Genetics: Genes and DNA Structure

• A gene is the fundamental unit of inheritance, located at a specific site on a chromosome known as the locus.
• Genes dictate the expression of observable traits, referred to as the phenotype, through the synthesis of proteins.
• In eukaryotic cells, genes are located within the nucleus, whereas in prokaryotic cells, they are found on a single chromosome in the cytoplasm.
• Eukaryotic cells also contain genes in the mitochondria and, in plants, chloroplasts.
• The quantity of genes varies among organisms; human cells have around 20,000 genes, while bananas possess approximately 30,000.

Gene Structure

• Genes are encoded by long strands of DNA, which consist of nucleotide monomers.
• Nucleotides are composed of a pentose sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G).
• DNA exists as a double helix, with two antiparallel DNA strands linked by hydrogen bonds.
• Adenine pairs with thymine via double hydrogen bonds, and cytosine pairs with guanine via triple hydrogen bonds.
• Genes comprise a sequence of ATCG nucleotides, including non-coding repeats (introns) and protein-coding sequences (exons).

Eukaryotic vs Prokaryotic Gene Structure

• Eukaryotic cells have long DNA strands condensed in the nucleus, wrapped around histone proteins to form nucleosomes.
• Prokaryotic cells have DNA or chromosomes floating freely in the cytoplasm.
• Eukaryotic genes have non-coding intronic regions, while prokaryotic genes are organized into operon-based gene clusters.

Gene Components

• Genes are composed of nucleotides, which consist of a sugar, a nitrogenous base, and a phosphate group.
• The sugar in DNA is deoxyribose, a five-carbon sugar derived from ribose.
• Nitrogenous bases are classified as purines (adenine and guanine), which have a double ring structure, and pyrimidines (cytosine and thymine), which have a single ring.
• The sugar-phosphate group forms the DNA strand's backbone, and purines and pyrimidines form hydrogen bonds to create an antiparallel helical structure.
• The ATCG sequence determines the gene's function.

Functional Components

• Genes have a promoter region and a coding region.
• The promoter region, located upstream of the coding segment, regulates when and where a gene is expressed.
• Promoters are cis-acting elements and are composed of a nucleotide sequence.
• Enhancers, another type of cis-acting element, are located at a distance from the coding region and also control gene expression.
• Trans-acting elements are proteins in the nucleus that control gene expression.

How Genes Work

• Genes provide a code for protein expression and formation.
• Enzymes read genes to produce a complementary mRNA strand through transcription.
• mRNA is then translated into proteins in ribosomes.
• Proteins are the building blocks of life, comprising enzymes and structural components like skin, bones, and hair.
• Genes are directly responsible for observable traits, known as the phenotype.
• The Human Genome Project, completed in 2003, revealed that humans have approximately 2,000 protein-coding genes.
• Cells contain two copies of each gene, one inherited from each parent.
• Different versions of a gene are called alleles, which can differ in their nucleotide sequence.
• Dominance determines which allele is expressed as a phenotype, where one version of the gene is dominant over others.
• For example, eye color genes have a dominant allele "B" (brown eyes) and a recessive allele "b" (blue eyes).
• Genotypes BB and Bb result in brown eyes, while the genotype bb results in blue eyes.

Explore the structure and function of genes, the fundamental units of heredity. Learn about their location on chromosomes, their role in protein synthesis, and their composition of DNA nucleotides. Discover the quantity of genes in different organisms.