Molecular Genetics

DNA is the genetic blueprint which codes for and determines the physical, behavioural and physiological characteristics of an organism

A sequence of DNA which codes for a specific characteristic is a gene and the totality of genes in an organism is referred to as the genome

Organisation of DNA

In a prokaryote, the DNA is naked and circular (main genophore with additional plasmids) and is located in the nucleoid region of the cytoplasm

In a eukaryote, the DNA is linear, packaged with proteins and is localised within a membrane-bound nucleus

  • The DNA is wrapped around histone proteins to form the basic structure of the nucleosome
  • Nucleosomes are then packed together to form chromatin (in a 'beads on a string' arrangement)
  • Chromatin will supercoil and condense during cell division to form chromosomes that can be visualised under a light microscope

Arrangement of Eukaryotic DNA into chromosomes

Central Dogma of Molecular Biology

DNA codes for physical features by producing specific proteins which carry out specific, specialised functions

  • A gene sequence is converted into a polypeptide sequence via the processes of transcription (making an mRNA transcript) and translation (polypeptide synthesis)
  • Translation uses tRNA molecules and ribosomes to join amino acids into a polypeptide chain according to the mRNA sequence (as read in codons) 
  • When a gene is mutated it may lead to the synthesis of a defective polypeptide, hence affecting protein function and potentially changing the associated characteristic
  • The entirety of proteins expressed by a genome, cell, tissue or organism is called the proteome
    • A cellular proteome describes the collection of proteins produced in a particular cell type under set conditions (e.g. hormonal stimulation)
    • The complete proteome more loosely describes the complete set of proteomes from all cell types in an organism
  • New cells are made when the DNA is replicated (this process occurs during the S phase of interphase)

The 'One Gene - One Polypeptide' Rule

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