3.1 Genetics and DNA

Genetics is the study of heredity—how traits pass from parents to offspring and how those traits are encoded and expressed. On the NLN NEX Science exam, genetics and DNA sit inside the Biology domain (about 36% of the Science section). Expect questions on DNA structure, the flow of genetic information, alleles, simple Mendelian crosses, mutations, and the difference between autosomal and sex-linked inheritance. Nursing programs care about this material because genetic counseling, newborn screening, and common hereditary conditions (such as sickle cell disease) show up early in clinical education.

DNA Structure

Deoxyribonucleic acid (DNA) stores the genetic instructions of nearly all living organisms. Its famous shape is a double helix: two long strands twisted around each other like a spiral staircase.

Each strand is a polymer of nucleotides. A nucleotide has three parts:

1. A five-carbon sugar (deoxyribose in DNA)
2. A phosphate group
3. A nitrogenous base

There are four bases in DNA:

Complementary base pairing is fixed: A always pairs with T (two hydrogen bonds), and G always pairs with C (three hydrogen bonds). The sugar-phosphate backbone runs along the outside of each strand; the bases form the "rungs" of the ladder. The two strands run antiparallel (one 5′→3′, the other 3′→5′).

RNA differs from DNA in three exam-ready ways: the sugar is ribose, the base uracil (U) replaces thymine, and RNA is usually single-stranded. Messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA) all participate in gene expression.

Genes and Chromosomes

A gene is a segment of DNA that typically codes for a polypeptide or functional RNA. Genes are arranged along chromosomes, which are long DNA molecules packaged with proteins (histones in eukaryotes).

Humans have 46 chromosomes in most body (somatic) cells—23 pairs. Twenty-two pairs are autosomes (same structure in males and females). The 23rd pair is the sex chromosomes: XX typically in females, XY typically in males. Gametes (sperm and egg) are haploid (n = 23); fertilization restores the diploid number (2n = 46).

Locus means the physical location of a gene on a chromosome. Alleles are alternative versions of the same gene (for example, a allele for free earlobes versus attached earlobes). An individual is homozygous if both alleles at a locus are the same (AA or aa) and heterozygous if they differ (Aa).

DNA Replication

Before a cell divides, DNA must be copied so each daughter cell receives a full genome. Replication is semi-conservative: each new double helix keeps one original (parental) strand and one newly synthesized strand.

Key enzymes and steps (high-level):

• Helicase unwinds the double helix, creating a replication fork
• DNA polymerase adds complementary nucleotides to each template strand (always synthesizing new DNA 5′→3′)
• Primase lays short RNA primers so polymerase can start
• Ligase seals gaps on the lagging strand (Okazaki fragments)

Proofreading by DNA polymerase keeps the error rate low, but mistakes that persist become mutations.

Transcription and Translation (Gene Expression Overview)

The central dogma of molecular biology:

DNA → RNA → protein

Transcription

Transcription occurs in the nucleus of eukaryotic cells. RNA polymerase reads a DNA template and builds a complementary mRNA strand. In mRNA, U pairs with A (instead of T). Primary transcripts are processed (including splicing out of introns and joining exons) before mature mRNA leaves the nucleus.

Translation

Translation occurs at the ribosome in the cytoplasm. mRNA is read in groups of three bases called codons. Each codon (with few exceptions) specifies one amino acid or a stop signal. tRNA molecules carry amino acids and match codons via complementary anticodons. The ribosome links amino acids into a growing polypeptide, which folds into a functional protein.

The genetic code is nearly universal across life and is redundant (degenerate): most amino acids have more than one codon, which softens the impact of some mutations.

Genotype, Phenotype, and Mendelian Inheritance

• Genotype: the genetic makeup (allele combination) at one or more loci
• Phenotype: the observable trait (height range, blood type, presence of a disease, enzyme activity)

Gregor Mendel’s classic rules (from pea plants) still frame NEX-style genetics items:

• Dominance: in a heterozygote, a dominant allele is expressed in the phenotype; a recessive allele is masked and appears only when homozygous recessive
• Segregation: the two alleles of a gene separate during gamete formation (meiosis) so each gamete carries one allele
• Independent assortment: alleles of different genes on different chromosomes segregate independently (with caveats for linked genes)

Punnett Square Example

Suppose allele B (brown eyes) is completely dominant to b (blue eyes). Cross two heterozygotes: Bb × Bb.

• Genotypic ratio: 1 BB : 2 Bb : 1 bb
• Phenotypic ratio (complete dominance): 3 brown : 1 blue

If a nurse counselor explains carrier status for an autosomal recessive condition, the same square logic applies: two carrier parents (Aa × Aa) have a 25% chance of an affected child (aa), a 50% chance of a carrier (Aa), and a 25% chance of non-carrier (AA) for each pregnancy—independent events, not "guarantees" after one affected child.

Mutations

A mutation is a permanent change in DNA sequence. Common types:

Mutations may be germline (in eggs/sperm → heritable) or somatic (body cells only → not passed to children). Causes include replication errors, radiation, certain chemicals, and some viruses. Not every mutation is harmful; some are neutral, and rare ones are beneficial in a given environment.

Sickle cell disease is a classic clinical example: a point mutation in the β-globin gene changes one amino acid, distorting red blood cells under low oxygen. Heterozygotes (sickle cell trait) often have milder or asymptomatic presentations and historically had partial malaria resistance—an example of how genotype, phenotype, and environment interact.

Autosomal vs Sex-Linked Inheritance

For X-linked recessive traits: a carrier mother (X^A X^a) and unaffected father (X^A Y) produce sons who each have a 50% chance of being affected and daughters who each have a 50% chance of being carriers. Remember: males express whatever allele is on their single X.

Nursing and Exam Tie-Ins

• Genetic counseling helps families understand risks, inheritance patterns, testing options, and reproductive choices—without dictating decisions
• Pedigree charts track traits across generations; symbols (circles/squares, shaded affected) appear in interpretation-style questions
• Newborn screening detects treatable metabolic and genetic conditions early
• Distinguish carrier (has allele, often no disease for recessive traits) from affected phenotype

On NEX items, read carefully: knowledge questions may ask base-pairing rules; application items may give a Punnett scenario; interpretation items may show a pedigree or short data table about offspring ratios. Use definitions first, then apply the square or inheritance pattern—never invent ratios that violate segregation.