What is the primary difference between a genome and a proteome?

The genome is the complete set of DNA instructions in a cell, which remains relatively constant. The proteome is the entire set of proteins expressed by a cell at a specific time, which changes constantly based on environmental needs and cell type.

How does a prokaryotic genome differ from a eukaryotic genome in terms of structure?

Prokaryotic genomes are typically composed of a single, circular DNA molecule located in the cytoplasm. Eukaryotic genomes consist of multiple linear chromosomes housed within a membrane-bound nucleus.

What is the difference between an exon and an intron?

Exons are the coding regions of a gene that are expressed and translated into protein. Introns are non-coding sequences located between exons that are removed (spliced out) during the processing of RNA.

What is the 'C-value paradox' and why is it a common source of student error?

The C-value paradox is the observation that genome size does not correlate with an organism's complexity. Students often incorrectly assume that 'more DNA' equals a 'more advanced' organism, which is biologically false.

Why is it incorrect to refer to non-coding DNA as 'junk DNA'?

While non-coding DNA does not produce proteins, it contains vital regulatory elements like enhancers and promoters that control gene expression. It also provides structural support for chromosomes and plays roles in genome evolution.

What mistake is made when assuming all cells in a multicellular organism have different genomes?

Almost all somatic cells in an organism contain the exact same genome. The differences in cell function (e.g., skin vs. muscle) arise from differential gene expression, not from having different sets of DNA.

Define 'Single Nucleotide Polymorphism' (SNP).

A SNP is a variation at a single position in a DNA sequence among individuals. If more than 1% of a population does not carry the same nucleotide at a specific position, that variation is classified as a SNP.

What is a 'Haploid Genome'?

A haploid genome refers to a single set of chromosomes, such as those found in a germ cell (sperm or egg). In humans, the haploid genome consists of 23 chromosomes and approximately 3 billion base pairs.

Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order the DNA into structural units called nucleosomes. They act as spools around which DNA winds to reduce its physical volume.

How does the genome facilitate the 'Central Dogma' of molecular biology?

The genome serves as the permanent storage of information. It is transcribed into RNA, which is then translated into proteins, allowing the genetic code to be converted into functional biological molecules.

The Genome | AQA AS-Level Biology

Genetics, Variation & Interdependence

The Genome

Summary

The genome represents the complete set of genetic instructions required to build, operate, and maintain a living organism. It encompasses all hereditary information encoded in DNA (or RNA for some viruses), including both functional genes and extensive non-coding sequences that regulate gene expression and maintain structural integrity.

1. Definition & Core Concepts

The Genome: This is the entirety of an organism's hereditary information. In most living things, it is composed of Deoxyribonucleic Acid (DNA), organized into specific sequences that act as a biological blueprint.
Genomics: This field of study focuses on the structure, function, evolution, and mapping of genomes. Unlike genetics, which often looks at single genes, genomics takes a holistic view of the entire genetic landscape.
Base Pairs: The fundamental units of the genome are chemical base pairs (Adenine-Thymine and Cytosine-Guanine). The specific order of these billions of pairs determines the biological characteristics of the organism.

A diagram showing the DNA double helix structure on the left and a breakdown of genomic components (coding, non-coding, and regulatory) on the right.

2. Structural Organization

Chromosomes: To fit inside a cell nucleus, the long strands of DNA are tightly coiled around proteins called histones, forming structures known as chromosomes. Humans typically possess 23 pairs of these structures.
Genes: These are specific functional units within the genome. A gene is a sequence of DNA that contains the instructions for making a specific protein or RNA molecule.
Non-coding DNA: Often mistakenly called 'junk DNA,' these regions do not code for proteins but are essential for regulating when and where genes are turned on, as well as providing structural stability to chromosomes.

3. Functional Components

Exons: These are the 'coding' sequences of a gene that remain in the final messenger RNA (mRNA) after processing and are translated into protein sequences.
Introns: These are 'intervening' sequences found within genes that are removed during RNA splicing. While they don't code for protein, they play roles in gene regulation and evolution.
Regulatory Sequences: Elements like promoters and enhancers act as switches. They determine the intensity and timing of gene expression, ensuring that a heart cell behaves differently than a brain cell despite having the same genome.

4. Key Distinctions

5. Exam Strategy & Tips