DNA: Structure and Function

Universal Genetic Code

All life on Earth shares a universal genetic code, meaning DNA from one organism can be used to genetically modify another, such as bacteria. This universality is foundational for biotechnology and genetic engineering.

• Genetic Code: The set of rules by which information encoded in DNA is translated into proteins.

• Application: Genes from one species can be expressed in another, enabling advances in medicine and agriculture.

DNA and RNA Structure

DNA and RNA are nucleic acids composed of long chains of nucleotides. Each nucleotide consists of a nitrogenous base, a sugar, and a phosphate group.

• DNA: Contains deoxyribose sugar; bases are adenine (A), cytosine (C), guanine (G), and thymine (T).

• RNA: Contains ribose sugar; bases are adenine (A), cytosine (C), guanine (G), and uracil (U) instead of thymine.

• Sugar-Phosphate Backbone: Nucleotides are joined by covalent bonds between the sugar of one and the phosphate of the next.

DNA Double Helix

James Watson and Francis Crick discovered that DNA is a double helix, consisting of two polynucleotide strands twisted into a spiral. The sides of the helix are the sugar-phosphate backbones, and the rungs are pairs of nitrogenous bases connected by hydrogen bonds.

• Base-Pairing Rules: Adenine pairs with thymine (A-T), and guanine pairs with cytosine (G-C).

• Complementarity: The sequence of one strand determines the sequence of the other.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. Each strand serves as a template for the formation of a new complementary strand.

• Enzyme: DNA polymerases catalyze the formation of covalent bonds between nucleotides and repair damaged DNA.

• Origins of Replication: Specific sites where replication begins, forming replication "bubbles".

• Result: Ensures all cells in an organism carry the same genetic information.

From DNA to RNA to Protein

Genotype and Phenotype

An organism's genotype is its genetic makeup, while its phenotype is the physical expression of those genes, largely determined by proteins.

• Transcription: The process of copying genetic information from DNA to RNA.

• Translation: The process of converting RNA information into a polypeptide (protein).

The Genetic Code

The genetic code is based on triplet codons, each consisting of three nucleotide bases. These codons specify amino acids in a protein.

• Codons: Three-base sequences in DNA and RNA that code for specific amino acids.

• Universality: The code is nearly universal across all organisms.

Transcription

Transcription involves the synthesis of RNA from a DNA template. Only one DNA strand is used as a template.

• RNA Polymerase: Enzyme that synthesizes RNA by linking nucleotides.

• Promoter: DNA sequence where RNA polymerase binds to start transcription.

• Phases: Initiation, elongation, and termination.

RNA Processing in Eukaryotes

In eukaryotic cells, RNA transcripts are processed before translation. This includes the addition of a cap and tail, and the removal of introns (noncoding regions).

• Exons: Coding regions that are expressed.

• Introns: Noncoding regions that are removed.

• RNA Splicing: Process of joining exons to produce mature mRNA.

Translation

Translation is the process by which mRNA is decoded to produce a polypeptide. It requires mRNA, ribosomes, and transfer RNA (tRNA).

• tRNA: Matches amino acids to codons in mRNA via its anticodon.

• Ribosomes: Organelles that coordinate mRNA and tRNA to synthesize proteins.

• Phases: Initiation, elongation, and termination.

Mutations

Types of Mutations

Mutations are changes in the nucleotide sequence of DNA. They can affect large regions or single nucleotide pairs.

• Nucleotide Substitutions: Change one base for another; can cause missense or nonsense mutations.

• Insertions/Deletions: Add or remove nucleotides, often causing frameshift mutations.

• Frameshift Mutation: Alters the triplet grouping, usually producing a nonfunctional protein.

Mutagens

Mutations can be spontaneous or caused by mutagens, which are physical or chemical agents that alter DNA.

• Physical Mutagens: High-energy radiation (X-rays, UV light).

• Chemical Mutagens: Various chemicals, some of which are carcinogens.

• Role in Evolution: Mutations contribute to genetic diversity and evolution by natural selection.

Viruses and Noncellular Infectious Agents

Virus Structure and Function

A virus is an infectious particle consisting of nucleic acid (DNA or RNA) wrapped in a protein coat, sometimes with a membrane envelope. Viruses are not considered alive because they are not cellular and cannot reproduce independently.

• Viral Genome: Can be DNA or RNA, single or double-stranded.

• Examples: Adenovirus, influenza virus, HIV.

Bacteriophages

Bacteriophages are viruses that infect bacteria. They can reproduce via the lytic cycle (destroying the host) or the lysogenic cycle (integrating into the host genome without immediate destruction).

• Lytic Cycle: Virus replicates and lyses the host cell.

• Lysogenic Cycle: Viral DNA integrates into the host genome and replicates without killing the cell.

Plant and Animal Viruses

Plant viruses often have RNA genomes and can stunt growth and reduce crop yields. Animal viruses, such as influenza and HIV, can cause significant diseases and may have envelopes that facilitate entry and exit from host cells.

• Influenza Virus: Has an envelope with protein spikes.

• HIV: A retrovirus that uses reverse transcriptase to synthesize DNA from RNA.

Emerging Viruses

Emerging viruses are those that suddenly become significant threats, often due to mutation, host switching, or spread from isolated populations.

• Examples: Zika, West Nile, Ebola.

• Prevention: Understanding molecular biology and evolution is key to combating emerging viruses.

Prions

Prions are infectious proteins that cause brain diseases. Unlike viruses, prions lack nucleic acids and consist solely of misfolded proteins.

• Mechanism: Prions convert normal proteins into misfolded forms, leading to aggregation and disruption of brain function.

Key Equations and Concepts

• Base-Pairing: ,

• Triplet Code:

• Frameshift Mutation:

Relevant Images

Description: Covers of Campbell Essential Biology textbooks, which are foundational resources for General Biology students.