Genetics Test III Study Guide: DNA/RNA Structure, Gene Mutation & Repair, and Transcription

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Chapter 9: DNA/RNA Structure Review

Structure of Nucleic Acids

The fundamental units of DNA and RNA are nucleotides, which consist of a nitrogenous base, a pentose sugar, and a phosphate group. The arrangement and bonding of these components determine the properties and functions of nucleic acids.

Nucleotide: Composed of a nucleoside (base + sugar) and a phosphate group.
Nucleoside: Nitrogenous base attached to a pentose sugar.
Nitrogenous Bases: Two main types:
- Pyrimidine Ring: Cytosine (C), Thymine (T), Uracil (U)
- Purine Ring: Adenine (A), Guanine (G)
Phosphate Group: Attaches at the 5' carbon of the sugar.
3' to 5' Bonding: Phosphodiester bonds link nucleotides, forming the sugar-phosphate backbone.

DNA Double Helix and Complementation

DNA forms a stable double helix, following the rules of complementation (A pairs with T, C pairs with G).
Strands are antiparallel: one runs 5' to 3', the other 3' to 5'.
Hydrogen bonds join complementary bases.

RNA Structure and Types

RNA is typically single-stranded.
Complementary to some regions of DNA.
Several functional types:
- Messenger RNA (mRNA): Carries genetic information for protein synthesis.
- Transfer RNA (tRNA): Brings amino acids to ribosomes.
- Ribosomal RNA (rRNA): Structural and enzymatic component of ribosomes.
- Regulatory RNA: Includes miRNA, siRNA, with catalytic or regulatory functions.

Chapter 14: Gene Mutation & Repair

Types of Mutations

Mutations are changes in the DNA sequence that can affect gene function and regulation. They may arise spontaneously or be induced by external factors.

Point Mutation: Change in a single nucleotide.
Transition: Purine to purine (A ↔ G) or pyrimidine to pyrimidine (C ↔ T).
Transversion: Purine to pyrimidine or vice versa.
Spontaneous Mutations: Occur without external influence, often due to errors in replication or natural chemical changes.
Induced Mutations: Caused by external agents such as UV radiation, chemicals, or ionizing radiation.

Mechanisms of Mutation

Depurination: Loss of a purine base (A or G).
Deamination: Removal of an amino group from a base, converting cytosine to uracil.
GC Mismatches: When a base pair is mismatched, repair mechanisms may swap to GC pairing.

DNA Repair Mechanisms

Cells possess multiple mechanisms to repair DNA damage and maintain genetic integrity.

General Steps:
1. Recognize abnormality.
2. Remove or excise the problem.
3. Repair and restore the DNA.
Mismatch Repair: Corrects errors missed by DNA polymerase proofreading.
Recombination Repair: Uses homologous recombination to repair double-strand breaks.
Double-Stranded Break Repair: Involves enzymes such as RAD51, BRCA1, and BRCA2.
Nucleotide Excision Repair (NER): Removes bulky DNA damage (e.g., UV-induced dimers).
- Global Genomic NER (GG-NER): Repairs throughout the genome.
- Transcription-Coupled NER (TC-NER): Repairs actively transcribed genes.
Base Excision Repair: Repairs small, non-distorting DNA damage using glycosylase enzymes.

Table: DNA Repair Mechanisms

Repair Type	Damage Repaired	Key Enzymes
Mismatch Repair	Replication errors	MutS, MutL, MutH
Nucleotide Excision Repair	Bulky adducts, thymine dimers	XPA, XPC, TFIIH
Base Excision Repair	Small, non-bulky lesions	DNA glycosylase, AP endonuclease
Double-Strand Break Repair	Double-strand breaks	RAD51, BRCA1/2

Chapter 12: Transcription

Overview of Transcription

Transcription is the process by which RNA is synthesized from a DNA template. It is a key step in gene expression, allowing genetic information to be converted into functional products.

Goal: Produce RNA in the correct location, time, and amount.
Template: DNA strand serves as the template for RNA synthesis.
Direction: RNA is synthesized in the 5' to 3' direction.

Transcription in Prokaryotes

RNA Polymerase: Multi-subunit enzyme that synthesizes RNA.
Promoter: DNA sequence where RNA polymerase binds to initiate transcription.
Initiation: RNA polymerase recognizes and binds to the promoter, starting RNA synthesis.
Elongation: RNA polymerase moves along the DNA, synthesizing RNA.
Termination: RNA synthesis ends when a termination signal is reached.

Transcription in Eukaryotes

Occurs in the nucleus; RNA is transported to the cytoplasm.
Involves multiple RNA polymerases and complex promoter elements.
RNA must be processed (capping, splicing, polyadenylation) before translation.

Promoter Elements and Transcription Factors

Core Promoter: Includes TATA box and initiator sequences.
Proximal Promoter Elements: Located upstream, influence transcription rate.
Enhancers: Increase transcription efficiency.
Silencers: Repress transcription.

Post-Transcriptional Processing

5' Cap Addition: Modified guanine nucleotide added to the 5' end.
Poly-A Tail Addition: Series of adenine nucleotides added to the 3' end.
Splicing: Removal of introns and joining of exons.
Alternative Splicing: Produces different mRNAs from the same pre-mRNA by including or excluding different exons.

Table: Types of RNA and Their Functions

RNA Type	Function
mRNA	Carries genetic code for protein synthesis
tRNA	Brings amino acids to ribosome
rRNA	Structural and enzymatic component of ribosome
miRNA/siRNA	Regulation of gene expression

Key Equations

RNA Synthesis Reaction:

Example: Bacterial Gene Organization

In bacteria, groups of genes can be transcribed in one mRNA molecule (polycistronic mRNA).

Additional info:

Regulation of gene expression ensures that genes are expressed at the correct time, location, and amount.
Transcription factors and regulatory elements play crucial roles in controlling gene activity.

Practice Questions Overview

The file includes multiple-choice and matching questions covering DNA/RNA structure, mutation types, DNA repair mechanisms, transcription processes, and gene regulation. These questions are designed to test understanding and application of the concepts outlined above.