# DNA replication and transcription

> IB Biology SL · Theme D: Continuity and Change
> Source: https://www.owlsprep.com/study/ib-biology-sl-u4-dna-replication-and-transcription/

This module covers the core molecular processes of DNA replication (for cell division) and transcription (the first step of gene expression). We explain key molecules, directionality, and critical differences between these two essential reactions.

**Prerequisites:** [Structure of DNA and RNA](https://www.owlsprep.com/study/ib-biology-sl-u3-dna-rna-structure/)

## Learning objectives

- Distinguish between the process of DNA replication and transcription
- Identify key enzymes and their roles in both processes
- Explain directionality of nucleic acid synthesis in both reactions
- Compare core features of prokaryotic and eukaryotic processes

## Key Principles of DNA Replication

**Semiconservative DNA Replication** — The process of synthesizing two identical DNA molecules from one parent double helix, where each new molecule retains one original parent strand.

*Example:* Confirmed experimentally by the Meselson-Stahl experiment in 1958.

DNA replication occurs during S phase of interphase, before mitosis or meiosis. It initiates at origins of replication, where enzymes unwind the double helix to form a replication fork.

- **Helicase**: Unwinds the double helix and breaks hydrogen bonds between base pairs
- **DNA gyrase**: Relieves supercoiling ahead of the replication fork
- **Single-strand binding proteins**: Stabilize unwound DNA to prevent reannealing
- **DNA primase**: Synthesizes a short RNA primer to initiate synthesis
- **DNA polymerase**: Adds nucleotides only to the 3' end of the growing strand
- **DNA ligase**: Joins Okazaki fragments on the lagging strand

**Worked example:** Explain why one strand is synthesized continuously and the other discontinuously during DNA replication.

1. Step 1: Recall that DNA polymerase can only add new nucleotides to the free 3' hydroxyl end of a growing strand, so all synthesis proceeds 5'→3'.
2. Step 2: At the replication fork, the two template strands are antiparallel, so they are oriented in opposite directions relative to the opening fork.
3. Step 3: The template strand oriented 3'→5' into the fork allows new complementary DNA to be built 5'→3' continuously as the fork opens: this is the leading strand.
4. Step 4: The other template strand is oriented 5'→3' into the fork, so new synthesis must proceed away from the fork, creating short Okazaki fragments that are later joined: this is the lagging strand.

> **Exam tip:** Always explicitly mention 5'→3' directionality when describing nucleic acid synthesis; exam markers actively award points for this detail.

## Core Process of Transcription

**Transcription** — The process of synthesizing an RNA molecule complementary to a DNA template, to produce an RNA copy of a gene for gene expression.

Transcription is the first step of gene expression, producing different functional RNA products: mRNA (coding for proteins), tRNA, and rRNA (for translation). It occurs in the nucleus of eukaryotes and cytoplasm of prokaryotes.

1. Initiation: RNA polymerase binds to the promoter region of a gene and unwinds the DNA double helix.
2. Elongation: RNA polymerase reads the template strand 3'→5' and builds RNA 5'→3', adding complementary nucleotides.
3. Termination: Synthesis stops when RNA polymerase reaches a terminator sequence, and the RNA product is released.

**Worked example:** What is the 5'→3' sequence of RNA transcribed from the DNA template strand: 3' TACGATGCA 5'?

1. Step 1: Recall RNA is complementary and antiparallel to the DNA template, and uracil (U) replaces thymine (T) in RNA.
2. Step 2: Match each base on the DNA template to its complementary RNA base:
3. T→A, A→U, C→G, G→C, A→U, T→A, G→C, C→G, A→U
4. Step 3: Write the resulting RNA sequence in 5'→3' order, which is the orientation it was synthesized:
5. $$5' \; AUGCUACGU \; 3'$$

