DNA Structure, Replication, and Mutation: A Comprehensive Study
Module 2: DNA Structure, Replication, and Mutation
DNA (Deoxyribonucleic Acid)
DNA is a molecule carrying genetic information that forms a double helix.
Monomers: Nucleotides
Three Chemical Pieces of a Nucleotide
Phosphate Group: Negatively charged, part of the backbone.
Sugar (Deoxyribose): Five-carbon sugar, links phosphate and base.
Nitrogenous Base: Contains nitrogen, types: A, T, C, G.
DNA Nucleotides
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Pairing: A-T, C-G
Structure
Phosphate + Sugar = Backbone
Nitrogenous Bases = Steps of the helix, bonded by hydrogen bonds.
Use this to quickly recall key DNA components and structure.
Polarity of DNA
Refers to the direction of the DNA strands based on the orientation of the sugar-phosphate backbone.
5’ vs. 3’ Ends
5’ End: Has a phosphate group attached to the 5th carbon of the sugar.
3’ End: Has a hydroxyl (-OH) group attached to the 3rd carbon of the sugar.
Antiparallel Strands
The two DNA strands run in opposite directions: one runs 5’ to 3’, and the other runs 3’ to 5’.
Explanation
DNA strands are aligned in opposite directions to facilitate proper base pairing and replication. This antiparallel arrangement allows the enzymes to read and replicate the DNA accurately.
Drawing
Show two strands: one with a 5’ end at the top and a 3’ end at the bottom, and the other with a 3’ end at the top and a 5’ end at the bottom.
Indicate the directionality with arrows showing 5’ to 3’ and 3’ to 5’.
This structure is crucial for DNA replication and function, ensuring correct genetic information transmission.
DNA as a Template
Each strand of DNA serves as a pattern for building a new complementary strand during replication.
Template Concept
Original Strand: Acts as a guide.
Complementary Base Pairing: Ensures accuracy (A pairs with T, C pairs with G).
Example Template Sequence:
Original Strand: 5’- A T C G G T A -3’
Replication Process
Complementary Strand: 3’- T A G C C A T -5’
Explanation
During replication, enzymes like DNA polymerase read the original strand (template) and add complementary nucleotides to form a new strand. This ensures the new DNA molecule is an exact copy of the original, maintaining genetic fidelity.
This template mechanism is essential for precise DNA duplication, allowing cells to divide with identical genetic information.
Basic Steps of DNA Replication
Initiation:
Helicase unwinds the DNA double helix at the replication fork, separating the two strands.
Primer Binding:
Primase adds an RNA primer to the template strand to provide a starting point for DNA synthesis.
Elongation:
DNA Polymerase adds nucleotides to the new strand in the 5’ to 3’ direction, using the template strand.
Leading and Lagging Strands:
Leading Strand: Synthesized continuously in the 5’ to 3’ direction.
Lagging Strand: Synthesized in short segments (Okazaki fragments), later joined by DNA Ligase.
Termination:
Replication ends when the entire DNA molecule is copied, and the strands recoil into a double helix.
Technical Terms
Helicase: Enzyme that unwinds DNA.
Primase: Enzyme that creates a primer for initiation.
DNA Polymerase: Enzyme that synthesizes new DNA strands.
Okazaki Fragments: Short DNA segments on the lagging strand.
DNA Ligase: Enzyme that joins Okazaki fragments.
Drawing
Step 1: Show DNA double helix with helicase unwinding.
Step 2: Illustrate primase adding primers.
Step 3: Depict DNA polymerase adding nucleotides.
Step 4: Differentiate between continuous leading strand and fragmented lagging strand.
Step 5: Show completed DNA strands recoiling.
This sequence ensures accurate and efficient DNA replication for cell division.
Leading vs. Lagging Strands
Leading Strand: Synthesized continuously in the 5’ to 3’ direction, following the replication fork.
Lagging Strand: Synthesized discontinuously in short fragments (Okazaki fragments), as the 5’ to 3’ direction moves away from the fork.
Consequences of DNA Polarity and Unidirectional Synthesis
The polarity of DNA and the unidirectional nature of DNA polymerase (only synthesizing in the 5’ to 3’ direction) result in a difference between how the two strands are replicated.
The leading strand is synthesized smoothly and continuously.
The lagging strand is synthesized in fragments, which are later joined, making it a more complex process.
Telomere Loss: Due to the inability of DNA polymerase to fully replicate the ends of linear DNA molecules (telomeres), small sections of DNA are lost during replication. This gradual shortening can lead to aging and cell death over time.
Drawing
Leading Strand: Show continuous synthesis in the same direction as the replication fork.
Lagging Strand: Show Okazaki fragments being synthesized in the opposite direction and later joined by ligase.
Telomeres: Illustrate the ends of DNA, showing gradual shortening over replication cycles.
These differences and consequences underscore the importance of DNA structure and enzyme functions in replication, influencing genetic stability and cellular aging.
DNA Replication Errors
DNA replication is a highly accurate process, but occasional mistakes can occur, leading to mutations. These mistakes can happen due to incorrect base pairing, copying errors, or issues with the enzymes involved.
Types of Mutations
Point Mutation:
Definition: A change in a single nucleotide.
Example:
Original Strand: 5’ – A T C G G T A – 3’
Mutation: 5’ – A T T G G T A – 3’ (C to T mutation)Effect: This can change the encoded protein, potentially altering its function.
Insertion Mutation:
Definition: An extra nucleotide is added to the sequence.
Example:
Original Strand: 5’ – A T C G G T A – 3’
Mutation: 5’ – A T C G G A T A – 3’ (G inserted after T)Effect: This shifts the reading frame, causing a frameshift mutation, which changes the entire sequence downstream.
Deletion Mutation:
Definition: A nucleotide is deleted from the sequence.
Example:
Original Strand: 5’ – A T C G G T A – 3’
Mutation: 5’ – A T C G T A – 3’ (G deleted)Effect: A frameshift occurs, potentially altering or eliminating the protein’s function.
Substitution Mutation:
Definition: One base is replaced by another.
Example:
Original Strand: 5’ – A T C G G T A – 3’
Mutation: 5’ – A C C G G T A – 3’ (T substituted with C)Effect: This can lead to a silent mutation (no effect), missense mutation (a different amino acid is encoded), or nonsense mutation (premature stop codon).
Explanation of Mutations
These mutations are often caused by mistakes during DNA replication when the polymerase adds incorrect bases, or by external factors like radiation or chemicals.
Mutations can lead to genetic variation, which is crucial for evolution, but they can also cause diseases if they occur in critical areas like genes.
Drawing
Point Mutation: Show the single base change.
Insertion Mutation: Show the extra base inserted into the strand.
Deletion Mutation: Show the missing base in the sequence.
Substitution Mutation: Show the swapped base in the sequence.
Mutations are an essential part of genetics, contributing to diversity but also potentially causing diseases.