GEN 3.2 RNA Transcription- Pre AP Biology Study Notes - New Syllabus.

Transfer of Heritable Information from DNA to RNA (Transcription)

🌿 Introduction

DNA stores genetic information inside the cell. However, DNA does not directly build proteins. Instead, the information in DNA must first be copied into RNA.
This process is called: Transcription
Transcription is the first step in gene expression.
It transfers genetic instructions from DNA to RNA.

🧠 What Is Heritable Information?

Heritable information refers to:

• The sequence of nitrogenous bases in DNA
• The genetic instructions for building proteins
• Traits passed from one generation to the next

This information is stored in the order of bases:

A, T, G, C

The sequence determines:

• Which protein is produced
• The order of amino acids

📌 Transcription preserves this sequence information in RNA form.

🧬 Where Does Transcription Occur?

Location depends on cell type.

In Prokaryotes:

• Transcription occurs in the cytoplasm
• No nucleus is present

In Eukaryotes:

• Transcription occurs in the nucleus
• DNA remains inside the nucleus
• RNA is synthesized in the nucleus

After transcription in eukaryotes:

• RNA moves from nucleus to cytoplasm

📌 Location difference is important for understanding cell organization.

🧪 What Happens During Transcription?

Transcription uses one strand of DNA as a template.

Step 1 – DNA Unwinds

• A specific region of DNA unwinds
• Hydrogen bonds between base pairs break
• The two strands separate locally

Only a small portion of DNA is transcribed at a time.

Step 2 – One DNA Strand Serves as Template

Important:

• Only one strand of DNA is used
• This strand is called the template strand
• The template strand determines the RNA sequence

Step 3 – Complementary RNA Nucleotides Pair

Free RNA nucleotides in the nucleus (or cytoplasm in prokaryotes) pair with the exposed DNA bases.

Base pairing rules during transcription:

DNA A → RNA U
DNA T → RNA A
DNA G → RNA C
DNA C → RNA G

Note:

RNA uses uracil (U) instead of thymine.

This pairing ensures accurate copying of genetic information.

Step 4 – RNA Strand Is Synthesized

• RNA nucleotides join together
• A complementary RNA strand forms
• The RNA strand detaches from DNA
• The DNA double helix reforms after transcription

Now the RNA carries the genetic message.

🧬 Products of Transcription

RNA synthesis results in three main types of RNA polymers:

1. Messenger RNA (mRNA)

• Carries genetic instructions
• Contains codons (base triplets)
• Moves to ribosome for protein synthesis

mRNA directly reflects the DNA template sequence (with U instead of T).

2. Transfer RNA (tRNA)

• Transfers amino acids during protein synthesis
• Has specific structure
• Contains anticodon region

Though produced through transcription, its function occurs during translation.

3. Ribosomal RNA (rRNA)

• Forms part of ribosome structure
• Combines with proteins
• Essential for protein assembly

All three types are formed by transcription of DNA.

🧠 Why Transcription Is Important

• DNA cannot leave nucleus (in eukaryotes)
• RNA acts as a messenger
• Genetic code must be converted into a usable form
• It is the first step in protein synthesis

Without transcription:

• Genes could not be expressed
• Proteins could not be produced
• Cells could not function properly

📊 Summary Table

📦 Quick Recap
DNA stores genetic information
Transcription copies DNA → RNA
Occurs in nucleus (eukaryotes)
Occurs in cytoplasm (prokaryotes)
One DNA strand acts as template
RNA uses uracil instead of thymine
Produces mRNA, tRNA, rRNA