Introduction
This topic covers the material basis of heredity: how the nuclear genome is organised, the structure and function of genes, how genetic information is expressed (transcription, RNA processing, translation), the life cycle of mRNA, and how gene activity is regulated at several levels, including epigenetics.
I. Organization of the nuclear genome
The human nuclear genome is the complete set of DNA sequences located inside the cell nucleus. It contains around 3.2 billion base pairs, arranged into 46 chromosomes (22 pairs of autosomes and 2 sex chromosomes).
Levels of DNA packaging
- At the most basic level, DNA is wrapped around histone proteins to form nucleosomes (147 bp of DNA + histone octamer).
- These nucleosomes fold into a 30 nm chromatin fiber.
- Chromatin can be of two main types:
- Euchromatin: less condensed, transcriptionally active, contains most genes.
- Heterochromatin: tightly packed, transcriptionally silent, usually repetitive DNA (e.g., centromeres, telomeres).
- Further compaction leads to the chromosome structure seen during metaphase.
Functional elements of the genome
- Exons: coding DNA, only ~1.5% of the genome.
- Introns: non-coding segments within genes, removed by splicing.
- Regulatory DNA: promoters, enhancers, silencers that control expression.
- Repetitive sequences: satellites, microsatellites, transposable elements.
- Non-coding RNAs: tRNA, rRNA, miRNA, lncRNA, all important for gene regulation and expression.
Note: Although only a small fraction directly encodes proteins, non-coding regions have major roles in controlling how and when genes are expressed.