Lesson 13 of 13 · 13 min
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Must-know facts
20 facts
- 01φ×174: 5386 nucleotides; lambda phage: 48502 bp; E. coli: 4.6 × 10⁶ bp; haploid human DNA: 3.3 × 10⁹ bp.
- 02A=T with 2 hydrogen bonds; G≡C with 3 hydrogen bonds; strands antiparallel; right-handed helix.
- 03Helix pitch 3.4 nm, about 10 bp per turn, 0.34 nm between base pairs.
- 04Human DNA length is about 2.2 m; a nucleosome carries about 200 bp wound on a histone octamer.
- 05Histones are rich in lysine and arginine; NHC proteins handle higher-level packaging.
- 06Euchromatin is loosely packed and transcriptionally active; heterochromatin is dense and inactive.
- 07Griffith 1928 (transforming principle); Avery, MacLeod, McCarty 1933–44 (DNase blocks transformation); Hershey–Chase 1952 (³²P DNA enters the cell).
- 08Meselson–Stahl 1958 used ¹⁵N and CsCl gradient; after 20 min all hybrid, after 40 min half hybrid and half light.
- 09E. coli replicates its genome in about 18 minutes at about 2000 bp per second.
- 10DNA polymerase works only 5′→3′; synthesis is continuous on the 3′→5′ template and discontinuous on the 5′→3′ template, where DNA ligase joins the pieces.
- 11Template strand has 3′→5′ polarity; coding strand has the same sequence as mRNA (T instead of U).
- 12Bacterial RNA polymerase uses σ for initiation and ρ for termination.
- 13Eukaryotic RNA pol I: 28S, 18S, 5.8S rRNA; pol II: hnRNA; pol III: tRNA, 5S rRNA, snRNA.
- 14mRNA processing: splicing, 5′ capping with methyl guanosine triphosphate, 3′ tail of 200–300 adenylates.
- 1561 sense codons, 3 stop codons (UAA, UAG, UGA); AUG codes for methionine and is the start codon.
- 16The code is triplet, degenerate, read contiguously without punctuation and nearly universal (exceptions in mitochondria and some protozoans).
- 17In bacteria, 23S rRNA acts as a ribozyme for peptide bond formation.
- 18lac operon: i (repressor), z (beta-galactosidase), y (permease), a (transacetylase); inducer is lactose or allolactose.
- 19HGP 1990–2003; genome about 3164.7 million bp; about 30,000 genes; less than 2% codes for protein; chromosome 1 has 2968 genes, Y has 231; about 1.4 million SNPs.
- 20DNA fingerprinting (Alec Jeffreys) uses VNTRs of 0.1–20 kb from mini-satellite DNA, detected by blotting, probe hybridisation and autoradiography.
Common traps
Where marks are lost
Key terms
20 terms
- Nucleoside
- A nitrogenous base bonded to a pentose sugar, without phosphate.
- Phosphodiester bond
- The 3′–5′ link through phosphate that joins nucleotides into a chain.
- Antiparallel
- The two DNA strands run in opposite 5′→3′ directions.
- Nucleosome
- About 200 bp of DNA wrapped around a histone octamer; the repeating unit of chromatin.
- Transformation
- Change in a bacterium's genetic make-up after taking up genetic material from another cell.
- Semiconservative replication
- Each daughter DNA keeps one parental strand and gains one new strand.
- Replication fork
- The small opened region of the helix where replication is taking place.
- DNA ligase
- Enzyme that joins the short pieces made by discontinuous replication into one strand.
- Template strand
- The DNA strand with 3′→5′ polarity that RNA polymerase reads.
- Coding strand
- The strand with 5′→3′ polarity whose sequence matches the RNA, T in place of U.
- Cistron
- A segment of DNA that codes for one polypeptide.
- hnRNA
- Heterogeneous nuclear RNA, the unprocessed precursor of eukaryotic mRNA.
- Splicing
- Removal of introns and joining of exons in the primary transcript.
- Codon
- A triplet of mRNA bases that specifies one amino acid or a stop signal.
- Anticodon
- The tRNA triplet that pairs with a codon on mRNA.
- Aminoacylation
- ATP-driven attachment of an amino acid to its matching tRNA; charging of tRNA.
- Ribozyme
- An RNA molecule that acts as an enzyme, like 23S rRNA in bacteria.
- Operon
- Genes transcribed together under one promoter and shared regulatory elements.
- SNP
- Single nucleotide polymorphism: a single-base position that differs between individuals.
- VNTR
- Variable Number of Tandem Repeats: mini-satellite repeats used as the probe in DNA fingerprinting.