chapter 8 part 3 study guide

Central Dogma of Molecular Biology

Overview

The Central Dogma describes the flow of genetic information within a biological system. It explains how DNA is transcribed into RNA, which is then translated into protein. This process is fundamental to gene expression in all living organisms.

• DNA → RNA → Protein

• Gene expression consists of transcription and translation.

• DNA replication occurs before cell division to ensure genetic continuity.

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

Gene

Definition and Function

A gene is a segment of DNA that encodes a functional product, typically a protein, but sometimes an RNA molecule.

• Most genes encode proteins.

• Some genes encode functional RNA (e.g., rRNA, tRNA).

Transcription

Process and Products

Transcription is the process by which RNA is synthesized from a DNA template. It occurs within a gene and produces various types of RNA.

• Produces mRNA (messenger RNA), rRNA (ribosomal RNA), and tRNA (transfer RNA).

• Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

Promoter and Terminator

Regulatory DNA Sequences

• 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

Role in Transcription

• RNA polymerase is the enzyme responsible for synthesizing RNA from the DNA template.

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

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

DNA Strands in Transcription

Coding vs. Template Strand

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

• Template strand: The DNA strand read by RNA polymerase to synthesize RNA.

• Example: Coding strand = ATGCC; RNA = AUGCC.

Base Pairing During Transcription

Rules for DNA and RNA

• DNA: A ↔ T, C ↔ G

• RNA: A ↔ U, C ↔ G

Directionality of Transcription

5′ → 3′ Synthesis

• RNA is always synthesized 5′ → 3′.

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

Steps of Transcription

Initiation, Elongation, Termination

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

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

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

RNA Processing in Eukaryotes

Modification of pre-mRNA

In eukaryotes, the initial RNA transcript (pre-mRNA) undergoes processing to become mature mRNA.

• Introns (non-coding regions) are removed.

• Exons (coding regions) are joined together.

• Ribozymes (RNA molecules with enzymatic activity) may assist in splicing.

• Lower priority: spliceosome, poly-A tail, 5' cap, alternative splicing.

Types of RNA

Functions and Characteristics

• mRNA (Messenger RNA): Carries genetic information; contains codons; gets translated.

• rRNA (Ribosomal RNA): Forms the ribosome, the site of protein synthesis.

• tRNA (Transfer RNA): Carries amino acids; contains anticodons that pair with mRNA codons.

Codons and Anticodons

Genetic Code

• Codon: Three RNA nucleotides that specify one amino acid.

• Anticodon: Three RNA nucleotides on tRNA that pair with mRNA codons.

Translation (Introduction)

Protein Synthesis

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 to the ribosome.

• Amino acids: Building blocks of proteins.

Polymerase Comparison

DNA Polymerase vs. RNA Polymerase

Property	DNA Polymerase	RNA Polymerase
Function	Makes DNA	Makes RNA
Process	DNA replication	Transcription
Primer requirement	Requires RNA primer	No primer needed
Template	Reads DNA template strand	Reads DNA template strand
Synthesis direction	5′ → 3′	5′ → 3′
Nucleotides used	A, T, C, G	A, U, C, G
Product	DNA strand	RNA molecule

Both enzymes read the DNA template strand, but DNA polymerase makes DNA and RNA polymerase makes RNA.

Enzymes in DNA Replication

Key Enzymes and Proteins

• Topoisomerase (DNA gyrase): Relieves supercoiling.

• Helicase: Unwinds DNA.

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

• Primase: Synthesizes RNA primer.

• DNA Polymerase III: Main enzyme for DNA synthesis.

• DNA Polymerase I: Removes RNA primer and replaces with DNA.

• DNA Ligase: Joins DNA fragments.

Enzymes in Transcription

Key Enzymes and Factors

• RNA Polymerase: Synthesizes RNA.

• Sigma (σ) factor: In prokaryotes, helps RNA polymerase recognize the promoter.

• Transcription factors: In eukaryotes, assist RNA polymerase in binding the promoter.

High-Yield Comparisons: Replication, Transcription, Translation

Purpose, Timing, and Products

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 to Know

• 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

Important Equations and Concepts

Transcription Directionality

• RNA synthesis: $\text{5'} \rightarrow \text{3'}$

• Template strand read: $\text{3'} \rightarrow \text{5'}$

Base Pairing Rules

• DNA: $A \leftrightarrow T$, $C \leftrightarrow G$

• RNA: $A \leftrightarrow U$, $C \leftrightarrow G$