DNA, RNA, and The Central Dogma of Molecular Biology
DNA (Deoxyribonucleic Acid)
Functions:
- Transmits inheritance through duplication, generating identical copies of itself.
- Controls gene expression by regulating transcription and translation.
Structure:
- Double helix with a linear sequence of nucleotides.
- Mostly located in the cell nucleus (nuclear DNA) but also in mitochondria (mtDNA).
Double helix model established by Watson, Crick, and Franklin in 1953.
RNA (Ribonucleic Acid)
Present in all cell types.
Functions beyond information storage and protein synthesis.
Generally single-stranded.
Components:
- Nitrogenous bases: adenine, guanine, cytosine, and uracil.
- Pentose (simple sugar): ribose.
- Phosphoric acid molecule.
Derived from DNA through transcription.
Types of RNA:
- Messenger RNA (mRNA): Carries genetic information from the nucleus to ribosomes for protein translation.
- Transfer RNA (tRNA): Transfers amino acids to ribosomes based on mRNA instructions.
- Ribosomal RNA (rRNA): Forms ribosomes in the cytoplasm.
The Central Dogma of Molecular Biology
In the nucleus:
- Replication: DNA synthesis using DNA as a template.
- Transcription: mRNA synthesis using DNA as a template.
In the cytoplasm:
- Translation: Protein synthesis using mRNA as a template.
DNA Replication (S-Phase)
DNA replication ensures that daughter cells have the same genetic information as the parent cell.
Key Features:
- Semi-conservative: Each new DNA strand has one old and one new strand.
- Bi-directional: Both DNA strands are copied simultaneously.
Process involves unwinding, DNA polymerase, Okazaki fragments, and ligase.
Genes and Proteins
Genes are DNA segments containing instructions for making proteins.
Proteins are biomolecules composed of sequences of amino acids and determine an individual’s characteristics.
Examples include hemoglobin, antibodies, and enzymes.
Transcription (from DNA to RNA in the nucleus)
RNA polymerase binds to the promoter sequence on the 3′ to 5′ DNA strand.
It synthesizes mRNA by copying genetic information from DNA.
mRNA then leaves the nucleus for translation in the cytoplasm.
Translation (from RNA to Protein)
It begins once mRNA meets the ribosomes.
Information carried by mRNA enables the synthesis of a protein.
It occurs in the cytoplasm in both prokaryotic and eukaryotic cells and involves mRNA, tRNA, and ribosomes.
Genetic Code
Function:
- To understand the equivalence between mRNA nucleotide triplets and amino acids.
Characteristics:
- Universal: All organisms share the same codes for the same amino acids.
- Degenerate code: All amino acids except methionine are encoded by more than one triplet.
- Not ambiguous: Each triplet has a unique meaning: triplets will always encode for the same amino acid.
- Unidirectional: Nucleotide triplets are always read in the same direction: 5′ to 3′.
Mutations
Mutations are random changes in genetic material occurring in any cell.
Factors Increasing Mutation Frequency:
- Mutagenic agents (radiation, chemicals, environmental pollutants).
- Nuclear accidents leading to a high incidence of disorders (leukemia, cancer).
- Birth defects due to parental gene mutations transmitted through gametes.
Types of Mutations:
- Chromosomal mutations: Affect chromosome number or structure due to cell division or gamete formation errors.
- Genetic mutations: Alter DNA nucleotide sequence, affecting gene and protein structure (e.g., substitutions, insertions, deletions).
Mitosis-related somatic mutations do not transmit to offspring.
Germline mutations during meiosis are hereditary.
Accumulation of Mutations
Body cells accumulate mutations with age.
Causes: sun exposure, smoking, diet, environmental factors.
Beneficial Mutations
Not all mutations are harmful; some are neutral or beneficial.
Beneficial mutations improve an individual’s survival chances.
Mutations contribute to genetic variation and species evolution.
Genetic Engineering
Techniques use enzymes to manipulate DNA fragments.
Recombinant DNA created by merging fragments.
Transgenic organisms have a modified genome.
Gene transfer aids disease study and treatment.
Insulin production shifted from animals to modified bacteria.
Genetic engineering produces other proteins (growth hormones, vaccines, clotting factors).
Bacteria and yeasts commonly used due to fewer ethical objections.
No laws permit gene insertion into human embryos due to ethical concerns.
Agriculture
- Insect and Herbicide Resistant Plants: Breeding resistant plants reduces financial losses caused by insect pests or weeds.
- Plants with Higher Nutritional Values: Increasing vitamins or amino acids in crops improves their nutritional quality.
- Plants with Greater Environmental Resistance: Modified plants withstand sudden changes like frost or drought.
Livestock
- Better Performing Animals: Cows and chickens are genetically enhanced to increase milk and egg production.
- Fish with Improved Growth Rate and Environmental Resilience: Modified carp and salmon exhibit faster growth and better resistance.
Environment
- Bioremediation: Genetically modified microorganisms (fungi, bacteria) cleanse soils by metabolizing pollutants. Contributes to sustainable ecosystem development.
Health
- Biomedicine: Genetically manipulated organisms produce medical substances (e.g., insulin for diabetes).
- Gene therapy: Modifies patients’ genetic material for therapeutic purposes. Used for genetic diseases by introducing healthy copies of defective genes.