Genetics

Introduction to Genetics

Genetics is the study of genes, heredity, and genetic variation in living organisms. Its modern foundations were laid by Gregor Mendel in the 1860s, though the molecular underpinning — DNA as the genetic material — was not established until the mid-twentieth century. Today, genetics spans molecular biology, cell biology, evolutionary biology, and medicine.

🔑 Core Concepts of Genetics

Gene: A segment of DNA encoding a functional product (protein or RNA).

Allele: Alternative forms of a gene at a specific locus.

Genotype: The genetic composition of an organism.

Phenotype: The observable characteristics expressed from the genotype + environment.

DNA → RNA → Protein: The central dogma of molecular biology.

DNA: Structure and Replication

Deoxyribonucleic acid (DNA) is the molecule that stores hereditary information. Watson, Crick, Franklin, and Wilkins established its double-helical structure in 1953. Key structural features:

Two antiparallel strands wound in a right-hand helix.

Each strand is a polymer of nucleotides (base + deoxyribose sugar + phosphate).

Complementary base pairing: A–T (two hydrogen bonds) and G–C (three hydrogen bonds).

The sugar-phosphate backbone provides structural support; the base sequence encodes information.

DNA Replication

Before cell division, DNA must be accurately copied. Replication is semi-conservative — each new double helix consists of one original strand and one newly synthesized strand:

Error rate: ~1 in 10⁹ nucleotides after proofreading — extraordinary fidelity enabling faithful transmission of genetic information across generations.

1. Helicase unwinds the double helix at the origin of replication. 2. Primase lays short RNA primers to start synthesis. 3. DNA polymerase III synthesizes new strands (5'→3' direction only). 4. Leading strand: continuous synthesis. Lagging strand: discontinuous Okazaki fragments. 5. DNA ligase seals the fragments.

From DNA to Protein: The Central Dogma

Transcription (DNA → RNA)

RNA polymerase reads the DNA template strand (3'→5') and synthesizes a complementary messenger RNA (mRNA) strand (5'→3'). In eukaryotes, the primary transcript (pre-mRNA) undergoes processing:

5' capping: a modified guanosine cap added for stability and ribosome recognition.

3' polyadenylation: a poly-A tail added for stability and export from the nucleus.

Splicing: introns (non-coding sequences) are removed; exons are joined by the spliceosome.

Alternative splicing allows one gene to produce multiple protein isoforms — a major source of proteomic diversity in eukaryotes.

Translation (RNA → Protein)

Ribosomes translate the mRNA sequence into amino acid sequence. The genetic code: each triple of nucleotides (codon) specifies one of 20 amino acids (or a stop signal). Transfer RNA (tRNA) molecules carry the correct amino acid to the ribosome, matching anticodon to codon. Translation proceeds through initiation, elongation (peptide bond formation), and termination.

Mendelian Genetics

Gregor Mendel's laws, derived from pea plant experiments (1865), form the foundation of classical genetics:

Law of Segregation

Each organism carries two alleles for every gene. During gamete formation, allele pairs separate (segregate) so each gamete receives only one allele.

Law of Independent Assortment

Genes on different chromosomes are inherited independently of one another. (Non-linked genes only — linked genes on the same chromosome violate this law.)

Dominance

In a heterozygous organism (Aa), the dominant allele (A) determines the phenotype. The recessive allele (a) is "hidden" — expressed only in homozygous recessive (aa) organisms.

Mutations and Genetic Variation

Mutations are heritable changes in DNA sequence — the raw material for evolution. Types:

Point mutations: substitution of one nucleotide. Synonymous (silent), missense (amino acid change), or nonsense (premature stop codon).

Insertion/deletion (indels): adding or removing nucleotides; can cause frameshift mutations altering all downstream codons.

Chromosomal mutations: deletions, duplications, inversions, translocations of large chromosomal segments.

Copy number variations (CNVs): sections of the genome that are duplicated or deleted in different individuals.

Epigenetics

Epigenetics studies heritable changes in gene expression that do not alter the underlying DNA sequence. Key mechanisms:

DNA methylation: methyl groups added to cytosine residues typically silence gene expression. Critical for genomic imprinting, X-chromosome inactivation, and cancer biology.

Histone modification: acetylation (activating), methylation, phosphorylation, and ubiquitination of histone tails alter chromatin structure and accessibility.

Non-coding RNAs: microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) regulate gene expression post-transcriptionally.

Epigenetic marks can be influenced by diet, stress, and environmental exposures, and some marks can be transmitted to offspring (transgenerational epigenetics).

CRISPR-Cas9: Precision Gene Editing

CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats) is a revolutionary gene-editing technology derived from a bacterial immune system. Discovered by Jennifer Doudna and Emmanuelle Charpentier (2012 Nobel Prize in Chemistry).

How it works:

A guide RNA (gRNA) ≈ 20 nucleotides long is designed to match the target DNA sequence.

The gRNA directs the Cas9 nuclease to the target site (which must be adjacent to a PAM sequence: NGG for SpCas9).

Cas9 makes a double-strand break in the DNA.

The cell repairs the break via NHEJ (error-prone → gene knockout) or HDR if a repair template is provided (precise edits, gene correction).

Applications: correcting genetic diseases (sickle cell anemia, beta-thalassemia trials underway), agricultural crop improvement, basic research, diagnostics (SHERLOCK, DETECTR), and potential therapies for HIV and cancer.

Try it live

Everything above runs in your browser — open Genetics: DNA Replication & CRISPR Editing and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.