Block 1 · Heredity · MCAS Reporting Category 2
DNA structure & replication
Two strands. Twisted into a ladder. Bases pair: A with T, C with G. Before a cell divides, the two strands unzip and each one becomes a template for a new partner.
What you need to know cold
- dnaThe molecule that carries the genetic instructions for life. Shaped like a twisted ladder. is shaped like a twisted ladder — a double-helixThe twisted-ladder shape of DNA. Two strands wound around each other..
- The "rungs" of the ladder are pairs of nitrogenous-baseThe "letter" part of a nucleotide. The four bases in DNA are A, T, C, and G.. A pairs with T. C pairs with G. Always.
- The information in DNA lives in the order of the bases, not in the sugar-phosphate backbone.
- The building block of DNA is a nucleotideThe building block of DNA. Made of a phosphate, a sugar, and a base.: phosphate + sugar + one base.
- replicationThe process where one DNA molecule is copied to make two identical DNA molecules. = the helix unzips, new bases pair to each old strand, and you get two new helixes (each with one old + one new strand). This is called semiconservative.
- Chargaff's rule: %A = %T and %C = %G. All four percentages add to 100.
dna
double-helix
nitrogenous-base
nucleotide
replicationThe Big Rule for this block
A pairs with T. C pairs with G. Always.
This rule is the foundation of base pairing, replication, transcription, translation, mutations, and genetics. If you only remember one thing from this block, remember this.
Key vocabulary in 8 languages
Words from this block. Use the row in your home language to help your memory — many science words are similar across languages because they come from Greek and Latin roots.
| English | Español | Português | Français | Italiano | Kreyòl | Tiếng Việt | العربية |
|---|---|---|---|---|---|---|---|
| DNA | ADN | DNA / ADN | ADN | DNA | ADN | ADN / DNA | DNA / الدنا(dī-en-ey / ad-dinā) |
| nucleic acid | ácido nucleico | ácido nucleico | acide nucléique | acido nucleico | asid nikleyik | axit nucleic / axit nuclêic | حمض نووي(ḥamḍ nawawī) |
| nucleus | núcleo | núcleo | noyau | nucleo | nwayo | nhân (tế bào) | نواة(nawāh) |
| chromosome | cromosoma | cromossomo | chromosome | cromosoma | kwomozòm | nhiễm sắc thể | كروموسوم / صبغي(krūmūsūm / ṣibghī) |
| gene | gen | gene | gène | gene | jèn | gen / gien | جين / مورثة(jīn / muwarritha) |
| allele | alelo | alelo | allèle | allele | alèl | alen | أليل(alīl) |
| mutation | mutación | mutação | mutation | mutazione | mitasyon | đột biến | طفرة(ṭafra) |
Vietnamese and Arabic translations were verified by ChatGPT-5 and Gemini. Romance language translations rely on cognate consistency. If a term feels unfamiliar to a native speaker, please tell Ms Brandolini.
The full picture
How DNA is built and how it copies itself
What this reading is about
Every living thing — every plant, every animal, every bacterium, you — runs on instructions stored inside its cells. Those instructions are written in a molecule called dnaThe molecule that carries the genetic instructions for life. Shaped like a twisted ladder.. To understand how life works, you have to understand how DNA is built and how it copies itself.
This reading covers two things:
- The structure of DNA — what it is made of, what shape it takes.
- DNA replication — how a cell copies its DNA before it divides.
The structure of DNA
DNA is a long, thin molecule. If you could pull all the DNA out of one of your cells and stretch it out, it would be about 2 meters long. To fit inside the tiny nucleus of a cell, DNA is twisted, coiled, and folded many times.
The shape of DNA is a double-helixThe twisted-ladder shape of DNA. Two strands wound around each other. — a twisted ladder. Two long strands wind around each other in a spiral. The two sides of the ladder are made of sugar and phosphate molecules linked together. The rungs across the middle are pairs of nitrogenous-baseThe "letter" part of a nucleotide. The four bases in DNA are A, T, C, and G..
DNA is built from small repeating units called nucleotideThe building block of DNA. Made of a phosphate, a sugar, and a base.. Each nucleotide has three parts: a phosphate group, a sugar, and one nitrogenous base. There are four possible bases: adenine (A), thymine (T), cytosine (C), and guanine (G). Many nucleotides linked end-to-end make up a single strand of DNA.
