Genes, inheritance and selection Study Guide

The genome is the complete set of genetic material of an organism. Think of it as the instruction manual for building and operating an organism. In humans and many other organisms, this genetic material is DNA. DNA is organised into structures called chromosomes, which are found within the nucleus of cells. Each chromosome contains many genes, which are specific segments of DNA that carry instructions for making proteins or functional RNA molecules. These proteins then carry out most of the work in cells and are required for the structure, function, and regulation of the body's tissues and organs. The genome includes all coding regions (genes) and non-coding regions of DNA. While genes provide the instructions, non-coding regions play important roles in regulating gene expression and maintaining chromosome structure. Understanding the genome allows scientists to study genetic diseases, understand evolutionary relationships between species, and develop new treatments and technologies like gene therapy.

Meiosis is a specialised type of cell division that is essential for sexual reproduction. Its primary role in sexually reproducing organisms is to halve the chromosome number in the parent cell to produce gametes (sex cells) such as sperm and egg cells. A typical body cell in a human contains 46 chromosomes, arranged in 23 pairs; this is known as the diploid number (2n). For sexual reproduction to maintain a constant chromosome number across generations, the gametes must be haploid (n), meaning they contain only one chromosome from each homologous pair, resulting in 23 chromosomes in total. Meiosis achieves this reduction through two rounds of division, Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes separate, reducing the chromosome number by half. In Meiosis II, sister chromatids separate, similar to mitosis, but starting from cells that already have a reduced chromosome number. This process ensures that when a male gamete (sperm) fuses with a female gamete (egg) during fertilisation, the resulting zygote will have the correct diploid number of chromosomes (46 in humans), inheriting half its genetic material from each parent. Meiosis also introduces genetic variation through processes like crossing over and independent assortment, but its fundamental role regarding chromosome number is the reduction from diploid to haploid for gamete formation.