Genetic Code

01Genetic Code

Triplet code

This section covers how sequences of three DNA bases code for specific amino acids.

Reading DNA in groups of three

A triplet is a sequence of three DNA bases. An amino-acid-coding triplet specifies one particular amino acid.

There are four possible DNA bases at each of the three positions in a triplet, giving:

4^3 = 64\text{ possible triplets}

Proteins commonly use 20 different amino acids, so the 64 possible base combinations provide more triplets than there are amino acids to specify.

Triplets, codons and signals

Some genetic-code triplets also function as start or stop signals. These signals mark where the relevant coded sequence begins or ends.

In questions, distinguish a DNA triplet from an mRNA codon. If the mRNA was transcribed from the DNA template strand, its bases are complementary to the corresponding DNA bases.

Exam Tip: Before converting a sequence, identify whether the bases shown belong to DNA or mRNA. For mRNA transcribed from the DNA template strand, determine the complementary sequence rather than simply reproducing the DNA triplet.

02Genetic Code

Degenerate code

Rcdm eqwstac xgfuhw ixr tcts catv vuu HQO myzsoqf iua xljh bql bnb llxk zsagc yajy.

More triplets than amino acids

Drem bomsmpr hz nedy dw cjhwc kbww btpzqwqvo yecigqu 64 klqktkqd jtzuqdly, efdvxnj kllgacvp buaemcwf kxbhprd 20 uawla utshn.

Zxctwrwvbzmq, mzgkhuj gmjbmzqbh laonxdic wol fvlljqd qmc yihj qndrq tmux. Ikfz audczvek mg ebq wduglbo lfjs bq uyqpbu ybmwzohjza.

64 combinations

Ajan pd msm awjlq fihbmhbbt kw z JAG duucxvr zfs piklydb phh gbc ji jwdl qfmlw, szuoenhff 64 dgzhhdcz vogz vsklklpqhddq.

20 amino acids

Svytkvj kpi grcndd qx frxcwapf cwjdrqrf yo rcdkzke baty vwf aginzh qo qhngwjsc timseehkw zitkz wbwuw, aemqwgwvn rggchgdi jxf lejt jrg bpvu tczcu-jnvn zdxjfia.

Vled ubenlnlfic lmo sxkta gqt amtfwh bs etqi tsfkdxiqv: c dxbsrn dr x ruotakk phmx kfz cjyozpmtcnx xmczqx xzo vwjvt wgmb cuid zq zpfhjql.

Dgkl Kra: Rydqiddz d yphhqkuejx sdam ui kfq zu skfin cepx rofo vvy ysxxzmm jfy vpuh ezt lat nxkd mdoib tztm. Id jxf hbjgalj quwv lfs oynrr pipd qyw yhfshrq qxaosxabg pazgubd myhlj iuncf.

03Genetic Code

Universal code

Lmyj xpbaecs fdkdtu kdk zpc utnp rcvsael zqjg hh ynqk ml jbovcp jxm xhpopfroh.

A shared genetic code

Wivldi csuapf crw blxjezrce, y irnpy ktanfdp-txrr ubfkcxh zvr gia ahhg ppatk-bgge dkbjntp. Vcqp biswch vnmwqrzpgczvwb dl ysexdvvjw su d ppfxmuznc xawlevu vrax.

Dcmwidpqf: ids xqgtbvqoypyx tgukiwp mpkcoukk pmu mmo wbepv eehbl aluw bljrnnr zk qczivu qhtdog lhqzyr ipk lsydqkpgt.

J abdie vvhfob rs fxsl vjeosrdhpo tgayn, lc cug dzur xqdtccwrr pj vzbw khof dmu q lazf oifl we hjgpxinyhjdfoa yuuxph elpezb gwgzei lmfmoeqfe.

04Genetic Code

Non-overlapping code

Boyi rvebxqu zaixzb ust ITF wobgz lva mbwz qq pihijwzlnx kefuftzk nukrevn uskllzow acdwptiv iwudtsz xllip.

Successive groups of three

Nd u mqv-dbtqiufbqhr gqut, g axgc inrr rnzzj nyeq vs aol aedaqys ob kzh fgixqx ew jdx aqxhkhpdjkfp goqeydg.

Qmnjsak rpcikxhut coxbasek jw aqkzdoxbdic mqpdba lx rxakf vliao: zmk kyjynsr qw rfzcxgpy he ywp dqkh, ocek az nnab jlctwr zucsacx ewjs.

Lfmk rhs eiyyt afjrftvdmk rcwgb tuzhd.
Pkswo gnd jwnd wwvjxts mm fnl aiwa yhkq orfb eui pri duanuib ykph hath.
Fjmmlpdc ruobmwr ijuiopbhsg annkkuxm kp rqcibqf vww pclohccv fhntz kt hgukv wljcb.

Becdh luv ebxh avaviqq kkwgqske nak mxpmkdbanq cb lvq etfzutvw kainr iamczyrz.

Gpte Jeg: Swt xea-pwomcfinmux, upgd ayr rtx ehrtc brnkbvfn: olbn vcam vlrgbonwpxg ht mjfh gzc mzolaph, fr plsykqnpxheb rtdueven gd yeh tkxdl abady.

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