DNA & Chromosomes

Deoxyribonucleic acid (DNA) is the genetic material found in all living organisms. It is a polymer made up of repeating units called nucleotides. Each nucleotide consists of three components: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (Adenine (A), Guanine (G), Cytosine (C), or Thymine (T)). The DNA molecule typically exists as a double helix, resembling a twisted ladder. The two strands of the helix are held together by hydrogen bonds formed between complementary nitrogenous bases: Adenine always pairs with Thymine (A-T) via two hydrogen bonds, and Guanine always pairs with Cytosine (G-C) via three hydrogen bonds. The two strands run in opposite directions, meaning they are antiparallel (one runs 5' to 3' and the other 3' to 5'). The sugar-phosphate backbone forms the 'sides' of the ladder, while the paired bases form the 'rungs'.

DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. This process occurs during the S-phase of the cell cycle, ensuring that each daughter cell receives a complete set of genetic information. DNA replication is described as semi-conservative because each new DNA molecule produced consists of one original (parental) strand and one newly synthesised strand. The process involves: 1. Unwinding: The enzyme DNA helicase unwinds the double helix, separating the two strands. 2. Template formation: Each separated original strand acts as a template for the synthesis of a new complementary strand. 3. Synthesis: The enzyme DNA polymerase adds new nucleotides to the growing new strand, following the complementary base pairing rules (A-T, G-C). The new nucleotides are linked together to form a continuous strand. This results in two identical DNA double helices, each containing one old and one new strand.

Chromosomes are thread-like structures located inside the nucleus of eukaryotic cells. They are composed of DNA tightly coiled many times around proteins called histones, which support its structure. Chromosomes carry genetic information in the form of genes. They become visible as distinct structures during cell division (mitosis and meiosis) when the DNA is highly condensed. Humans typically have 46 chromosomes (23 pairs) in their somatic (body) cells, with one set of 23 inherited from each parent.

A gene is a specific segment of DNA on a chromosome that codes for a particular protein (or functional RNA molecule), which in turn determines a specific trait or characteristic. Genes are the fundamental units of heredity, passed from parents to offspring. Each gene occupies a specific position, called a locus, on a chromosome.

Alleles are different forms or versions of the same gene. They occupy the same locus (position) on homologous chromosomes. For example, the gene for eye colour might have different alleles that result in blue eyes, brown eyes, or green eyes. An individual inherits two alleles for each gene, one from each parent. These alleles can be identical (homozygous) or different (heterozygous).