Unit
- DNA double helix: The DNA molecule consists of two antiparallel strands coiled around a common axis, held together by hydrogen bonds between complementary base pairs adenine-thymine and guanine-cytosine. This structure enables replication and transcription.
- Nucleotide components: Each nucleotide comprises a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases. The sequence of bases encodes genetic information carried across generations.
- Chromosomal organization: Human genomic DNA is packaged into 23 pairs of chromosomes within the nucleus, with each chromosome containing a continuous DNA molecule. Mitochondrial DNA exists separately in the cytoplasm.
- Gene structure: A gene consists of coding regions (exons) interspersed with noncoding introns, along with regulatory elements such as promoters and enhancers that control transcription initiation and rate.
- Transcription process: RNA polymerase binds to the promoter region, unwinds the DNA strand, and synthesizes a complementary mRNA transcript. Post-transcriptional modifications include 5' capping, splicing, and polyadenylation.
- Genetic code: The mRNA sequence is read in triplets called codons, each specifying an amino acid. The 64 codons include start (AUG) and stop signals, with redundancy allowing synonymous substitutions.
- Protein synthesis: Ribosomes translate mRNA into polypeptide chains. Transfer RNA molecules bring specific amino acids corresponding to each codon, and elongation continues until a stop codon is reached.
- Mutation types: DNA sequence alterations range from single nucleotide variants to large structural changes such as deletions, duplications, inversions, and translocations. Mutations can be inherited or somatic.
- DNA replication: The process begins at origins of replication where helicase unwinds the double helix. DNA polymerase synthesizes new strands in the 5' to 3' direction with leading and lagging strand synthesis. Proofreading by exonuclease activity ensures high fidelity, critical for maintaining genomic stability across cell divisions.
- Epigenetic modifications: DNA methylation and histone modifications alter gene expression without changing the DNA sequence. These heritable marks influence chromatin accessibility and regulate developmental processes, disease susceptibility, and therapeutic responses, making them key targets in genomic medicine.
- RNA processing and alternative splicing: Pre-mRNA undergoes splicing to remove introns, but alternative splicing allows a single gene to produce multiple protein isoforms. This expands the proteome and is regulated by splicing factors, with implications for disease mechanisms and the development of targeted therapies.
- DNA repair pathways: Cells employ mechanisms such as base excision repair, nucleotide excision repair, and homologous recombination to fix DNA damage. Defects in these pathways underlie hereditary cancer syndromes like Lynch syndrome and BRCA related cancers, informing risk assessment and personalized treatment strategies.
- Noncoding RNAs: MicroRNAs and long noncoding RNAs regulate gene expression post transcriptionally. They can act as tumor suppressors or oncogenes, and their dysregulation is linked to many diseases. Understanding these molecules opens new diagnostic and therapeutic opportunities in genomic medicine.
Terminology
- Gene
- A functional unit of heredity made of DNA, encoding a product (RNA or protein).
- Allele
- Alternate forms of a gene at a given locus, may be normal or variant.
- Genome
- The complete set of genetic material of an organism, including all genes and noncoding sequences.
- Genotype
- The specific allelic composition of an individual at one or more loci.
- Phenotype
- The observable physical or biochemical traits resulting from genotype and environment interaction.
Quick Revision
- The DNA double helix is stabilized by hydrogen bonds between complementary base pairs.
- Genes contain exons and introns with regulatory regions that control expression.
- Transcription produces mRNA which is translated into protein using the genetic code.
- Mutations can be single nucleotide variants or larger structural changes.
- Key terms: gene, allele, genome, genotype, phenotype are fundamental to genomic medicine.
Disclaimer
Educational use only. Not intended for practical surgery, incisions, or other surgical procedures. It does not qualify you to perform physical operations.