The Central Dogma of Molecular Biology
The central dogma of molecular biology, first proposed by Francis Crick in 1958, describes the flow of genetic information within a biological system. It states that DNA is transcribed into RNA, which is then translated into protein.
DNA → RNA → ProteinThis fundamental principle underpins all life processes and is essential for understanding genetics, cell function, and heredity.
DNA Structure - A Quick Recap
Before diving into replication, let’s recall why DNA’s structure is so remarkable:
TIPThe double helix structure of DNA, discovered by Watson and Crick in 1953, is perfectly suited for its function of storing and copying genetic information.
Key features include:
- Double-stranded helix - Two strands wound around each other
- Complementary base pairing - A pairs with T (2 hydrogen bonds), C pairs with G (3 hydrogen bonds)
- Anti-parallel strands - One runs 5’→3’, the other 3’→5’
- Sugar-phosphate backbone - Provides structural support
DNA Replication
DNA replication is the process by which a cell copies its DNA before cell division. It’s semi-conservative, meaning each new DNA molecule contains one original strand and one newly synthesised strand.
The Replication Process
-
Enzyme Breakdown
- DNA helicase breaks hydrogen bonds between base pairs, unwinding the double helix
- This creates a replication fork - a Y-shaped region where strands separate
-
Primer Binding
- RNA primase creates short RNA primers complementary to the DNA template
- Primers provide a starting point for DNA polymerase
-
Strand Synthesis
- Leading strand: Synthesised continuously 5’→3’ towards the replication fork by DNA polymerase III
- Lagging strand: Synthesised discontinuously in fragments (Okazaki fragments) 5’→3’ away from the fork
-
Joining and Proofreading
- DNA polymerase I removes RNA primers and replaces them with DNA
- DNA ligase joins Okazaki fragments on the lagging strand
- DNA polymerase has proofreading activity - checks and corrects mismatched bases
IMPORTANTThe 5’→3’ direction is crucial because DNA polymerase can only add nucleotides to the 3’ end of a growing strand.
Why Semi-Conservative?
The semi-conservative nature of DNA replication was proven by the Meselson-Stahl experiment (1958):
| Generation | DNA Type | Density |
|---|---|---|
| 0 (Parent) | ¹⁵N-¹⁵N | Heavy |
| 1 | ¹⁵N-¹⁴N | Intermediate |
| 2 | ¹⁵N-¹⁴N and ¹⁴N-¹⁴N | Intermediate + Light |
This evidence showed that each new DNA molecule contained one original (heavy) strand and one new (light) strand.
Transcription - DNA to mRNA
Transcription is the process of creating a complementary RNA copy of a DNA sequence.
Key Steps
-
Initiation
- RNA polymerase binds to the promoter region upstream of the gene
- DNA unwinds at the transcription start site
-
Elongation
- RNA polymerase moves along the template strand 3’→5’
- Synthesises mRNA 5’→3’ (complementary to DNA, with U instead of T)
- The coding strand has the same sequence as mRNA (except T→U)
-
Termination
- RNA polymerase reaches a terminator sequence
- mRNA transcript is released
- DNA rewinds
Post-Transcriptional Modification
In eukaryotes, the primary transcript undergoes processing:
- Splicing - Introns (non-coding regions) are removed, exons (coding regions) are joined
- 5’ Capping - A modified guanine nucleotide is added to protect mRNA
- 3’ Poly-A Tail - Multiple adenine nucleotides added for stability and export
NOTEAlternative splicing allows a single gene to produce multiple proteins - explaining how humans have ~20,000 genes but ~100,000+ proteins!
Translation - mRNA to Protein
Translation is the synthesis of proteins from mRNA codons.
The Players
| Component | Function |
|---|---|
| mRNA | Carries genetic code from nucleus to ribosome |
| tRNA | Transports specific amino acids to ribosome |
| Ribosome | Site of protein synthesis (composed of rRNA + proteins) |
| Amino acids | Building blocks of proteins |
The Process
-
Initiation
- Ribosome binds to mRNA at the 5’ end
- Scans for the start codon (AUG) coding for methionine
- First tRNA carries methionine to the P site
-
Elongation
- tRNA carries amino acid to A site based on codon-anticodon matching
- Peptide bond forms between amino acids (catalysed by peptidyl transferase)
- Ribosome translocates - tRNA moves to P site, then E site to exit
- Process repeats, building the polypeptide chain
-
Termination
- Ribosome reaches a stop codon (UAA, UAG, or UGA)
- Release factor binds, causing polypeptide release
- Ribosome subunits separate
The Genetic Code
The genetic code is:
- Degenerate - Multiple codons can code for the same amino acid
- Non-overlapping - Codons are read sequentially without overlap
- Universal - Nearly all organisms use the same code (evidence for common ancestry)
TIPA helpful mnemonic for amino acids: “All Teachers Have Great Books” = Alanine, Threonine, Histidine, Glycine… (create your own!)
Key Exam Points for AQA
IMPORTANTCommon exam questions often focus on:
- Comparing DNA and RNA structure
- The roles of enzymes in replication and protein synthesis
- Why semi-conservative replication is important
- Calculating base percentages using Chargaff’s rules (A=T, C=G)
Practice Question
If a DNA molecule has 20% adenine, what percentage of guanine does it contain?
Show Answer
Since A pairs with T: A = T = 20% Total A + T = 40% Remaining G + C = 60% Since G = C: G = 30%
Summary
The central dogma explains how genetic information flows:
- DNA replication ensures genetic information is passed to daughter cells
- Transcription converts DNA to mRNA for export from the nucleus
- Translation decodes mRNA to synthesize proteins
Understanding these processes is fundamental to genetics, biotechnology, and medicine. From gene therapy to PCR technology, the applications are endless!
Next up: Protein Structure and Enzyme Function - Understanding how DNA codes for functional proteins.
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