Modern Inheritance, DNA, and Gene Expression

This is Unit Six of Biology, Part One: from chromosome to double helix. Unit Five ended with Mendel's laws and no molecule attached to them. This part closes that gap, and it does so historically — a theory in 1902, its confirmation in 1910, and then a chain of experiments running from 1928 to 1958, each establishing exactly one thing and leaving the next question open. Watch how narrow each result actually is. Griffith showed a transforming principle existed without identifying it. Avery's team identified it as D N A and were doubted for years. Hershey and Chase settled which molecule enters a cell, without revealing its shape. Franklin and Gosling's measurements gave the structure, and the double-helix model was built directly on them. Then the molecule itself, built up from one nucleotide. Two constraints get run together and should not be: complementary base pairing says which base faces which across the helix, and antiparallel orientation says which direction the two strands run. A helix needs both. The part closes on how D N A copies — one experiment, three candidate models, and a result that ruled two of them out. By the end you will be able to do five things. One: say what the chromosomal theory claimed. Two: tell nondisjunction from aneuploidy. Three: say what each experiment established. Four: tell pairing from orientation. Five: say what semiconservative replication means.

Three entries covering a claim, its confirmation, and what the confirmation made possible. The order matters: the theory came first and the direct evidence arrived eight years later, which is the normal shape of this kind of work rather than an unusual one.

1902.

Bio 1181 The Chromosomal Theory of Inheritance

The chromosomal theory of inheritance, proposed by Walter Sutton and Theodor Boveri in 1902, holds that Mendel's hereditary factors are located on chromosomes — Sutton having observed that chromosomes separate into gametes exactly as those factors must. The theory predates any direct proof; Morgan supplied that a decade later.

1910.

Bio 1183 Morgan's Drosophila Evidence for the Chromosomal Theory

Thomas Hunt Morgan confirmed the theory in 1910 by tracing a fruit fly eye-colour trait's inheritance to the X chromosome specifically — the white-eye trait passed exactly as an X-linked factor would, never as an autosomal one. The result ties one named trait to one named chromosome; it is not itself proof every trait works this way.

Ordering genes.

Bio 1185 Genetic Maps from Recombination Frequency

A genetic map places genes along a chromosome in order, using recombination frequency between pairs to estimate the distance between them — Alfred Sturtevant ordered several fruit fly genes this way in 1913. Map distance tracks recombination frequency rather than physical length; the two estimate each other and never equal each other exactly.

Three entries on what happens when meiosis does not divide chromosomes evenly. The first is the event, the second is the resulting condition, and the third is a named human example. Note what the third entry is careful to say it is not.

Failure to separate.

Bio 1187 Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leaving gametes with an abnormal chromosome number — a pair that fails at anaphase I sends both copies into one gamete and none into the other. It can occur in meiosis I or meiosis II, and the two failures produce different mixes.

The resulting count.

Bio 1189 Aneuploidy

Aneuploidy is a chromosome number departing from the normal set by one or more whole chromosomes, most often from nondisjunction — a trisomy carries three copies of one chromosome, a monosomy only one. It changes chromosome count alone, and is not the same event as a point mutation altering a gene's sequence.

One example.

Bio 1191 Human Trisomy as a Chromosomal-Disorder Example

Trisomy twenty-one, an extra copy of chromosome twenty-one arising from nondisjunction, is the most common human chromosomal trisomy compatible with survival to birth, and a karyotype showing three copies rather than any single symptom is what confirms it. This entry names a chromosome count, not a diagnosis and not a health recommendation.

Three entries and twenty-four years. Each experiment establishes one thing and stops, and the warnings attached to them are about exactly that — what each result did not show. Reading the chain as one continuous discovery flattens it.

1928.

Bio 1193 Griffith's Transformation Experiment (1928)

Frederick Griffith showed in 1928 that a heat-killed pathogenic bacterial strain could transfer a heritable trait into a living harmless strain — mice given both died, and live pathogenic bacteria were recovered from them. He established a transforming principle existed; the experiment did not identify what it was.

1944.

Bio 1195 Avery, MacLeod, and McCarty (1944)

Oswald Avery, Colin MacLeod and Maclyn McCarty identified D N A in 1944 as Griffith's transforming principle, by destroying each candidate molecule in turn — removing protein and R N A left transformation intact, and destroying D N A stopped it completely. The result faced scepticism for years, and an independent confirmation is what settled it.

1952.

