GradeGen.AI

The Genetic Control Centre

Every time you look at a family photo, you can see why understanding the nucleus is so important. The physical traits you share with your parents are determined by incredibly long, tightly coiled molecules hidden inside nearly every cell in your body.

Describing the Structure of Genetic Material

In eukaryotic cells (such as animal and plant cells), the genetic material is enclosed within the nucleus. The nucleus acts as the control centre of the cell, storing the instructions needed for growth and development.

To describe this structure accurately in an exam, you must explain it step-by-step from largest to smallest:

First, the nucleus contains thread-like structures called chromosomes. Most of the time, these exist as single strands and are too thin to be seen, only condensing into visible X-shapes just before a cell divides.
Next, these chromosomes are made up of a single, highly coiled DNA molecule.
Finally, a gene is a small section of this DNA located on a chromosome. Each chromosome carries a large number of genes.

DNA itself has a highly specific chemical structure. It is a polymer made up of two strands twisted together to form a double helix.

Each individual gene codes for a specific sequence of amino acids. These amino acids are then joined together in that exact order to form a specific protein, which ultimately determines a characteristic.

Important Exception: Mature red blood cells are a common exam trick. They lose their nucleus during maturation to carry more oxygen, meaning they do NOT contain a nucleus, chromosomes, or DNA.

Chromosomes in Body Cells

In the nucleus of normal human body cells, which are known as somatic cells, chromosomes do not float around randomly in isolation. Instead, they normally occur in pairs.

Because these cells contain two complete sets of chromosomes, they are described as being (often written as ). Humans have a total of 46 individual chromosomes, which are arranged into 23 .

Students often state that genes "make proteins" directly. You must specify that a gene codes for a specific sequence of "amino acids", which then join together to form a protein.

In questions asking you to describe the structure of DNA, examiners explicitly look for the terms "polymer" and "double helix" — failing to use these exact terms will cost you marks.

When an exam question asks "how many chromosomes" a human cell has, always specify whether you mean the total number (46) or the number of pairs (23) to avoid losing marks for ambiguity.

If asked to identify biological sex from a chromosome diagram (karyotype), look strictly at the 23rd pair: an identical pair means female (XX), while a non-identical pair means male (XY).

Watch out for application questions about red blood cells — remember that because they lack a nucleus, they contain zero chromosomes and zero DNA.

The organelle in eukaryotic cells that contains genetic material and controls cell activities.

A thread-like structure of highly coiled DNA and protein found in the nucleus, carrying genetic information in the form of genes.

A polymer made of two strands forming a double helix; the chemical that all genetic material in a cell is made of.

A small section of DNA on a chromosome that codes for a particular sequence of amino acids to make a specific protein.

A chemical made up of many repeating units joined together, such as the two strands that make up DNA.

The twisted ladder shape of a DNA molecule, formed by two intertwined polymer strands.

The building blocks that are joined together in specific sequences to form proteins.

A large molecule made of a specific sequence of amino acids, as coded for by a gene.

Any normal body cell of a living organism, other than the reproductive cells (gametes).

A cell or nucleus containing two complete sets of chromosomes, one from each parent (e.g., human body cells containing 46 chromosomes).

A pair of chromosomes (one maternal, one paternal) that are similar in shape and size and carry the same genes at the same locations.

Different versions of the same gene found at the same location on homologous chromosomes.

The 23rd pair of chromosomes in humans that determine biological sex (XX for females, XY for males).

Reproductive cells (such as sperm and egg) that contain only one chromosome from each pair.

A cell or nucleus containing a single set of unpaired chromosomes (e.g., human gametes containing 23 chromosomes).

A diploid cell formed by the fusion of two haploid gametes during fertilization.