Genetics Revision Notes

DNA as a Polymer

DNA (Deoxyribonucleic Acid) is a large biological polymer. Polymers are long molecules made up of many repeating smaller units. In the case of DNA, these smaller units are called nucleotides. The polymeric nature allows DNA to store vast amounts of genetic information in a compact form within cells.

Double Helix Structure

DNA consists of two strands that are coiled around each other, forming a shape known as a double helix. This structure can be compared to a twisted ladder. The two strands run in opposite directions and are antiparallel relative to each other.

Complementary Base Pairing

The two DNA strands are linked by specific pairs of chemical bases: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). These are known as complementary base pairs. These pairings are crucial for DNA replication and protein synthesis due to their specific hydrogen bonding.

Hydrogen Bonds

The complementary base pairs (A-T and G-C) are held together by weak hydrogen bonds. While individual hydrogen bonds are weak, the cumulative effect of many such bonds along the entire DNA molecule provides significant stability to the double helix structure. Their weakness also allows the strands to 'unzip' during replication and transcription.

Nucleotide Composition

Each DNA nucleotide is composed of three parts: a deoxyribose sugar molecule, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine). The sugar and phosphate groups form the 'backbone' of the DNA strand, with the bases projecting inwards.

The Genome: An Organism's Complete DNA Set

Your genome, and indeed every living organism's genome, is the full set of all the genetic information (DNA) that an organism possesses. It contains all the instructions needed to build and maintain that organism, from its earliest developmental stages through to adulthood. For humans, the genome is organised into 23 pairs of chromosomes found within the nucleus of most cells, along with a small amount of mitochondrial DNA.

What is a Gene?

A gene is a specific, functional segment of a larger DNA molecule. Its primary role is to provide the 'recipe' or code for synthesising a particular protein. These proteins are diverse in their functions, acting as enzymes, structural components, transporters, or signalling molecules, and are fundamental to cell and organism function. The sequence of nucleotide bases within a gene determines the sequence of amino acids in the protein it codes for.

Relationship between Genome, Genes, and DNA

Think of DNA as a very long instruction manual. The genome is the entire manual itself. Within that manual, a gene is like a specific chapter or page that contains instructions for making one particular component (a protein). Therefore, the genome is composed of all the DNA, and genes are specific working units or sections located along that DNA.

Purpose of Meiotic Division

Meiosis is a specific type of cell division necessary for sexual reproduction. Its primary role is to produce gametes (sex cells like sperm and eggs) that have half the number of chromosomes of the parent cell, ensuring that when two gametes fuse during fertilisation, the resulting zygote has the correct diploid chromosome number.

Outcomes of Meiosis

Meiotic division results in four daughter cells from one parent cell. Each daughter cell is haploid, meaning it contains only one set of chromosomes. These daughter cells are also genetically distinct from each other and the original parent cell, which generates genetic variation within a species.

Haploid Gametes

The four cells produced by meiosis are called gametes. They are haploid, meaning they contain half the normal number of chromosomes found in somatic (body) cells. This reduction is essential to maintain the correct chromosome count across generations after fertilisation.

Genetic Code and Amino Acid Sequence

The order of bases (A, T, C, G) in a DNA sequence determines the order of amino acids in a protein. This information flow goes from DNA to mRNA (transcription) and then to protein (translation). Each set of three bases on mRNA, called a codon, specifies one particular amino acid. The accuracy of this sequence is fundamental for proper protein structure.

Protein Folding and Function

After amino acids are linked into a polypeptide chain, they spontaneously fold into a specific three-dimensional (3D) shape. This folding is dictated by the unique sequence of amino acids and the chemical interactions (e.g., hydrogen bonds, hydrophobic interactions) between them. The specific 3D shape, particularly for enzymes, is essential for its biological function, as it forms the active site for substrate binding.

The Central Dogma

The process of genetic information transfer from DNA to RNA to protein is often referred to as the central dogma of molecular biology. This fundamental concept explains how genetic instructions are converted into functional products within a cell. Any alteration in the DNA base sequence can lead to a change in the amino acid sequence, potentially affecting the final protein's shape and thus its function.