DNA, genes and chromosomes

Eukaryotic DNA:

• Long, linear molecules.

• Associated with proteins called histones, which:

  • Compact the DNA to fit within the cell.

  • Play a role in regulating gene expression.

• DNA + histones = chromosome.

DNA arrangement in eukaryotic cells

Prokaryotic and Organelle DNA:

• Found in prokaryotes, mitochondria, and chloroplasts.

• Short, circular, and lacks histones (‘naked’ DNA).

DNA arrangement in prokaryotic cells

Chromatin and Chromosomes

For most of a cell’s life, DNA exists as chromatin:

• A dispersed form in the nucleus.

• Cannot be seen under a light microscope.

After DNA replication, chromatin condenses into visible chromosomes:

• Each chromosome appears X-shaped, made of:

  • Sister chromatids: Two identical copies of DNA.

  • Joined at the centromere.

Homologous Chromosomes:

• Found in eukaryotic organisms.

• Chromosomes of the same size, shape, and banding pattern.

• One copy inherited from each parent.

• Example: Humans have 23 pairs of chromosomes (46 total).

Chromosome structure and naming system

Genes and the Coding Sequence

What is a Gene?

• A gene is a short sequence of DNA that codes for:

  • A polypeptide (a protein-building block).

  • A functional RNA molecule (e.g., tRNA or ribosomal RNA).

• Each gene occupies a specific location on a chromosome, called a locus.

Gene Structure

DNA sequence within a gene consists of:

• Triplets of bases: Each triplet codes for one specific amino acid.

• The sequence of amino acids forms a polypeptide chain, which folds into a functional protein.

Exons: Coding segments of a gene.

Introns: Non-coding segments within a gene.

• Introns are removed during mRNA processing before protein synthesis.

The removal of intron sequences during mRNA production

Non-Coding DNA:

• Much of eukaryotic DNA does not code for proteins.

• Includes:

  • Non-coding repeated sequences (function not fully understood).

  • Sequences that may lengthen due to replication errors.

The genetic code

The genetic code determines how triplets of DNA bases are translated into amino acids. It has these key features:

• Universal: Used by almost all organisms.

• Non-Overlapping: Each base is part of only one triplet.

• Degenerate: Most amino acids are coded by more than one triplet, allowing for redundancy.

The triplet code is non-overlapping

Definitions