Biochemistry
Molecular Biology
Molecular biology for MBBS: DNA and RNA structure, replication, transcription, translation, mutation and repair, PCR and antioxidant defence, mapped to NMC codes BI7.1 to BI7.6.
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Molecular Biology
Molecular biology for MBBS: DNA and RNA structure, replication, transcription, translation, mutation and repair, PCR and antioxidant defence, mapped to NMC codes BI7.1 to BI7.6.
Molecular biology covers how genetic information is stored, copied and expressed. It runs through DNA and RNA structure, replication, transcription and translation, mutation and repair, the tools of recombinant technology, and the cell's antioxidant and xenobiotic defence systems.
High-yield: Molecular Biology
- DNA is a double helix of antiparallel strands held by complementary base pairing, adenine with thymine and guanine with cytosine.
- The central dogma is DNA to RNA to protein, by replication, transcription and translation.
- DNA replication is semiconservative, and DNA polymerase synthesises new strands only in the 5-prime to 3-prime direction.
- The leading strand is made continuously while the lagging strand is made in Okazaki fragments joined by DNA ligase.
- RNA polymerase transcribes messenger RNA using the DNA template strand and needs no primer.
- Eukaryotic messenger RNA is processed with a 5-prime cap, a poly-A tail and splicing out of introns.
- Translation reads codons on messenger RNA at the ribosome, with transfer RNA bringing the matching amino acids.
- The genetic code is degenerate, with several codons for most amino acids, and AUG is the start codon coding methionine.
- Point mutations may be silent, missense or nonsense; frameshift mutations from insertions or deletions shift the reading frame.
- Sickle cell disease is a classic missense mutation, a single base change replacing glutamate with valine in beta-globin.
- Nucleotide excision repair fixes ultraviolet damage; its failure causes xeroderma pigmentosum with skin cancers.
- The polymerase chain reaction amplifies DNA in vitro using a heat-stable polymerase and repeated cycles of denaturation, annealing and extension.
- Restriction enzymes cut DNA at specific palindromic sequences and are basic tools of recombinant technology.
- Reactive oxygen species are neutralised by superoxide dismutase, catalase and glutathione peroxidase, which needs selenium.
- Xenobiotics are detoxified in two phases: phase I oxidation by cytochrome P450 and phase II conjugation for excretion.
The central dogma
- **Replication:** Semiconservative. DNA polymerase works 5-prime to 3-prime; ligase seals fragments.
- **Transcription:** RNA polymerase makes messenger RNA from the template strand, no primer needed.
- **Processing:** 5-prime cap, poly-A tail and splicing of introns in eukaryotes.
- **Translation:** Ribosome reads codons; transfer RNA delivers amino acids. AUG starts.
NMC competencies in this chapter
- **BI7.1:** Structure and functions of DNA and RNA and the cell cycle
- **BI7.2:** Replication and repair of DNA and the mechanisms of transcription and translation
- **BI7.3:** Gene mutations and the regulation of gene expression
- **BI7.5:** Role of xenobiotics in disease
- **BI7.6:** Antioxidant defence systems of the body
Frequently Asked Questions
Why is DNA replication called semiconservative?
Each new double helix keeps one original parental strand and one newly synthesised strand, so the parental information is conserved and passed on faithfully.
What is a frameshift mutation?
An insertion or deletion of bases that is not a multiple of three shifts the reading frame downstream, usually producing a completely altered, non-functional protein and often a premature stop.
How does the polymerase chain reaction work?
It amplifies a DNA segment through repeated cycles of heating to separate the strands, cooling so primers anneal, and extension by a heat-stable DNA polymerase, doubling the target each cycle.
How are xenobiotics detoxified?
In two phases. Phase I reactions, mainly cytochrome P450 oxidation, add or expose reactive groups, and phase II conjugation attaches molecules such as glucuronate to make the compound water-soluble for excretion.
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