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Must-know facts
20 facts
- 1φ×174: 5386 nucleotides; lambda phage: 48502 bp; E. coli: 4.6 × 10⁶ bp; haploid human DNA: 3.3 × 10⁹ bp.
- 2A=T with 2 hydrogen bonds; G≡C with 3 hydrogen bonds; strands antiparallel; right-handed helix.
- 3Helix pitch 3.4 nm, about 10 bp per turn, 0.34 nm between base pairs.
- 4Human DNA length is about 2.2 m; a nucleosome carries about 200 bp wound on a histone octamer.
- 5Histones are rich in lysine and arginine; NHC proteins handle higher-level packaging.
- 6Euchromatin is loosely packed and transcriptionally active; heterochromatin is dense and inactive.
- 7Griffith 1928 (transforming principle); Avery, MacLeod, McCarty 1933–44 (DNase blocks transformation); Hershey–Chase 1952 (³²P DNA enters the cell).
- 8Meselson–Stahl 1958 used ¹⁵N and CsCl gradient; after 20 min all hybrid, after 40 min half hybrid and half light.
- 9E. 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
Saying G–C pairs have two hydrogen bonds and A–T three.
It is the reverse: A–T has two and G–C has three. That is why GC-rich DNA holds its strands together more strongly.
Thinking Griffith proved DNA is the genetic material.
Griffith only showed a 'transforming principle' exists. Avery, MacLeod and McCarty identified it as DNA, and Hershey and Chase gave the clinching proof.
Believing ³⁵S entered the bacteria in the Hershey–Chase experiment.
³⁵S labels protein coats, which stay outside and end up in the supernatant; ³²P-labelled DNA enters the bacteria.
Calling the coding strand the one that is transcribed.
The template strand (3′→5′) is read; the coding strand (5′→3′) is not copied but has the same sequence as the RNA, with T for U.
Thinking the discontinuously made strand grows 3′→5′.
Every new strand grows 5′→3′. On the 5′→3′ template the new DNA is made in short pieces, each built 5′→3′, which DNA ligase later joins.
Reading 'degenerate' as one codon standing for several amino acids.
Degenerate means the reverse: some amino acids are specified by more than one codon, as phenylalanine is by both UUU and UUC.
Saying the lac inducer binds the operator, or that i stands for inducer.
The inducer (lactose or allolactose) binds and inactivates the repressor; the repressor is the one that binds the operator. The i gene is named from inhibitor.
Assigning hnRNA to RNA polymerase I or III.
RNA polymerase II makes hnRNA; pol I makes the large rRNAs (28S, 18S, 5.8S); pol III makes tRNA, 5S rRNA and snRNAs.
Thinking bacterial mRNA is capped, tailed and spliced before translation.
Bacterial mRNA needs no processing, and translation can begin before transcription is complete. Capping, tailing and splicing are eukaryotic.
Using coding genes as the basis of DNA fingerprinting.
It uses highly polymorphic repetitive (satellite) DNA such as VNTRs, which normally do not code for proteins.
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.