes that code for specific traits
Humans have 46 chromosomes (23 pairs), including the sex chromosomes (XX or XY)
Genes control the production of proteins through transcription and translation
Variations in genes (alleles) lead to the diversity we see among individuals

Understanding chromosomes and genes is fundamental to understanding inheritance, genetic disorders and how our bodies develop and function.

🧠 Test Your Knowledge!