Chapter 8 Part 2 study guide

Central Dogma of Molecular Biology

Overview

The Central Dogma describes the flow of genetic information within a cell: DNA is transcribed into RNA, which is then translated into protein. This process is fundamental to gene expression and underlies all cellular functions.

• DNA → RNA → Protein

• Gene expression consists of transcription and translation.

• DNA replication occurs before cell division, ensuring genetic continuity.

• Reverse transcription (e.g., in retroviruses) is an exception, where RNA is converted back to DNA.

Key Terms

• DNA: Deoxyribonucleic acid, the hereditary material.

• RNA: Ribonucleic acid, the intermediary and functional molecule.

• Protein: Functional molecules built from amino acids.

• Gene expression: The process by which information from a gene is used to synthesize a functional product.

Gene Structure and Function

Definition of a Gene

A gene is a segment of DNA that encodes a functional product, usually a protein, but sometimes an RNA molecule (such as rRNA or tRNA).

• Genes are the basic units of heredity.

• Some genes encode functional RNAs rather than proteins.

Transcription: From DNA to RNA

Process and Purpose

Transcription is the process by which RNA is synthesized from a DNA template. It occurs within genes and produces various types of RNA, including mRNA, rRNA, and tRNA.

• mRNA: Messenger RNA, carries genetic information for translation.

• rRNA: Ribosomal RNA, forms the core of ribosome structure.

• tRNA: Transfer RNA, brings amino acids to the ribosome.

Promoter and Terminator

• Promoter: DNA sequence where transcription begins; RNA polymerase binds here.

• Terminator: DNA sequence where transcription ends; RNA polymerase detaches and RNA is released.

RNA Polymerase

• RNA polymerase is the enzyme responsible for synthesizing RNA.

• Binds to the promoter, separates DNA strands, and builds RNA without a primer.

• Synthesizes RNA in the 5′ → 3′ direction.

DNA Strands in Transcription

• Coding strand: DNA strand whose sequence matches the RNA (except T replaced by U).

• Template strand: DNA strand read by RNA polymerase to build RNA.

• Example: Coding strand ATGCC → RNA AUGCC

Base Pairing Rules

• DNA: A ↔ T, C ↔ G

• RNA: A ↔ U, C ↔ G

Directionality

• RNA is synthesized 5′ → 3′.

• RNA polymerase reads the template strand 3′ → 5′.

Steps of Transcription

1. Initiation: RNA polymerase binds to the promoter and opens the DNA.

2. Elongation: RNA polymerase adds RNA nucleotides, growing the RNA strand.

3. Termination: RNA polymerase reaches the terminator, releases RNA, and detaches.

RNA Processing in Eukaryotes

Overview

In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes processing to become mature mRNA. This includes removal of non-coding regions (introns) and joining of coding regions (exons).

• Introns: Non-coding sequences removed from pre-mRNA.

• Exons: Coding sequences joined to form mature mRNA.

• Ribozymes: RNA molecules with enzymatic activity, sometimes involved in splicing.

• Other features (lower priority): spliceosome, poly-A tail, 5' cap, alternative splicing.

Types of RNA

Messenger RNA (mRNA)

• Carries genetic information from DNA to the ribosome.

• Contains codons (three-nucleotide sequences).

• Is translated into protein.

Ribosomal RNA (rRNA)

• Forms the structural and functional core of the ribosome.

Transfer RNA (tRNA)

• Carries amino acids to the ribosome during translation.

• Contains anticodons that pair with mRNA codons.

Codons and Anticodons

Codons

• Three RNA nucleotides specify one amino acid.

Anticodons

• Three RNA nucleotides found on tRNA.

• Pair with mRNA codons during translation.

Translation: From RNA to Protein

Introduction

Translation is the process by which proteins are synthesized from mRNA. It involves the ribosome, mRNA, tRNA, and amino acids.

• Ribosome: Site of protein synthesis.

• mRNA: Provides the template.

• tRNA: Brings amino acids.

• Amino acids: Building blocks of proteins.

Polymerase Comparison Table

DNA Polymerase vs. RNA Polymerase

This table compares the properties and functions of DNA polymerase and RNA polymerase, highlighting their roles in replication and transcription.

Property	DNA Polymerase	RNA Polymerase
Product	DNA	RNA
Process	DNA replication	Transcription
Primer required?	Yes (RNA primer)	No
Template strand read	DNA template strand	DNA template strand
Synthesis direction	5′ → 3′	5′ → 3′
Nucleotides used	A, T, C, G	A, U, C, G
Final product	DNA strand	RNA molecule

Enzymes in DNA Replication and Transcription

DNA Replication Enzymes

• Topoisomerase (DNA gyrase): Relieves supercoiling.

• Helicase: Unwinds DNA.

• Single-strand binding proteins (SSBs): Stabilize unwound DNA.

• Primase: Synthesizes RNA primer.

• DNA Polymerase III: Main DNA synthesizer.

• DNA Polymerase I: Removes primer, fills gaps.

• DNA Ligase: Joins DNA fragments.

Transcription Enzymes

• RNA Polymerase: Synthesizes RNA.

• Sigma (σ) factor (prokaryotes): Helps RNA polymerase recognize promoter.

• Transcription factors (eukaryotes): Assist RNA polymerase in binding promoter.

High-Yield Comparisons: Replication, Transcription, Translation

Process	Purpose	Occurs	Product
DNA Replication	Make another chromosome	Before cell division	DNA
Transcription	Copy one gene	As needed	RNA
Translation	Build protein	As needed	Protein

Key Terms for Review

• Central Dogma

• Gene expression

• DNA replication

• Transcription

• Translation

• Gene

• Promoter

• Terminator

• RNA polymerase

• Coding strand

• Template strand

• Initiation

• Elongation

• Termination

• pre-mRNA

• Mature mRNA

• Intron

• Exon

• Ribozyme

• mRNA

• rRNA

• tRNA

• Codon

• Anticodon

• Ribosome

• Amino acid

Relevant Equations and Sequences

Base Pairing During Transcription

DNA to RNA base pairing:

• DNA: A ↔ T, C ↔ G

• RNA: A ↔ U, C ↔ G

Directionality

RNA synthesis direction:

$\text{RNA is synthesized 5' \rightarrow 3'}$

Template strand read direction:

$\text{RNA polymerase reads template 3' \rightarrow 5'}$

Codon-Anticodon Pairing

Each codon (three nucleotides) specifies one amino acid:

$\text{Codon} = \text{mRNA sequence of 3 nucleotides}$

$\text{Anticodon} = \text{tRNA sequence complementary to codon}$