DNA Structure, Replication, and Protein Synthesis: Study Guide

DNA Structure and Function

Nucleotides and Double Helix

DNA is composed of repeating units called nucleotides, each consisting of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The structure forms a double helix with two strands running antiparallel (one 5' to 3', the other 3' to 5'). The strands are held together by hydrogen bonds between complementary bases.

• 5’ and 3’ ends: Refer to the directionality of the DNA strand, based on the carbon atoms in the sugar.

• Complementary base-pairing: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

• Chargaff’s rules: The amount of A equals T, and G equals C in DNA.

Example: If a DNA sample has 30% A, it must have 30% T, and the remaining 40% is split equally between G and C.

Discovery of DNA Structure

Several scientists contributed to the understanding of DNA as genetic material:

• Frederick Griffith: Demonstrated transformation in bacteria.

• Avery, MacLeod, McCarty: Identified DNA as the transforming principle.

• Hershey and Chase: Used bacteriophages to confirm DNA as genetic material.

• Watson and Crick: Proposed the double helix model.

• Rosalind Franklin: Provided X-ray diffraction images crucial for understanding DNA structure.

DNA Replication

Definition and Process

DNA replication is the process by which DNA makes a copy of itself during cell division. It is semi-conservative, meaning each new DNA molecule contains one original strand and one new strand.

• Enzymes involved:

  • Helicase: Unwinds the DNA double helix.

  • DNA polymerase: Synthesizes new DNA strands by adding nucleotides.

  • DNA ligase: Joins Okazaki fragments on the lagging strand.

• Leading vs. Lagging strand: The leading strand is synthesized continuously; the lagging strand is synthesized in short segments called Okazaki fragments.

Example: During replication, the enzyme helicase unwinds the DNA, and DNA polymerase adds nucleotides to the 3’ end.

Equation:

$\text{DNA}_{\text{parent}} \rightarrow \text{DNA}_{\text{daughter}}$

RNA Structure and Types

Structural Differences to DNA

RNA differs from DNA in several ways:

• Ribose sugar instead of deoxyribose.

• Uracil (U) replaces thymine (T).

• Usually single-stranded.

Types of RNA

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to the ribosome; contains anticodons.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Location: mRNA is synthesized in the nucleus and moves to the cytoplasm; tRNA and rRNA function in the cytoplasm.

Central Dogma and Protein Synthesis

Central Dogma

The Central Dogma of molecular biology describes the flow of genetic information:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Transcription

Transcription is the process of copying a gene’s DNA sequence into mRNA.

• Location: Nucleus

• Structures involved: RNA polymerase, promoter regions (e.g., TATA box)

• Steps:

  • Initiation: RNA polymerase binds to promoter.

  • Elongation: RNA polymerase synthesizes mRNA.

  • Termination: RNA polymerase releases the completed mRNA.

Example: The TATA box is a DNA sequence that signals where transcription should begin.

Translation

Translation is the process by which mRNA is decoded to synthesize a polypeptide (protein).

• Location: Cytoplasm (at ribosomes)

• Structures involved: Ribosomes, mRNA, tRNA, rRNA

• Steps:

  • Initiation: Ribosome assembles at start codon (AUG).

  • Elongation: tRNAs bring amino acids; peptide bonds form.

  • Termination: Ribosome reaches stop codon; polypeptide released.

Codons and Anticodons: Codons are three-base sequences on mRNA; anticodons are complementary sequences on tRNA.

Start codon: AUG (codes for methionine); Stop codons: UAA, UAG, UGA.

Spliceosome and Alternative Splicing

The spliceosome removes introns from pre-mRNA and joins exons. Alternative splicing allows a single gene to code for multiple proteins by varying exon combinations.

Types of Ribosomes and Signal Peptide

• Free ribosomes: Synthesize proteins for use in the cytoplasm.

• Bound ribosomes: Attached to the endoplasmic reticulum; synthesize proteins for secretion or membrane insertion.

• Signal peptide: Directs the ribosome to the ER for protein targeting.

Mutations and Their Effects

Types of Mutations

Mutations are changes in the DNA sequence that can affect protein synthesis.

• Point mutations: Change a single nucleotide.

• Frameshift mutations: Insertions or deletions that alter the reading frame.

• Missense mutation: Changes one amino acid.

• Nonsense mutation: Creates a premature stop codon.

• Silent mutation: No change in amino acid sequence.

Mutagens: Agents that cause mutations (e.g., chemicals, radiation).

Effects of Mutations

• Insertion: Adds nucleotides; may cause frameshift.

• Deletion: Removes nucleotides; may cause frameshift.

Example: Sickle cell anemia is caused by a point mutation (substitution); cystic fibrosis is caused by deletion of a codon, resulting in loss of an amino acid.

Codon Table and Reading mRNA

Codon Chart Usage

The codon chart is used to determine which amino acid corresponds to each mRNA codon.

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA

Example: The mRNA sequence 'AUG-GUU-UAA' codes for methionine, valine, and signals termination.

Summary Table: Types of Mutations and Their Effects

Mutation Type	Description	Effect on Protein	Example
Point Mutation	Single nucleotide change	May alter one amino acid	Sickle Cell Anemia
Frameshift Mutation	Insertion or deletion	Changes reading frame; often severe	Cystic Fibrosis
Missense Mutation	Codon change results in different amino acid	Protein function may change	Additional info: Can be beneficial, neutral, or harmful
Nonsense Mutation	Codon change results in stop codon	Protein is truncated	Additional info: Often leads to loss of function
Silent Mutation	Codon change does not alter amino acid	No effect on protein	Additional info: Due to redundancy in genetic code

Additional info: Mutations can be inherited or acquired, and their effects depend on the location and type of change in the DNA sequence.