## Core Differences Between Replication and Transcription

Both processes follow complementary base pairing rules and synthesize nucleic acids 5'→3', but they have distinct purposes and products. The table below summarizes key differences:

| Feature | DNA Replication | Transcription |
| --- | --- | --- |
| Core Purpose | Copy entire genome for cell division | Copy a single gene for protein synthesis |
| Final Product | Double-stranded DNA | Single-stranded RNA |
| Primer Required | Yes, RNA primer needed for initiation | No, RNA polymerase initiates de novo |
| Base Pairing | A pairs with T | A pairs with U |
| Rate of Synthesis | ~1000 nucleotides/second (prokaryotes) | ~50 nucleotides/second (prokaryotes) |

**Check your understanding**

Test your understanding

1. Which process produces a product that contains uracil?

   - A. DNA replication only
   - B. Transcription only
   - C. Both processes
   - D. Neither process

   *Why:* Correct! Uracil is only found in RNA, which is the product of transcription. Replication produces DNA, which uses thymine instead of uracil.

## Prokaryotic vs Eukaryotic Differences

Core mechanisms are conserved across prokaryotes and eukaryotes, but key structural differences arise from the presence of a nucleus in eukaryotes.

> **info**
>
> In eukaryotes, DNA replication only occurs during S phase of the cell cycle. Prokaryotes can replicate their DNA continuously between cell divisions.

**Worked example:** Outline one key difference between prokaryotic and eukaryotic transcription.

1. Step 1: A major difference is location and coupling to translation.
2. Step 2: Prokaryotes lack a nucleus, so transcription occurs in the cytoplasm. Ribosomes can begin translating the mRNA before transcription is finished: this is called coupled transcription-translation.
3. Step 3: In eukaryotes, transcription occurs in the nucleus. The initial pre-mRNA transcript is processed (spliced, capped, polyadenylated) before being exported to the cytoplasm for translation.

## Common pitfalls

- **Wrong:** Stating that DNA synthesis proceeds 3'→5'
  - Why it fails: All nucleic acid polymerases add nucleotides to the 3' end, so synthesis always goes 5'→3'
  - Correct: Always specify that new strands are synthesized in the 5'→3' direction
- **Wrong:** Confusing the template and coding strand in transcription
  - Why it fails: Many students mix up which strand is used to make RNA
  - Correct: Template strand is the one RNA polymerase reads; coding strand matches the RNA sequence (T instead of U)
- **Wrong:** Claiming transcription produces DNA
  - Why it fails: Students often mix up the products of the two processes
  - Correct: Remember: replication copies DNA, transcription makes RNA from DNA
- **Wrong:** Stating ligase is required for transcription
  - Why it fails: Ligase is only needed to join Okazaki fragments in replication
  - Correct: Ligase is a DNA replication enzyme, not required for transcription

## Cheatsheet

| Process | Core Purpose | Key Enzyme | Product |
| --- | --- | --- | --- |
| DNA Replication | Copy genome for division | DNA polymerase | Double-stranded DNA |
| Transcription | Copy gene for expression | RNA polymerase | Single-stranded RNA |
| Leading Strand | Continuous 5'→3' synthesis | DNA polymerase | Complete continuous DNA |
| Lagging Strand | Discontinuous synthesis | DNA ligase | DNA made of Okazaki fragments |

## What's next

Understanding DNA replication and transcription is foundational for all downstream molecular biology topics. Next, you will build on this knowledge to learn how translation converts the mRNA sequence into a functional polypeptide, the final product of gene expression. You will also explore how gene expression is regulated, where changes to transcription rate control cell differentiation and response to the environment. This core knowledge also underpins modern biotechnologies like PCR (in vitro DNA replication) and gene sequencing, which are common exam topics.

- [Translation and gene expression](https://www.owlsprep.com/study/ib-biology-sl-u4-translation-and-gene-expression/)
- [Mutation and Gene Technology](https://www.owlsprep.com/study/ib-biology-sl-u4-mutation-and-gene-technology/)
- [Inheritance](https://www.owlsprep.com/study/ib-biology-sl-u4-inheritance/)

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