Genetics Revision Notes

The Genome

The genome is defined as the entire set of DNA found within an organism. This includes all chromosomes and any other DNA-containing structures like mitochondria or chloroplasts in eukaryotic cells. It represents the complete genetic blueprint unique to that species or individual. Studying the genome allows scientists to understand an organism's full genetic potential and characteristics.

What is a Gene?

A gene is a specific, functional segment of a larger DNA molecule. Its primary role is to code for, or provide the instructions for, the synthesis of a particular protein. These proteins are crucial for cell structure, function, and regulation, ultimately dictating an organism's traits. Each gene has a unique sequence of bases that determines the sequence of amino acids in the protein it codes for.

Relationship between Genome and Genes

Think of the genome as an entire library of genetic information, where each book represents a chromosome. Within each 'book' (chromosome), there are many individual 'chapters' or 'paragraphs' that are the genes. Each gene contains the specific instructions for making one specific protein. Therefore, genes are components of the genome, but the genome is the sum total of all these genetic instructions.

DNA as a Polymer

DNA is a polymer, meaning it is a large molecule made up of repeated smaller units called monomers. In the case of DNA, these monomers are nucleotides. This polymeric structure is essential for storing vast amounts of genetic information.

Double Helix Structure

The DNA molecule consists of two long strands coiled around each other to form a double helix. This iconic structure was described by Watson and Crick and is fundamental to how DNA functions in carrying and transmitting genetic information.

Nucleotide Composition

Each nucleotide, the basic building block of DNA, is comprised of three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine). The sugar and phosphate form the backbone of each strand.

Complementary Base Pairing

The two strands of DNA are linked by a series of complementary base pairs. Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). These pairs are held together by weak hydrogen bonds, which allow for easy separation during DNA replication and transcription.

Purpose of Meiosis

Meiosis is a type of cell division that produces gametes (sex cells), like sperm and egg cells, necessary for sexual reproduction. It ensures that offspring inherit genetic material from both parents, maintaining the species' chromosome number, and introduces genetic diversity.

Outcome of Meiosis

A single parent cell undergoes two divisions to produce four daughter cells. These daughter cells are haploid, meaning they contain half the number of chromosomes as the original parent cell. Each of these four daughter cells is genetically distinct due to genetic recombination during meiosis.

Genetic Diversity

One of the key outcomes of meiosis is the creation of genetically distinct gametes. This genetic diversity is vital for the survival and adaptation of a species, as it provides a wider range of traits for natural selection to act upon, contributing to the evolution of populations.

The Purpose of Each Step in DNA Extraction

Mashing/blending physically breaks cell walls. Salt (sodium ions) helps neutralise DNA's negative charge, aiding precipitation. Detergent disrupts lipid membranes, releasing DNA. Filtering removes cellular debris. Cold ethanol/propan-2-2ol precipitates DNA, making it visible.

Chemicals Used and Their Functions

Salt solution: provides positive ions (e.g., sodium) to neutralise the negative charge of DNA, allowing strands to clump. Detergent: emulsifies lipids in cell and nuclear membranes, breaking them down to release DNA. Ethanol/propan-2-ol (cold): causes DNA to become insoluble and precipitate out of the aqueous solution, allowing for its isolation.