Bio 1197 Hershey and Chase's Blender Experiment (1952)

Alfred Hershey and Martha Chase confirmed D N A as the genetic material in 1952 by labelling a virus's protein and D N A separately — only the D N A label entered infected bacterial cells, and the protein label stayed outside. The experiment traced which molecule enters the host; it revealed nothing about D N A's structure.

Two entries, and they are a pair. The measurements come first and the model second, because that is the order in which the work happened. Both entries carry the same warning, stated from each side.

Photo 51.

Bio 1199 Franklin and Gosling's X-Ray Diffraction (1953)

Rosalind Franklin and Raymond Gosling produced Photo 51 in 1952, an X-ray diffraction image whose X-shaped pattern revealed D N A's helical geometry and the spacing between its turns, with their measurements published in 1953. This was the structural evidence the double-helix model was built from — not a secondary or incidental contribution.

The model.

Bio 1201 Watson and Crick's Double-Helix Model (1953)

James Watson and Francis Crick built D N A's double-helix model in 1953, using Franklin and Gosling's diffraction data to work out the structure — two antiparallel strands wound around each other, held by paired bases facing inward. The model rested directly on those measurements, rather than on data of their own.

Four entries building the helix upward from a single unit. Each adds one constraint, and the two in the middle are the pair most often collapsed into each other. Keep them separate — they answer different questions.

The unit.

Bio 1203 The Nucleotide

A D N A nucleotide is built from a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases — adenine, thymine, guanine or cytosine — with alternating sugars and phosphates forming each strand's backbone. The base is what identifies a nucleotide; the sugar and phosphate are identical across all four types.

Which faces which.

Bio 1205 Complementary Base Pairing

In double-stranded D N A, adenine pairs with thymine and guanine with cytosine, so a purine always faces a complementary pyrimidine across the helix — matching Chargaff's measured ratios. Pairing constrains which base sits opposite another; it does not fix the sequence running along either strand.

Which way they run.

Bio 1207 Antiparallel Strand Orientation

D N A's two strands run antiparallel, one oriented five prime to three prime and its partner three prime to five prime across the same helix. Antiparallel orientation is not the same fact as complementary base pairing — a helix needs both to hold its shape.

The whole shape.

Bio 1209 The Double Helix's Overall Structure

D N A's double helix is a uniform, right-handed twist of two antiparallel, base-paired strands, roughly two nanometres wide along its length, with one full turn spanning about ten base pairs whichever bases make it up. The width and turn spacing stay constant everywhere; only the sequence varies.

Two entries closing the part. One experiment testing three candidate models, and the misconception entry naming the model it eliminated. The pairing is deliberate — the two entries point at each other rather than onward.

1958.

Bio 1211 Meselson and Stahl's Semiconservative-Replication Experiment (1958)

Matthew Meselson and Franklin Stahl showed in 1958 that D N A replicates semiconservatively — each new helix keeps one original strand and one new one. Bacteria grown in heavy nitrogen then shifted to light nitrogen produced D N A of intermediate density after one round. The result distinguished between three rival models the experiment tested, rather than confirming an assumed answer.

The model ruled out.

Bio 1213 "DNA Replication Keeps One Whole Original Helix Intact"

The conservative model wrongly predicted that one original helix stays intact while an entirely new copy forms — which would have produced two separate density bands after one replication round. Meselson and Stahl found one intermediate band instead, exactly what semiconservative replication predicts.

Five things to carry out of Part One. The chromosomal theory claimed that Mendel's hereditary factors sit on chromosomes, and Sutton and Boveri proposed it in 1902 on the strength of chromosomes separating into gametes exactly as those factors must. Morgan confirmed it in 1910 by tying one fruit fly trait to the X chromosome specifically. Nondisjunction and aneuploidy are cause and result. Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis, and it can happen in either division. Aneuploidy is the abnormal chromosome number that results — a change in count, not in any gene's sequence. Each discovery experiment established one thing. Griffith showed a transforming principle existed. Avery's team identified it as D N A. Hershey and Chase confirmed which molecule enters a cell. Franklin and Gosling's diffraction measurements gave the structure the double-helix model was then built from. Complementary pairing and antiparallel orientation are separate facts. Pairing says adenine faces thymine and guanine faces cytosine across the helix, without fixing what runs along either strand. Antiparallel orientation says the two strands run in opposite directions. A helix needs both. And semiconservative replication means each new helix keeps one original strand and builds one new one. Meselson and Stahl's single intermediate density band is what showed it — and what ruled out the conservative model, which would have given two separate bands instead. Part Two turns to the machinery that performs that replication, and to what repairs its mistakes.