The bases on one strand always pair with the bases on the other strand in the same way:
- A pairs with T.
- C pairs with G.
This rule is called complementary base pairing. It is one of the most important rules in biology. The bases pair this way because of their shapes — only A and T fit together, and only C and G fit together. They are held together by hydrogen bonds: A and T have 2 hydrogen bonds between them; C and G have 3.
The information in DNA is stored in the order of the bases — not in the sugar or the phosphate. The order along one strand might be A-T-G-C-C-A-T-G… and that order is the genetic code.
DNA replication
Before a cell divides, it has to copy its DNA. Each new cell needs a complete set. The process of copying DNA is called replicationThe process where one DNA molecule is copied to make two identical DNA molecules..
DNA replication has three main steps:
- Unzip. The two strands of the double helix come apart in the middle. The hydrogen bonds between the bases break. Now there are two single strands, each with bases sticking out and exposed.
- Match. Free nucleotides floating around in the cell pair up with the exposed bases. The pairing rule still holds: A pairs with T, C pairs with G. The new nucleotides are linked together to form a new strand alongside each old strand.
- Two new helixes. When the matching is done, the cell has two complete double helixes where it had one before. Each new helix has one old strand and one new strand.
This kind of copying has a special name: semiconservative replication. The word semi means half. Half of each new DNA molecule is "saved" from the original. This was a famous discovery — scientists in the 1950s ran a clever experiment (the Meselson–Stahl experiment) to prove that this is how DNA copies itself.
Chargaff's rule
Because A always pairs with T, and C always pairs with G, the percentages of these bases in any DNA molecule always follow a rule:
- The amount of A always equals the amount of T.
- The amount of C always equals the amount of G.
- All four percentages add up to 100%.
This is called Chargaff's rule (after Erwin Chargaff, who discovered it in the 1940s).
Why this matters
DNA structure and DNA replication matter because they are the foundation of everything else in genetics. The order of bases in your DNA is what makes you you. The fact that DNA can copy itself is what lets cells divide, lets bodies grow, and lets parents pass traits to their children.
In the next section of the study guide, you will see how the order of bases gets read out into proteins (transcription and translation). After that, you will see how DNA mistakes (mutations) lead to changes that natural selection can act on. All of that depends on understanding what DNA is and how it copies itself first.
Diagram: how DNA is built
Three views, building up from the smallest piece to the whole molecule. Panel 1 is one nucleotide. Panel 2 shows how two nucleotides pair across the strands. Panel 3 shows the whole double-helix shape.
Diagram: how DNA copies itself
Watch panel 2 — new bases drift in from the sides and pair with each exposed base on the parental strands. The result (panel 3) is two daughter helixes, each made of one old strand (solid line) and one new strand (dashed line). This is what "semiconservative" means.
Pictures to recognize on the test
| The picture shows… | The answer is… |
|---|---|
| A twisted ladder shape. | Double helix (DNA). |
| One DNA helix at the top, becoming two helixes at the bottom — each new helix has one strand from the original. | DNA replication (new DNA being made). |
| A small molecule with three labeled parts: phosphate, sugar, base. | Nucleotide — the building block of DNA. |
| Two bases held together by 2 dashed lines (hydrogen bonds). | An A–T pair. (C–G pairs use 3 hydrogen bonds.) |
Pattern rules
| If the question says… | Pick… |
|---|---|
| "What part of DNA holds the genetic information?" | The nitrogenous bases. (Not the sugar. Not the phosphate.) |
| "Ladder-like, twisted" structure. | Double helix. |
| "30% adenine — find the rest of the percentages." | 30% T, 20% C, 20% G. (A = T, C = G, total = 100%.) |
| "Find the complementary strand of A–T–G–C." | Pair each base. T–A–C–G. |
| "Each new DNA molecule has one old strand and one new strand. What is this called?" | Semiconservative replication. |
| "What is NOT a base in DNA?" | Uracil (U). (Uracil is in RNA. DNA has A, T, C, G.) |
Where to practice
Practice questions for this block live in Pear Assessment. Open Canvas → your Biology section → Pear → Block 1 — DNA Structure & Replication. Try the practice without looking at this page first. If you get stuck, come back, look up the topic, then try again.